CP33

CONTENTSPage Foreword CODE OF PRACTICE 6 Section One. Scope and general 1.1 1.2 1.3 1.4 Scope Application 7 - 7 7 7 Referenced documents Definitions Section Two. Analysis of need for protection 2.1 2.2 2.3 2.4 General Need for protection 12 - 12 18 18 Need for personal protection Need for protection of persons and equipment within buildings Section Three. Protection of buildings Scope of section Zones of protection 22 - 22 23 30 31 Methods of protection Matters to be considered when planning protection Materials Form and size of conductors Joints Fasteners Air terminations Downconductors Test links Earth terminations Earthing Electrodes - 36 38 38 39 41 42 42 45 Page 3.14 Metal in and on a structure - 46 Section Four. Protection of miscellaneous structures and property 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 Scope of section Structures with radio and television aerials Structures near trees Protection of trees Chimneys, metal guy-wires or cables Protection of mines Protection of boats Fences Miscellaneous structures Protection of houses and small buildings - - - 4.10 Section Five. Protection of structures with explosive or highly flammable contents 5.1 5.2 5.3 5.4 5.5 Scope of section General considerations Areas of application - Equipment application Specific occupancies - Section Six. Installation and maintenance practice 6.1 Work on site - - 6.2 6.3 6.4 Inspection and testing Records Maintenance - Section Seven. Protection of persons within buildings 7.1 7.2 7.3 Scope of section Need for protection - Modes of entry of lightning impulses Page General considerations for protection Protection of persons within buildings - 72 73 APPENDICES The nature of lightning and the principles of lightning protection (informative) Notes on earthing electrodes and measurement of earth impedance (informative) The calculation of lightning discharge voltages and requisite separation distances for isolation of a lightning protection system (informative) Waveshapes for assessing the susceptibility of equipment to transient overvoltages due to lightning (informative) Protection of equipment within or on structures against lightning (informative) Assessment of risk of damage to electronic equipment within or on structures due to lightning (informative) Behavioural precautions for personal safety (informative) - 77 88 104 113 118 125 164 SINGAPORE STANDARD CODE OF PRACTICE FOR LIGHTNING PROTECTION FOREWORD This Code of Practice is a revision of Singapore Standard CP 33 : 1985 and was prepared by the Technical Committee on the Code of Practice for Lightning Protection under the direction of the Electrical Industry Practice Committee. This Code is intended to give guidance on the principlesand practice that experience has shown to be important in protecting structures against damage from lightning. It examines the characteristics of the lightning phenomenon and indicates the statistical nature of the evidence on which assessments for protection are based. It also provides guidance on the need for protection to be provided for structures in general. The Committee considered methods for artificially increasingthe range of attraction of a lightning conductor but on the evidence available, was unable to make a recommendation. It is noted that none of the reference codes used in the drafting of this Code recommends the use of such methods. Guidance on protection of electronic equipment against lightning is included. It is emphasized that Appendices E and F are included for information only, and that compliance with Appendices E and F are not necessary for compliance with CP 33 as a whole unless invoked in a contract. In preparing this Code, reference was made to the following overseas publications : 1. 2. AS 1768 : 1991 BS 665'1 : 1992 Lightning protection Code of practice for protection of structures against lightning Figure D.3 is reprinted from IEEE 1100 : 1992 'IEEE recommended practice for powering and grounding sensitive electronic equipment' Copyright o 1992 by the Institute of Electrical and Electronics Engineers, Inc. The IEEE disclaims any responsibilrty or liability resulting from the placement and use in this publication. Information is reprinted with the permission of the IEEE. Acknowledgement is made for the use of information from the above publications. NOTE: 1. Singapore Standards are subject to periodical review to keep abreast of technological changes and new technical developments. The revisions of Singapore Standards are announced through the issue of either amendment slips or revised editions. Compliance with a Singapore Standard does not exempt users from legal obligations. 2. SECTION ONE SCOPEANDGENERAL 1.1 SCOPE This Code sets out guidelines for the protection of persons and property from hazards arising from exposure to lightning. The recommendations specifically cover the following applications: (a) The protection of a variety of buildings or structures, including those with explosive or highly-flammablecontents, and mines. The protection of persons, both outdoors, where they may be at risk from the direct effects of a lightning strike, and indoors, where they may be at risk indirectly as a consequence of lightning currents being conducted into the building. The protection including sensitive electronic equipment from overvoltages resulting from a lightning strike to the building or its associated services. (b) (c) The nature of lightning and the principles of lightning protection are discussed and guidance is given to assist in the determination of whether protective measures should be taken. The recommendations in this Code do not apply to the protection of large scale power or communication systems, nor do they apply to the protection of special structures such as oil and gas platforms. APPLICATION 1.2 This Code does not override any statutory requirements but may be used in conjunction with such requirements. Compliance with the recommendations of this Code will not necessarily prevent damage or personal injury due to lightning but will reduce the probability of such damage or injury occurring. REFERENCED DOCUMENTS The documents referred to in this Code are listed in the back pages of this Code. 1.3 1.4 DEFINITIONS For the purpose of this Code the definitions below apply. 1.4.1 Air Termination. A conductor or rod of a lightning protection system, positioned so as to intercept a lightning discharge, which establishes a zone of protection. 1.4.2 Air Termination Network. A network of air terminations and interconnecting conductors which forms the part of lightning protection system which is intended to intercept lightning discharges. 1.4.3 Base Conductors (Base Tapes). Conductors placed around the perimeter of a structure near ground level interconnected to a number of earth terminations to distribute the lightning currents amongst them. 1.4.4 Bond (Bonding Conductor). A conductor intended to provide electrical connection between the lightning protection system and other metalwork and between various metal parts of a structure or between earthing systems. 1.4.5 Downconductor. A conductor which connects an air termination with an earth termination. 1.4.6 Earth Impedance (2). The electrical impedance of an electrode or structure to earth, derived from the earth potential rise divided by the impulse current to earth causing that rise. It is a relatively complex function and depends on : (a) (b) (c) the resistance component (R) as measured by an earth tester; the reactance component (X), depending on the circuit path to the general body of earth; and a modifying (reducing) time-related component depending on soil ionization caused by high current and fast rise times. 1.4.7 Earth Potential Rise (EPR). The increase in electrical potential of an earth electrode or earthed structure, with respect to distant earth, caused by the discharge of current to the general body of earth through the impedance of that electrode or structure. Earthing Boss (Terminal Lug). A metal boss specially designed and welded to process plant, 1.4.8 storage tanks, or steelwork to which earthing conductors are attached by means of removable studs and nuts or bolts. 1.4.9 Earthing Conductor. The conductor by which the final connection to an earth electrode is made. 1.4.10 Earthing Electrodes (Earth Rods, Ground Rods, Earthing Tapes, Meshes etc). Those portions of the earth termination which make direct low resistance electrical contact with the earth. The resistance of the lightning protection system to the general mass of earth, as measured from a test point. 1.4.11 Earthing Resistance. 1.4.12 Earth Termination (Earth Termination Network). That part of a lightning protection system which is intended to discharge lightning currents into the general mass of the earth. All parts below the lowest test link in a downconductor are included. 1.4.13 Explosive Gas Atmosphere. A mixture of flammable gas, vapour or mist with air in atmospheric conditions in which, after ignition, combustion spreads throughout the unconsumedmixture that is between the upper and lower explosive limits. NOTE. The term refers exclusively to the danger arising from ignition. Where danger from other causes such as toxicity, asphyxiation, and radioactivity may arise, this is specifically mentioned. 1A.14 Finial. A vertical projection from a horizontal air termination system. 1.4.15 Hazardous Area. An area where an explosive atmosphere is, or may be expected to be present continuously, intermittently or due to an abnormal or transient condition (see SS 254). 1.4.16 Joint. A mechanical and electrical junction between two or more portions of a lightning protection system. 1.4.17 Lightning Flash (Lightning Discharge). An electrical discharge in the atmosphere involving one or more electrically charged regions, most commonly in a cumulonimbus cloud, taking either of the following forms: (a) Ground flash (earth discharge). A lightning flash in which at least one discharge channel reaches the ground. Cloud flash. A lightning flash in which the discharge channels do not reach the earth. (b) 1.4.18 Lightning Flash Density. The number of lightning flashes of the specified type occurring on or over unit area in unit time. This is commonly expressed as per square kilometre per year (km-'year'). The ground flash density is the number of ground flashes per unit area and per unit time, preferably expressed as a long-term average value. 1.4.19 Lightning Protection System. A system of conductors and other components used to reduce the injurious and damaging effects of lightning. 1.4.20 Lightning Strike. A term used to describe the lightning flash when the attention is centred on the effects of the flash at the attachment point, rather than on the complete lightning discharge. 1.4.21 Lightning Strike Attachment Point. The point on the ground or on a structure where the lower end of the lightning discharge channel connects with the ground or structure. 1.4.22 Lightning Stroke. complete ground flash. A term used in this Code to describe an individual current impulse in a 1.4.23 Side Flash. A discharge occurring between nearby metallic objects or from such objects to the lightning protection system or to earth. 1.4.24 Striking Distance (d,). The distance between the tip of the downward leader and the eventual strike attachment point at the moment of initiation of an upward intercepting leader. 1.4.25 Structure Or Object. Any building or construction, process plant, storage tank, tree, or similar, on or in the ground. 1.4.26 Surge Arrestor. A protective device, usually connected between any conductor of a system and earth, which limits surge voltages by diverting surge current to earth when a given voltage is exceeded. 1A.27 Test Link. A joint designed and situated so as to enable resistance or continuity measurements to be made. A calendar day during which thunder is heard at a given location. The 1.4.28 Thunderday. international definition of lightning activity is given as the number of thunderdays per year (also called 'isoceraunic level' or 'ceraunic level'). 1.4.29 Zone Of Protection. The portion of space within which an object or structure is considered to be protected by a lightning protection system against direct stroke. 1.4.30 Electronic Equipment. Communications equipment, telemetry, computer, control and instrumentation systems and power electronic installationsand similar equipment incorporatingelectronic components. 1.4.31 Equipment Transient Design Level (ETDL). The level of transients to which an equipment has been satisfactorily tested. NOTE. ETDL is sometimes known as 'immunity level'. 1.4.32 Transient Control Level (TCL). The maximum level of transient occurring in a protected system, achieved by design of protection (screening etc.) or by use of surge suppressor. 1.4.33 Self Inductance. The property of a wire or circuit which causes a back emf. to be generated when a changing current flows through it. NOTE 1. The back e.m.f. developed across the self inductance of a wire or circuit is given by: where V is the back e.m.f., in volts; is the self inductance, in henries; is the rate of change of current, in amperes per second. L di dt NOTE 2. See Appendix F 1.4.34 Mutual Inductance. The property of a circuit whereby a voltage is induced in a loop by a changing current in a separate conductor. NOTE 1. The induced voltage developed in a loop with mutual inductance of a circuit is given by: where V is the induced voltage in a loop, in volts; is the mutual inductance, in henries; is the rate of change of current in a separator conductor, in amperes per second. M di dt NOTE 2. See Appendix F 1.4.35 Transfer Inductance. The property of a circuit whereby a voltage is induced in a loop by a changing current in another circuit, some part of which is included in the loop. NOTE 1. The induced voltage developed in a loop with transfer inductance of a circuit is given by: where V Mtt is the induced voltage in a loop, in volts; is the transfer inductance, in henries; is the rate of change of current in another circuit, in amperes per second. di dt NOTE 2. See Appendix F 1.4.36 Lightning ElectromagneticPulse (LEMP). Voltages or currents induced into cables and other conductors by the radiated field from a lightning flash some distance away. NOTE. LEMP may have nuisance value to electronic systems, but rarely gives transients of high voltage or high energy. The voltage common to all conductors of a group as measured between that group at a given location and an arbitrary reference (usually earth). 1.4.37 Common Mode (CM). NOTE. Common mode is sometime known as 'longitudinal mode' 1.4.38 Differential Mode (DM). The voltage at a given location between two conductors of a group. NOTE. Differential mode is sometimes known as 'transverse mode'. 1.4.39 Local Area Network (LAN). A data communications systems supporting layers 1 and 2 of the IS0 Reference Model for Open Systems Interconnectionshaving a geographic coverage up to 1 km endto-end and possessing sufficient performance to support the aggregate data throughput required by the stations (Data Terminal Equipment) being used. 1.4.40 Let Through Voltage. The maximum peak voltage occurring within 100 as after the application of the test wave. 1.4.41 Data Line. A cable carrying information as distinct from power. NOTE. Examples of data lines are telephone lines, telemetry control and signal lines. SECTION TWO ANALYSIS OF NEED FOR PROTECTION 2.1 GENERAL Before proceeding with the detailed design of a lightning protection system, the following essential steps should be taken : (a) It should be decided whether or not the structure needs protection and, if it does what the special requirements are (see Clause 2.2 and Section 3). A close liaison should be ensured between the architect, the builder, the lightning protection system engineer and the appropriate authorities. (b) (c) 2.2 The procedures for testing, commissioning and future maintenance should be agreed. NEED FOR PROTECTION 2.2.1 General. Structures with inherent explosive risks, e.g. explosives factories, stores and dumps and fuel tanks usually need the highest possible class of lightning protection system and recommendations for protecting such structures are given in Section 5. For all other structures, the standard of protection recommended in the remainder of this Code is applicable and the only question remaining is whether protection is necessary or not. In many cases, the need for protection may be self-evident, for example: (a) (b) (c) (d) (e) Where large numbers of people congregate; Where essential public services are concerned; Where the area is one in which lightning is prevalent; Where there are very tall or isolated structures; Where there are structures of historic or cultural importance; Where there are structures containing explosive or flammable contents. (9 However, there are many cases for which it is not so easy to make a decision. In these areas, .. reference should be made to 2.2.2 to 2 2 8 where the various factors affecting the risk of being struck and the consequential effects of a strike are discussed. However, some factors cannot be assessed and these may override all other considerations. For example, a desire that there should be no avoidable risk to life or that the occupants of a building should always feel safe may decide the question in favour of protection, even though it would normally be accepted that there was no need. No guidance can be given in such matters but an assessment can be made taking account of the exposure risk (that is the risk of the structure being struck) and the following factors: (a) (b) Use to which the structure is put; Nature of its construction; (c) (d) (e) Value of its contents or consequential effects; The location of the structure; The height of the structure (in the case of the composite structures, the overall height). 2.2.2 Estimation Of Exposure Risk. The probable number of srikes to the structure per year is the product of the 'lightning flash densrty' and the 'effective collection area' of the structure. The lightning , , flash density, N is the number of flashes to ground per km2 per year. Values of N vary from place to place. In Singapore the best estimate for the average annual density can be taken to be 12.6 flashes to ground per km2 per year. The effective collection area of a structure is the area of the plan of the structure extended in all directions to take account of its height. The edge of the effective collection area is displaced from the edge of the structure by an amount equal to the height of the structure at that point. Hence, for a simple rectangular building of length L, width W and height H (in m), the collection area has length (L + 2 H) m and width (W + 2H)m with four rounded corners formed by quarter circles of radius H (in m). This gives a collection area, A, (in m2), of: The probable number of strikes to the structure per year, P, is as follows: It should first be decided whether this risk P is acceptable or whether some measure of protection is thought necessary. This is shown in Figure 2.1 I I 7 Hm - I I _ I IWm I I 1r 4 1 / Figure 2.1 Plan of collection area I I I ' Boundary of c o l l e c t ~ o na r e a Risks Associated Wih Everyday Living. To help in viewing the risk from lightning in the context of the risks associated with everyday living, Table 2.1 gives some figures based on BS 6651 : 1992. The risk of death or injury due to accidents is a condition of living and many human activities imply a judgement that the benefits outweigh the related risks. Table 2.1 is intended simply to give an appreciation of the scale of risk associated with different activities. Generally, risks greater than (1 in 1000) per year are considered unacceptable. With risks of 10" (1 in 10 000) per year, it will be normal for public money to be spent to try to eliminate the causes or mitigate the effects. Risks less than (1 in 100 000) are generally considered acceptable, although public money may still be spent on educational campaign designed to reduce those risks which are regarded as avoidable. 2.2.3 2.2.4 Suggested Acceptable Risk. On the basis of Subclause 2.2.3, the acceptable risk figure has been taken as 10" per year, i.e. 1 in 100 000 per year. Table 2.1. Comparative probability of death for an individual per year of exposure (order of magnitude only) Risk Activlty Smoking (10 cigarettes per day) All accidents Traffic accidents Leukaemia from natural causes Work in industry, drowning Poisoning Natural disasters Rock climbing for 90 s*, driving 50 miles by road* Being struck by lightning * These risks are conventionally expressed in this form rather than in terms of exposure for a year. NOTE. The source of this table is BS 6651 : 1992 2.2.5 Overall Assessment Of Risk. Having established the value of P, the probable number of strikes to the structure per year (see Subclause 2.2.2), the next step is to apply the 'weighting factors', as given in Tables 2.2 to 2.6. This is done by multiplying P by the appropriate factors to determine whether the result, the overall risk factor, exceeds the acceptable risk of Po = lw5 per year. 2.2.6 Weighting Factors. In Tables 2.2 to 2.6, the weighting factor values are given under the headings A to E and denote a relative degree of importance or risk in each case. Tables 2.2 to 2.6 are mostly self-explanatory. Table 2.4 gives the weighting factor for contents or consequential effects. The effect of the value of the contents of a structure is clear, the term 'consequential effects' is intended to cover not only material risks to goods and property but also such aspects as the disruption of essential services of all kinds, particularly in hospitals. The risk to life is generally very small but, if a building is struck, fire or panic can naturally result. All possible steps should therefore be taken to reduce these effects, especially among children, the old and the sick. For multiple use buildings, the value of weighting factor A applicable to the most severe use should be used. Table 2.2. Weighting factor A (use of structure) - Use to which structure is put Houses and other buildings of comparable size Houses and other buildings of comparable size with outside aerial Factories, workshops and laboratories Office blocks, hotels, blocks of flats and other residential buildings other than those included below Places of assembly, e.g., churches, halls, theatres, museums, exhibitions, department stores, post offices, stations, airports, and stadium structures Schools. hos~itals. children's and other homes Value of factor A Table 2.3 Weighting factor B (type of construction) Type of construction Reinforced concrete or steel frame with metallic roof Membrane structure with metallic frames Reinforced concrete or steel frame with non-metallic roof Timber or masonry with non-metallic roof Timber or masonry with metallic roof Any building with a thatched roof Value of factor B 0.4 0.8 1.o 1.4 1.7 2.0 NOTE. A structure of exposed metal which is electrically continuous down to ground level is excluded from the table as it requires no lightning protection, beyond adequate earthing arrangements. Table 2.4 Weighting factor C (contents or consequential effects) Contents or consequential effects Ordinary domestic or office buildings, factories and workshops not containing valuable or specially susceptible contents Industrial and agricultural buildings with specially susceptible* contents Power stations, gas installations, telephone exchange, radio stations Key industrial plants, ancient monuments and historic buildings, museums, art galleries or other buildings with specially valuable contents Schools, hospitals, children's and other homes, places of assembly Value of factor C * This means specially valuable plant or materials vulnerable to fire or the result of fire. Table 2.5 Weighting factor D (degree of isolation) Degree of isolation Structure located in a large area of structures or trees of the same or greater height, e.g. in a large town or forest Structure located in an area with few other structures or trees of similar height Structures completely isolated or exceeding at least twice the height of surrounding structures or trees Value of factor D 0.4 Table 2.6 Weighting factor E (type of terrain) Type of terrain I Flat land at any level ( On hillside I On hilltop I 1 Value of factor E 0.3 1.o I I I 1.3 I Interpretation Of Overall Risk Factor. The risk factor method given in this Code is intended 2.2.7 to give guidance on what can, in some cases, be a difficult problem. If the result obtained is considerably less than lu5(1 in 100 000) then, in the absence of other overriding considerations, protection does not appear necessary; if the result is greater than lo-', say for example l o 4 (1 in 10 O O, then sound reasons would be needed to support a decision not to give protection. O) When it is thought that the consequential effects will be small and that the effect of a lightning strike will most probably be merely slight damage to the fabric of the structure, it may be economic not to incur the cost of protection but to accept the risk. Even though this decision is made, it is suggested that the calculation is still worthwhile as giving some idea of the magnitude of the risk being taken. Structures are so varied that any method of assessment may lead to anomalies and those who have to decide on protection have to exercise judgement. For example, a steel framed building may be found to have a low risk factor but, as the addition of an air termination and earthing system will give greatly improved protection, the cost of providing this may be considered worthwhile. A low risk factor may result for chimneys made of brick or concrete. However, where chimneys are free-standing or where they project for more than 4.5 m above the adjoining structure, they will require protection regardless of the factor. Such chimneys are, therefore, not covered by the method of assessment. Similarly, structures containing explosives or flammable substances are subject to additional consideration (see Section 5). Results of calculations for different structures are given in Table 2.7 and a specific case is worked through in Subclause 2.2.8. NOTE. Table 2.7 should be read in conjunction with Figure 2.2. Sample Calculation Of Overall Risk Factor. A hospital is 10 m high and covers an area of 2.2.8 70 m x 12 m. The hospital is located on flat land and isolated from other structures. The construction is of brick and concrete with a non-metallic roof. To determine whether or not lightning protection is needed, the overall risk factor is calculated, , Number of flashes per km2 per year. The value for N is 12.6 flashes per km2 per year. Collection area. Using the first equation in 2.2.2 the collection area, A in m2,is given by: , A , = (70 x 12) + 2(70 x 10) + 2(12 x 10) + ( A X100) A = 840 , A , = + 1400 + 240 + 314 2794 m2 Probability of being struck. Using the second equation in 2.2.2 the probable number of strikes per year, P, is given by: P = A x N x 10.~ , , 2794 m2x 12.6 x 3.5 x 10.' approximately P P = = Applying the weighting factors. The following weighting factors apply: factor A = factor B = factor C = factor D = factor E = 1.7 1.0 1.7 2.0 0.3 = The overall multiplying factor Therefore, the overall risk factor protection is necessary. = Ax Bx Cx Dx E = 1.7 The conclusion is, therefore, that 1.7 x 3.5 x 10-2= 5.95 2.3 NEED FOR PERSONAL PROTECTION A hazard to persons exists during a thunderstorm. Each year, a number of persons are struck by lightning particularly when outdoors in an open space such as an exposed location on a golf course, or when out on the water. Other receive electric shocks attributable to lightning when indoors. In built-up areas protection is frequently provided by nearby buildings, trees, power lines or street lighting poles. Persons within a substantial structure are normally protected from direct strikes, but may be exposed to a hazard from conductive materials enteringthe structure (e.g. power, telephone, or TV antenna wires) or from conductive objects within the structure which may attain different potentials. Measures for the protection of persons within buildings or structures are set out in Section 7. Lightning strikes direct to a person or close by may cause death or serious injury. A person touching or close to an object struck by lightning may be affected by a side flash, or receive a shock due to step, touch or transferred potentials, as described in Appendix A. When moderate to loud thunder is heard, persons out of doors should avoid exposed locations and should seek shelter or protection in accordance with the guidance for personal safety provided in Appendix G, particularly if thunder follows within 15 s of a lightning flash (corresponding to a distance of less than 5 km). 2.4 NEED FOR PROTECTION OF PERSONS AND EQUIPMENT WITHIN BUILDINGS As explained in Clause 2.3, persons and equipment within buildings can be at risk from lightning currents and associated voltages which may be conducted into the building as a consequence of a lightning strike to the building or associated services. Some equipment (e.g. electronic equipment, including computers) is especially susceptible to damage from overvoltages transferred from external connections caused by lightning and such damage may occur even when the lightning strike is remote from the building, e.g. from a surge conducted into the building via the power and telecommunication cables. Measures may therefore need to be taken to protect persons and equipment within buildings and Section Seven provides further advice on this subject. The measures recommended in Section Seven can be implemented even when a lightning protection system for the building structure has not been provided. The decision as to whether to provide protection specifically directed to equipment will depend on the value placed on that equipment and on the cost and inconvenience which might result from the equipment being out of service for an extended period. The risk factor determined from Clause 2.2 will provide guidance on the likelihood of a building being subject to a lightning strike with consequent risk of damage occurring to equipment within the building. However, since damage to equipment can result from lightning strikes to adjacent properties or to power or signal lines some distance away, the index value may not be a sufficient indicator of the risk. The incidence of damage occurring to similar equipment within buildings in the vicinity may provide a better guide to the need to protect. -- Reference General arrangement :ollection area and method o f calculation (C 1 (d Ac=7X8+2(6X 7)+ng2+ + 10 (approx.) f o r areas i n black A, = 405 m 2 \ ' w- - (e 1 (f) A l l dimens i o ris are i n metres. NOTE. This figure should be used in conjunction w i t h table 27 I - - Figure 2.2 Details of structures and collection areas Table 2.7 Examples of calculations tor evaluating the need for protectlon 11 Description of structure Risk of being struck, P Weighting factors 12 Recommendation Collection area, A, Flash density, An apartment, built with reinforced concrete and brick and Is having nonmetallic roof An office building, built with reinforced concrete and is having nonmetallic roof 12.6 N9 A Use of structure (Table 2.2) 1.2 B C Type of Contents or construction consequential (Table 2.3) effects (Table 2.4) 1.o D Degree of isolation (Table 2.5) 0.4 E Type of country (Table 2.6) 0.3 Overall Overall risk multiplying factor factor (product a (products columns 5 of columns to 11) 6 to 10) Protection recommended - Protection recommended - - A school, built with reinforced concrete and brick and is having nonmetallic roof Protection recommended Table 2.7 Examples of calculations for evaluating the need for protection Ref. ir Figure 2.2 Description of structure Risk of being struck, P Weighting factors Collectior area, 4 Flash density, N , A Use of structure (Table 2.2) 0.3 B Type of constructior (Table 2.3) 1.7 C Contents or consequential ~ffects (Table 2.4) 0.3 D Degree of isolation (Table 2.5) 0.4 E Type of country (Table 2.6) Overall multiplying factor (products ?f columns 6 to 10) Overall risk factor (product of columns 5 to 11) Recommendation A two storey detached bungalow, built with reinforced concrete and brick and is having nonmetallic roof A factory, built ~ i t reinforced h concrete and steel framed encased and is having metallic roof A security guard post of 3mx3mx 3m, built with reinforced concrete and brick and is having metallic roof 405 0.3 0.02 Protection recommended Protection recommended Protection not required VOTE. The risk of being struck, P (column 5 ) , is multiplied by the product of the weighting factors (columns 6 to 10) to yield an overall risk factor (column 12). This should be :ompared with the acceptable risk (10'~) guidance on whether or not to protect. Risks less than for require protection; for do not generally require protection; risk greater than lu4 protection is recommended (see Subclause 2.2.3 to 2.2.8) isks between and SECTION THREE PROTECTION OF BUILDINGS 3.1 SCOPE OF SECTION This Section sets out recommendations for installation practices and for the selection of equipment to prevent or to minimize damage or injury which may be caused by a lightning discharge. The recommendations apply generally to the protection of buildings and structures. Recommendations for the protection of particular structures and property are given later in this Code. 3.2 ZONES OF PROTECTION 3.2.1 Basis Of Recommendations. Some parts of a structure are exposed to direct lightning strikes while other parts lie within zones of protection established by higher parts of the structure. Protection against direct lightning strikes is achieved by installing a lightning protection system in such a way that its air terminations establish zones of protection enclosing the whole structure. The recommendations that follow are based on the 'rolling sphere' technique of determining zones of protection. Using this technique a sphere of specified radius is theoretically brought up to and rolled over the total building. All sections of the building which the sphere touches are considered to be exposed to direct strokes. Sections of the building which cannot be touched by the sphere are considered protected by other sections of the building. A sphere of 45 m radius has been selected to provide a high degree of protection to conventional buildings, this being designated as 'standard protection'. A sphere of smaller radius may be used to establish zones of protection where a higher degree of protection is desired. NOTE. Asphere of 20 m radius is recommended for the protection of structures with explosive or highly flammable contents (see Subclause 5.2.2). The influence of variation in sphere radius on the protection provided is discussed in A.7. For unusual or complex building forms, the 'rolling sphere' technique may be used directly in determining both zones of protection and air termination configurations. 3.2.2 Required Protection. Air terminations should be installed on parts of the structure most likely to be struck such as the outermost edges of the roof (especially the corners of an elevated roof), at tops of towers, and on parapets, ridges and chimneys which protrude above the general roof level, in accordance with Subclause 3.9.2. The zones of protection established by air terminations on higher parts of a structure should be determined having regard to the following: (a) Air terminations which do not exceed 45 m above ground are consideredto protect lower sections of structure where these lie in the space beneath an arc of 45 m radius and where the arc passes through the highest point of the building and to the ground (see Figure 3.1 (a)). (b) Air terminations or structures in excess of 45 m are considered to protect only those lower sections of the structure which lie in the space beneath an arc of 45 m radius which is tangential both to the air termination or side of the building and to the ground, as shown in Figure 3.1 (b). For buildings in excess of 45 m, direct strikes to the side of the structure above the 45 m level may be anticipated. However, these are less probable than strikes to the top of the building and are also likely to be of a lesser magnitude. Roofs of structures and protruding parts of structures which do not lie within the zones of protection established by air terminations on higher parts of the structure should be protected by additional air terminations. Air terminations of height h above a flat roof or horizontal plane are considered to protect points on that plane up to a horizontal distance r from a horizontal air termination conductor or to a horizontal radius r from a vertical air termination rod, where r is given by: where r and h are in metres. A simple array of such vertical rods at spacing distances d metres from the nearest adjacent rods on a flat roof or horizontal plane is considered to protect the whole surface within the boundary of the array provided that d 5 r. fi. Table 3.1 and Figure 3.2 illustrate the protective zones established by air terminations on a flat non-conducting roof with a parapet on one side. 3.3 METHODS OF PROTECTION 3 3 1 Structural Steel-framed Buildings. Buildings with structural steel framing may be protected .. by the installation of metal air terminations at the high parts of the building, the air terminations being connected to the steel framing and the framing earthed in the vicinity of the foundation. A typical system is shown in Figure 3.3 (see also Subclause 3.14.1). . Table 31 Height and spacing of air termination to protect a flat roof Height of air termination h (metres) 0.5 Horizontal distance for which roof is protected r (metres) 6.7 Maximum spacing distance for array d (metres) 9.5 Protected zone 7 (a) Structures up to 45 m high 1 1 >15 m ~ 4 m high 5 Not prptected >45 r high n (b) Structures in excess of 45 m high Figure 3.1 Zone of protection established by air termination as on the higher parts of a structure Zone p r o t e c t e d by handrail a s parapet conductor 7 /Vertical air terminations .Vertical air terminat ions Horizontal air termination terminations -/ (a) Top view Handrail a s parapet Vertical air terminations -, I zones 1 A (b) Sectional front view I Note. The hatched areas shows the zones of protection established by each air termination. In the top view the zones are in the plane of the roof. Figure 3.2 Zones of protection on a flat roof Conductors on parapet wall or handrailing used as air termination Metallic roofing used as air termination Connection of air termination 7-system to downconductors Structural steel or electrically continuous steel reinforcement in column employed as downconductors ALTERNATIVELY Electrically continuous metallic cladding may be used NOTE: Parapet conductor should be connected to steel in each external column I Concrete footings employed as earth electrodes providing that satisfactory earth resistance can be achieved: otherwise separate Y : ~ . earth electrodes connected to reinforcement shall be provided Large metal window frames, metal cladding, balcony railing, vent ~ i ~ etc.s to e be bonded to bownconductor system Electrically continuous railing employed as air termination <;.<, -, I yl}~ y - -.;' < 8.: , w .," . ,./' 111 Connection to structural steel .. ically continuous steel I Example of parapet railing employed as air termination In reinforced concrete buildings 332 .. Building Without Structural Steel Frames 3 3 2 1 General. The required conditions of protection for non-metallic buildings are generally met by ... placing metal air terminations on the uppermost parts of the building or its projections, with conductors connecting the air terminations to each other and to ground. By this means a relatively small amount of metal properly positioned and distributed can be made to afford a satisfactory degree of protection and, if desired, the material may be placed so as to give minimum interference to the appearance of the building. A typical system is shown in Figure 3.4. Additional methods utilizing the individual characteristics of particular types of building construction are given in Subclauses 3.3.2.2 to 3.3.2.4, and in Figure 3.3. 3 3 2 2 Structures w t continuous metal. Structures containing continuous metal, e.g. metal within ... ih a roof, wall, floor or covering may, if the amount and arrangement of the metal is adequate in terms of the recommendations of Clauses 3.10 to 3.14, utilize such metal as part of the lightning protection system. 3 3 2 3 Metal-roofed buildings. For buildings which are roofed, or roofed and clad, with metal, it may ... be possible to dispense with some air terminations and to cater for any upper portions of the building which are susceptible to damage by earthing such metal. 3 3 2 4 Reinforced concrete buildings. The following recommendations apply to the use of steel ... reinforcement in reinforced concrete buildings as part of the lightning protection system (see also A.5.5.2): (a) General. As far as possible, the steel reinforcement should be made electrically continuous in all concrete elements having a structural purpose, e.g. columns, beams and also in non-structural concrete elements, e.g. concrete wall panels, where the element, or a part of it, if dislodged, could endanger persons below. Where steel reinforcing elements are not in physical contact with each other, lightning discharges may cause cracks in the vicinity of the gaps in reinforcement. Where insulating gaps cannot be avoided, the building should be treated in the same way as one gf non-conducting materials. Where the steel reinforcement is used as the downconductor system, an effective electrical connection should be made from the air termination system to the steel reinforcement at the top of the building. Such connections should be made, by means such as welding or clamping, to the vertical and horizontal bars in as many places as necessary to ensure a multiplicity of conductive paths for the discharge of lightning current. NOTE. Steel reinforcement which is overlapped and tied by means of wire is not considered to provide an effective electrical connection for this purpose but such joints are acceptable as part of the downconductor system where current sharing is assured. Modern reinforced concrete structures frequently involve several structural techniques including in situ reinforced concrete, prestressed reinforced concrete and precast concrete; recommendations for these are listed in Items (b), (c) and (d). Air termination network in a mesh of size 10 m by 20 m Shaded areas indicate parts of building requiring protection on exposed edges of and corners of building air terminati conductors Downconductors. spacing 30 m max. ditional downconduc tor alleviate dead end Base conductor interconnection of earth terminals desirable \ - - ~ a r t h electrodes as necessary to achieve combined maximum 10R resistance Protected zone 1 End view indicating zone of protection utilizing rolling sphere principle Figure 3.4 Typical system employing horizontal and vertical air termination network (b) In situ reinforced concrete. The metal rods in the columns of a reinforced concrete structure cast in situ are occasionally welded at splice points, thus providing definite electrical continuity. Where very tall columns are involved, a spliced connection between rods is frequently achieved by a mechanical clamping device or threaded ferrule which also provides a high degree of electrical continuity. Most frequently, however, the rods are tied together by steel .tie wire at splice points, but despite the fortuitous nature of the metallic connection, the very large number of rods and crossing points assures a subdivision of the total lightning current into a multiplicity of parallel discharge paths. Experience shows that with this splicing techniques the rods can also be readily utilized as part of the lightning protection system without thermal or mechanical damage to the structure. Particular recommendations on the size, material or number of tie wires are not given in this Code, normal building practice being relied upon to provide adequate continuity. Normal building practice also ensures the multiple conducting paths continue into the building foundations (see Note). The foundations are deep in the mass of earth and the resistivity of concrete is generally comparable with that of clay or other moderately conductive ground. Hence, except in soils of low resistivity, the resistance to ground from the foundation reinforcement is often lower than can be obtained economically with driien rods, becauseof the much greater surfacearea. Concrete foundations themselves constitute a satisfactory earth termination network but their use, as such, precludes the inclusion of base conductors. It is desirable, however, that a metallic connection to the reinforcing be installed, in a position suitable for the bonding of metallic services associated with the building. NOTE. Conductive paths may not be ensured if special building techniques are used, e.g. grouting reinforcing bars into drilled holes in concrete after it has set, using an insulating epoxy-based material. (c) Prestressed reinforced concrete. Prestressed reinforced concrete is used most commonly in the horizontal structural elements in a building, such as the beams and floor slabs, and only rarely in vertical elements such as columns. Consequently, the principal reason for avoiding insulating gaps in prestressed concrete relates to side flashing rather than to the ability of the reinforcement to carry a lightning discharge to ground. Details of the treatment of prestressed concrete in order to avoid side flashing are given Clause 3.14, and the principles described in that clause should be used in the rare instance where vertical prestressed elements, such as prestressed columns, occur in a building. Although prestressed concrete affords a large reduction in the cross-sectional area of steel reinforcement compared with conventionally reinforced concrete, calculations indicate that prestressed cables of 10 mm diameter or larger, will not be damaged thermally by lightning and that thermal effects become negligible when several cables are connected in parallel. (d) Precast concrete. Where electrical continuity is required through precast concrete elements, the structural connection details, e.g. attachment plates, threaded ferrules, bolt or dowel connections, should be carefully examined from an electrical continuity standpoint. In most cases, the attachment device will be a metallic one and continuity can be achieved by simply welding the attachment device to electrically continuous reinforcement within the precast concrete element. 3.3.3 Structures With Flammable Or Explosive Atmosphere. Structures in which very small induced sparks present an appreciable element of danger, such as structures which contain explosive atmospheres of flammable vapour or gas and structures in which easily ignitable fibres or materials producing combustible flyings are stored, e.g. cotton, grain or explosives, usually required much more than the standard protection. Such structures can be protected by tall conducting masts earthed at the bottom end, by bonding as detailed in Subclause 3.14.2.2 or by overhead earthed wires (for further details see Section 5). 3.4 MATERS TO BE CONSIDERED WHEN PLANNING PROTECTION 3.4.1 Structures To Be Erected. For structures that are to be erected, the matter of lightning protection should be considered in the planning stage, as the necessary measures can often be effected in the architectural features without detracting from the appearance of the building. In addition to the aesthetic consideration, it is usually less expensive to install lightning protectionduring constructionthan afterwards. 3.4.2 Design Consideration 3.4.2.1 General considerations. The structure or, if the structure has not been built, the drawings should be examined with due regard to all the relevant details of this Code and in particular to the Metal used in the roof, walls, framework or reinforcement above or below ground, e.g. sheet piling, to determine the suitability of such metal in place of, or for use as, components of the lightning protection system. For a non-metallic roof, the position of any conduit, piping, water mains or other earthed metal NOTE. immediately beneath the roof should be noted, as this may inadvertently attract a discharge if not shielded by an adjacent roof or structure, or downconductor on or above the roof. Available positions for downconductors providing the required number of low impedance paths from the air termination network to the earth termination; this is particularly important for internal downconductors. The nature and resistivity of the soil as revealed by trial bore holes for foundation purposes or soil resistivity tests with, where economically practicable, the driving and testing of a trial earth rod electrode with the object of designing a suitable earth termination. Services entering the structure above and below ground. Radio and television receiving aerials. Flag masts, roof level plant rooms, e.g. lift motor rooms, ventilating plant and boiler rooms, water tanks and other salient features. The construction of roofs to determine methods of fixing conductors with special regard to maintaining weatherproofing of the structure. Possible penetrationof waterproofing membranewhere earth terminations are to be sited beneath the structure. The provision of hdes through, or fixing to, reinforced concrete. The provision of bonding connections to steel frame, reinforcement rods or internal metalwork, and for any holes through the structure, parapets, cornices, and the like, to allow for the free passage of the lightning conductor. The choice of metal most suitable for the conductor, e.g. aluminium conductors for structures where aluminium is employed externally. Accessibility of test joints; protection by non-metallic casing from mechanical damage or pilferage and hazard to persons; lowering of flagmasts or other removable objects; facilities for periodic inspection, especially, on tall chimneys. The preparation of an outline drawing incorporating the foregoing details and showing the positions of the main components to form a basis for the record drawing recommended in Clause 6.4. Requirements for the coordination of the structure's lightning protection earthing and the earthing of power and communication services. 3.4.2.2 Route for conductors. Conductors should be installed with a view to offering the least impedance to the passage of discharge current between the air terminals and ground. The most direct path is the best (see Subclause 3.10.2). The impedance to earth is approximately inversely proportional to the number of widely separated paths, so that from each air terminal there should be as many paths to earth as practicable. The number of paths is increased and the impedance is decreased by connecting the conductors to form a cage enclosing the building. 3.4.2.3 Trouble-free installation. Since a lightning conductor system as a general rule, is expected to remain in working condition for long periods with little attention, the mechanical construction should be strong, and the materials used should offer resistance to corrosion. 3.4.2.4 Economy of installation. Economy of installation can be effected by keeping the variety of equipment to a minimum, avoiding the use of unusual air terminal ornaments and similar features, and taking advantage of constructional features of the building as far as practicable. 3.5 MATERIALS 3.5.1. General. Copper is recommended for its conductivii and durability; however, alternative materials may be used if suitable for the environment in which they are installed and are otherwise satisfactory for the purpose (see Clause 3.6). Typical materials from which the components parts of lightning protection systems may be chosen are given in Table 3.2 (see also Subclause 3.5.2). Where insulating coatings are used, due regard should be given to their durability and flammability. For the protection of conductors at the tops of chimneys, see Subclause 3.5.2.2(a). 3.5.2. Corrosion 3.5.2.1 Basic considerations. The materials used in lightning protection systems should be resistant to corrosion resulting from the environment in which they are installed. This includes the effects of atmospheric, soil or water-borne electrolytes or contaminants, and of contact with those metals or alloys which will lead to galvanic corrosion in the presence of moisture. CP33 : 1996 Corrosion resulting from contact of dissimilar metals can exist where a conductor is held by fixing devices on or against external metal surfaces of a building or structure. Corrosion of this nature can also arise where water passes over a relatively cathodic metal such as copper carrying small amounts of copper corrosion product which is deposited as a fine film of metallic copper on relatively anodic metals such as aluminium, zinc or steel. This causes destructive galvanic corrosion of the latter metals which are commonly used in building cladding or roofing. The metallic components of the lightning protection system should therefore be compatible with the metals used externally on the structure over which these components pass or with which they may make contact. The components of lightning protection systems may be constructed from a variety of materials as described in Subclauses 3.5.2.2and 3.5.2.3. 3.5.2.2 Air terminations and downconductors. Specific recommendations for air terminations and downconductors are given in Clauses 3 9 and 3.10 respectively. Account should be taken of the . principles outlined in Subclause 3 5 2 1 in the selection of materials for those components. ... Where there is a risk of metallic building elements being contaminated by corrosion products, e.g. from copper conductors, the use of insulated conductors should be considered. Such insulation may need protection against ultraviolet radiation, e.g. by enclosure in conduit or by the application of appropriate paints or coatings. Where insulated cables are used as downconductors, bonding should be effected at the specified intervals and bonding connections should be sealed against the ingress of moisture. Where structural steel or reinforcing bars form part of the downconductor system no further corrosion protection will normally be required. With the common conductor materials, several specific precautions are necessary as follows: (a) Bare copper. Copper should be of the grade ordinarily used for commercial electrical work. NOTE. Where any part of a copper protective system is exposed to the direct action of chimney gases or other corrosive gases, it should be protected by a continuous coating of tin, lead or other material suitable for the environment to which it is exposed. Such a coating should extend at least 500 mm below the top of the chimney. The coating should not be removed at joints. (b) Bare alloys. Alloys of metals should be substantially as resistant to corrosion as copper under similar conditions. Galvanised iron may be used as part or the whole of the downconductor system provided it has adequate current-carrying capacity and is fastened with fiings having compatible corrosion characteristics. The galvanized iron may comprise the structural or decorative elements of the building subject to these requirements. Bare aluminium or aluminium alloys. Care should be taken not to use aluminium in contact with concrete, mortar, the ground, or in other situations where moisture may be retained causing the aluminium to deteriorate. Precautions should be observed at connections with dissimilar metals. In aluminium lightning protection systems, copper, copper-covered and copper alloy fixtures and fiings should not be used. Aluminium or aluminium alloy fixtures and fiings or non-metallic components of adequate strength and durability are required. Special arrangements will be needed at the ground termination for this class of system. Other materials may be used to the extent recommended elsewhere in this Code. (c) CP33 : 19% Table 3.2 Recommended materials for the manufacture of lightning protection components Materials and processes Ingots for cast components Leaded gunmetal Aluminium silicon bronze Aluminium alloy Cast iron Malleable iron Forgings and stampings (hot or cold formed) Copper Naval brass Aluminium Pressings and fabrication (from strip, coil, foil and sheet) Annealed copper Aluminium Aluminium Naval brass Stainless steel Steel (for galvanizing) Steel (for galvanizing) Bars, rods and tubes (for machined components and fiings) Hard drawn copper Annealed copper Copper silicon Phosphor bronze Aluminium bronze Aluminium Naval brass Aluminium Steel (general use) Steel (for galvanizing) Stainless steel (general use) Stainless steel (austenitic) BS 2874 BS 2874 BS 2874 BS 2874 BS 2875 : Part 3 BS 1471 BS 2874 BS 1474 BS 970 : Part 1 BS 970 : Part 1 BS 970 : Part 1 BS 970 : Part 1 ClOl , C102, C103 C101. C102, C103 cs101 PB102-M CAI02 6082TF cz112 6082TF All grades BS 2870 BS 1474 BS 1470 BS 2870 BS 1449 : Part 2 BS EN 10025 BS 1449 : Part 1 BS 2872 BS 2872 BS 1474 BS 970 : Part 1 C101, C102, C103 CZl12 6082TF All grades Grade or type I Table 3.2. Recommended materials for the manufacture of lightning protection components (Cont'd) Materials and processes I Grade or type Nuts, bolts, washers, screws and rivet fixings for use on copper Phosphor bronze Naval brass Copper silicon Nuts, bolts, washers, screws and rivet fixings for use on aluminium I Aluminium alloy Stainless steel Galvanized steel galvanized to BS 729 BS 1473 BS 3111 : Part 2 BS 3111 : Part 1 1 Solid rounds, flats and stnnded conductors Copper Annealed copper Hard drawn copper Copper (stranded) Copper (flexible) Hard drawn copper strand and copper cadmium Aluminium Aluminium strip/rod Aluminium strip/rod Aluminium Aluminium (steel reinforced) Aluminium alloy Aluminium Steel Galvanized steel Galvanized strip (See note 1) NOTES C101, C103 c101, C102, C103 Insulated BS 2897 BS 2898 BS 3988 BS 215 : Part 2 BS 3242 BS 215 : Part 1 BS 302 : Part 2 BS 1449 : Part 1 I 1. The recommended finish is galvanizing in accordance with BS 729, which has to be done after manufacture or fabrication. Stainless steel in contact with aluminum alloys is likely to cause additional corrosion to the latter materials (see PD 6484). In these cases, it is important to take protective measures, e.g. the use of inhibiters. This table applies to finials, adornments and projections such as crosses or weather-vanes on churches which are used as part of the air termination network. Materials/alloys other than those listed above may be used provided they have a minimum content of 75% copper and exhibit similar tensile and corrosion resistance properties. 2. I 3 . 4. 3.5.2.3 The earth electrode system. The design of the earth electrode system should assume that the earth electrode will be bonded, directly or fortuitously, to the following: (a) (b) (c) (d) (e) The earthed neutral of the electricity supply (see SS CP 5). The building structural steelwork or reinforcing material. The communication service earth(s), if any. The water supply pipes, if metallic. Pipelines for gaseous or liquid fuels, if metallic. Some supply authorities attempt to isolate services (d) and (e) from (a), for galvanic corrosion control reasons, by inserting insulating spacers at the pipe entry. Consideration should be given to the fitting of surge arrestors across the insulating spacers, in consultation with the supply authority, to prevent arc discharge without prejudicing the corrosion control measures. The earth electrode system should be capable of satisfactory performance for the expected life of the lightning protection system under the conditions existing at the site when bonded to: (a) (b) (c) (d) copper-based earthing systems (in most electrical installations); steel-based structural material; communication service earths which may be stainless steel, galvanized iron, copper or lead; and other metallic services, e.g. steel or copper pipes for water or gas. There are two hazards which arise from the bonding of other electrodes or service lines to the earthed neutral of the electrical supply. Firstly, if the earthing system of the electrical supply is copperbased (as is mostly ?hecase) it will cause progressive galvanic destruction of less cathodic metals such as steel, to which it is connected. Secondly, the electricity supply has many loads connected to it that generate a direct current component; this direct current is an electrolytic hazard to other earthing systems to which the supply system earth is bonded. The amount of direct current which can be generated by each appliance is limited by AS 3100 and NZS 6200, but it is still sufficient to place at risk some types of electrodes. In particular, steel rods clad with copper or stainless steel suffer premature failure when this small amount of direct current perforates the cladding, initiating a process of selfdestruction of the rod core. It will be clear that the selection of any metal or alloy for the earth electrode system, places either itself or other systems or services at some risk from galvanic corrosion. For lower-cost installationsthe use of one of the common metals or alloys may be satisfactory. A list of these, with comments relating to their corrosion performance, is provided in Table 3.3. The extent to which the material combination 'can be damaging' is related to soil moisture, the type and nature of electrolytes present, and area and resistance relationships. Inherently, if such materials are used, a maintenance checking routine is essential (see B.9). Where soil conditions are particularly aggressive from a corrosion viewpoint (soil resistivity typically below 30 am, especially if combined with pH value of less than 5 4 , such as may exist in reclaimed marine areas, the use of an inert anode material may be necessary. Where such soil conditions exist, the selection of an appropriate earth electrode system should be sought. Table 3.3 Corrosion performance of metals and alloys used as earthing electrodes - - Deleterious effect of this metal/alloy on other bonded underground ferrous metals Galvanized iron or steel Solid copper Copper-clad Solid stainless steel or nickel iron alloy Stainless-steel-clad steel Bronze Brass Zinc Aluminium Magnesium Nil Deleterious effect on this metal/alloy from bonding to earthed neutral (copper-based) systems Damaging ! I ! I Damaging Damaging Generally acceptable ! I ! I Nil Can be damaging may be acceptable Can be damaging may be acceptable I Generally acceptable Generally acceptable Can be damaging Nil Nil Nil I Can be damaging May be acceptable May be acceptable Can be dezincified Damaging Extremely damaging Extremely damaging 3.6 FORM AND SIZE OF CONDUCTORS 3.6.1 Factors Influencing Selection. The form and size of the conductors of the lightning protection system should be selected having regard to their: (a) (b) electrical and thermal characteristics and; mechanical strength, if required, and the likelihood of corrosion. Dimensions of current carrying components of lightning protection systems are given in Tables 3.4 and 3.5. 3.6.2 Mechanical Strength And Corrosion Consideration. Conductors of larger cross-section than those recommended in Table 3.4 may be needed where: (a) (b) a significant reduction of cross-section area is likely to be experienced in service due to the effects of corrosion: or an increase in cross-sectional area of different shape (e.g. tubular instead of solid) is required to provide adequate mechanical strength, e.g. for air terminations (see Subclause 3.9.1). Consideration should also be given to the use of a larger cross-sectional area than that recommended in Table 3.4 in situation where inspection or repair of the conductor is unusually difficult. Table 3.4 Minimum dimensions of components parts Component Dimensions Area (mm2) Air terminations Aluminium and copper strip Aluminium, aluminium alloy, copper and phosphor bronze rods Stranded aluminium conductors Stranded copper conductors Down conductors Aluminium and copper strip Aluminium, aluminium alloy and copper rods Stranded aluminium conductor Stranded copper conductors Earth terminations Hard-drawn copper rods for direct driving into soft ground Hard-drawn or annealed copper rods for indirect driving or laying in ground Phosphor bronze rods for hard ground Copper-clad steel rods (see Notes) for harder ground Fixed connections (bonds) in aluminium, aluminium alloy and copper External strip Rods Internal strip Rods Stranded flexible connections (bonds) External, aluminium External, annealed copper Internal, aluminium Internal, annealed copper NOTES 1. 20 x 3 10.0 dia 20 x 3 10.0 dia 12.0 dia 10.0 dia 12.0 dia 16.0 dia 20 x 3 10.0 dia 20 x 1.5 6.5 dia For copper-clad steel rods, the core should be of low carbon steel with a tensile strength of approximately 600 ~/mm'. The cladding should be of 99.9% pure electrolytic copper molecularly bonded to the steel core. The radial thickness of the copper should not be less than 0.25 mm. Couplings for copper-clad steel rods should be made from copper silicon alloy or aluminium bronze alloy in with a copper content of 80%. The use of internal phosphor bronze dowels may give a lower resistance than external couplings of diameter greater than the rod. 2. 3. Table 3.5 Minimum thickness for sheet metal used for roofing and forming part of the air termination network Material Steel, galvanised Stainless steel Copper Aluminium and zinc Lead 3.7 JOINTS Minimum thickness (mm) 0.5 0.4 0.3 0.7 2.0 3.7.1 Effectiveness Of Joints. The lightning protection system should have as few joints as possible. Joints and bonds should be mechanically and electrically effective, e.g. clamped, screwed, bolted, crimped, riveted or welded. Where overlapping joints are used, the length of the overlap should be not less than 20 mm for all types of conductor. Contact surfaces should first be cleaned then inhibited from oxidation with a suitable corrosion-inhibiting compound. 3.7.2 Protective Covering. Joints and bonds may be protected with bitumen or embedded in plastics compound. Particular attention should be given to joints of dissimilar metals. 3.8 FASTENERS Conductors should be securely attached to the building or other object upon which they are placed. Fasteners should be substantial in construction and not subject to breakage, and should be together with the nails, screws, or other means by which they are fixed, of the same material as the conductors, or of such nature that there will be no serious tendency towards galvanic corrosion in the presence of moisture because of contact between the different parts. Fasteners should be spaced so as to give adequate support to the conductor. Recommended fiing centres for conductors are in Table 3.6. The method of fastening should not result in a reduction of the conductor cross-section below the minimum recommended in Clause 3.6. Table 3.6 Recommended fiing centres for conductors Arrangement Horizontal conductors on horizontal surfaces Fixing centres (mm) 1000 I Horizontal conductors on vertical surfaces I Vertical conductors Vertical conductors over 20 m Vertical conductors over 25 m II I 500 1000 II I 500 3.9 3.9.1 AIR TERMINATIONS General Requirements. An air termination may consist of a vertical rod as for a spire, a single horizontal conductor as on the ridge of a small dwelling, or a system of horizontal conductors with or without vertical rods for the protection of roofs of large horizontal dimensions (see Figure 3.4). Protection may also be provided with a horizontal overhead.wire supported if necessary, independently of the building to be protected or by a vertical air termination network (see Figure 3.2). Salient points of the structure should be incorporated in the air termination network. The upper portions of the downconductors on tall buildings should be regarded as a continuation of the air termination network and should be positioned so as to intercept side strikes to the building. Preference should be given to placing downconductors as near as possible to the exposed outer vertical corners of a building. All metallic projections, on or above the main surface of the roof should be bonded to, and form part of, the air termination network. In the case of aerials which have to be insulated from earth, a spark gap connection to earth or surge arrestor should be provided. If portions of a structure vary considerably in height, any necessary vertical air termination or air termination network of the lower portions should, in addition to their own downconductors, be bonded to the downconductors of the taller portions (see Figures 3.1 and 3.4). Air terminations may be of any form provided the section used and the means of attaching it to the building structure have adequate mechanical strength to withstand the expected wind loading and natural harmonic resonances. Where copper rod is used as vertical air termination, Table 3.7 gives guidance on the maximum height which should be adopted. Table 3.7 Guide to maximum height for vertical air terminations comprising copper rods Diameter of rod (mm) 2 10 216 2 21 2 26 Recommended maximum height (m) 1.o 1.5 2.0 2.5 < 16 < 21 < 26 < 31 Protection Of Roofs. The parts of roofs most likely to be struck by lightning are parapets, the edges of flat roofs, chimneys, and the ridges and eases of sloping roofs. Preference should be given to positioning the air terminations so as to protect these highly exposed parts. 3.9.2 The height of a vertical air termination rod should be such that the tip will be not less than 0.3 m above the object to be protected. On large flat and gently sloping roof areas a number of vertical rods of greater than 0.3 m in height may be needed to establish a zone of protection over the whole roof area in accordance with Clause 3.2. Horizontal air termination conductors may be used to protect a planar roof surface. The roof will be deemed to be protected by air termination network in a mesh of size no bigger than 10 m by 20 m on the roof surface, provided that the highly exposed edges or ridges forming the boundary of the surface are fully protected by the air termination network. Horizontal and vertical air termination conductors and interconnecting conductors of the air termination network should be located so as to constitute, as nearly as local conditions permit, an enclosing network which joins each air termination to each other and to all downconductors. Metal objects, such as gutters, should be bonded to the air termination network. In special circumstances, such as where it is desired to preserve the appearance of a historic building, the air termination conductors may be installed immediately underneaththe cladding (e.g. tiles) of a nonconductive roof. However, it should be noted that, in the event of a lightning strike to the roof, the cladding will be punctured and may suffer some damage. 3.9.3 Protection Of The Sides Of Tall Buildings 3.9.3.1 Influence of forms of construction. Many buildings will have perimeter columns in which the reinforcement (or structural steel) is used as a part of the downconductor system. Where these columns on the external facade are no further than 10 m apart, no further protection will be required in respect of strikes to the side of the building. In the event of a strike to such a column or to isolated metal components such as small window frames, it is likely that a section of masonry cladding material may be dislodged. Where the risk of this is unacceptable, conductors should be installed on the external faces of the columns to receive the strikes. These conductors will take the form of lightning air terminations/downconductorsand should be bonded at the bottom into the lightning protection system. Where suitable columns do not exist to receive strikes to the sides of buildings, vertical conductors should be installed for this purpose. These conductors should be spaced around the perimeter of the building at a spacing not exceeding 10 m or 30 m if the conditions of Subclause 3.9.3.3 apply. 3.9.3.2 Curtain wall construction. It has become common place for tall buildings to have external glass curtain walls, with the curtain wall external to perimeter columns. The majority have major glass elements contained (and restrained) within a metallic framework. This framework is often inherently connected, electrically, to the metal in the building structure via the standard connection details used to mechanically f c the curtain wall structure to the structural frame of the building itself. o Where this inherent connection occurs and where the frame of the building is incorporated into the lightning protection system, no further bonding of the curtain wall to the lightning protection system is necessary. Some curtain wall designs incorporate a metallic framework which is not exposed externally. This framework is then not inherently available to receive direct strikes to the side of the building. For the curtain wall section of this type above 45 m, where direct strikes of the side of the building are anticipated, special design and detailing modifications to the curtain wall should be made. The objective of these modifications should be to achieve a performance of receiving direct strikes equivalent to a curtain wall with an exposed framework. The modifications should provide exposed metalwork, suitable for receiving direct strikes, spaced at intervals of not more than 10 m vertically and 10 m horizontally, or 30 m horizontally if the conditions of Subclause 3.9.3.3 apply. This exposed metalwork should be located to occur particularly at comers of the curtain wall where the probability of direct strikes is the highest. The provision of exposed metalwork for this purpose at less than 45 m above ground is not necessary, however, the curtain wall framework should be bonded to the lightning protection system at intervals not exceeding those recommended above. 3.9.3.3 Dispensation for large flat surfaces. For tall buildings, application of the rolling sphere in accordance with Clause 3.2 will indicate that protection should be provided for the sides of the building above the height of the sphere radius (see Figure 3.1). However, large flat surfaces which are vertical or near vertical are less likely to form attachment points for lightning discharges than are external corners or other projections which provide electric field enhancement. Consequently, notwithstanding the protection that may be inferred as necessary for such surfaces in accordance with Clause 3.2, surfaces that are protected in accordance with the following recommendations will be deemed to be protected for the purposes of this Code : (a) Downconductors should be provided on external comers and other external changes of direction where the plane of the principal surfaces subtends an angle of greater than 20" (see sketch). Provide downconductor if angle e x c e e d s 2 ' 0 Top view of change of direction Additional downconductors should be provided at the intervals necessary to comply with Subclause 3.10. 1. (b) Surfaces that are inclined at 45' or more from the vertical should be treated as roofs and protected in accordance with Subclause 3.9.2. 3.10 DOWNCONDUCTORS 3.10.1 General. The number of downconductors should be one for every 30 m of perimeter. The perimeter should be measured by the 'taut-string' method (see Figure 3.4). A non-metallic structure exceeding 30 m in height should have at least two downconductors symmetrically spaced, bonded by a metal cap or by a conductor around the top. 3.10.2 Route. The route followed by downconductors should be in accordance with the following recommendations: (a) Downconductors should be distributed around the outside walls of the structure. It is undesirable to locate downconductors in areas where persons are liable to congregate. The walls of light wells may be used for fixing downconductors, but lift shafts should not be used for this purpose. Where the provision of suitable external routes for downconductors is impracticable or inadvisable, e.g. buildings of cantilever construction from the first floor upwards, downconductors may be housed in an air space provided by a non-metallic, noncombustible internal duct. Any covered recess or any vertical service duct running the full height of the building may be used for this purpose, provided that it does not contain any unarmoured or non-metal-sheathed service cable (see Subclause 3.14.2.3). Any extended metal running vertically through the structure should be bonded to the lightningdownconductor at the top and bottom unless the clearances are in accordance with Clause 3.14. (b) (c) (d) A downconductor should follow the most direct path possible between the air termination and the earth termination. Right angle bends may be used when necessary but deep reentrant loops should be avoided. A structure on bare rock, protected in accordance with Subclause 3.12.3.1, should be provided with at least two downconductors equally spaced. (e) NOTE. The positioning and spacing of downconductors on large structures has often to be decided in practice by architectural considerations. However, their number should be governed by the recommendations above. It is now recognized that sharp bends in a downconductor, such as arise at the edge of a roof, do not significantly impede the discharge of a lightning current, nor are the mechanical forces produced by a lightning current likely to endanger the conductor or its fixings. In contrast, re-entrant loops in a conductor can produce high inductive voltage drops so that the lightning discharge may jump across the open side of the loop. As a rough guide it can be stated that this risk may arise when the length of the conductor forming the loop exceeds 8 times the width of the open side of the loop. It follows from the above that there is no need to round the path of the downconductors at the edge of a roof and that an upturn within the limits stated is acceptable. Where large re-entrant loops as defined cannot be avoided, e.g. for some cornices or parapets, the conductor should be arranged in such a way that the distance across the open sides of the loop complies with the principles given above. Alternatively, such cornices or parapets should be provided with holes through which the conductor can pass freely (see Figures 3.5 and 3.6). An exception to the above practice is necessary for a building cantilevered out from the first storey upwards. The downconductors in this case should be taken straight down to the ground since, by following the contour of the building, a hazard could be created to persons standing under the overhang formed by the cantilever. In such a case, the use of internal ducts for downconductors is recommended (see Figure 3.7). Where any part of a lightning protection system is exposed to 3.10.3 Mechanical Damage. mechanical damage it should be protected by covering it with moulding or tubing preferably of nonconductive material. If metal is used, the conductor should be electrically connected to both ends of the protective covering. 3.1 1 TEST LINKS Where practicable, test links should be provided to enable the continuity of each individual parallel path of the lightning conductor system to be measured. Where a driven or buried earthing electrode is provided as part of the lightning protection system, test links should be provided to permit measurement of the resistance of the individual earth terminations, in such a position that, while not inviting unauthorised interference, is convenient for use when testing. Such resistance measurements are indicative only and provide the basis of comparison to determine whether any deterioration in the earthing system has occurred in service (see also Appendix B). 3.12 EARTH TERMINATIONS 3.12.1 General Principles. Each downconductor should be connected to an earth electrode or to the earth termination network. The design of earth terminations should be such that lightning currents are discharged into the earth in a manner that will minimise touch and step potentials and the risk of side flashing to metal in or around a structure. This can be achieved by ensuring that the potential with respect to the general mass of the earth at each earth termination is limited by a sufficiently low resistance to earth and that the discharged current flows uniformly in all directions away from the structure. 8d maximum Exceeding 8d : ... ( . . . ..... . .. . , > . . .. . . . - . ,' _.. . . . .... .... .._.. .. . 6 .. . .. . '. . . 4.a - . . . . . _ ...;. . . '.. . .. . . .. ..&'.I . :... ..-. ._. ... . . 1. _ . I -.. .'. -. ' ..., ' . ' - . . A:. .. . . .., t. . _.. . f . . . . . 8 . . .!. *!.:-... ... . . . +. . (a) Acceptable . , .d. .'.' .. '.:. (bl N o t permissible Figure 3.5 General principles of a reentrant loop in a conductor taken over a parapet wall Figure 3.6 Acceptable method of taking a conductor through a parapet wall I Not permissible I p-Permissible I I 1 I I I I I I I I I I I I I I , I Figure 3.7 Routes for downconductors in a building with a cantilevered upper floors Ionization of the soil near an earth electrode carrying lightning current tends to reduce the potential of the electrode relative to remote earth to a lower value than that which would be calculated using the earth resistance measured at low currents. Appendix B provides information on the effectiveness of various forms of earth electrode systems for lightning protection purposes and on the associated calculation/measurement procedures. 3.12.2 Resistance To Earth 3.12.2.1 Basis for measurements. The term earth resistance is used in this clause and elsewhere in this Code because the most common measuring instruments available are low frequency devices. A more appropriate measurement for lightning protection purposes is that of earth impedance and such measurements are preferred when suitable high frequency or impulse type instruments are available. 3.12.2.2 Recommended values. In general, the whole of an interconnected lightning protection system should have a resistance to earth not exceeding 10 n before any bonding is effected to services which are not part of the lightning protection system. In addition, each earth electrode of an interconnected lightning protection system which is not interconnected at or below ground level should have a resistance to earth not exceeding the product obtained by multiplying 10 n by the number of downconductors. Where the installation has two or more air termination networks not directly interconnected, such as a twin-tower NOTE. building, then for the purpose of determining the required earthing resistance, it should be considered as consisting of separate lightning protection systems. A reduction of resistance to earth can be achieved by extending or adding to the electrodes or by interconnectingthe individual earth terminations of downconductors. Where reinforced concrete footings are used as earthing electrodes for a building, compliance with the recommended maximum resistance values should be determined by the measurement of resistance of typical footings which support the building structure. The measurements should be made at the stage of building construction when the footings are structurally isolated and may be treated as independent earthing electrodes. 3.12.3 Common Earthing Electrode And Potential Equalization 3.12.3.1 Common earthing electrode. Where conditions permit potential equalization techniques to be used, a common earthing electrode may be installed to serve the lightning protection system and other appropriate services. The earth electrode should comply with the recommendations in this Code and with any regulations which may govern the appropriate services. The resistance to earth should be the lowest required by any of the regulations for such services. Where isolation is required, a common earthing electrode should not be used but the separate earthing electrodes should be bonded via a spark gap or surge arrestor to minimize potential differences between the earthing systems in the event of a lightning strike. 3.12.3.2 Communications protective earths. Where a communications protective earth is installed at a dwelling or similar small building, that earth should be connected to other earths present (see A.5.6). However, the protective earth of some older types of telex systems carries direct current and, for such systems, the protective earth should be bonded to other earths through a normally non-conducting protector or surge arrestor. 3.13 3.13.1 EARTHING ELECTRODES General Considerations. An earthing electrode may be of any type provided : (a) (b) it achieves a low resistance to the general body of earth, as recommended in this Code; it has adequate mechanical strength and corrosion resistance to ensure the desired service life will be achieved when installed in the environment concerned; and it has adequate current-carrying capacity for the discharge of lightning surges without sustaining damage that might jeopardize its continued effectiveness. (c) NOTES 1. 2. Electrode earth resistance may be measured by standard methods (see 8.10). If the soil resistivity is known, the electrode earth resistance may be calculated as shown in 8.3. It should be noted, however, that such calculations are only approximate and it is important that the electrode earth resistance should in fact be determined by field test. It is fairly easy to determine soil resistivity by test as set out in 8.10.1 3. The selection and design of the earthing system should therefore take account of the following: Soil resistivity; The corrosion aggressiveness of the soil; The physical structure of the soil (rocks, obstructions and other services); The corrosion compatibility of the electrode system with other structures to which it will be, or may become, bonded; The options available for installation at the site (trenching, driving, drilling, land excavation or use of structural metalwork); The effects which it may have on other systems (electrical or communications). 3.13.2 Connections To Electrodes 3.13.2.1 Mechanical protection. Where conductors which are connected to electrodes are accessible to the public, such conductors should be protected against mechanical damage. Where conductors connecting driven electrodes in parallel are not kept above the ground, they should be buried not less than 500 mm below the surface. 3.13.2.2 Selection of materials. Care should be exercised in the selection and application of materials for connections to electrodes to avoid the possibility of galvanic corrosion, e.g. because of differences between the materials of such connections and the electrodes. 3.13.2.3 Joints. Joints between earthing conductors and electrodes should be of adequate strength and current-carrying capacity, and be arranged so as to ensure that there will be no failure of the connection under conditions of use or exposure that can reasonably be expected. Clamps and similar mechanical connections should be designed and constructed so that the connection will not slacken off in use. If test links are inserted in earthing conductors connected to electrodes, they should be either bolted or sweated in the closed position and be arranged so that the opening of any one link does not interfere with earth connections other than the one under test. 3.13.3 3.13.2.4 Test links. Inspection And Testing Of Electrodes. The resistance of earthing electrodes should be determined by test both at the time of installation and regularly during the life of the installation. For details of inspection and testing, see Section 6. METAL IN AND ON A STRUCTURE 3.14 The term 'metal in or on a structure' includes all metal such as reinforcement rods and bars, pipes, metal chimneys, NOTE. corrugated iron and tubing containing electric wiring. Metal hidden from view should not be overlooked. Tubing containing electric conductors, or cable sheaths, is, for instance, often embedded in an external wall and may be quite close to the lightning protection system. 3.14.1 Use Of Metal In Or On A Structure As A Part Of The Lightning Protection System. Where a structure contains electrically continuous metal, e.g. continuous metal frame, or metal within a roof, wall, floor or covering, this metal suitably bonded in accordance with Subclause 3.14.2.3may be used as part of the lightning protection system, provided that the amount and the arrangement of the metal render it suitable for use, as recommended in Subclauses 3.9 to 3.13 inclusive. Where a structure is simply a continuous metal frame without external covering, it requires no air termination or downconductor; it is sufficient to ensure that the conducting path is electrically .) continuous and that the base is adequately earthed (see Figure 3 8 . A reinforced concrete structure or a reinforced concrete frame structure may have sufficiently low inherent resistance to earth to form part of the lightning protection system and, if connections are brought out from the reinforcement at the highest and lowest points during construction, a test may be made to verify this on completion of the structure (see Clause 3.12). If the resistance to earth of the steel frame of a structure or the reinforcement of a reinforced concrete structure is found to be satisfactory for the purpose, a horizontal air termination should be installed at the top and bonded to the steel frame or to the reinforcement. Where regular inspection is not possible, it is recommended that a corrosion-resistant material be used for bonding to the steel or to the reinforcement and that this be brought out for connection to the air termination. Downconductors and earth terminations will of course be necessary ifthe inherent resistance of the structure is found to be unsatisfactory when tested. 3.14.2 Prevention Of Side Flashing 3.14.2.1 Methods of prevention. When a lightning protection system is struck, its electrical potential with respect to earth is raised and, unless suitable precautions are taken, the discharge may seek alternative paths to earth by side flashing to other metal. Two methods exist to prevent side flashing: bonding and isolation. Bonding is the procedure whereby metal parts are positiiely connected to one another so as to prevent inadvertent electrical connection occurring due to side flash. Isolation is the separation or insulation of metal parts in such a way that electrical breakdown or side flash to them is prevented. Isolation may be achieved by separation of the lightning protection system from the structure protected or by separating metal parts and services in a non-conductive structure from the lightning protection system. Bonds column drilled and tapped /INota: Inhibitor to be used in all clamp and bonds) Test clamp 2 I Earth electrode inspection chamber I (inconduit) i I NOTE: This diagram is not to scale Figure 3.8 Earth electrode connection 47 Bonding effectively eliminates any local potential difference between the metal parts that are bonded together. However, it is possible to obtain large potential differences for very short times between adjacent metallic objects which are connected together at a remote location. These potential differences could be hazardous ifthe bonding system is inadequate. Many structures can be effectively bonded so as to eliminate any hazard, however care should be taken to prevent subsequent installation of a metallic service creating a hazard. It should be noted that any conductive element which is bonded into the lightning protection system can be expected to carry a proportion of the lightning current. With isolation, it is often difficult to obtain and to maintain the necessary safe clearances, and to prevent connection of an 'Isolated' lightning protection system back to the structure via ground and buried metallic services. To achieve isolation it may be necessary to utilize a protection system that is completely separate from the protected structure, and is remotely earthed. If the structure is constructed with conductive materials such as reinforced concrete or steel frames, isolation of a protection system mounted on the structure requires the use of high impulse strength, high voltage insulation. In general isolation can be achieved at low cost, using a protection system mounted on the structure, for small structures only. 3.14.2.2 Bonding. The conditions under which bonding should be effected are as follows: (a) Where practicable, all structural steel and metallic reinforcement in a structure, if not used as a part of the lightning protection system, should be bonded to that system. As indicated in Subclause 3.3.2.4(b), metal rods in in situ reinforced concrete may be consideredto be electrically continuous. Consequently, bonding may be achieved with a reasonable number of connections to the rods, a bonding connection to each rod being unnecessary. Where prestressed concrete elements are involved it has been found the prestressing cables frequently remain electrically isolated from other structural metal at the completion of the stressing process. Such cables should be bonded at both ends to the lightning protection system (see Note) particularly where the structural element is exposed to the weather. NOTE. This bonding is not recommended out of concern for a side flash causing immediate structural damage, but rather to avoid the chance of the side flash causing cracking of the corrosion protecting concrete grout used around the cable. Prestressing cables under stress are highly susceptibleto corrosion. Where metal exists in a structure, such as reinforcement in a precast concrete spandrel panel, which cannot be bonded into a continuous conducting network and which is not or cannot be equipped with external earthing connections, its presence should be disregarded. The danger inseparable from the presence of such metal can be minimized by keeping it entirely isolated from the lightning protection system. (b) Where the roof structure is wholly or partly covered by metal, care should be taken that such metal is provided with a continuous conducting path to earth. Metal which is attached to the outer surface of a structure should preferably be bonded as directly as possibleto the lightning protection system. Where bonding is difficult and where the consequences of side flashing to isolated metalwork is not considered serious, bonding may be omitted. Where such metal has considerable length, e.g. cables, pipes, gutters, stairways, and runs approximately parallel to a downconductor or column, it should be bonded at each end and at intervals of not more than 20 m. In curtain wall construction, where the framework would otherwise be electrically isolated, the frame should be made electrically continuous and should be bonded to the lightning protection system at intervals not exceeding 10 m around the perimeter of the building. This should occur at the top and bottom of each curtain wall and at levels separated by not more than 20 m vertically, includingthose sections which are less than 45 m above ground. Where there is insufficient clearance from the lightning protection system, metal entering or leaving a structure in the form of sheathing, armouring or piping for electric, gas, water, telephone, steam, compressed air or other services, should be bonded as directly as possible to the earth termination at the point of entry or exit of the structure. In this operation, the appropriate standards and any regulations which may apply to such services should be observed. Masses of metal in a building, such as a bell-frame in a church tower, should be bonded to the nearest downconductor by the most direct route available. Isolation. The necessary separation distance from any point on the lightning protection system depends on the electric potential, or voltage, generated at that point by the lightning discharge. To achieve a sufficiently low probability of side-flash, the responses of the protection system to a range of severe stroke current waveshapes have to be considered. Because the time for a lightning stroke current waveshape to significantly change its steepness is similar to the time taken by the incident wave to travel from the point of strike to the earth termination, travelling wave techniques are used to calculate the voltage waveforms generated. However, an approximatevoltage waveform sufficient to estimate the required separation distance can generally be calculated from the resistive and inductive voltage drops in the system; the calculation procedures are outlined in Appendix C. 3.14.2.3 For conventional lightning protection systems using typical bare metal downconductors, the separation distance in air at a given point on the protection system is required to be not less than D, where D, in metres, is the greater of D, and D, as defined below and shown in Figure 3.9. D, is the required clearance associated with the discharge voltage of the design first stroke of a severe lightning flash and takes account of the design maximum lightning current. D, is defined only for H/n < 30. D is the required clearance associated with the discharge voltage of the design subsequent , stroke of a severe lightning flash and takes account of the design maximum steepness of the current wavefront. To take account of systems with a common earthing electrode it is necessary to separate D, into two components as follows: where : H = length of downconductor from the point considered to earth, in metres; n = number of downconductors connected to a common air termination; R = combined earthing resistance of lightning protection system, in ohms. Di is the component of the first stroke separation distance associated with potential difference generated within the structure. D, is the component of the first stroke separation distance associated with local earth potential rise, and is independent of the point on the lightning protection system considered. This term is applicable to any remotely earthed objects, such as services entering the building, which do not share a common earthing electrode with the lightning protection system, and to any long unearthed objects within a relatively non-conductive structure. Where a common earthing electrode in accordance with Subclause 3.12.3 is used, the term D, may be neglected (R = 0 in Figure 3.9). Where it is applicable, the clearance D, should be maintained throughout the structure and thus determines the minimum separation distance at the base of the structure. The required clearance for steep-fronted surges, D,, may be read from the dotted curve given in Figure 3.9. As the separation distance D, varies with the length of downconductor from the point considered to earth, Di normally determines the required separation in the upper parts of tall structures. The shortest separation distance over the surface of non-conductive structural material should be 20 for protected dry surfaces and 3 0 for external surfaces. The separation distance through solid non-conductive structural material should exceed 0.50. NOTES 1. For a substantial reinforced or structural steel frame building which utilizes the structure as part of the lightning protection system, the separation distance may be obtained from Figure 3.9 by taking n to be 1.5 times the number of reinforced or steel columns. The term D, may be neglected for these buildings by assessing D for R = 0 except when considering remotely earthed services entering the building. The extent to which uninsulated services may be considered to be affected by local earth potential rise can be determined by a test in which a known current is injected into the lightning protection system and potential differences to the system earth are surveyed. 2 . 3.14.2.4 Effects of bonding on cathodically-protected metal. In the bonding of adjacent metalwork to the lightning protection system, careful consideration should be given to the possible effects such bonding would have upon metalwork which may be cathodically-protected. 3.14.2.5 Bonding of underground s e ~ c e s . In the ground, bonding between the earth termination network of any structure and buried metal setvice pipes is essential, unless the service can be effectively isolated. If this is not done, an electric breakdown can occur through the soil between these systems and the electric arc can cause structural damage or may puncture a service pipe (see also Subclause 3.14.2.2 (d)). REQUIRED SEPARATION DISTANCE FOR ISOLATION D , m SECTION FOUR PROTECTION OF MISCELLANEOUS STRUCTURESANDPROPERTY 4.1 SCOPE OF SECTION This Section provides recommendations for the protection of a variety of structures and property against lightning where such protection is deemed necessary (see Section 2). The recommendations of Sections 3 and 7 should be observed except where otherwise indicated. 4.2 STRUCTURES WITH RADIO AND TELEVISION AERIALS Structures protected against lightning in accordance with the 4.2.1 Indoor Aerial System. recommendations of this Code may be equipped with indoor radio and television receiving aerials without further precautions, provided that the clearance between the aerial system, including its down leads or feeders, and the external lightning protection system or any of its internal sections is in accordance with the values in Clause 3.14. 4.2.2 Outdoor Aerials On Protected Structures. Structures protected against lightning in accordance with the recommendations of this Code may be equipped with outdoor radio and television aerials without further precautions, providedthat every part of the aerial system, including any supporting metalwork, is within the zone of protection of the lightning protection system (see Clause 3.2j. Where these conditions cannot be fulfilled, precautions should be taken to ensure that the lightning current can be discharged to earth without damage to the structure and its occupants as follows: (a) With an aerial system fitted directly onto a protected structure. This can be accomplished by connecting the aerial bracket structure to the lightning protection system at the nearest point accessible below the aerial installation. Wih an aerial system f i e d on a metallic support structure which projects above the lightning protectionsystem. This can be accomplished by connecting the aerial support structure to the lightning protection system at the nearest point accessible below the aerial installation. (b) Consideration should be given to the fitting of surge arrestors in the conductors connected to the aerial system. 4.2.3 Aerials On Unprotected Structures. Before installing an aerial on an unprotected structure, the need to provide a lightning protection system should be assessed as described in Section 2. 4.2.4 Earthing Of Radio Systems. The earth electrode of the lightning protection system may also be used for the purpose of earthing a radio system. 4.3 STRUCTURESNEARTREES When a tree is struck by lightning, a voltage drop develops along its branches, trunk and roots. The side flash clearances between the tree and adjacent structures are set by taking 100 kV/m as the flash-over strength of unseasoned wet timber and 500 kV/m as the breakdown strength of air. If the tree does not exceed the height of the structure its presence can be disregarded. If the tree is taller than the structure, the following clearances between the structure and the tree may be considered as safe: (a) For normal structures; one-third of the height of the structure. (b) For structures with explosive or highly flammable contents; the height of the structure. If the clearances cannot be met then the structure should be f i e d with lightning protection in such a manner that the side flash always terminates on the protection system. If the tree is f i e d with a lightning protection system, no further protection will be necessary for the structure provided that the conditions for the zone of protection and separation are fulfilled. 4.4 PROTECTION OF TREES The protection of trees against the effects of lightning needs to be considered only where the preservation of the tree is desired for historical or other reasons. For such cases the following recommendations are made: A main downconductor should be run from the topmost part of the main stem to the earth termination and should be protected from mechanical damage near ground level. Large upper branches should be provided with branch conductors bonded to the main conductor. In the fixing of the conductors, allowance should be made for swaying in the wind and the natural growth of the tree. Test joints may be waived. The earth termination should consist of two rods driven into the ground on opposite sides of, and close to, the trunk of the tree. A strip conductor should be buried to a depth of 0.3 m to encircle the roots of the tree at a minimum distance of 8 m radius from the centre of the tree or at a distance equal to 1 m beyond the spread of the foliage, whichever is the greater. This conductor should also be bonded to the rods by two radial conductors. The earth terminations and resistance should comply with Clause 3.13. Where two or more trees are so close together that their encircling earth conductors would intersect, one conductor adequately connected to the earth rods should be buried so as to surround the roots of all the trees. The recommended earth termination network is designed to protect the roots of the tree and to reduce the potential NOTE. gradient, in the event of a lightning discharge to the tree, to a safe value within the area bounded by the outer buried strip conductor. 4.5 CHIMNEYS, METAL GUY-WIRES OR CABLES 4.5.1 General. Metal guy-wires or cables attached to steel anchor rods set in earth may be considered as sufficiently earthed. Other guy-wires should be earthed. For means of securing conductors to structures, see Clause 3.8. Metal chimneys or flues need no protection against lightning other than that afforded by their construction, except that they should be properly earthed. If the construction of the foundation does not provide ample electrical connection with the earth, ground connections should be provided similar to those recommended for chimneys made of materials other than metal (see Clauses 3.12 and 3.13). 4.5.2 Metal Ladders And Metal Linings. Where chimneys have a metal ladder or lining they should be connected to the lightning protection system at their upper and lower ends. Reinforced Concrete Chimneys. Chimneys consisting partly or wholly of reinforced concrete 4.5.3 should comply with the recommendations of Clauses 3.3, 3.8, 3.10 and 3.14, and, in addition, the reinforcing metal should be electrically connected together and electrically connected to the downconductors at the top and bottom of the concrete. In existing chimneys, the reinforcement of which may be electrically continuous, it is recommended that additional NOTE. connections be made at points where the reinforcing rods are accessible. 4.6 PROTECTION OF MINES 4.6.1 Factors Influencing Need For Protection. In mining operations, electric shocks, possible premature detonation of explosives and ignition of flammable gases from the effects of lightning are recognized additional hazards. Because these hazards are associated with the effects of lightning at or below the ground surface, factors additional to the risk index values of Section 2 influence the need to provide lightning protection. These additional factors are associated with ground resistivity, depth of the mining operation, presence of personnel and the presence of flammable gas or explosives. The degree of hazard is regarded as greater the shallower the depth of the operation and the higher the resistivity of the ground involved. Generally, these additional factors will indicate that lightning protection should be provided or precautionary work procedures adopted. Object Of Recommendations. The following recommendations for lightning protection for mining operations are aimed at reducing the risk of electric shock and premature detonation of explosives. While the recommendations will also reduce the risk of ignition of flammable gases from the effects of lightning, flammable gas ignition is best prevented by ensuring that flammable concentrations of gases do not occur. 4.6.2 The intent of the recommended lightning protection system is to reduce the possibility of substantial voltages appearing between conducting structures and between conducting structures and earth in their immediate vicinity. Absolute protection against the effects of lightning cannot, however, be guaranteed with the recommended protection system alone; consequently, recommendations are also given for operational procedures for the use of explosives when lightning occurs close to the mine site. In surface workings, premature detonation of explosives, both directly and through electric detonators, are considered possible,while in underground operations prematuredetonation of explosives is considered possible only through electric detonators. 4.6.3 4.6.3.1 Underground Workings General. The following recommendations apply particularly to underground workings where electric detonators are used as the means of initiating explosives. 4.6.3.2 Electric detonators. Detonators specially designed to reduce the risk of ignition by electrical discharge across the fuse head should be used. 4.6.3.3 Shot firing circuit. Requirements for circuit equipment and procedures to be adopted for firing explosives electrically are set out in AS 2187.2 or NZS 4403. Additional to those requirements, where fixed wiring is used as part of the firing circuit, the conductors should be enclosed in metal screening, armouring or conduit. This metal screening, armouring or conduit should be connected to the electrical system earthing and bonded to other metallic structures as described in Subclause 4.6.3.7. Overhead power lines. To minimize the magnitude of incoming lightning surges on overhead power lines, overhead earth wires should be provided on all overhead lines within 1.5 km of the mine. 4.6.3.4 Additionally, surge arrestors should be installed at the termination of the overhead line for protection of connected cables or equipment. 4.6.3.5 Lightning protection should be provided on all structures above Surface structures. underground openings, such as winder head frames. Wih other structures and buildings the need to protect or not should be determined from Section 2. Lightning protection of surface structures should be carried out in accordance with Section 3 and, where these buildingshave explosive or highly-flammablecontents, the additional recommendations of Section 5. Various conductive structures such as metallic enclosures of air, water and electricity services reinforcing steel concrete in foundations, are laid in or on the ground and advantage should be taken of these to reduce the earthing resistance of the lightning protection system by interconnecting and bonding these structures together and to the lightning protection earthing system. The sizes of bonding conductors are given in Table 3.4. 4.6.3.6 Bonding of surface metalwork. All metal structures entering openings to underground workings of a mine should be bonded together at the point of entry to the opening and connected to the earthing system of structures above the opening. This includes any reinforcing steel in the shaft, concrete lining, shaft steel work, guides and ladders, armouring and sheathing of electrical cables, air, water and ventilating pipes, rails and bell rope attachments. The sizes of bonding conductors are given in Table 3.4. 4.6.3.7 Bonding of underground metalwork. In addition to the bonding recommended in Subclause 4.6.3.6, metal structures and services in underground access shafts should also be bonded together at intervals of not more than 75 m. Rock-bolted support structures are deemed to provide an adequate earth for this purpose. Winding ropes, guide ropes and balance ropes cannot be bonded to other structures except at fixing points and, possibly ineffectively, through conveyances. High voltages relative to their surrounds could occur during lightning activity. Further precautions. The degree of hazard in any mine, both from electric shock and initiation of electric detonators, is related to the depth of the operations. This relationshipis inadequately defined at present. 4.6.3.8 Shaft sinking and driiing are particular operationswhere lightning is a recognized hazard. With these operations all work associated with electrical blasting should be suspended and personnel withdrawn to a safe distance when an electrical storm is approaching. A conservativeapproach would requirethat the precautions applied to shaft sinking and drifting be applied to all underground operations. 4.6.4 Surface Workings 4.6.4.1 General. The following recommendationsapply to surface mining operationswhere any type of explosive is used in the mining operation. 4.6.4.2 Equipment. For many surface workings involving blasting operations, action need only be taken in the immediate vicinity of the area where blasting takes place. This is because no interconnection by metallic structures, such as air/water/electricity services, exists with distant structures or ground. Where these services exist the recommendations apply to these services for underground workings, i.e. Subclauses 4.6.3.3 to 4.6.3.5 and, where practicable, the bonding recommendations of Subclause 4.6.3.7 should also apply. Where electric detonators are used, electric detonators of the type described in Subclause 4.6.3.2 should be used. On-site precautions. All work associated with blasting operations should be suspended and 4.6.4.3 personnel should be withdrawn to a safe distance from explosives when an electric storm is approaching. High equipment such as drilling rigs, shovels and draglines which may increase lightning attraction should be moved to a safe distance from the area where blasting is to take place prior to explosives being brought to the site. Lightning Detector. Speciallydesigned lightning detectors should be provided to warn of 4.6.5 approaching electrical storms so that the precautions set out in Subclauses 4.6.3.8 and 4.6.4.3 may be taken. 4.7 PROTECTION OF BOATS A boat should be considered to be at risk both because of its method of 4.7.1 General. construction (except for metal-hulled boats) and because it forms a marked protrusion above the surrounding water surfaces. Overseas statistics show that in excess of 10 percent of fatalities occurring on cruising sailing boats are due to lightning. While the principles to be applied will not differ from those for land-based structures, the methods employed will depend on the form of construction and the type of boat to be protected. 4.7.2 Elements Of The Protection System 4.7.2.1 Air termination. A metal mast or the metal fitting on a timber mast will act as an adequate air termination. 4.7.2.2 Downconductors The mast, if metallic or if provided with a metal track, and stays will both act as downconductors and each should be connected to an earth termination. Stays as small as 3 mm diameter steel wire will serve as effective downconductors, but may be damaged under severe lightning discharges. 4.7.2.3 Earthing. Any metal surface which is normally submerged in the water will provide adequate earthing. Propellers, metal rudder surfaces, metal keels, or the earth plate for the radio transmitter may be used. A metal or a ferro-cement hull constitutes an adequate earth. 4.7.2.4 Metallic objects. Metallic objects which are permanent parts of the boat and whose function would not seriously be affected by earthing should be made part of the lightning protection system by interconnection with the downconductor system. The purpose of interconnecting the metal parts of a boat with the conductor is to prevent side flashes to metal NOTE. objects which could form part of an alternative path to earth or which could bridge out a substantial length of the downconductor. A general rule is that if the non-conducting part of the alternative path through such object is less than one-eighth the length of downconductor bridged out, then that object should be electrically interconnected with the downconductor. 4.7.2.5 Radio transceivers. A whip antenna consisting of a fine wire embedded in a glass fibre tube cannot be considered as a satisfactory lightning conductor and should be folded down during a lightning storm. All radio equipment or other navigational equipment with exposed transducers such as radar, wind speed/direction indicators, and the like, should be fitted with effectively-earthed spark gaps or surge arrestors. Alternatively, input cabling should be disconnected from the equipment if there appears to be imminent danger of the boat being struck by lightning, 4.7.2.6 Corrosion. Care should be taken that the design of the lightning protection system does not promote the occurrence of electrolytic corrosion. Bonding of dissimilar metals and interconnection of the earth terminals of different pieces of electrical equipment should not be undertaken without expert ..) knowledge of the possible problems involved (see also Subclause 3 5 2 . 4.7.3 Installation Recommendations 4.7.3.1 Protection of boats with masts. Sailing or power boats which have a mast or masts of sufficient height to give an adequate zone of protection in accordance with Clause 3.2 may be protected by earthing the lower ends of the standing rigging and the base of a metallic mast, or the lower end of a continuous metal sail track on a timber mast. Where the mast of a boat is stepped on deck, particular care should be taken to ensure that the conductor from the base of the mast follows a direct route if it passes through the accommodation section of the boat, otherwise a situation analogous to that shown in Figure 3.7 may occur. A typical small sailing boat with an aluminium mast stepped on deck, a glass fibre hull with the metal ballast encapsulated in glass fibre (or unballasted and with a non-metallic centreboard) and with chainplates moulded into the hull presents a problem. In such cases it is suggested that some protection be provided when necessary by temporarily connecting the mast and stays together at deck level by a length of chain or other flexible conductor and allowing a short length of the conductor to hang in the water at each chainplate. 4.7.3.2 Protection of boats without masts. Boats without masts do not constitute as high a risk as boats with masts. However, where the size of the boat is such as to cause a marked protrusion above the surrounding water surfaces, such boats should be fitted with air terminations which will give at least the protection recommended for land-based structures in Section 3. 4.7.4 PrecautionsFor Persons And Maintenance Suggestions. To the extent consistent with safe handling and navigation of the boat during a lightning storm, persons should remain inside a closed boat and avoid contact with metallic items such as gear levers or spotlight control handles. Persons should stay as far as practicable from any parts of the standing rigging or other items forming part of a downconductor. No person should be in the water or dangle arms or legs in the water. If a boat has been struck by lightning, compasses and navigation instruments should be checked for calibration. Protective coatings on steel hulls and glass fibre sheathing over ballast keels should also be checked for damage. All standing and running rigging and associated fttings should be checked in detail. 4.8 FENCES If an extended metal fence is struck it is raised momentarily to a high potential relativeto earth. Persons or livestock in close proximity to, or in contact with, such fencing at the time of a lightning discharge to the fencing may therefore be exposed to danger. Fences which give rise to the most risk are those constructed with posts of poor conducting material, such as wood or concrete. Fences built with metal posts set in earth are less hazardous, especially if the electrical continuity is broken. Breaking the electrical continuity prevents a lightning stroke from affecting the entire length of a fence, as it can if the stroke is direct and the fence continuous, even though earthed. Thus it is desirable to limit the length of fencing by the provision of gaps, and also to provide several earth electrodes in each section so as to facilitate the discharge to earth of the lightning current. In addition, persons or livestock can be endangered by potential differences in the ground in the proximity of fences (see Figure 4.1). The risk is greatest on rocky ground. No value can be given for the earth termination resistance, since this must be largely governed by the physical conditions encountered, but the lower the resistance to earth the less risk will result to persons and livestock. In this connection, it should be borne in mind that because of large body spans and bare contact areas many types of livestock are more susceptible to electric shock than humans. I/ = voltage due t o lightning current in ground Figure 4.1 Equipotential lines near metal fence caused by lightning discharge to fence MISCELLANEOUS STRUCTURES 4.9 4.9.1 Large Tents And Marquees. Where large temporary structures of this type are used for such purposes as exhibitions and entertainments involving large numbers of people, consideration should be given to their protection against lightning. In general such structures are manufactured from nonmetallic materials and the simplest form of protection will usually consist of one or more horizontal air terminations suspended above them and connected solidly to earth. A nonmetallic extension of the vertical supports provided for such structures may, if convenient and practicable, be used for supporting a system of horizontal air terminations but a clearance of not less than 1.5 m should be maintained between the conductor and the fabric of the enclosure. Downconductors should be arranged outside the structure away from exits and entrances and be connected to earthing rods which in turn should be connected to a ring conductor in such a manner as to be inaccessible to the general public. Those types of tented structure which have metal frameworks should have these efficiently bonded to earth at intervals of not more than 30 m of perimeter. 4.9.2 Small Tents. For small tents, no specific recommendations can be given. Metal ScaffoldingAnd Similar Structures, Including Overbridges. Where metal scaffolding 4.9.3 is readily accessible to the general public, particularly when it is erected over and on part of the common highway or may be used in the construction of public seating accommodation, it should be efficiently bonded to earth. A simple method of bonding such structures consists of running a strip of metal other than aluminium, 20 mm x 3 mm size, underneath and in contact with the base plates carrying the vertical members of the scaffolding and earthing it at intervals not exceeding 30 m. With public seating accommodation only the peripheral members of the structure need bonding to earth. Other steel structures, such as those used for pedestrian bridges over main trunk roads, are frequently sited in isolated situations where they may be prone to lightning strikes and should therefore be bonded to earth, particularly at the approach points. 4.9.4 Tall Metal Masts, Towers, Cranes And Revolving And Travelling Structures. Masts and their guy-wires, floodlighting towers and other similar structures of metallic construction, particularly those to which the general public have access, should be earthed in accordance with this Code. Cranes and other tall lifting appliances used for building construction purposes, shipyards and port installations also require bonding to earth. For cranes or revolving structures mounted on rails, efficient earthing of the rails, preferably at more than one point, will usually provide adequate lightning protection. In special cases, where concern is felt regarding possible damage by lightning to bearings, additional measures may be justified. Mobiletowers, portable cranes and similar structures mounted on vehicles with pneumatictyres can be given a limited degree of protection against lightning damage by drag chains or tyres of conducting rubber such as are provided for dissipating static electricity. PROTECTION OF HOUSES AND SMALL BUILDINGS 4.10 4.10.1 General Considerations. The application of this clause is intended to be restricted to relatively small buildings, such as houses or similar buildings, of a smaller size than those envisaged in Section 3 of this Code. Lightning protection for a house or small building in complete accordance with the recommendations of Section 3 may be difficult to justify on economic grounds. However, there may be a need to provide some degree of protection against lightning damage. Houses and small buildings vary greatly in the degree to which their construction provides inherent lightning protection. Small buildings with mainly non-metallic materials offer little or no inherent protection against lightning, whereas a building with a metallic roof, metallic gutters, and metallic downpipes leading into the ground have a high degree of inherent protection, since the main elements of a lightning protection system are already present. If lightning strikes a house with little or no inherent lightning protection, the lightning is likely to penetrate through the roof and attach to electrical wiring in the roof area. This will usually result in damage to electrical equipment in the house, and in extreme cases, may result in a fire, or in hazard to persons within the house. The objective in protecting small buildings should be to provide conductors to intercept the lightning, to provide a low-resistance path to ground., and to provide at least two earth stakes or equivalent earthing electrodes for conveying the lightning current into the earth. Air Termination For The Building. If the building roof consists mainly of metallic materials, 4.10.2 then it will serve as the air termination. It is necessary to ensure that there is electrical continuity between the various parts of the roof. Adequate continuity will often be provided by the way in which the metallic parts are overlapped and fastened. If the building roof consists mainly of non-metallic materials, then separate air termination conductorswill have to be provided. Suitable materials are listed in Clause 3.5. Copper wire and copper strip are recommended for their durability. At least one conductor should be run along the highest parts of the roof, for example, the highest ridge of the building. If the roof has a complicated shape, it may be necessary to run additional conductors along the highest parts of each section of the roof. All conductors should be joined together. To be in accordance with this Code the cross-sectional area of the conductors should be at least 35 mm2, achieved, for example, by copper strip 25 mm x 1.5 mm. However, it should be realised that much thinner conductors are able to carry most lightning currents without damage, and that almost any conductor would be better than none. For a large, more or less flat roof of non-conducting material, the simplest form of air termination may be a series of vertical metallic rods above the roof level, all connected together. The zone of protection provided by a vertical rod may be estimated using the information in Clause 3.2. Metallic gutters may become a strike attachment point. If there are metallic gutters around the roof, these should be connected to the air termination conductors. Wih metallic roofs, these connections may already exist in the fastenings of the guttering to the roof. With non-metallic roofs, the guttering should be connected to the air termination conductor at no less than two points. Provision Of Downconductors For The Building. There should be at least two lowresistance paths to convey towards the ground the current from any lightning strike to the roof. Metallic downpipes from metallic gutters may be used for this purpose, provided that they afford a direct lowresistance path for the lightning current. In the absence of any low-resistance path from roof to ground, at least two conductors should be provided to serve as downconductors. These may be continuations of the conductors foiming part of the air termination, and the same recommendations apply as in Subclause 4.1 0.2. 4.10.3 4.10.4 Provision Of Earthing Electrodes. A path to earth for the lightning current should be provided at no less than two well-separated points, for example, at opposite ends of the house. Preference should be given to areas that are usually damp, such as gardens. Each downconductor should be connected to an earthing electrode by the shortest possible route, with the provision that downconductors and earthing electrodes should not be placed close to entry doors, or places where persons are likely to stay for long periods. For example, earthing electrodes should not be placed close to swimming pools. Earthingelectrodes and their connected conductors should be examined periodically to ensure that they are intact, and not suffering corrosion or mechanical damage. SECTION FIVE PROTECTION OF STRUCTURES WITH EXPLOSIVE OR HIGHLY-FLAMMABLE CONTENTS 5.1 SCOPE OF SECTION This Section provides a guide to the protection of structures containing explosives, or highlyflammable solids, liquids, gases, vapours or dusts, from lightning or induced discharges, and indicates ways of protecting those structures that are not inherently self-protecting. Reference should be made to the AS 2430 or NZS 6101 series for information on areas that are likely to have an explosive atmosphere. GENERAL CONSIDERATIONS 5.2 5.2.1 Acceptable Risks. An acceptable risk may be present when the quantity of dangerous material is strictly limited, as in a laboratory or small store, or where the structure is specifically designed to restrict the effects of a catastrophe, or is sited in an isolated position. Circumstances may also arise in which the dangerous materials are not exposed but are completely encased in metal of an adequate thickness, and under these conditions lightning protection may not be necessary. In other situations the risk to life and property may be so obvious that the provision of every means possible for protection from the consequences of a lightning discharge is essential. Protection Required. The presence of explosives or highly flammable material in a structure may increase the risk to persons and to the structure in the event of a lightning discharge. For this reason, except in the circumstances described in Subclause 5.2.1, the recommendationsin this Section should be followed for structures in which explosives or highly-flammable solids, liquids, gases, vapours or dust are manufactured, stored or used, or in which highly flammable or explosive gases, vapours or dusts may accumulate, i.e. in those areas which may be classified as hazardous. 5.2.2 Because of the increased risk, a rolling sphere of 20 m radius should be used when determining zones of protection in accordance with Clause 3.2, in lieu of the sphere of 45 m radius recommended for general application. Electrostatic Shielding. The electrostatic induced voltage on isolated objects in the field of 5.2.3 a storm cloud may cause sparks to ground when a lightning discharge occurs to some adjacent object. Isolated objects within a structure that is adequately shielded will themselves be electrostatically shielded. If the structure is not shielded or is only partly shielded, then the isdated objects should be earthed to prevent electrostatic sparks. For further discussion on the earthing of isolated internal objects, see Section 7. AREAS OF APPLICATION Protection should, in all cases, be provided for the following structures: (a) (b) Tanks and vessels containing flammable solids, liquids, vapours or gases, or highlyflammable or explosive dusts. All metallic pipes and power and communication service lines at the point where they enter or leave a hazardous area. 5.3 Piping which is not in electrical contact with its associated tank or vessel, such as an open discharge line into a water tank, should be bonded to the tank or vessel by a flexible conductor, and earthed. Cathodic protection may justify the insertion of an insulating flange which will interrupt the electrical continuity of the total length of line. Where flexible connections between pipelines and tanks do not incorporate an earthcontinuity conductor, a separate conductor for earthing should be provided. No pipeline should be used for earth-continuity purposes as a substitute for the recommended earthing conductor. Buildings in explosive areas which may contain explosive or large quantities of highly flammable materials, or nominated buildings which may, in emergency, be taken into use for the storage of explosives. Buildings in explosive areas which may contain small quantities of highly flammable material or a large quantity of combustible material if sited within 50 m of a building specified in Item (c). Any structure sited within 30 m of a building containing explosives, which thus constitutes a projectile hazard to this building in the event of dislodgment of masonry and the like by lightning. Any structure sited within 30 m of a building containing explosives which if struck by lightning might constitute a subsequent fire hazard. 5.4 EQUIPMENT APPLICATION 5.4.1 Earth Bosses. Earth bosses should be made from low carbon steel, tapped to receive a bolt or stud. Pressurevessels should be provided by the manufacturerwith a suitable boss to take the earth connection. Welding of bosses in excess of 40 mm in diameter on site may necessitate stress-relieving of the weld. Earth bosses should be about 50 mm long, but extended types, from 100 mm to 500 mm long, are used for fire-proofed steelwork and lagged vessels. In order to avoid corrosion, earth bosses should be installed at not less than 500 mm above ground level. 5.4.2 Bonding Conductors. Where various items of process plant or a number of vessels are mounted on an extensive concrete plinth which elevates the equipment above ground level, bonding conductors should be provided to form a common earth connection for all the downconductors from the plant. Copper tape should be installed along two opposite sides of the plinth, fastened to the walls not less than 500 mm above ground level to avoid corrosion. Tee-joints may be used between down and bonding conductor. Diagonally opposite ends of the base conductor should be provided with a test link from which connection is made to the earth termination network, preferably to earth busbars which provide alternative earth connections. Where one bonding conductor only is installed, test links and earth connections should be provided at each end. Sizes Of Tapes. Sizes of tapes should be in accordance with Table 3.4. For common 5.4.3 earthing systems, larger sizes may be needed depending on the fault current. These should be selected in accordance with SS CP 5. Downconductors (see Clause 3.10). All high salient structures within a process area should be provided with at least two downconductors unless they are of welded construction or electrically continuous down to base level. 5.4.4 Wherever possible, downconductors should be installed remote from stairs and operational walkways and ladders. Downconductorsshould preferably be installed at diagonally opposite comers of the structure which provide the shortest possible path for connection to the earth termination network; they should be installed on the outside of the structure and should not pass through it. Earth tape should be used for downconductors and while, wherever possible, it should be in a continuous length, test links may be attached for connection of down or base conductors at various levels. Where structural steelwork or columns do not require the installation of an air terminal, the downconductor should extend from above the highest point of the structure. Provision should be made for thermal expansion of the earthing conductor and associated structure. A test link should be installed in the downconductor in accordance with Clause 3.11, not less than 500 mm above ground level. Each downconductor from the highest point or points within the process area should take the shortest possible path direct to earth and should be equipped with its own set of earth electrodes to provide a path of minimum impedance for a lightning discharge. The earth electrodes should be interconnected below ground level with the bonding conductor belonging to the earthing system. 5.4.5 Air Terminations (see Clause 3.9). All high salient structures which are not electrically continuous and which are not within the zone of protection of an adjacent protected structure should be equipped with air terminations in accordance with the recommendations of this Code. Where two or more air terminations are employed they should be interconnected by roof conductors for connection to at least two downconductors as follows : (a) (b) Roof conductors. Copper tapes should take the shortest salient route between the various air terminations and with fasteners spaced as for downconductors. Air termination network. Buildings which are protected by an air termination network should be provided with at least two downconductors, which should be directly connected to the most widely-spaced parts of the air termination network. 5.5 5.5.1 SPECIFIC OCCUPANCIES Protection Of Steel Tanks 5.5.1.1 General precautions. The following precautions should be taken to minimize the effects of lightning discharge on tanks containing petroleum products, including tanks with fixed roofs and tanks with floating roofs: (a) The shells of all tanks intended for the storage of highly flammable liquids which can produce an explosive gas atmosphere should be permanently and effectively earthed. Other tanks, such as water tanks, if located in a hazardous area should also be permanently and effectively earthed. The combined earth resistance of permanent earth connections to the tank should not exceed 10 ohm. The recommended method of earthing is by means of earth electrodes as detailed in Clause 3.13, but in some installationssoil conditions and the earth resistance of the tank when isolated from associated pipelines may in themselves constitute permanent and effective earthing. In such cases the necessity for tank earth electrodes should be considered with particular reference to site measurements of earth resistance. (b) The minimum number of individual earth electrodes on storage tanks will depend upon the diameter and soil condition, and should be in accordance with the following schedule for single tanks: Diameter of tank Minimum number of independent electrodes For a group of small tanks, earth electrodes common to the group may be installed, provided that each tank has two independent paths to earth. One of these paths may be through the pipeline earthing system. NOTE. The reason for the minimum of two earth electrodes is that during testing of one electrode the tank will remain earthed by the other electrode. Earth electrodesfor a tank may be interconnectedaround the periphery of the tank, and where two or more connections are used they should be spaced symmetrically round the tank. (c) Each earthing conductor should terminate in an approved design of cable lug and be attached to a steel boss welded to the tank body and tapped to receive a bolt or stud, preferably 10 mm diameter. Lock washers should be used on the connecting assembly. Soldered connections should be avoided. It is suggested that the boss be welded on the tank at a minimum height of 500 mm above the bottom of the tank. When a pipe or rod earth electrode is driven into the ground, mechanical protection should be given to the head of the electrode. NOTE. It is the practice of some organizations to enclose all earth stake heads in a pit, where they are associated with 'special' earthing, such as lightning protection or static earthing. (d) (e) Steel tanks with floating roofs according to their location should be protected by one of the measures described below: 0) Multiple shunt connections between the floating roof and' the tank shell, in particular those designs incorporating mechanical linkage in the seal assembly. This is the most effective method of discharging induced static charges on the floating roof caused by atmospheric conditions; under this arrangement it is not necessary to bond across internal drainpipe joints or external moving staiway joints. Overhead earth wires or other suitable forms of interception protection in accordance with Clause 3.2 (see also Subclause 5.2.2). This may be appropriate in areas where there is a known high thunderday level. (ii) 5.5.1.2 Above-ground steel tanks containing flammable liquids at atmospheric pressure. The contents of steel tanks with steel roofs or riieted, bolted, or welded construction, with or without supporting members, used for the storage of flammable liquids, are considered to be reasonably well protected against lightning if the tanks comply with the following recommendations: (a) (b) All joints between steel plates should be riveted, bolted, or welded. All pipes entering the tank should be metallically bonded to the tank at the point of entrance. All vapour or gas openings should be closed. The metal tank and roof should have adequate thickness so that holes will not be burned through by lightning discharges (5 mm sheet steel roofs on tanks are considered adequate for this purpose*). The roof should be continuously welded to the shell, or bolted, or riveted and caulked, to provide a gastight seam and electrical continuity. (c) (d) (e) 5.5.1.3 Steel tanks with non-metallic roofs. Steel tanks with wooden or other non-metallic roofs are not considered to be self-protecting, even if the roof is essentially gastight and sheathed with thin metal and with all gas openings closed or flameproofed. Such tanks should be provided with air terminals of sufficient height and number to receive all discharges and keep them away from the roof. The air terminals should be thoroughly bonded to each other, to the metallic sheathing, if any, and to the tank. Isolated metal parts should be avoided, or else bonded to the tank. In lieu of air terminals any of the following may be used: (a) (b) (c) 5.5.2 Conducting masts suitably spaced around the tank; Overhead earth wires; A combination of masts and overhead earth wires. Installations Handling Crude Oil And Products 5.5.2.1 Jetties for marine tankers and barges. The following recommendations should be observed as applicable: (a) General. All pipelines to jetties and structural steelwork plant and bollards on jetties together with associated dolphins, walkways, and shore bollards should be connected to the earthing system. Electrical equipment on a jetty should be connected to an earthing system as specified in SS CP 5 and SS CP 16. Dependent upon site and operating conditions, it may be possible to obtain overall protection by using one earthing system. Where it is considered that one common earthing system may be adapted to comply with all the requirements, it is necessary to ensure that the value of earth resistance does not exceed 1 ohm. Where steel or steel box piles are not employed, an earthing conductor should be installed to enter the water below low water mark to provide a direct path for lightning discharge. * This value is based on a recommendation in ANSI/NFPA 78. 65 (b) Jetties with cathodic protection. It is recommended that the following precautions be taken where jetties are protected by either sacrificial anodes or power-impressed systems to prevent sparking at the tanker manifold when loading lines are being connected or disconnected : (0 Install an insulating flange at the jetty end of each loading line between jetty and vessel whereby all flanges shore-side of the insulating flange are earthed to the jetty earthing system and all flanges to the seaward side are earthed via the vessel. Ensure that the insulating flange cannot inadvertently be short-circuited by the electrical connection of exposed metallic flanges on the seaward side of the insulating flange to the jetty structure either by direct contact or by hosehandling equipment. Where sacrificial anodes are installed, it may be necessary to use manilla mooring ropes or straps to extend the life of the anodes and minimize current flow between jetty and vessel. (ii) (iii) (c) Ship/shore bonding cables. An independent cable bonding connection between ship and jetty with or without cathodic protection, is not considered as serving any useful purpose in : (i) (ii) the dispersal of static electricity; or minimizing possible current flow in conductive type loading hoses. 5.5.2.2 Bulk rail car loading and discharging. For the bulk loading and discharging of rail cars reference should be made to Institute of Petroleum, Model Code of Safe Practice in the Petroleum Industry - Part I : Electrical. 5.5.3 Aircraft Fuelling And De-fuelling Aircraft fuelling and de-fuelling should be suspended when electrical storms are in the vicinity. 5.5.4 Structures With Explosive Or Highly Flammable Contents 5.5.4.1 Methods of protection. Structures with explosive or highly flammable contents should be protected in one or more of the ways detailed in Subclauses 5.5.4.2 to 5.5.4.5 and in accordance with the recommendations of Subclauses 5.5.4.6 to 5.5.4.15, as appropriate. 5.5.4.2 Air termination network. An air termination network should be suspended at an adequate height above the area to be protected (see Clause 3.2). Where a suspended conductor crosses chimneys or vents which emit explosive dusts or gases under forced draught, the suspended conductors should be at least 5 m above the chimney or vent. 5.5.4.3 Network of horizontal conductors. Where the expense of the method described in Subclause 5.5.4.2 cannot be justified, and where no risk is involved in discharging the lightning current over the surface of the structure to be protected, a network of horizontal conductors with a mesh n between 3 r and 8 m according to the risk, should be fixed to the roof of the structure. Each separate structure protected as above should be equipped with twice the number of downconductors recommended in Clause 3.10. 5.5.4.4 Vertical conductors. A structure or a group of structures of small horizontal dimensions may be protected by one or more vertical lightning conductors (see Clause 3.2). 5.5.4.5 Below-ground structures. A structure which is wholly below ground and which is not connected to any services above ground can be protected by an air termination network as described in Subclause 5.5.4.2 by virtue of the fact that soil has an impulse breakdown strength which can be taken into account when the risk of flashover from the protection system to the structure to be protected, including its services, is being determined. Where the depth of burying is adequate, the air termination network may be replaced by a network of earthing strips arranged on the surface in accordance with expert advice. Where this method is adopted, the recommendations on bonding between metal in, or metal conductors entering, the structure, given in Subclauses 5.5.4.7 to 5.5.4.1 1 should be ignored. Where the underground structure has a reinforced concrete roof at or immediately below soil level, the reinforcement may be used as a protectior~ system provided that the reinforcement is welded so that rectangular electrical conducting paths are formed with sides not exceeding 2 m in length. Where the underground structure has a roof which is not reinforced or where the reinforcement is not electrically continuous, a buried conductor network located above the structure and buried not less than 500 mm below the soil level may be used. Where the structure is such that protection cannot be provided by use of the reinforcement and the depth of soil above the roof is less than 500 mm, air terminations may be mounted on suitable bases above the structure at soil level and interconnected by a roof conduction network of closed mesh of between 3 m and 8 m. In structures containing nitroglycerine, the combined use of the systems described in Subclauses 5.5.4.3 and 5.5.4.4 is recommended. 5.5.4.6 Interconnection of earth terminations. The earth terminations of the earth protective system should be interconnected by a ring conductor. This ring conductor should preferably be buried to a depth of at least 500 mm and be at least 2 m from the walls of the structures unless other considerations, such as the need for bonding other objects to it, testing or risks of corrosion, make it desirable to leave it exposed. The resistance value of the earth termination network should be maintained permanently at 10 n or less. If this value proves to be unobtainable, the methods recommended in Clause 3.12 should be adopted, or the ring conductor should be connected to the ring conductor of one or more neighbouring structures until the above value is obtained. 5.5.4.7 Bonding of structural metal. All major metal forming part of the structure, including continuous metal reinforcement and services should be bonded together and connected to the lightning protection system. Such connections should be made in at least two places and should, as far as is possible, be equally spaced round the perimeter of the structure at intervals not exceeding 15 m. 5.5.4.8 Bonding of internal metal. lightning protection system. Major metalwork inside the structure should be bonded to the 5.5.4.9 Electrical conductors entering structure. Electrical conductors entering the structure should be metal-cased. The metal casing should be electrically continuous within the structure; it should be earthed at the point of entry outside the structure on the supply side of the service and bonded directly to the lightning protection system. 5.5.4.10 Electrical conductors connected to overhead supply line. Where the electrical conductors are connected to an overhead electricity supply line, a length of buried cable with metal sheath or arrnouring should be inserted between the overhead line and the point of entry to the structure, and a surge protective device, e.g. of the type containing voltagedependent resistors, should be provided at the termination of the overhead line. The earth terminal of this protective device should be bonded direct to the cable sheath or arrnouring. The spark over voltage of the lightning protective device should not exceed half the breakdown withstand voltage of the electrical equipment in the structure. In this operation, the appropriate Code and any regulations which may apply should be observed. 5.5.4.1 1 Metal not continuously earthed. Metallic pipes, electrical conductor sheaths, steel ropes, rails or guides not in continuous electrical contact with the earth, which enter the structure, should be bonded to the lightning protection system. They should be earthed at the point of entry outside the structure and at two points, one about 75 m away and one a further 75 m away. 5.5.4.12 Adit or shaft. For a buried structure or underground excavation to which access is obtained by an adit or shaft, the recommendations in Subclause 5.5.4.1 1 as regards extra earthing should be followed for the adit or shaft at intervals not exceeding 75 m as well as outside the structure. 5.5.4.13 Fences and retaining walls. The metal uprights, components and wires of all fences, and bf retaining walls in close proximity to the structure, should be connected in such a way as to provide continuous metallic connection between themselves and the lightning protectionsystem. Discontinuous metal wire fencing on non-conducting supports or wire coated with insulating material should not be employed. 5.5.4.14 Avoidance of tall components. Structures with explosive or highly flammable contents should not be equipped with tall components such as spires and flagstaffs or radio aerials on the structure or within 15 m of the structure. This clearance applies also to the planting of new trees, but structures near existing trees should be treated in accordance with Clause 4.3. 5.5.4.15 Tests of system. Tests should be carried out in accordance with Clause 6.2 at intervals of not more than 1 year. The test equipment used should be certified for use in the particular hazardous area. In some cases, non-certified testing equipment may be used provided that the location where the tests are to be conducted has been proven to be free of combustible gases or vapours by competent persons. SECTION SIX INSTALLATION AND MAINTENANCE PRACTICE 6.1 WORK ON SITE Throughout the period of erection of a structure, all large and prominent masses of metalwork, such as steel frameworks, scaffolding and cranes, should be effectively connected to earth. Once work has commenced on the installation of a lightning protection system, an earth connection should be maintained at all times. During the construction of overhead power lines, overhead equipment for railway electrification and the like, the danger to persons can be minimized by ensuring that an earthing system is installed and properly connected before any conductors other than earth wires are run out. Once the conductors are run out and insulation installed, they should not be left 'floating' while men are working on them but should be connected to earth in the same manner as when maintenance is being carried out after the line is commissioned. INSPECTION AND TESTING 6.2 All lightning protection systems should be inspected by a competent person after completion, alteration or extension, in order to verify that they are in accordance with this Code. On the completion of the installation or of any modification to it, the resistance to earth of the whole installation and of each earth termination should be measured, and the electrical continuity of all conductors, bonds and joints and their mechanical condition verified. The testing should be carried out in accordance with Appendix B. If the resistance to earth of a lightning protection system, when so determined, exceeds the specified value for the particular applications the value should be reduced to be in accordance with the recommendations of this Code. If the resistance is less than the recommended value but significantly higher than the previous reading, the cause should be investigated in accordance with Appendix B. This test should be repeated at intervals not exceeding one year. A slightly shorter period than one year would have the advantage of varying the season of the year at which the test was done. The condition of the soil, the procedure adopted, details of chemical or other soil treatment, and the results obtained should all be recorded as listed in Clause 6.3. RECORDS 6.3 The following records should be kept on site, or by the persons responsiblefor the upkeep of the installation: (a) (b) (c) (d) Scale drawings showing the nature and position of all component parts of the lightning protection system. The nature of the soil and any special earthing arrangements. Date and particulars of any chemical or special treatment. Test conditions and results in accordance with Clause 6.2. (e) Alterations, additions or repairs to the system. The name of the persons responsible for the installation or its upkeep. (9 NOTE. Detection of the occurrence of lightning flashes to the structure and the magnitude of the discharge current may be estimated by magnetic links, magnetic tape strips or other current monitoring devices. 6.4 MAINTENANCE If the general recommendations of this Code have been duly observed, little maintenance should be needed. The periodic inspection and tests described in Clause 6.2 will indicate what maintenance, if any, should be undertaken. Particular attention should be paid to earthing, to any evidence of corrosion and to any alterations or additions to the structure which may affect the lightning protection system. Examples of such alterations or additions are as follows: (a) (b) (c) Changes in the use of a building. Installation of fuel oil storage tanks. The erection of radio and television receiving aerials. SECTION SEVEN PROTECTION OF PERSONS WITHIN BUILDINGS SCOPE OF SECTION 7.1 This Section sets out recommendations for the protection of persons within buildings from the effects of lightning. These recommendations may be applied irrespective of whether a lightning protection system for the building structure is provided in accordance with other sections of this Code. 7.2 NEED FOR PROTECTION Whilst persons and equipment within buildings may be protected from a direct lightning strike, many circumstances arise where the effects of lightning are transmitted within the building, by various means as described bdow, placing persons and equipment at risk. Communicationsand electronic equipmentare particularlysusceptible to damagefrom lightning impulses and such damage may occur at energy levels well below those needed to cause injury to persons. In addition, there is a significant fire risk associated with impulse failure of many types of electrical and electronic equipment. 7.3 MODES OF ENTRY OF LIGHTNING IMPULSES There are three principal modes of entry of lightning impulses into buildings, as described below, and these may occur separately or in combination : (a) Directly by the interception of lightning on exteriormetal work. Lightning impulses may be transmitted to within the building as a consequence of a strike to exterior metal which has a direct conductive connection to the interior of the building, e.g. via communications antennas, plumbing fittings and the like. This mode of entry is characterized by a series path for the full impulse energy and is capable of conveying the full destructive effect of the lightning discharge. The waveshape of the lightning impulse is usually not significantly modified. Indirectly by the interception of lightning on other structures or services. A lightning strike to other structures or services which have conductive connection to the building, e.g. the low voltage electricity distribution system or other services, may result in an impulse being transmitted into the building. The impulse is characterized by a lower energy level compared to that involved in Item (a), being a shunt path to the interior of the building served by the low voltage mains. It is an earth potential rise (EPR) effect originated by the lightning impulse passing to ground through the neutral/earth conductor resulting in a increase in potential by ordinary ohmic means. The magnitude of the impulse at the structure is governed by the neutral/earth impedance at the interception point, the length of the service line, the number of earth features per unit length on the line adjacent to the interception point and, lastly, the electrical characteristics of the lightning discharge. The impulse wave is normally modified by the transmission path in the EPR mode by distributed electrostatic capacity and transmission line effects. This reduces the severity of the impulse but prolongs the conduction time of protection equipment. (b) Although the energy levels involved in an EPR impulse are substantially less than those which apply for ltem (a), they may still be of a high order. Based on sparks or arcs observed in incidents involving personal injury or equipment damage under EPR conditions, voltages of the order of 100 000 V are not uncommon and cases involving voltages of about 1 000 000 V have been observed. Impulse currents in this mode can range from a few amperes to several thousand amperes. It should be noted that EPR conditions can arise singly or as a combination of occurrences. In addition to lightning intercepting the low voltage overhead distribution, other lightning leaders may intercept trees, clothes lines, sheds or other nearby structures, giving rise to a quite complex overall EPR condition. (c) Inductively by electric and magnetic field coupling. In general, this mode of entry involves low energy levels and is of limited incidence in comparison with the mode in ltem (b). Induction occurs when a lightning strike to ground gives rise to electromagnetic and electrostatic fields. These fields induce an impulse in conductors that intercept them. The conductors which are most affected by this mode of entry are electricity reticulation and telecommunications lines. Commonly the former is not damaged but the impulse may be transmitted to customer terminals and appear as a lower level EPR type impulse. This may damage or disable some forms of communications equipment. These three modes of lightning impulse entry to a building are illustrated in the examples given in Figure 7.1. Protection systems designed to counteract EPR impulses will normally provide adequate protection against impulses arising from entry modes (b) and (c). 7.4 GENERAL CONSIDERATIONS FOR PROTECTION Because of the many variables involved, each building will require specific consideration of the protective measures which should be applied. Particular attention should be given to possible entry and exit points for lightning impulses which may include one or more of the following : Roof top or external structures (e.g. TV antennas, communication antennas, metallic flues and ventilation outlets) or other exposed metal work not protected by the lightning protection system for the building structure (e.g. metallic guttering and downpipes, metallic fences). These features will invariably be possible entry points for a lightning discharge. The electricity service entry. This will normally be an entry point for lightning if the service is aerial or overhead. It may be either an entry point or an exit point if the service is underground. The communication services entry. This may be an entry point if the service is overhead using a drop wire or aerial cable. The service is more commonly underground and in such cases could be either an entry point or exit point. Gas supply systems. This may either be an entry point or exit point if the service is underground. Water supply systems. This may either be an entry point or exit point if the service is underground. Other conductive services. This may either be an entry point or exit point if the service is underground. Earthing systems (often there are several). These are almost always exit points for lightning but may occasionally present an EPR entry condition. The lightning protection system for the building (if provided). By design these systems provide both an entry and exit for a lightning discharge but, because of bonding, will present an EPR condition to other services. An illustration of the possible entry and exit points for a lightning discharge is provided in Figure 7.2. PROTECTION OF PERSONS WITHIN BUILDINGS 7.5 Objectives Of Protection. The principal objective of measures for the protection of persons 7.5.1 within buildings is to prevent hazardous potential differences between conductive parts with which the person(s) may be in contact. This is normally achieved by applying equipotential bonding between any conductive path into and out of the building, i.e. the entry points and exit points referred to in Clause 7.4. If such bonding has been installed it does not matter l a person is subject to an earth potential rise with respect to distant earth as all conductive materials in the vicinity will be at approximately the same potential. An important consideration in the installation of equipotential bonding is how to install such bonding without adversely affecting the operation of the various services involved, particularly the protection systems associatedwith the respective systems. This is explained further in Subclause 7.5.2. 7.5.2 Installation Of Equipotential Bonding. All possible points of entry and exit for the lightning discharge should be electrically bonded together in as direct a manner as practicable. The route taken by the bonding conductors is important. If incorrectly routed the bonding conductors themselves may damage other circuits or equipment from induction or side flashing as currents of the order of tens of kiloamperes and voltages of the order of several thousand volts with respect to distant earth may be involved. Consequently, bonding conductors should not be grouped with other cables which are sensitive to induction unless they are bonded to the lightning protection system. If the bonding conductor is long (some tens of metres) it must be considered as an impulse transmission line, in which mode the protection afforded by the bonding will be limited. Some specific recommendations applicable to bonding of the entry and exit points referred to in Clause 7.4 are given Mow: (a) Roof top antennas and communicationshardwzre. The bonding conductor should be attached to the most substantial part of the structural metal supporting the equipment consistent with it fulfilling the requirements of an air termination for the lightning protection system of a building. The bonding conductor to the antenna or communications hardware should be insulated to at least the level required in SS 358: Part 5 and IEC 245-4 if run within the building, but may be uninsulated if run externally. The cross-sectional area of the bonding conductor should be at least 16 mrn2 of copper. (b) The electrici2y supply service entry. There are two distinct considerations which apply. Firstly, the supply earth should be bonded to the lightning protection system earth with a conductor dimensioned in accordance with Chapter 54 of SS CP 5, or to a copper conductor of 6 mm2 cross sectional area, whichever is the greater. Secondly, surge arrestors should be installedfor each active conductor. If the surge arrestor is mounted on, or in, the building its earthing system should be bonded to the lightning protection system by a conductor having a cross-sectional area of not less than that utilized for its own earthing conductor. Where the surge arrestor equipment is separated from the building (e.g. mounted on an electricity supply service pole), the surge arrestor earth should not be used as the earthing system for the building lightning protection system. However, the lightning protection system earth and the surge arrestor earth may be bonded together, if desired. The telecommunications service entry. This may be either aerial (overhead) or underground. If aerial, the service should be regarded as a potential entry point for lightning and a telephone protector should be fied, subject to the requirements of the telecommunications regulatory authority. The earthing conductor and earthing system should be bonded either to the electricity supply earthing system, or to the lightning protection system earth. The cross-sectional area of the bonding conductor need not exceed 4 mm2of copper. If the telecommunications service is underground, the service will act essentially as an exit for lightning. In this mode it may be necessary to fit a telephone protector to the service(s) to provide a bonding point for potential equalization. This bonding may also require a local earth, as determined by the telecommunications regulatory authority. The bonding conductor should have a cross-sectional area of not less than 4 mm2of copper. (c) (d) Water and gas supply systems. Metallic water and gas supply systems should be bonded to the lightning protection system and connected to the electrical supply earth. However, some water and gas supply authorities f i insulating spacers or ferrules for corrosion control at customers' installations. These may require bridging, particularly in thecase of gas services, by a surge arrestor as determined in consultation with the gas supply authority. Bonding conductors to these services should have a crosssectional area of not less than 4 mm2of copper. If calculation or local experience indicates that the water service is of very low resistance to ground (e.g. less than 0.5 a), it may well be the principal exit for the lightning impulse. In such circumstances, consideration should be given to upgrading the current capacity of the bonding conductor between the lightning protection system earth and the water service to a cross-sectional area of at least 16 mm2 of copper. (e) Other service lines. Specific considerations may apply for some structures. For example, a radio telephone tower should be bonded to its associated equipment building; similarly, a pump station should be bonded to an elevated water tower. For both examples given, the bonding conductor is likely to carry the full lightning current and should therefore have a cross-sectional area of at least 16 mm2of copper. The earthing systems. Buildings frequently have several earthing systems that may be installed independently at different times. These include the electricity supply earth, telecommunications earth (sometimes more than one), lightning protection earths and other special purpose earths. (9 It is generally desirable to bond all such earths but there may be specific reasons for not doing so. Direct-current-carryingearths, e.g. older telex systems, should usually be isolated to prevent corrosion damage to other services and earths. In such cases consideration should be given to bonding these earths through a polariition cell, to facilitate the protection of persons from lightning surges. This type of cell can be used where there is a corrosion-based objection to bonding, e.g. copper-based earths to galvanized iron earths or structures, of which the latter would suffer galvanic corrosion. If 50 Hz or audio frequency bonding is not needed, a gas discharge arrestor may serve the purpose. Bonding conductors between earthing systems should have a crosssectional area not less than 4 mm2of copper. (g) The lightning protection system earth. Where a lightning protection system is in place all of the above services should be bonded to the lightning protection system earth. + (a) Direct (b) Indirect Overhead l i n e s ( ~ u l s e induced), \\ (c)Induced Figure 7.1 Modes of entry of lightning impulses 75 TV antenna Communications antenna Air termination LEGEND: Possible entry or inlet @ Possible exit or drain @ NOTES 1. See Clause 7.4 for further information and qualifications concerning entry and exit points. Equipotential bonding which may be necessary for protection is not shown. 2. Figure 7.2 Possible entry and exit points for a lightning discharge APPENDIX A THE NATURE OF LIGHTNING AND THE PRINCIPLES OF LIGHTNING PROTECTION (Informative) A.l SCOPE OF APPENDIX This appendix deals with the nature of the phenomena involved in a study of lightning protection and the basic principles of designing such protection. A brief description of various elements of a lightning protection system and their function is also provided. Recommendations for systems to protect against the direct or indirect effect of lightning are given in the body of this Code. A.2 NATURE OF LIGHTNING A.2.1 Nature Of Lightning. Thunderstorms occur under particular meteorological conditions, and partial separation of electrical charges within the thundercloud usually results in regions with net negative charge mainly in the lower parts of the thundercloud, and regions with net positive charge mainly in the upper part. Lightning is an electrical discharge between differently charged regions within the cloud (cloud flash) or between a charged region, nearly always the lower negatively charged region, and earth (ground flash). A complete ground flash consists of a sequence of one or more high amplitude short duration current impulses, or strokes. In some ground flashes low amplitude long duration currents (sometimes termed continuing currents) flow between the strokes or after a sequence of strokes. The currents are unidirectional and usually negative, i.e. a negative charge is injected into the object struck. For all practical purposes the stroke can be considered to be generated by a current source whose waveshape and magnitude are unaffected by the characteristics of the ground termination. A.2.2 The Lightning Attachment Process. The first stroke of a ground flash is normally preceded by a downward-progressing low-current leader discharge which commences in the negatively charged region and progressestowards the earth, depositing negative charge in the air surrounding the channel. When the lower end of the leader is roughly 100 m from the earth, electrical discharges (streamers) are likely to be initiated at protruding earthed objects, and to propagate towards the leader channel. Several streamers may start, but usually only one is successful in reaching the downcoming leader. The high current phase (return stroke) commences at the moment the upward moving stream meets the downcoming leader. The position in space of the lower portion of the lightning channel is therefore determined by the path of the successful streamer, i.e. the one which succeeded in reaching the downcoming leader. The primary task in protecting a structure is therefore to ensure a high probability that the successful streamer originates from the lightning protection conductors, and not from a part of the structure that would be adversely affected by the lightning current that flows subsequently. As the path of the successful streamer may have a large horizontal component, e.g. many tens of metres, as well as a vertical component, an elevated earthed conductor will provide protection for objects spread out below it. It is therefore possible to provide protection for a large volume with a relatively small number of correctly positioned conductors. This is the basis for the concept of a zone of protection provided by an elevated earthed conductor, and provides the basic principle underlying interceptionlightning protection. Thus the basic protection system consists of air termination electrodes to provide launching points for streamers, and downconductors and earth electrodes to deliver the lightning current into the earth. A.2.3 Thunderstorm And Lightning Occurrence. Thunderstorm occurrence at a particular location is usually expressed in terms of the number of calendar days in a year when thunder was heard at the location, average over several years. In Singapore this number of thunderday per year (isoceraunic level) is estimated to be 190. The corresponding ground flash density, N is estimated to be about 12.6 , ground flashes per sq km per year. The occurrence will be higher than the average on high ground, e.g. ridges, and lower than average on nearby low ground. In some cases, a large topographical feature such as a high mountain may interact with prevailingmeteorological conditions to cause a concentration of thunderstorms and ground flashes. On a smaller scale, tall objects, e.g. roof of a building, tree top or overhead conductor, tend to divert lightning flashes to themselves, as explained in A.2.2, thus shielding a certain surrounding area from direct strikes. A.3 EFFECTS OF LIGHTNING The principal effects of a lightning discharge to an object are electrical, thermal and mechanical. These effects are determined by the magnitude and waveshape of the current discharged into the object. Statistical distributions of some characteristics of ground flashes are given in Table A.1. When the lightning current flows through the building or its lightning protection system, the electrical potential of the building may rise to a high value with respect to remote earth (this terminology is usually adopted despite the fact that the potential is usually negative with respect to remote earth). It may also produce around the earthing electrodes a high potential gradient which can be dangerous to persons and to livestock. The rate of rise of current in conjunction with inductance of the discharge path produces a voltage drop that will vary in time depending upon the current waveshape. As the point of strike on the lightning protection system may be raised to a high potential, there is also the risk of a flashover from the lightning protection system to nearby metal objects. This is called a side-flash. The risk of side-flash is increased at any deeply re-entrant bend or loop in a downconductor due to the local increase in inductance. If such a flashover occurred, part of the lightning current would be discharged through internal installations with consequent risk to the occupants and the fabric of the building. The amount of energy deposited in any object carrying lightning current may be calculated by multiplying the action integral by the electrical resistance of the object. From this, the temperature rise may be calculated. It should be noted however that the resistance of most objects other than metallic conductors, e.g. wood, masonry or earth, is very non-linear for the large currents associated with lightning. It should also be noted that the passage of lightning current through moist resistive materials such as masonry or wood can convert the moisture to high-pressure steam, causing the material to explode or shatter. The thermal effect of a lightning discharge is confined to the temperature rise of the conductor through which the lightning current is discharged. Although the amplitude of a lightning current may be high, its duration is so short that the thermal effect on a lightning protection system, or on the metallic parts of a structure where this is included in the lightning protectionsystem, is usually negligible. This ignores the fusing or welding effects which occur locally consequent upon the rupture of a conductor which was previously damaged or was of inadequate cross-section. In practice the cross sectional area of a normal lightning conductor is determined primarily by mechanical and secondarily by thermal considerations. Table A.l Summary of the frequency distributions of the main characteristics of the lightning flash to ground Percentage of events having value of characteristic greater than value shown below (See Note 1) Unit Item No. Lightning characteristics Number of common strokes Time interval between strokes First stroke peak , current I , Subsequent stroke peak current I, , First stroke (di/dt. m a Subsequent stroke (dm) m Total charge Continuing current charge Continuing current 99 1 2 3 4 5 6 7 90 75 2 35 50 3 55 25 10 7 1 12 1 10 5 1 25 5 90 50 150 400 130 ms kA 12 6 n 10 15 25 6 30 15 25 45 15 80 30 3 6 6 20 30 40 70 kA 10 15 3 10 50 100 3xld 40 100 70 70 '31s w s 80 40 40 150 200 1 200 100 C C A ms 8 6 30 50 20 30 100 9 80 250 lo3 200 900 400 1500 5x16' I 10 11 m , Overall duration of flash Actionintegral(see Note 2) 400 5xld 600 3x10~ l d 105 AS ' NOTES : 1. The values shown in this Table have been derived from a number of sources, and have been rounded in accordance with the accuracy with which these data are known. Values at the 1 percent and 99 percent levels are very uncertain, and are given only to indicate an order of magnitude. The action integral, defined as i2dt for the whole flash, is equivalent to the energy deposited in a oneohm resistor by the passage of the entire c ent for the duration of the flash. 2. L At the point of attachment of a lightning discharge channel to a thin metal surface, a hole may be melted in the surface. In this case, some thermal energy will be deposited directly in the metal from the hot plasma of the discharge channel, as well as the thermal energy caused by the passage of current through the metal. The size of the hole melted in the sheet depends on the material, the thickness of the sheet, and the charge delivered. For example, a moderately severe lightning flash delivering a charge of 70 C would melt a hole about 100 mm2 in area in a sheet of galvanized iron 0.38 mm thick. The passage of lightning current through a conductor causes a force on the conductor given by the equation : F = Bxlxi where : F B the force on the conductor, in newtons (N); the component of the magnetic flux density perpendicular to the conductor, in teslas (T); the length of the conductor, in metres (m); the current through the conductor, in amperes (A). A.4 POTENTIAL DIFFERENCES CAUSED BY LIGHTNING A.4.1 General. A lightning flash to a building or structure, or a flash to ground near a building or structure will cause a potential rise in the vicinity of the strike attachment point, and may also cause a potential rise of objects remote from the point of strike. For example, a lightning strike to a service conductor (power or communications, or other metallic service) can cause current to be transmitted to the building, thus raising the potential of the building. A lightning flash to ground can also induce voltages and currents in remote conductors by electric and magnetic coupling (see also Section 8 and Appendix D). A.4.2 Earth Currents. At the point where the lightning current enters the ground the current density is high. Hazardous earth potential gradients may be generated. Earth electrodes should be distributed more or less symmetrically, preferably outside and around the circumference of a structure, rather than be grouped on one side. This will help to minimize earth potential gradients near the building, and tend to cause the lightning current to flow away from the building rather than underneath it. In addition, with earth connections properly distributed, the current from a lightning flash to ground near the building will be collected at the outer extremities. Thus current flow underneath the building, as well as ground potential gradients, will be minimized. A.4.3 Side-flash. If a lightning conductor system is placed on a building and there are unbonded metal objects of considerable size nearby, there will be a tendency for side-flashing to occur between the conductors of the lightning protection system and the unbonded metal objects. To prevent damage from side-flash, interconnecting conductors should be provided at all places where side-flashes are likely to occur. This is referred to as equipotential bonding, although complete equalization of potential is never achieved. As the currents required to equalize potentials are considerably less than the full lightning current, conductors of relatively small cross-section are adequate for this purpose (see also Subclause 3.14.2). A.4.4 Potential (Voltage) Differences. The impedance of the earth termination networkto the rapidly changing lightning current influences the potential rise of the lightning protection system. This in turn affects the risk both of side-flashing within the structure to be protected, and of dangerous potential gradients in the ground adjacent to the earth termination network. The potential gradient around the earth termination network, on the other hand, depends on the physical arrangement of the electrodes and the soil resistivii. In Figure A.l a lightning flash is assumed to occur to the lightning protection system of a building. For the purposes of the illustration, no equipotential bonding is shown although such bonding is required in accordance with this Code. As the lightning current is discharged through the downconductor and the earthing electrode, the conductor system and the surrounding soil are raised, for the duration of the discharge, in potential with respect to the general mass of the earth. The resulting potential differences as shown by 'step', 'touch' and 'transferred' potentials in Figure A.l may be lethal; hence the importance of keeping the impedance of the earth termination network low, and of preventinglarge local potential gradients by equipotential bonding, and by the manner in which the earth electrodes are arranged. AS PRINCIPLES OF LIGHTNING PROTECTION A.5.1 Purpose Of Protection. The purpose of lightning protection is to protect persons, buildings and their contents, or structures in general, from the effects of lightning, there being no evidence for believing that any form of protection can prevent lightning. InterceptionOf Lightning. The function of an air termination electrode in a lightning protection A.5.2 system is to divert to itself the lightning discharge which might otherwise strike a vulnerable part of the object to be protected. It is generally accepted that the range over which an air termination electrode can attract a lightning discharge is not constant, bul increases with the severity of the discharge. The path of a lightning discharge near a structure is determined by the path of the successful streamer (see A.2.2) which will usually be initiated from a conducting part of the structure nearest to the downcoming leader. The initiation of streamers is also influenced by the local electric field. The upper outer edges and corners of buildings or structures, and especially protruding parts, are likely to have higher local electric fields than elsewhere, and are therefore likely places for the initiation of streamers. When the downcoming leader is within about 200 m of the building, the electric field at these protruding parts and comers will exceed the breakdown field strength of air, resulting in corona currents that cause these parts to be surrounded by ionized air. The resulting space charges influence the electric field in such a manner that the field is limited to the breakdown strength of air. However, these compicating factors do not alter the fact that the most probable strike attachment point on a building is the edge, corner, or other protruding part closest to the downcoming leader. This is the basic reason why the rolling sphere method gives a reliable guide to the most probable strike attachment points. Hence, if air terminations are placed at all locations where high electric fields and streamer initiation are likely, there will be a high probability that the discharge will terminate on some portion of the lightning protection system. A.5.3 Determination Of Lightning Strike Attachment Points To Buildings A.5.3.1 The rolling sphere method. The procedure for determining lightning strike attachment points is based on the rolling sphere method whereby a sphere of specified radius (45 m for standard level of protection, see A.7) is imagined to be rolled across the ground towards the building, up the side, and over the top of the building, and down the other side to ground. This can be carried out in various orientations with respect to the building. Any point on the building touched by the sphere is a possible lightning strike attachment point. NOTES: 1. Person X is in contact with the ground at a and b; Person Y is in contact with the ground at c and the conductor at d; Person Z is in contact with the conductor at e and a metallic handrail f shown grounded at g. Person X is subject to 'step' potential. Person Y is subject to 'touch' potential. Person Z is subject to 'transferred' potential. The potential depends on the current magnitude and the impedance of the path of the lightning discharge. Step potential increases with the size of the step a-b in the radial direction from the conductor and decreases with increasing distance between Person X and the conductor. The transferred potential increases with increasing radial distance between the downconductor and the ground g. The diagram does not show equipotential bonding which may be necessary to protect persons from hazardous potential differences of the type described in this diagram (see Sections 3 and 7). 2. 3. 4. 5. 6. 7. 8. Figure A.l Instantaneous potential differences during a lightning flash to an earthed conductor The physical basis for this method is as follows. As the lightning leader stroke approaches the ground, the electrical field at various salient points, such as the upper comers of buildings, will help to launch electrical discharges, or streamers, which progress upwards toward the tip of the down-coming leader stroke. The position in space of the lightning discharge channel, and the location of the strike attachment point, is determined when the leader and one of the upward streamers join to complete the lightning discharge path. The upward streamer which determinesthe strike attachment point is generally that launched from the salient point or earthed conductor closest to the downcoming leader. The rolling sphere will tend to touch those salient points, and the method therefore provides a geometric means of identifying such points. A.5.3.2 The striking distance. The striking distance, d, , is the distance between the leader tip and the eventual strike attachment point at the moment when it has become inevitable that the gap, of dimension d, , will be bridged by the discharge channel. The rolling sphere method is closely related to the electrogeometric method developed for predicting lightning attachment to electric power lines, whereby the lightning leader is supposed to progress until it comes within the distance ds , of an earthed object, when the final discharge path is determined to that object. There are theoretical and observational grounds for a relationship between d,, and the i-, where i,, is the peak return stroke current. The following relationship has been proposed: where: d, I,, = = the striking distance, in metres (m); the peak current of the return stroke, in kiloamperes (kA) The advantage of the rolling sphere method is that it is relativelyeasy to apply, even to buldings of complicated shape. The limitation of the method is that no account is taken of the influence of electric fields in initiating return streamers, and the method therefore does not distinguish between likely and unlikely lightning strike attachment points. In particular, the enhancement of electric field at the upper outer corners of a building makes these comers the most probable strike attachment points, whereas return streamers are unlikely to be initiated from a flat surface away from a comer or edge, even if on the r o d and touched by the sphere. Some qualitative indication of the probability of strike attachment to any particular point can be obtained if the sphere is supposed to be rolled over the building in such a manner that its centre moves at constant speed. Then the length of time that the sphere dwells on any point of the building gives a qualitative indication of the probability of that point being struck. Thus for a simple rectangular building with a flat roof, the dwell time would be large at the comers and edges, and small at any point on the flat part of the roof, correctly indicating a high probability of the comers or edges being struck, and a low probability that a point on the flat part of the roof will be struck. The rolling sphere method needs to be applied with electric field enhancement effects in mind, so that high priority is given to providing air termination electrodes at the more probable attachment points. For a building of more or less rectangular shape with a flat roof, this means g'Ning top priority to providing air termination electrodes around the periphery of the roof. This could take the form, for example, of a metallic perimeter handrail. A.5.4 Protection Of The Sides Of Tall Buildings. When the rolling sphere method is applied to a building of height greater than the assumed sphere radius, then the sphere touches the sides of the building at all points above a height equal to the sphere radius. This indicates the possibility of strikes to the sides of the building, and raises the question of the need for air termination conductors on the sides of the building. Practical experience indicates that strikes to the sides of tall buildings do occur but are uncommon. There are theoretical reasons for believing that only flashes with low i,, and consequently low d, values are likely to be able to penetrate below the level of the roof of the building and strike the sides. The consequences of a strike to the sides of a building are likely to be dislodgement of masonry, as the current penetrates to the building reinforcing steel. If it is decided that some protection for the sides of a building is justified, then conductors should be provided at the most probable lightning attachment points on the sides of the building. The most probable attachment points are at protruding corners and vertical edges of the sides of the building, including changes of direction that are determined as requiring protection in accordance with Subclause 3.9.3.1. The conductors will generally serve both as air termination conductors and downconductors and will in general be connected to the roof air termination conductors at their upper ends, and to the earthing network at their lower ends. The conductors may be made flush with the surface, and should be placed as near as practicable to the vertical edge to be protected. Where the building construction includes extensive metal objects on the vertical outer surfaces, such as large metallic window frames, then such objects can form part of the interception protection system. It is necessary to provide electrical connections between adjacent metal objects both in the horizontal and vertical directions, and to provide periodic connections between the surface metalwork and the reinforcing steel, or the downconductors if separate from the reinforcing. This provides multiple paths for the lightning current from any point on the surface metalwork to earth, and local potential differences will be reduced to an acceptable level. Return streamers are more likely to develop from a good conductor on the surface than from a poor conductor in a similar position. Thus if a wall consists mainly of a poorly conducting material, and there are isolated.objects with earth connections made of a highly conductive material distributed over the surface, then the return streamers will tend to originate preferentially from the highly conducting objects with earth connections. A wall of poorly conducting material can therefore be substantially protected by earthed metallic studs placed on a grid pattern flush with the surface of the wall. Alternatively, protection can be provided by a system of metallic strips flush with the surface. Even if the lightning happens to strike a point on the non-conducting surface away from an earthed conducting point, it is likely that it will track across the non-conducting surface and terminate on the earthed conducting point. A.5.5 Safe Discharge To Earth A.5.5.1 General. If the air termination network is adequately connected to earth the current will pass to ground without damage to the structure. Metallic parts of a building or structure may usefully be made part of the lightning conductor system, provided that the passage of lightning current will not cause harm (for bonding of metal in or on a structure, see Subclause 3.14.2.2). A.5.5.2 Use of reinforcing steel as a downconductor. It is sometimes suggested that overlapped and tied reinforcing rods do not provide good electrical connections, and are therefore not suitable for carrying lightning currents. However, the situation differs greatly from that in which a conducting path for power currents is required. Even if there are thin films of iron oxides and cement between the bars, the voltage required to cause breakdown of these films would be less than 1000 V. Once breakdown has occurred, there would be localized arcing between the steel bars, with a voltage drop of a few tens of volts. The initial breakdown across the oxide and cement films would occur during the first few microseconds of the first stroke when there is a large inductive voltage drop from top to bottom of the building; this vdtage would be very much larger than the voltage required to break down the oxide and cement films between bars. Thus there are good reasons for relying on the reinforcing bars to act as downconductors, even when no special precautions have been taken (such as welding the bars together) to ensure electrical continuity. The localized arcing referred to above would produce relatively small amounts of energy in relation to the thermal capacity of typical reinforcing bars, so heating effects should be negligible. Where the structural steel reinforcement of the building is to be utilized as the downconductor system, it is important that there be an effective electrical connection between the air termination system and the steel reinforcement. Such connections should be made as close as practicable to the top of the building and preferably at a number of points around the building perimeter. Tall metal structures, such as chimneys, provide an adequate conducting path, but care must be taken to ensure that they are also suitably earthed. Special precautions are needed for the protection of structures containing explosives, highly flammable materials and gases. The principles involved in such protection systems are given in Section 5. A.5.6 Potential Equalization As explained in A.4, lightning strikes may give rise to harmful potential differences within a building. Of particular concern is the occurrence of potential differences which may exist between incoming conductors such as metallic water services, telecommunication systems, power systems, and local earth. Reduction of these potential differences may be achieved by a system of coordinated bonding of all affected conductors contained in the building. This includes all incoming metallic services, protection earths associated with power and communication systems, and the building lightning protection earthing system (if provided). Potential equalization (understood to imply approximate potential equalization) may be effected by including in the bonding scheme earthed building metalwork such as reinforcement metals and metal framework, ifany. In cases where the presence of dissimilar metals may create corrosion problems or for other reasons, the commoning path may be effected by using suitably rated overvdtage protection devices. A.5.7 Protection In Open Spaces As amended Feb 1999 A.5.7.1 Although nobody should remain in an open space during a thunderstorm, there are situations where people may happen to be in the open. If there are lightning strikes and they are unable to take shelter in time they will be exposed to the risk of direct strikes and/or be subject to the effect of potential difference developed in the ground which may cause injury or fatality. Such areas may need to be provided with additional protection. A.5.7.2 Designated protected zone When identifying the open areas for lightning protection, the size of the open spaces, the nmber of people using these spaces and the frequency of usage will be considered. Based on these considerations, two types of designated areas have been identified. Type A: Small defined open spaces where full protection can be provided. Some examples of these are the playgrounds and hard courts in residential estates. Type B: Large open spaces where full protection is not practicable. Some examples of these are: a) b) c) d) e) school fields; public parks; beaches; golf courses; vacant lands used for ad-hoc activities such as fun fairs, trade fairs, etc. A.5.7.3 Type A open spaces are usually small defined areas which are surrounded by tall buildings and are inherently protected from direct strikes. However, because of the proximtty of such areas to the lightning conductors of the surrounding buildings, the risks of step potentials caused by lightning striking a nearby building is probable. To minimise the danger of step potentials to people, an effective solution is to provide equipotential netting in the designated area. This equipotential netting comes in the form of a network of ground conductors electrically bonded together to provide a common ground. A.5.7.4 In some situations, a Type A area may be far from any building and is hence exposed to the risk of direct strikes. For example, a children playground may be located amidst an open ground away from the residential blocks in a housing estate. In such a case, it is necessary for the playground to be provided with overhead protection as well as underground equipotential netting to protect its users from both direct strikes and step potentials. A.5.7.5 For Type B areas such as parks, beaches, golf courses, etc. which are frequented by large numbers of people, it is recommended that protected shelters be erected at suitable locations and intervals to provide protection for users/visitors during and pending thunderstorms. In conjunction, a localised lightning detection and warning device should also be provided to alert users of approaching storms. A.5.7.6 For those Type B areas where ad-hoc activities such as fun fairs, trade fairs and outdoor events are organised occasionally, the provision of a lightning detection and warning device would suffice. On being alerted of an approaching storm, the visitors or participants could be advised through the public address system to disperse and to take shelter in nearby building or structure. A.5.7.7 The required mesh size for equipotential netting depends principally on the magnituje of lightning stroke current and resistivtty of the soil. In the absence of information on soil resistivtty, a reasonable mesh size could be 1 m by 1 m and with cross sectional area of conductor of not less than 20 mm2. Some guidance on the design of earth meshes for safety is given in IEEE Std 80 entitled IEEE Guide for Safety in AC Substation Grounding. In many cases, floorings are already provided with reinforced steel meshes embedded in concrete. In such situations, it is necessary only to ensure that these meshes are electrically continuous. Where such equipotential netting is directly buried in soil, the issue of corrosion should be adequately addressed. A.5.7.8 For school fields, a lightning detection and warning device would be sufficient as the fields are invariably next to the school buildings. A.6 ELEMENTS OF A PROTECTION SYSTEM The main parts of a typical lightning protection system for a building or structure may be summarized as follows, noting that not all parts will be present in all systems. Air terminations are placed so as to achieve interception lightning protection, ensuring a high probability that lightning will attach to the air termination network, and not to parts of the protected object that could be damaged by lightning current. Existing metalwork should be used as far as possible, supplemented by carefully positioned air terminations giving priority to high probability strike attachment points. These are the upper outer corners and edges of the buildingand any d e n t or protruding objects on the roof. The form of air termination should be chosen for simplicity and low cost consistent with adequate mechanical strength, durability and aesthetic acceptability. Downconductors are used to convey lightning current towards the earth. Existing building metalwork should be used as far as possible, especially steel frames and reinforcing steel in reinforced concrete columns, supplemented where necessary by external downconductors. If these downconductors are also to serve as part of the air termination network for the sides of a tall building, they should preferably follow the outer vertical corners of the building. Where the number of downconductors required exceeds the number of vertical corners, the remaining downconductors should be placed uniformly between the ones at the comers. Test links may be required between the downconductors and the earth electrodes to facilitate the testing of the lightning protection system. The earth termination ne2work consisting of one or more earth electrodes, and any interconnecting conductors between earth electrodes, serves the purpose of delivering the lightning current into the general mass of the earth. The footings of large reinforced concrete buildings will generally provide a better earth connection than can be provided by driven electrodes around the periphery. Where the superficial layers of the earth have high resistivity, deep driven electrodes may be needed to reach low-resistivity regions, and achieve an acceptable earth resistance. Equipotential bonding is used to reduce or prevent hazardous potential differences between any pair of extended conducting objects in the building or structure. Equipotential bonding becomes particularly important in buildings or structures having a high earth resistance. In the extreme situation where an acceptable connection to earth cannot be achieved, it would be necessary to rely entirely on equipotential bonding to protect persons and equipment against hazardous potential differences caused by lightning. Equipotential bonding may, in some situations, be achieved by means of overvoltage protection devices, where direct connection of the conducting parts results in an unwanted effect, for example, corrosion of metals. Overvoltage protection is achieved by using various types of overvoltage protection devices (e.g. spark gaps, gas-filled surge arrestors or metal oxide varistors) to prevent hazardous potential differences being applied to persons or equipment, while allowing correct operating potentials to exist (see Section 7 and Appendix D). The protection recommended in Section 3 (see Clause 3.2) is referred to as the standard level of protection. This protectionwill be satisfactoryfor a wide range of buildings and structures. However, there are some situations in which a higher level of protection is desirable, for example, where there is vulnerable rand expensive equipment in the building, or where the building contains explosive or flammable materials (see Subclause 5.2.2). There are some situations in which economics may prevent the installation of standard lightning protection, but a reduced level of protection at a lower cost can be justified. These variations from the standard level of protection may be achieved as follows. The rolling sphere radius, specified as 45 m for standard protection, should be reduced to give enhanced protection, and increased to give reduced protection. For example, a sphere radius of 20 m will result in more closely spaced air termination conductors, giving improved protection against lowcurrent discharges. Conversely, a sphere radius of 70 m will result in a protection system with a higher probability that low-current discharges will penetrate past the air termination conductors and strike an unprotected part of the structure. The consequences of such a protection failure need to be weighed against the economic saving made by choosing a reduced level of protection. The effect of varying the rolling sphere radius is indicated in Table A.2. Table A.2 Effect of varying the radius of the rolling sphere Rolling sphere m Correspondence peak current (approximate) kA Approximate percentage of events with lower peak current % A sphere radius of 70 m is included for illustrative purposes. A maximum sphere radius of 45 m is recommended for the design of lightning protection systems in accordance with this Code. The shielding failure rate for a lightning protection system may be defined as the proportion (or percentage) of all flashes to the structure that bypass conductors of the air termination system and strike an unprotected part of the structure. The shielding failure rate for a particular structure cannot be directly determined from the information given in Table A.2. For example, if the lightning protection system for a structure were designed using a rolling sphere radius of 45 m, one could not infer that all flashes with a striking distance less than 45 m would bypass the protection system and strike an unprotected part of the structure. Therefore one could not infer a shielding failure rate of 7%. In an actual situation, the shielding failure rate would be considerably less than 7%. The most that might be inferred is that there is a small probability that flashes with a peak current less than 10 kA will bypass the lightning protection system. Similarly, the protection provided for the sides of tall buildings can be increased or reduced respectively by reducing or increasingthe horizontal and vertical intervals that conductive connections are provided to the lightning protection system, relative to the intervals recommended in Subclause 3.9.3.2. APPENDIX B NOTES ON EARTHING ELECTRODES AND MEASUREMENT OF EARTH IMPEDANCE (Informative) GENERAL B.l B.l.l Function Of Earth Electrode System. The function of an earthing electrode is to provide an earthing connection to the general mass of earth. The characteristic primarily determining the effectiveness of an earth electrode or group of earth electrodes is the impedance which it provides between the earthing system and the general mass of earth. 8.1.2 Factors Influencing Earth Impedance. The impedance of the earth electrode to lightning currents varies with time and the magnitude of the current, and is dependent on: (a) (b) (c) (d) the resistance and surge impedance of the earth electrode system and the connecting conductors; the contact resistance between the earth electrode and the surrounding soil; the resistivity of the soil surrounding the electrode; and the degree of soil ionization. The resistance of the metallic conductors in the earthing system can generally be neglected. In addition there are often fortuitous paths to earth, e.g. via bonded electricity reticulation low voltage neutrals. These can mask the electrode impedance by paralleling other routes of high surge impedance but low d.c. or low-frequency impedance to earth. It is essential to utilize measurement techniques, referred to later, to discriminate between these conditions. Measures For Reducing Earth Impedance. Lightning current is considered to be a high 8.1.3 frequency phenomenon with current rise times in the order of 101° amperes per second. In these circumstances, an earthing system can best be regarded as a 'leaky' transmission line. Each conductor has resistance, inductance and capacitance to ground and leakage through non-insulated contact. An examination of earth conductors using transmission line equations will show that earth impedance is lowered by the following: (a) The use of flat tape rather than circular conductors. This increases surface area, reduces high frequency resistance due to skin effect, increases both capacitive coupling and the ground contact area for a given cross-section of conductor. The use of a centre point feed to create the effect of two parallel connected transmission lines is also effective. This concept can be further enhanced by using up to six radial conductors emanating from the injection point. The use of short-length multiple conductors is preferred over long buried systems. For a lightning discharge with a rise time of 0.4 cs and a propagation velocity of 0.75 times the speed of light, the peak current will be injected before the leading edge has travelled more than 90 m along a conductor. (b) (c) In areas of low to moderate soil resistiv'i, vertical electrodes will, for an electrode of given dimensions, usually be more effective in providing a low surge impedance. When trench (horizontal) electrodes are installed, the initial surge impedance of two or more paralleled wires or strips will be less than the equivalent length laid as one single unit. However, the multipled electrode will be of higher d.c. or low-frequency resistance due to electric field interaction between the individual electrode segments. The optimum surge performance for a single horizontal electrode will usually be achieved when the downconductor attaches to its midpoint. The contact resistance between the electrode and the soil can be up to about 10 percent of the total resistance of the earth electrode system. This resistance may be reduced by ionization and arcover in the soil in contact with the electrode. The major part of the earth resistance of an electrode arises from the resistance of the earth in the immediate v i c i n i of the earth electrode. The value of this resistance depends upon the shape, size, and position of the electrode and the resistivii, moisture content and degree of ionization of the soil in the vicinity of the electrode. The ratio of resistance at peak impulse current to resistance at low current depends on the number and arrangement of the electrodes, the peak current and soil resistivity. Examples are given in Table B.1. 8.2 RESISTIVITY OF SOIL 8.2.1 General. Soil resistivii is another term for the specific resistance of soil. It is usually expressed in ohm metres (symbol am), i.e. the resistance in ohms between opposite faces of a cube of soil having sides 1 m long. The resistivity of the soil depends on the chemical and mechanical composition of the soil, and the moisture content and temperature of the soil. In view of this there is a very large variation in resistivity between different types of soils and with different moisture contents. This is illustrated in Tables 8.2, 8.3 and B.4. NOTE. Electrodesshould not be located near brick kilns or other installations where the soil can be dried out by the operating temperatures involved. Table 8.1 Examples of reduction of resistance of earth connection under impulse conditions No. of rods and arrangement Soil resistance characteristics Soil resistivity, a m Resistance at low current, n Resistance at current peak, n Ratio of resistance at current peak/resistance at low current NOTE. The table depends on the following earthing dimensions and conditions: Diameter of rods Peak current injected One isolated rod 1o2 30 11.3 1o3 300 54 Four rods at corners of square, 3.05 m sides 1o2 10.5 1o3 105 37 6.8 = 10 mm = 80 kA Depth in earth Time to current crest = 3.05m = 4ps Table 8.2 Resistivity values for various materials Resistivii, n.m Material Salt sea water Damp clay Inland lake water, reservoirs River banks, alluvium Clay/sand mixture Concrete (see Note) Secondary rock Typical 0.2 1 10 20 25 30 100 3 000 1 ! I I Usual limits 0.15 to 0.25 2 to 12 10 to 500 10 to 100 20 to 200 40 to 1000 1 000 to 50 000 NOTE. Values of resistivity for concrete apply to the cast material and do not include the effect of any reinforcement bars. The values given will assist in determining the discharge resistance from steel reinforcement to the general body of ground. Table 8.3 Variation of soil resistivity with moisture content Typical value of resistivii, n.m Moisture content (percent by weight) Clay mixed with sand Sand Table 8.4 Variation of resistivity with temperature in a mixture of sand and clay with a moisture content of about 15 percent by weight Temperature 20 10 0 (water) 0 (ice) -5 -15 Typical value of resist'bity, n.m 72 99 138 300 790 3 300 8.2.2 Artificial Reduction Of Soil Resistivity. Chemical additives can be used to reduce soil resistivii. These additives generally take the form of fully ionizable salts such as sulphates, chlorides or nitrates. Such chemical additives should not be used indiscriminately as: (a) (b) the benefit that they provide will lessen with time due to leaching through the soil; and they may increase the rate of corrosion of the electrode material. Some of the chemical additives are also objectionable from an environmental viewpoint. A bland back fill material such as calcium or sodium bentonite clay, or montmorillonite with finely ground gypsum will reduce resistivii for a considerable period in high resistivii soils, maintain some moisture adjacent to the electrode system, and provide a uniform and non-corrosive environment for the electrodes. For further information see the recommendations in AS 2239 relating to the backfilling of galvanic anodes. 8.2.3 Determining Soil Resistivity By Test. It is fairly easy and useful to determine soil resistivii by test before commencing to install earth electrodes. Testing procedures are given in 6.10.1. 8.3 CALCULATION OF EARTH RESISTANCE OF AN ELECTRODE If the soil resistivity is known (see B.2.3), the earth resistance R in ohms can be calculated as follows: (a) Single vertical rod of length L and diameter d metres, top of rod level with surface: where : R Q = = resistance, in ohms; soil resistivity, in ohm metres; buried length of electrode, in metres; diameter of electrode, in metres. L = = d (b) As above, but top of rod h metres below surface: (c) Thin circular plate, diameter D metres, on surface: (d) Thin circular plate buried h metres below surface: For a vertical plate, h is measured from the centre of the plate. In the case of a square plate, the diameter can be replaced with 1.13 times the side of the square. (e) Straight horizontal wire of length L and diameter d metres, on surface: For a thin strip electrode, the diameter can be replaced with a half-width of the strip. (9 As for Item (e), but buried h metres below surface: (g) Radial wires, number of wires n, on surface: where : or for When the wires are buried at a depth of h metres then the diameter of the wire should be replaced by the equivalent diameter: (h) Ring of wire, radius of ring r metres: or in terms of the circumference I When the wires is buried at a depth h, the diameter of the wire should be replaced by the equivalent diameter determined from Equation B.3(9). NOTES. 1. The above equations assume that the longitudinal resistance of the electrodes can be neglected. The resistance for an earthing system, of such dimensions that the voltage drop along the electrodes or buried conductors must be considered, may be obtained as follows: = R when R, /R 5 0.4 = (RR,)' when R, /R 2 2 where R is the resistance calculated from the relevant equation and R, is the longitudinal resistance of the total length of wire. 2. The above equations also assume that the electrodes on the surface are buried to half their thickness or diameters; the lengths of rods and wires are much greater than their diameters; the thickness of the plates is much smaller than the plate diameters and the diameter smaller than the depth of burial; and that the angles between the radial wires are equal. Because of interaction, the obtainable earth resistance of two or more radial electrodes is higher than that for a single wire of the same length. The increase in resistance is approximately as follows: For two wires at right angles, energized at the joint, the earthing resistance is: 3 . where R is the resistance of a single straight wire of the same total length and energized at one end. For a three-point star it is: and for four, six and eight-point stars, all energized at the centre, the resistances are R + 12R, R 100 + 42R 65 R -and R + -respectively 100 100 4. The equations apply to direct current or power-frequencyalternating current energisation. For all practical purposes the resistance to a lightning surge of a typical structure earthing system discussed in this Code can be considered as being somewhat lower than its direct current or power frequency alternating current value. 8.4 USE OF ELECTRODES IN PARALLEL In situations where a desired earthing resistance cannot be achievedwith one earthing electrode, a number of electrodes may be used in parallel. The combined resistance of parallel electrodes is a complex function of a number of factors, some of the more important being the number of electrodes, their dimensions, the separation between the electrodes, the soil resistivity and the configuration of the electrodes. Where the desired resistance can be achieved with only a few additional electrodes and if the separation between the electrodes is larger than their lengths (see Note), then the resultant resistance may be calculated by using the ordinary equation for resistances in parallel. In other situations the combined resistance owing to the mutual interaction between the electrodes, will be always higher than given by this equation. NOTE. For practical purposes, the separation between vertical electrodes can be taken as twice the length of the electrode. For example, the combined resistance of two parallel electrodes of diameter d separated by a distance, s, which is small compared with the electrode's length L is given by the following equation where : R = = = = resistance, in ohms; soil resistivity, in ohm metres; buried length of electrode, in metres; equivalent radius of the electrode at the surface, in metres, determined from the equation; [(dh)%(ss')? where : = Q L a' a' = [dh~s']"~ ....B.4(2) d = h = diameter of electrodes, in metres; buried depth of electrode, in metres; distance between two parallel electrodes, in metres; distance from one electrode to the image of the other electrode, in metres. An equation for radial conductors is given in B.3@). s S' = = NOTE. The term () s' ' is the effective separation, and s' = (4h2 + s?'. 8.5 DRIVEN OR DRILLED ELECTRODES B.S.1 General. The use of driven or drilled electrodes combines economy of surface space with efficiency of performance, and accesses clays and other conductive layers at depth. In consequence it is a preferred method of electrode installation. 8.5.2 Safety. The indiscriminate driving of electrodes or drilling for their placement can lead to damage of other services and, in the case of electric power cables, to the creation of a significant hazard to the operator. As normally the drill or driven rod will not be earthed to a low impedance system, it will usually remain live and dangerous if it contacts a live conductor. High pressure gas or hydrocarbon pipes also create a significant hazard, quite apart from service failure aspects. Consequently, appropriate searches for such services should be made before drilling or driving. Installation. The extent of the earth electrode installation needed will depend on the variation in soil resistivity with depth, and the resistance to be achieved. Electrodes may be driven direct into the ground or f i e d in predrilled holes. In the latter case a bentonite/gypsum slurry or other drilling mud would normally be used as a permanent resistance and soil contact medium. 8.5.3 8.5.4 Materials For Electrodes. Electrodes should be made of metals not liable to be materially affected by corrosion. Subclause 3.5.2 describes the considerations involved in selecting electrode materials to minimize corrosion in service. 8.5.5 Electrode Diameter. Although the resistance between the earth and the electrode depends to a certain extent on the area of the electrode in contact with the soil, a large electrode of, say, 50 mm diameter does not decrease the resistance materially compared with electrodes of 13 mm or 20 mm diameter, which need to be only slightly deeper to achieve the same resistance. For a driven electrode, the minimum diameter is determined by mechanical rather than electrical considerations. The usual practice is to select a diameter which will give enough strength to enable the electrode to be driven into the soil of a particular location without bending or splitting. Large diameter electrodes are more difficult to drive than small diameter electrodes. For deep drilled electrodes the size is selected in the light of available drill diameters, requirements for connections (if any) and economy. Strip electrodes are commonly used. Depth Of Installation. The depth to which an electrode is installed is usually the most important factor affectingits earth resistance, first because the area of soil contacted increases directly with the length of electrode below the surface, and secondly because the soil resistivity usually decreases with depth. 8.5.6 This is shown by the measurements plotted in Figure B.l for a number of sites. For curves 1 and 2 it was known, by tests, that the soil down to a depth of between 6 m and 9 m consisted of ballast, sand and gravel, below which was clay. The rapid reduction in resistancewhen the electrode penetrated the latter is very marked. The mean resistivii up to a depth of 7 m in one case was 150 am, at 10 m the mean value for the whole depth was 20 n.m due to the low resistivity of the clay stratum. Similarly for curve 4, the transition from gravelly soil to clay at a depth of about 1.5 m is very effective. For curve 3, however, no such marked effect occurred although there is a gradual reduction in average resistivity with increase in depth, as can be seen by comparison with the dotted curves, which are calculated on the assumption of uniform resistivity. 8.5.7 Sleeving Of Exposed Part Of Vertical Electrode. Where side-flashing or step and touch potentials are a design problem on a driien electrode, it is good practice to sleeve the upper part of the electrode with a non-conductingpipe or heat shrink tubing of adequate weather resistance and electrical insulation properties. For example, this could take the form of 2 mm thickness of polyethylene or 4 mm thickness of PVC covering the upper 2 m to 3 m of the electrode. 8.5.8 Comparison With Other Electrode Types. The simplicity of driving an electrode compared with making excavations for the burying of plates or strips is obvious. The problem of the adequate packing of the soil around the electrode does not arise. The space occupied is small and, unlike plate electrodes, connections may be above ground. Moreover, where the permanent moisture level or layer of low resistivity soil is available only at considerable depth below the surface, electrodes may be driven to depths which would be far beyond that which would be practicable or economical for buried plate electrodes. -- - Calculated assuming uniform soil resistivity of value indicated-p 0.m LENGTH OF DRIVEN ELECTRODE, m Figure B.l Calculated and measured curves of resistance of 13 mm diameter driven rod electrodes However, where soil resistivity increases with depth, there is no point in driving an electrode any deeper as better results may be obtained by connecting a number of electrodes in parallel or by using a buried strip electrode. B.6 BURIED STRIP ELECTRODES Buried strip electrodes provide a solution to the problem of obtaining a low resistance earth connection in locations where soil resistivity is high, particularly where there is a superficial layer of soil over a stratum of rock and it is impracticable to drive an electrode. For a given cross-section, strip electrodes have the advantage of a greater surface area in contact with the soil. The material for such electrodes should be selected having regard to corrosion compatibilitywith the protected structure (see Subclause 3.5.2). For example, for a galvanized steel tower, a 35 mm x 3 mm galvanized steel strip would be preferred. Where the electrode is totally isolated from other metals, e.g. on an isolated stone or timber structure, a variety of materials may be used. These include copper, galvanized iron, steel, stainless steel resist. The last two materials offer some additional corrosion resistance in aerated soils, but with the disadvantage of higher electrical resistance and cost. Backfilling and compacting the trench will enhance the early resistance performance of the electrode. The cross-section of the conductor has very little effect on the resistance of the earth connection so that the strip or cable size is not important provided it affords reasonable protection against mechanical damage and corrosion, and is of adequate current surge capacity. The economics of depth of burial versus resistance performance do not warrant laying strip electrodes below 0.5 m, unless.the risk of mechanical damage requires this additional protection. The optimum resistance for a given amount of electrode materedis achieved if the electrode is buried in a straight single trench or in several trenches radiating from a point. If laid in parallel lines, the trenches should be widely separated (see 8.1 and the Notes to 8.3). 8.7 BURIED PLATE ELECTRODES This form of electrode is now mainly restricted to tower footings or the like where the civil works for the structure to be protected provide the facilities for the laying of the electrode. The performance/cost relationship does not support other than such specific applications. B.8 CONCRETE FOOTING ELECTRODES This form of electrode is one of the most effective both in cost and electrical performance of currently available electrode systems. For a given site it provides a permanent, distributed, low resistance electrode at very little cost above the structural civil work. The vital part of the exercise is the planning, design and supervision of the construction, as after concrete is poured it is impractical to address design deficiencies. If a sectional measurement of earth resistance is desired, this will only be feasible at the appropriate stage of construction. As this electrodewill likely be bonded deliberately or fortuitously to the electricity supply earthing system, and perhaps to other systems, it is important to consider the possibility of corrosion arising from contact with dissimilar metals (see Subclause 3.5.2). It may be necessary to address corrosion problems of rock anchors by cathodic protection. Also, the deliberate or fortuitous bonding of on-site fuel tanks, now relatively common adjacent to building foundations, should be taken into account in the design of lightning electrodes to ensure the earth discharge is from the electrode system, i.e. the fuel tanks are electrically screened. B.9 INSPECTION AND MAINTENANCE OF ELECTRODES The scheduling of electrode maintenance inspections is the prerogative of the system owner. However, the frequency of testing and the associated considerationsare listed below as a guide to good engineering practice. Data on the layout, materials of construction data and electrical measurements pertaining to the original design should be prepared and preserved as a guide to later performance. Ground resistivity data are likewise useful for future comparison. (a) (b) Inspections should be both physical and electrical. The inspections should be carried out at intervals of not more than 1 year unless there is a significant reason for more frequent inspection. Examples of the need for more frequent action would be if the electrode system is in a marine environment, subject to a high rate of corrosion. The physical inspections should address corrosion or mechanical damage to visible parts of the whole system, structural alterations that may have prejudiced the design or operation of the system, or changes in the usage of the structure, e.g. fuel storage added. Electrical tests should cover the continuity of the downconductors, the integriity of bonding arrangements, and the resistance to ground of the electrodes, preferably individually as well as collectively. Methods of testing resistance to ground are discussed in B.lO. The continurty of downconductors should preferably be checked by a high-current testing system (approximately 25 A) in order to detect reduced current carrying capacity resultingfrom fractures or other damage which may be obscured from view. (c) (d) . (e) The enhancement or replacement of electrodes to achieve a specified resistance may be necessary, and if this is done it should be recorded along with other test results. Restoration records of clamps, joints and fittings on downconductors or electrode terminations should also be kept as a future maintenance guide. Where uncertainty exists about the validity of inspection test results, comparison with original design figures and data, together with the historical test records, will often serve to indicate the extent of deterioration of the electrodes. The change in soil resistivii with rainfall can at times be particularly misleading, and test results should be viewed with some suspicion if a significant reduction is observed in resistance figures. (f) B.10 MEASUREMENT OF SOIL RESISTIVITY, EARTH ELECTRODE RESISTANCE AND EARTHING SYSTEM IMPEDANCE 8.10.1 Determination Of Soil Resistivity By Test B.lO.l.l Method of derivation. The Wenner or four-pin method of soil resistivii measurement is commonly used. It involves the use of four spikes (electrodes) equally spaced in a straight line and driven to the same depth d, not exceeding 5 percent of their separation s and not more than 1 m in any case (see Figure 8.2). If a known current Iis passed between the outer electrodes, and the voltage drop V between the inner electrodes is measured, the ratio V/I gives a resistance R. If the earth were perfectly homogeneous, i.e. of a constant resistivityQ, then: where s = = electrode separation, in metres; average soil resistivity to a depth of s metres, in ohm metres The soil is rarely homogeneous and the value Q calculated from Equation B.lO.l.l (called the apparent resistivity) will be found to vary with the electrode separations. It is from these variations that deductions can be made as to the variation in the nature of the underlying soil. Account should also be taken of seasonal variations of the soil. By repeating the measurements with different values of electrode separation, the average resistivity to various depths can be found and the results will indicate whether an advantage is to be gained by installing deepdriven earth electrodes to reach strata of low resistivity. In practice an indicating ohmmeter or null-reading bridge is used to measure the resistance R from which soil resistivity Q is calculated by using Equation B.lO.l.l. 8.10.1.2 Instrumentation Four-pin testing. Early instruments developed for resistivity measurements were many and varied. The traditional instrument, patented by Evershed and Vignoles, the Series 1 Earth Megger became an industry standard, and utilized a cross-coil analogue readout giving it immunity from a number of errors and electrical interferenceconditions previously experienced by other devices. It is no longer available due principally to its cost, weight, and operator inconvenience. However, because of its extreme robustness, a number of these instruments are still in use. Essentially it is a d.c. generator, hand driven, with a synchronous commutator for a.c. output and measurement purposes. It operates using a slipping clutch, activated at about 133 Hz, producing an output of several hundred volts. Although more modern instruments are now available, experienced technical staff often find it useful to retain a cross-coil hand generator to resolve readings in areas with high stray 50 Hz harmonic soil currents. The deflection of the cross-coil readout is insensitive to currents not commutated at the 133 Hz generator rate. - The subsequent generation of instrumentswere vibrator sourced square-wavedevices operating generally around 108 Hz to 110 Hz. In general, such instruments have a lower output than hand generator devices and are subject to a synchronous operation in areas where high a.c. stray currents are experienced. They also can give erroneous readings in circuits of significant reactance. The most recent instruments for this task are all semiconductor devices, generally relying on a pair of bipolar transistors as the square-wave generator, with appropriate synchronized semiconductor devices as the detector, in a null-bridge mode. Both analogue null and digital readout systems are available. Hybrid hand generator null balance-bridges are also available. Higher powered instruments operating normally in the 10 Hz to 40 Hz range and requiring rechargeable batteries of substantial capacity are available in this category to accommodate long 's' spacings, and for special ranges in three-pin testing (see 8.10.2). Such higher powered devices are less prone to indeterminate readouts due to instrument burden, or load insensitivity. The connections for all four-pin test instruments are shown in Figure 8.3. 8.10.2 Electrode Resistance Testing Three-pin Method 8.10.2.1 General procedure. If the P, and C,, electrodes of Figure 8.3 are superimposed it will be apparent that the mathematics of the test simplity to a virtual Ohm's law condition, i.e. R = V/A. This could be achieved by use of an altemating current supply, with an ammeter and voltmeter (sometimes called the 'fall of potential' method) as shown in Figure 8.4. Individual meters are not normally used, and the several generations of instruments described , , in 8.10.1.2 are invariably fried with facilities to link C, and P, terminals and C and P terminals respectively to serve three-pin and other testing requirements. A few instruments have been made with P, and C, permanently linked, but this practice has been discontinued in recent years. - LEGEND: s = electrode separation; d = depth of test spike; this must be small in relation to s, i.e. not greater than and not in any case greater that 1 m. & NOTES 1. The above configuration will give a reading for Q , by calculation, that is equivalent to the resistiv'ty at depth 0.75s. 2 . 3. If it is required'to determine average resistivity to various depths at a given point 'O', centre point 0 of the the electrode system is kept fixed and the electrode separation s increased outward from that point. As the effects of d.c. polarization on the test pins would give a superimposed error on V, of the same order as the small voltage being recorded, it is necessaryto use an alternating power source, or if d.c., a cyclically-reversedsource. The latter would also require a synchronous reversal of the indicating system. Figure 8.2 Four-pin method of soil resistivity measurement Figure B.3 Connections for earth resistivity test using an indicating ohmmeter or null balance bridge Figure 8.4 Basic ammeter and voltmeter method of measuring electrode resistance Later instruments, being either null-reading bridges or electronic detector/readout units do not present a circuit load or burden problem. However, the placement of electrodes is quite important for several reasons. Considering a section of the electrical gradient in Figure 8.4, reproduced as Figure 8.5 for homogeneous ground, it will be observed that the fall of potential follows the relationship: where D I = is the distance of potential probe from the electrode, in metres; = current to, or from, the electrode, in amperes; = soil resistivity, in ohm metres; V = the voltage at distance D from the electrode, in volts. Consequently it is important that P be sited on the 'flat' part of the curve in Figure 8.5. If C is not sufficiently distant from E, then there will be no flat part of the curve. This can be established by moving P and retesting. If it varies, then C is too close to E. As a general rule, C should be not less than 10 times the length of electrode E, for homogeneous ground, and P about half the distance from E to C. Figure 8.5 Fall of potential around electrode 8.10.2.2 Test lead considerations. Because of the inhomogeneity or layering of the soil it is prudent practice to use as long a lead from E to C as practical. In fact, for extreme conditions, such as a mountain top, where the site may be on a volcanic core, it is not uncommon practice to use a 500 m or 1000 m test lead of physically substantial construction. Testing with short leads merely gives an electrode resistance to that small volume of ground encompassed by the electric field between E and C, in a roughly hemispherical volume. With very long leads a significant hazard arises. Earth potential rise from power fault currents or lightning pulse can give rise to dangerous voltages between different parts of the earth's surface. The handling of leads of 500 m or greater should be accompanied by the careful use of insulating gloves suitable for working at voltages of up to 500 V a.c. Test instruments may require fitting with radio frequency suppression devices to prevent the pick up of high-frequency radio communication signals damaging the electronic detection equipment, or producing erroneous readings. Another condition requiring very long leads is the situation of an electrode of considerable dimensions, e.g. a 500 m strip electrode, especially in high resistivity ground such as a mountain top or sandy plain. In this case, the concept of 'resistance to the body of ground encompassed by the electric field' above, requires a lead to C of the order of 500 m, at right angles to the run of the electrode. The same safety considerations apply. 8.10.3 Isolation Of Surge Impedance Of An Electrode System From Other Fortuitous Earth Paths. The measurement of a lightning electrode system requires that the electrical condition specific to a lightning pulse be addressed. Typically the rise time of a substantially unmodified lightning pulse is around 1 1s. This means that fortuitously-bonded path lengths of more than a few tens of metres will present a reactive component that prejudices the ability of such paths to divert a significant portion of the total earth path current. This in t u n requires the measuring system to reject such paths and to measure the residual path - usually that to immediately adjacent electrodes or earth features. This can be achieved fairly simply but at some cost by utilizing a relatively high frequency source of power for the three-pin test. Several excellent commercial instruments are available that operate in the 25 kHz to 50 kHz range. These are used in the same manner as other three-pin (and four-pin) test sets. The difference in readings between these and the low-frequencytest sets will often be quite spectacular and will point up obvious reasons for observed catastrophic failure from lightning in systems thought to be adequately earthed. In particular, the electricity supply neutral/earth connection bonded for 50 Hz equipotential protection to lightning earthing system can give a grossly misleading sense of security if it is read with a 108 Hz test set. Typically, a one ohm reading can in reality be 100 ohms surge impedance, as measured by a high freqllency test set. If a high frequency test set is unavailable, an alternative method with reasonable accuracy may be used, based on the fall of potential curve around the electrode (see B.10.2.1), similar to that used for testing substation earth mats. Alternating current is applied between the electrode (or its downconductor) and an auxiliary test electrode (see Figure B.6). The portion of the current passing to earth via the electrode under test is measured by a clamp ammeter placed between the current injection point and the electrode entry to ground. The voltage to which this drives the electrode is measured by a flying lead voltmeter to a pin sited in the 'flat' portion of the fall of potential curve. The resistance of the electrode is a simple R = V/I relationship, and is a good approximation to the surge impedance. V Current injection point NOTE. No connections to bonding conductors should exist on the earth side of the current injection point. Figure B.6 Measurement of electrode surge impedance APPENDIX C THE CALCULATION OF LIGHTNING DISCHARGE VOLTAGES AND REQUISITE SEPARATION DISTANCES FOR ISOLATION OF A LIGHTNING PROTECTION SYSTEM (Informative) ! C.l GENERAL In the first one or two micro seconds of a lightning discharge, transient voltages occur on the air termination network and on the downconductors, which may be far greater than the discharge voltages which apply during the remainder of the discharge. This is because the discharge energy injected into the air termination network at any instant is momentarily trapped on the conductors of the protection system prior to discharging into the general mass of earth via the earth termination network. This transient voltage frequently determinesthe separation distance required for isolation of the lightning protection system, if it is desired to isolate in accordance with Subclause 3.14.2.3 as the preferred method of protection against side-flash (see Subclause 3.14.2.1). The peak values of the transient vdtages appearing at various points of the protection system differ according to location, and increase with distance from the earth termination measured along the route of the discharge through the protection system. At points very near to a compact earth termination, the transient voltages are suppressed by the discharge to ground and minimum values of discharge voltage and required clearance can be readily calculated (see Note 1). These lower limits apply only at the base of the structure and are given by the equations: where : V , =. the discharge voltage at the base of the structure due to local earth potential rise, in kilovolts; = the combined earth termination resistance, in ohms; R D, = the required clearance in air at the base of the structure, in metres. The complete transient vdtage waveshape at all points of the protection system can be calculated using travelling wave techniques and a computer, however substantial simplifications allowing helpful easily-calculated estimates can frequently be made (see C.2). An estimate of the transient voltage at any one point of a lightning protection system can often be made using a conventional circuit theory approach (see C.3). This is possible because the transient voltages can often be neglected due to the high insulation strength of air to extremely brief voltage stresses. Transient vdtages at points remote from the earth termination depend both on the lightning stroke current waveshape and the characteristics of the protection system. As indicated in A.2.2 and Table A1 of Appendix A, lightning flashes generally have a number of component strokes with differing waveshapes. The critical voltage may correspond to the highest peak current of a first stroke or to the steepest wavefront (di/dt), of a subsequent stroke. The design first and subsequent strokes used in this Code are shown in Figure C.l (see Note 2). The more severe of these cases was adopted in arriving at the required clearances in Subclause 3.14.2.3 (see Note 2). Peak at 4.6#s Design first stroke 40 ' TIME, ps Figure C.l Idealized lightning stroke currents adopted for design purposes The electrical breakdown strength of air depends on the polarity of the applied voltage and on the duration and shape of the voltage surge. In the studies conducted for this Code, the required clearances were estimated using the breakdown strength of air, shown in Figure C.2, which neglects some of these variables and should only be regarded as very approximate. TIME TO BREAKDOWN, r s Figure C.2 Effective electrical breakdown strength of air (approximation for the purpose of this Code only) In some cases where the transient voltage surges differed significantly from the waveshapes (chopped and triangular waves) upon which the graph is based, a further adjustment was made. NOTES. 1. The lower limit of design discharge voltage is based on the assumed peak lightning current of 150 kk The corresponding required clearance is based on a minimum electrical breakdown strength of 500 kV/m in air. The lightning flash is assumed to have a first stroke with I of 150 kA and (di/dt), , of 32.6 kA/@ and a steepest , of 40 kA and (dildt), I of 200 kA/p. A more severe case would occur not more than subsequent stroke with, once in each hundred lightning strikes to the building, that is about once in a thousand years for a single 60 m high structure in a locality with a moderate level of lightning activity (one strike per square kilometre per year). 2. C.2 TRANSIENT VOLTAGE CALCULATIONS BY TRAVELLING WAVE ANALYSIS C.2.1 Simplified Travelling Wave Characteristics. An electric charge injected into one end of a conductor propagates along the conductor as a travelling wave with velocity v given by: where : v = velocity, in metres per second; L, = the inductance per unit length, in henries per metre; C, = the capacitance per unit length, in farads per metre. For a single bare conductor of radius r at a distance h above a perfect ground in free space, L, and C,are given by: where p and E, are the permeability and permittii'Q of free space and the conductor is , assumed to be non-magnetic, i.e. i, = 41 x lo-' H/m, and E , = 8.85 x 10.12 F/m In air, p and E differ only slightly from p, , and E, and the velocity of the travelling wave becomes the velocity of light, c, in metres per second: In an insulated cable with continuously earthed sheath and dielectric or relative permittiiity k (where k is typically 3 to 6), the velocity is reduced by the factor 1 /Jk and is typically 0.5 c. For lossless surge propagation, the voltage e generated by a travelling wave with current iis given by: \b.) I., where : z Z = (((LIIc,) is called the surge impedance of the conductor, and in free space is given by: = 6 0 I n -2h r ....C.2.1(6) where Z is in ohms When a travelling voltage wave in a conductor arrives at an electrical discontinuity, such as an intersection of downconductors or the connection of a downconductor to an earth termination, part of the travelling wave is transmitted and part is reflected. If Z, is the surge impedance of the conductor on which the wave is travelling prior to reaching the discontinuity, and Z, is impedance seen at the termination or combined parallel surge impedance of conductors continuing beyond the junction or other discontinuity, then the reflected surge v', I' is . related to the incoming surge by the equations: ....C.2.1(8) ' and the combined transmitted wave v",i beyond the discontinuity or at the termination is given by: where : b = the reflection coefficient; a = the transmission coefficient. The lightning protection system is C.2.2 Surge Voltage Calculation By Lattice Diagram. represented by a simplified model in the form of nodes and branches. The nodes are placed at junctions or impedance discontinuities. Each branch has a surge impedance, and a travel time determined from its length and the surge velocity. The earthing resistance at each earth termination is treated as a branch with surge impedance equal to the earthing resistance in ohms and of infinite travel time. Lattice diagram calculations are usually carried out by computer. Generally the program calculates the response of the system to a unit step function current and the response to any given input current wave is calculated as the superposition of the responses to the succession of such step functions of various magnitudes, polarities and input times whose sum closely approximatesthe desired input waveshape. For manual calculations only very simple models can be handled but triangular waveshapes can be readily used. It is therefore necessary to consider whether parallel downconductors can be represented by a single downconductor. Example C.1 A building 35 m high is protected by four air termination rods placed at each comer of the flat roof and interconnected around the perimeter of the roof to four downconductors of surge impedance 480 n which run vertically to ground at each corner. Each earth termination has two to four driven earth stakes 3 m deep and 6 m apart to achieve a test resistance of 9 n to 10 a and a combined earthing resistance of 2.5 n. Calculate the first voltage peaks of the response to stroke currents i,(t) waveshapes A and B of Figure C.1, respectively. and i,(t) having The system may be modelled as shown in Figure C.3. The four air terminations and downconductors are represented by a single branch of surge impedance 120 n and length 40 m (say) terminated in a resistance (or reflectionless infinite branch) of impedance 2.5 n. Nodes 1 and 2 lie at the top and the base of the building respectively. For convenience of representation take i(t) as positive and adopt units of kiloamperes (kA), kilovolts (kV), and microseconds (FS). Then the surge velocity C may be taken as 300 m / ~ s and the branch (1, 2) travel time is T = 0.133 BS. The reflection coefficient b for surges arriving at node 2 is evaluated as -0.95 so that 5 percent of the incident surge current at node 2 leaks to earth with the remainder initially trapped on the protection system. The reflection coefficient for surges returning to node 1 from 2 is evaluated as unity (the surge impedance of the lightning stroke channel is neglected for this example). The lattice diagram is developed as indicated in Figure C.3(b). TlME 40 m branch TIME (1 (a) Model + b)i bC 1 + b)i b2(l + b ) ; I (b) Lattice diagram ( c ) Initial responses Figure C.3 Calculation of surge voltage by lattice diagram The surge voltage v(t) at node 1 depends simply on the incident current i(t) until the first reflection wave arrives at 0.266 ps (27) and is given by: , v(t) = Zi(t), for 0 s t s 2T ....C.2.1(11) Because b is negative (- 0.95) the first reflected voltage wave bv is negative and is doubled on arrival at the open-circuited node 1, i.e. from the instant 2T, the first reflected wave bv is again reflected as bv. The voltage at node I is varied by 26v, and is given by: v(t) = Z i(t) + 2bZi (t - 2T), for 2T c t s 4T = 120[i(t) - 1.9 i(t - 27)], for 2T < t s 4T ....C.2.1(12) , For the first stroke current i,(t), (where i,is in kiloamperes and t is in microseconds) the current waveshape in the period 0 < t s 4T is a ramp of uniform slope: i,(t) = 32.6t ....C.2.1(13) The response is a triangular wave given by: for which the peak value occurs at t = 2T as shown in Figure C.3(c). For the subsequent stroke current i,(t), the current waveshape reaches its crest value at t = 0.2 1s which is prior to the arrival of the first reflection wave at t = 2T. The peak value occurs at t = 0.2 ps, and as the waveshape to this time is a ramp of uniform slope (i,(t) = 200t, t s 0.2), the peak value is 4800 kV and the response takes the form shown in Figure C.3(c). It should be noted that the transient oscillatory response is damped by the discharge to earth occurring at node 2. A travelling wave analysis for each stroke current was carried out by computer for a 10-metre structure with combined earthing resistance of 5 n, and for a case similar to the above. The downconductor surge impedance was arbitrarily reduced to 400 n to allow for corona and a lightning stroke channel surge impedance of 1500 n (a minimum value) was used, increasingthe damping of the transient oscillations. The response up to 6 ps is shown in Figure C.4. The travelling wave analysis permits calculation of the voltage response at any point on the lightning protection system because of the distributed constant representation of the system. C.3 SURGE VOLTAGE CALCULATIONS BY LUMPED CIRCUIT APPROXIMATIONS The lumped circuit approximation precludes any assessment of the transient voltage oscillations associated with travelling waves generated on the protection system, however, elementary calculations generate the base lines about which any transient oscillations occur. Example C.2 A building 35 m high is protected in a similar manner to that of the example in C.2.2. Each of four downconductors is assessed as having a length of 40 m and the inductance in microhenries per metre is given by: where : h = r = the average height above ground of the four downconductors, in metres; typical radius of the four downconductors, in metres. TIME, ps (a) H = l O r n . n = 2 . R = S n Curves A: Response to design first stroke waveshape A of Figure C l Curves 6: Response to design subsequent stroke waveshape B of Figure C1 Figure C.4 Voltages on lightning protection systems illustrative cases calculated by simplified travelling wave analysis - The total inductance of the protection system is given by: where : H = the average length of downconductors from the point struck to earth, in metres (this differs from the definition in Subclause 3.14.2.3 because the voltage at intermediate points cannot be calculated for multiple downconductors using lumped circuit approximations) the number of downconductors connected to a common air termination (spacing of downconductors is assumed large enough for mutual effects to be neglected) n = The capacitance of the system is also neglected. The equiiralent circuit for calculating the voltage at roof level (node 1) and ground level (node 2) is shown in Figure C.5. Figure C.5 Simplified lumped equivalent circuit The voltage at node 1 with respect to remote earth is given by : The response to the idealized design stroke currents i (t) and i,(t), calculated from this equation, , is shown in Figure C.6. It can be seen by comparison with Figure C.4 that the simplified lumped circuit method is an extremely useful tool in estimatingsystem responses to the various lightning stroke currents. In the case of first strokes, for which the transient oscillations have effectively been damped by the time of the current peak, the voltage waveform calculated by this method is also an adequate basis for estimating the required clearances for isolation. 0 1 2 3 4 5 6 TIME, ps (a) H = 10 m. n = 2 R = 5 Q. L, = 1.5 ctHlm . (b) H = 40 m. n = 4, R = 2.5 l2, L, = 1.5 pHlm . Curves A: Response to design first stroke waveshape A of Figure C1 Cuwes B: Response to design subsequent stroke waveshape B of Figure C1 Fgure C.6 Vottages on lightning protection systems - illustrative cases calculated by simplified lumped circuit analysis APPENDIX D WAVESHAPES FOR ASSESSING THE SUSCEPTIBILITY OF EQUIPMENT TO TRANSIENT OVERVOLTAGES DUE TO LIGHTNING (Informative) Due to the random nature of lightning disturbances and the variable characteristics of the transmission media (such as power lines, telephone lines and coaxial cables), these transients exhibit wide waveshape variations. However, field and laboratory measurements, confirmed by theoretical calculations, have led to the selection of a small number of waveshapes that are representative of the majority of transients encountered in practice. The value of these standard waveshapes lies in the uniform specification of transient protection equipment. By using the same waveshapes and conditions to test the equipment, manufacturers can quote results which may be directly compared between brands, enabling the user to select an appropriate device. The three most common waveshapes used to represent transients on power lines are the 1.21 50 ps and 8/20 ps unidirectional impulses and the 0.5 ps, 100 kHz ring wave. The waveshapes are shown in Figures D.l and D.2, and Table D.l indicates their recommendedapplications and magnitudes. Guidance on the use of these waveshapes in the testing of equipment is given in IEC 60-1 or IEEE C62.41. It is important to note that the waveshapes of Figures D.l and D.2 represent line input conditions expected under practical conditions. Purchasers of protection equipment should ensure that equipment side output voltages are reduced to be within the input tolerance envelope of the specified equipment. Tests have shown that the input voltage variations to electronic equipment can be both time and magnitude dependent. Figure D.3 shows two magnitudeltirne curves derived for computing equipment. Long period variations can generally be corrected by line conditioners while fast transients due to lightning need special devices. These usually comprise non-linear devices to clamp overvoltages and subsequent filtering stages to modify the residual waveshape. The purpose of such devices is to bring the residual voltages from a lightning surge to within the safe operating zone. The test voltages of Figures D.l and D.2 represent input levels to protection devices. The residual voltage level is that seen at the output, or equipment side of the protection device, when the impulse is applied at the crest of the a.c. voltage. This voltage level will be a function of both the input pulse characteristics and the technical performance of the device under test. The residual voltage level should be specified to the nearest 100 V above the actual recorded performance. The rating should be provided in conjunction with the test impulse voltage and current category. For example, if a device recorded 746 V the residual voltage rating should be specified as follows: Residual voltage level800 V (Category A, 6 kV/200 A), or Residual voltage level800 V (Category B, 6 kV/3000 A). q,= 1.2 p s (a1 Open-circuit voltage waveshape . T,= 8ps (b) Discharge current waveshape Figure D.l Standard unidirectional waveshapes Figure D.2 0.5 ps 100 kHz ring wave (open-circuit voltage) - CP33 : 1996 Table D.l Recommended application for waveshapes of Figures D.l and D.2 Location (see Figure D.4) Category Description Long final subcircuits and Dower outlets Major submains, short final subcircuits and load centres External services, overhead lines to detached buildings Waveshape Figure D.2 Figure D.2 Figure D.1 (a) Figure D.1(b) Figure D.2 Figure D.2 Figure D.1 (a) Figure D.1(b) Figure D.1(b) Medium exposure peak amplitude Type of load high impedance low impedance high impedance low impedance high impedance low impedance high impedance low impedance high exposure and critical locations, e.g. mountain top telecommunications site BR-oWNA CONCERN LACK OF STORED Copyright O 1992. IEEE. All rights reserved. As amended Feb 1999 Figure 0.3 Typical vottagepime tolerance of computing equipment POWER OUTLETS AND LONG FINAL SUBCIRCUITS All power outlets more than 10 m from Category B All power outlets more than 20 m from Category C MAJOR SUBMAINS AND SHORT FINAL SUBCIRCUITS Distrlbutlon board devlces Submalns systems In industrial plants Heavy appliance outlets with short connection to the service entrance Lighting systems in commercial buildings OUTSIDE AND SERVICE ENTRANCE Service drop from pole to building entrance Run between meter and dlstrlbutlon board Overhead line to detached buildings Underground lines to well pumps Figure D.4 Location categories for application of the waveshapes in Table D l . APPENDIX E PROTECnON OF EQUIPMENT WITHIN OR ON STRUCTURES AGAINST LIGHTNING (Informative) E.l GENERAL Lightning induces overvoltages in both power and telephone or data lines. Cables carrying these circuits usually have different points of entry to a structure and may have protective devices connected to different earths. Equipment overvoltages may be experienced in the following ways: (a) (b) (c) On telephone lines. Transients occur both line to line, and line to earth. On power lines. Transients may occur line to earth, line to neutral, or linelneutral to earth. Simultaneously on both power and telephone lines exposed to the same event. On one or both of the protective earthing systems for the above services. (d) The protection of equipment against overvoltages involves the provision of appropriate voltage limiting (clamping) devices either at the point of entry of the service@) to which the equipment is connected (primary protection) or within the equipment (secondary protection), or both. Protective measures relevant to primary protection and secondary protection are described in Clauses E.2 and E.3 respectively. E.2 PRIMARY PROTECnON The primary protection of equipment sensfwe to overvdtages may be achieved by use of protective devices connected to a common earthing system at the service point of entry. However, in many existing structures separate earthing systems may have been provided for power, telephone, computer and lightning protection. Significant earth potential rises can occur on only one of these earths which, for equipment using multiple services (e-g. facsimile machines), stresses electrical insulation and may result in breakdown to the protective earth which remains at normal potential. In such cases special protection devices at the equipment location are usually required. Equipotential bonding is primarily provided for the protection of persons by preventing breakdown of earthed systems to other systems. Such bonding will also contribute to the protection of equipment but may not of itself be sufficient to prevent equipment damage. Also, line to neutral voltage transients on the electricity supply can cause damage to or malfunction of some electronic equipment. Figure E1 shows a multistage protection system where a computer or signal earth cannot be . commonedto other earths for electrical noise or other operational reasons. Primary protection is placed at the point of entry for power, telephone, signalling and data circuits. Secondary protection is placed adjacent to the main installation and is referenced to the special equipment earth. E.3 SECONDARY PROTECTION E.3.1 General. Electrical equipment will break down at the point of lowest impulse dielectric strength. Telecommunications line circuits may present sufficient electrostatic capacity to local earth to allow an impulse discharge to them as a consequence of an EPR condition, i.e. the earth terminal of the equipment is 'live' and the line circuits are subjected to a fault discharge across conductor strips on a printed circuit board with a breakdown voltage lower than the EPR impulse. Protection of sensitive equipment utilizing both power and telephoneldata lines is best carried out at or within the equipment. Specialized protection equipment may guard against common and transverse mode impulses on all lines entering equipment. This alone may not be sufficient to protect the equipment if their reference earths vary in potential. In this case, clamping devices which limit earth potential variation become essential for the security of the equipment. Classes of equipment prone to damage from earth potential rise include electronic PABX, modems and facsimile machines. E.3.2 Protective Devices. (a) Protective devices usually fall into one of the following categories : Gas discharge devices. These devices usually consist of glass or ceramic tubes filled with an inert gas sealed at each end with a metal electrode. They have breakdown voltages in the range 70 V to 15 kV with surge current ratings up to 60 kA. The strike time and firing voltage of these devices is dependent on the rate of increase of voltage. Typical strike times are in the range 10 ns to 500 ns. Unlike most other devices, gas discharge devices conduct at a much lower voltage than their firing voltage. This conduction voltage is typically below 30 V. Gas discharge devices are available in both two electrode and three electrode configurations. The latter provide a means of clamping a pair of wires to earth regardless of which conductor was subjected to the overvoltage. (b) Varistors. These devices are voltagedependent resistors. The earlier forms of varistors were constructed from carbon or silicon carbide but most modern devices are made from metal oxide and are known as metal oxide varistors (MOVs). The resistance of varistors drops significantlywhen the voltage exceeds a limit thus clamping the voltage near the limit. Varistors are used on circuits operating at voltages between 10 V and 1 kV. They can handle surges up to several kiloamperes and respond in tens of nanoseconds. Becausethe performance of MOVs deteriorates with repeated operation, it is usual to allow a high safety margin in the selection of the device rating in lightning prone areas. Alternatively, facilities should be provided to give an indication of device failure. Solid state devices. These devices consist of special Zener diodes which exhibit voltage limiting characteristics. The breakdown voltages of such devices are typically in the range 5 V to 200 V. They have current ratings up to several hundred amperes and response times of the order of 10 picoseconds. (c) Information on waveshapes which may be used to specify the performance of these devices is given in Appendix D. E.3.3 Application Of Protective Devices. With any signal or power transmission system employing two lines and a separate protection earth, two types of transients can occur. The first type appears as a difference between the two lines, independent of their potential differences to earth; this is known as a differential mode transient (also called transverse mode or normal mode). It is illustrated in Figure E.2 where the transient voltage source is superimposed onto the normal signal carried by the lines. The second type appears as a transient between each line and the earth, and is known as a common mode transient (sometimes called a longitudinal transient). It is illustrated in Figure E.3 where the transient voltage sources are superimposed onto the normal potentials between the lines and earth. This mode is that commonly experienced by twisted pair circuits as each wire is equally exposed to the transient voltage source. The use of two non-earthed lines is common. The a.c. mains use the line and neutral conductors to supply power, with an accompanying earth line for protection. Telephone lines use two wires over which the signal is transmitted, with neither line tied to earth. RS-422 signalling for computer data uses two lines for each data channel, which is known as balanced-pair signalling. When protective equipment is connected to such lines, both differential and common mode transients must be suppressed. Placing a protective device across the two signalling lines alone is not sufficient. The high potentials to earth created by common mode transients can cause insulation breakdown and arc-over, and can damage electronic components. The use of opto-isolators for signalling lines does not necessarily eliminate this problem. Opto-isolators suitable for printed board mounting are rated as high as 5000 V isolation between input and output but transients caused by lightning can easily exceed this value resulting in breakdown of the isolator, with transients 'punching through' and damaging subsequent circuitry. However, special purpose fibre optic opto-isolators are available with significantly higher isolation ratings. Protection against transients is best achieved by the provision of voltage clamping or diversion devices between the lines, and between the lines and earth. These will shunt common mode transients to earth before they are allowed to reach breakdown potentials. When used to protect equipment the gas discharge devices will normally handle the largest amount of energy with the solid state devices handling the least amount of energy. Robust equipment such as electromechanical equipment is normally protected by the addition of only gas discharge devices while sensitive electronic equipment may require all three types of device in combination. The typical method of combining these devices in a signal line can be seen in Figure E.4. Much modern equipment already has the varistor and solid state devices incorporated in its design and only the-high energy gas discharge device and its isolation impedance needs to be used. It is therefore important to match the protection device to the equipment. Gas discharge devices are generally not suited to the protection of mains supplied a.c. equipment because of the fold back nature of their operation. Metal oxide varistors (MOVs) are normally used in mains protection circuits. When configured as shown in Figure E.5 they provide essential clamping against both differential and common mode transients. These MOVs are usually specified to initiate clamping at an effective rms. voltage of 275 V. However, high-current surges may still produce peak voltages exceeding 1200 V within the rating of the device. Equipment may be subject to rates of rise of thousands of volts per microsecond prior to the clamping device becoming effective. In Figure E.6 a filter is added to condition the residual transient and to reduce the high rate of voltage rise observed immediately prior to damping. It is important to note that radio frequency interference filters may not be suitable for power circuit protection. Transient current levels may cause inductor saturation which will degrade the filter action. Figure D.3 in Appendix D shows a typical voltage/time tolerance curve for electronic equipment. Hybrid devices should be used to absorb the energy levels shown in Figure D.l and Table D.l according to their exposure. The residual transient after clamping and filtering should lie within the tolerance curve for the equipment being protected. A major cause of equipment breakdown has been traced to earth voltage differentials. The past practice of forced separation of power and telephone earths has allowed significant potentialsto occur inside equipment. Figure E.7 shows how the use of an earth differential clamp limits potential difference. I Telephone or data I I I I I I I I I I I I I I I I I I Power line filter and data line protection * Terminal I Secondary protection -I I I I I I I I < a.c.~ - Secondary protection - Computer C I - - I Computer common I - i l Power ! L ! N - Equipotential bonding conductor - Figure E.l 'Floating' computer common is reference earth for secondary protection Line 1 o @voltage Transient Line 2 C I I Equipment Figure E.2 Differential mode transient Equipment Figure E.3 Common mode transient Line Equipment Figure E.4 Multi-stage protection for telephone and signalling for telephone and signalling circuits Figure E.5 Surge diverter protec m for power circuls -L Line Filter -N Equipment E NOTE. Most standard radio frequency interference filters are not suitable for this application Figure E.6 Low-pass filter acts to reduce rate of rise of voltage after clamping NOTE. Combination units are of particular value where power, telecommunication, computer and lightning protection earths may not be bonded. Figure E.7 Combination units f i e d with differential clamp are suitable for equipment such as electronic PABX, computer modems and facsimile machines APPENDIX F ASSESSMENT OF RISK OF DAMAGE TO ELECTRONIC EQUIPMENT WITHIN OR ON STRUCTURES DUE TO LIGHTNING (Informative) F.l GENERAL This appendix gives advice on the assessment of the risk of damage to or maloperation of electronic equipment within or on structures due to lightning, and guidance on the design of systems for the protection against lightning of such equipment. Throughout this appendix the term 'electronic equipment' has the meaning given in Subclause 1.4.30. The implementation of the advice given may also give some level of protection against transients from other origins. A conventional lightning protection system is designed and installed only to protect the fabric of a structure. However, with the increasing reliance of industty and commerce on sensitive electronic equipment there is now a need to give an insight into the problems and advice on methods of protecting such equipment and associated data (software etc.) from the effects of a lightning strike. The complexities of the lightning strike phenomena to buildings, the lightning current flow through them and the coupling mechanisms giving rise to transients which cause damage to equipment and corruption of data are outlined. The risk of occurrence of transient disturbances from lightning is covered in Clause F.4 but there are many factors which can dictate the need for such protection, for example the need to: (a) (b) (c) (d) minimize fire risks and electric shock hazards; prevent extended stoppages in industry and commerce with the inherent financial implications; prevent health and safety hazards resulting from plant instability after loss of control; safeguard essential services such as fire alarms, communications and building management systems; prevent costly repair programmes to computer and instrumentation systems. (e) The advice given in this section is of a general nature and its application to a specific protection system should take into account the particular conditions pertaining to that system. In cases of difficulty, specialist advice should be sought. Figure F.l illustrates how lightning current may enter industrial plant and associated control systems following a lightning strike to buildings, control rooms or the surrounding ground. It is emphasized that even where protection is provided it can never be completely effective in eliminating the risk of damage to equipment or corruption of data. WARNING Attention is drawn to the danger of installing or carrying out maintenance work on lightning protection systems or surge protection devices during a storm. Small percentage lightning current w i l l flow down or data link T a result of strikes to any buildings or ground - Figure F.l Strike location points of industrial installations which could affect electronic system F.2 APPLICATION OF THIS APPENDIX When applying the advice in this appendix the following procedure should be adopted. (a) Decide whether there is a need to protect the structure against lightning (see Clause 2.2). If the answer to (a) is yes, consider the design specification required to protect the structure, before proceeding to (c). If the answer to (a) is no, then proceed directly to (c)Decide whether there is a need to protect the electrical and electronic installations within or on the structure against lightning (see Clause F.4 and F.5). If the answer to (c) is yes, then consult Clauses F.3, F.7 and F.13. If the answer is no, then no further action is required. (b) (c) (d) NOTE. Useful background information relating to various aspects of lightning protection is given in Clauses F.8 and F.9. F.3 BASIC CONSIDERATIONS OF ELECTRONIC SYSTEM LIGHTNING PROTECTION Exposure Levels. Before dealing with detailed design of protectionfor electronic equipment, F.3.1 the basic protection provided by the building should be considered. When considering the protection of electronic F.3.2 Protection Provided By The Building. equipment in a building and the need for such protection, it is necessary to take into account whether or not the structure of that building is already or will be equipped with a lightning protection system in accordance with sections three, four, five and six. Furthermore, the risk analysis (Clause F.4) of the need to provide protection for electronic equipment will often give a different result from that obtained by the risk analysis in Section Two. However, it is worth bearing in mind that many of the aspects of the protection of building structures and electronic equipment in buildings may already be required for some other reason, such as earthing and bonding to comply with SS CP 5. The type of structure which affords ideal lightning protection is a building with metal cladding on the walls and the roof providing a 'screened room' environment for the electronic equipment. If all the cladding and roofing is satisfactorily bonded together it enables the lightning current from a strike anywhere on the structure to flow as a 'sheet of current' all over the surface and down to the earth terminations. Many steel framed or reinforced concrete buildings with metal cladding approximate to this ideal. Such buildings only require attention to the prevention of transients brought in on the supply mains or other services (see Figure F.2). Hence care should be taken to obtain a low impedance bond to the lightning earth termination system from the metal armouring of the mains feeders, gas, water and other services. The method of power supply entry as shown in Figure F.3 is recommended, with surge suppressors being provided if required by the risk assessment. Wih such 'screened room' buildings, electronic installations totally within the building are very well protected. Where buildings are constructed from reinforced concrete or steel-framed with no metal cladding, lightning currents can flow in internal stanchions and advice is given in F.7.2 regarding precautions for computer location and wiring layout. If the construction materials of a building are substantially free of metal, it may be necessary to treat it as a high risk structure and given enhanced conventional building lightning protection (see F.7.1). In general, surge suppression devices should be f i e d as close to the point of entrylexit to the structure as practical. F 3 3 Current Routes In Buildings. Current flow in a 'screened room' building has been referred .. to in F.3.2 where it was noted that current principally flows as a 'sheet of current' distributed all over the surfaces of roof and walls and down to the earth termination. Minor resistance variations in different parts of the surface have little effect because current flow paths are determined by inductance and not resistance owing to the fast impulsive nature of the lightning return stroke and restrikes. A similar tendency for current to flow on external conductors occurs in steel framed or reinforced concrete structures, having the configurations shown in Figures F.4 and F.5 where the example of current confined to 15 discrete paths is given. It should be noted that the internal stanchions labelled A, B, C in Figure F.5 carry a very small percentage of the current and so give minimal magnetic fields inside. Thus the lightning protection afforded to electronic equipment within a building is considerably improved by having many down conductors preferably around the periphery of the building; the more down conductors that are available to carry current on the periphery, the weaker are the magnetic fields inside and the less the likelihood of transient interference into electronic equipment. From the above it can be seen that a single down conductor installed in a building is unacceptable from both the lightning transient injection and side flashing aspects. F.3.4 Effect On The Magnitude Of Lightning Transients With Different System Configurations. The ideal type of arrangement for buildings and electronic systems within them, which minimizes the risk of lightning discharge currents causing damage or upset to the systems is shown in Figure F.2(a). In such circumstances measures are taken to protect against lightning induced, transient voltages in the mains power supplies to the buildings. This is the arrangement described in F.3.2 where the building structures are well protected against lightning. Electronic systems in non-metallic buildings without external lightning protectionare most at risk and careful consideration of the method of protection of such buildings and their contents is required. An explanation of some of the risks is given in the following paragraphs of this subclause and advice on protection against them is given in F.7.1 and F.7.2. An example of a type of situation where there can be considerable risk is a building which contains electronic equipment and which may have associated equipment such as radio or radar aerials, meteorological apparatus or, in the case of a process plant, sensors mounted externally. This associated equipment may be mounted on the sides or top of in adjacent mast, radio tower, process vessel or conventional building as illustrated in Figure F.2(b). The roof or mast equipment is outside the protected environment of the building with its lightning protection system but cables leading from the equipment into the protected building can introduce severe transient voltages into the electronic equipment in the building if the roof or mast equipment is struck by lightning, no matter how good the lightning protection on the building. Furthermore, parts of the equipment mounted on the roof or mast may be susceptible to damage caused by a direct lightning strike or, at the very least, by the very large induced voltages or currents from lightning discharges flowing in and around sensors and their wiring. The foregoing example shows that the possibility of transient voltages being introduced into electronic equipment in buildings depends not only on the lightning protection of the building itself but the installation details of the wiring and sensors on the tower and the route to the electronic equipment in the building. Guidance on the measures to protect against these risks is given in Clause F.7. A further example of a common problem that can give rise to severe transient-voltagesis shown in Figure F.2(c). There is a tendency for lightning discharge current to follow the conducting paths formed by cables which interconnect buildings so that the current may flow from a building that has been struck into a building which has not itself been struck. Currents of tens of kiloamps may flow in such links and protection against this phenomenon is essential. Suitable protection is described in Clause F.7. This is a major factor in determining the risk to buildings and electronic systems. Key ---a Terminals, sensors etc. Aerials, meteorological sensors Mains suppressors A semi-shielded building J( D provides good protection of electronic equipment but there i s a need t o suppress mains transients ----Mains ---- (a) Electronic equipment contained within semi-shielded building * External sensors create a ' serious transient problem. Internal protection is satisfactory Electronic equipment ----Mains L-_L--- - - (b) Electronic equipment contained within serni-shielded building connected to external sensors Electronic equipment \ \ . -a Data line interconnection L)- ---Mains ---------- , Very serious transient problem from the data line interconnection. Surge protection a t both ends is recommended - ---- - - L - (c) Electronic equipment contained within semi-shielded buildings with data line interconnection Figure F.2 Configurations involving electronic equipment Main earthing terminal I Insulator Service pipes Figure F.3 Diagram showing bonding to service (gas, water and electricity) Lightning strike 0 0 0 0 10.3 6.2 5.4 Section A-A 6.2 10.3 NOTE. The figure shorn against each stanchion is the percentageof thetotal lightningcurrent flowing in that particular stanchion. Figure F.4 Lightning current distribution in a fifteen stanchion building NOTE 1. Transfer inductance (MM) contours are as follows: (1) 0.015 pH/m ( 2 )0.02 pWm (3) 0.03 pWrn (4) 0.04 pWm (5)0.05 pWm (6) 0.07 pWm ( 7 ) 0.09 pWm NOTE 2. The internal stanchions (A, B and C carry only 3.1 % 2 3 %and 3.1 % respectivelyof the total lightning current I , NOTE 3. The mutual inductance to a loop in vertical plane is obtained by subtractingthe value of transfer inductance at the position of one vertical leg from the value a t the other position (ignoring any resultant negative signs). The transfer inductanceto a wire on the stanchion is zero. Example: di p: For the 2 m high loop shown in the figure and a rate of rise of lightning current - of 50 W s dt Mutual inductance (M) = (0.03-0.015) -0.015 pWm Figure F.5 Plan view of fifteen stanchion building showing resulting field line plot (transfer inductance contours) for a lightning pulse. F4 . RISK ASSESSMENT F 4 1 Decision To Install Lightning Protection. The decision whether to install protection for .. electrical and electronic installations against the secondary effects of lightning, depends on: (a) (b) the probable number of lightning strikes to the area of influence (F.4.2); the vulnerability of the system configuration (F.4.3). F42 .. The Probable Number Of Lightning Strikes F 4 2 1 Effective collection area. The probable number of lightning strikes to the effective collection ... area in any one year, is given by the product of 'lightning flash density' and the effective collection area. The effective collection area, A, (in m2) is given by: 4= (Area of structure) + (surroundingarea collection ground Of F 4 2 2 Area of the structure. This area is the plan area of the structure. ... F 4 2 3 Collection area of the surrounding ground. A lightning strike to ground or a structure causes ... a localised increase in ground potential. Any cables (mainsldata lines) entering the area of raised ground potential will be subject to a common mode transient overvoltage. The effect of a ground strike will diminish as the distance between the perimeter of the structure and the strike point increases. There will be a certain distance beyond which a strike will have negligible effect. This is the collection distance D in metres. For typical 100 a-m resistivity soil the distance D should be taken to be 100 m. For soil with other values of resistpity the distance D should be taken to be numerically equal to the soil resistiv'i value up to a maximum value of 500 m for a soil resistivity of 500 a-m or greater. The collection area of surrounding ground is the area between the perimeter of a structure and a line defined by a distance D away from it. Where the height of the structure (h) exceeds D, the collection distance is assumed to be h. F 4 2 4 Collectionarea of adjacent associated structures. The collectionarea of adjacent associated ... structures which have direct or indirect electrical connections to the electrical or electronic equipment in the structure being considered, should be taken into account. Typical examples are external lighting towers supplied from the main buildings electrical installation, other buildings with computer terminals, control and instrumentation equipment etc. and transmission towers. At a site where several buildings are conductively connected and are spaced at a distance less than 20,the collection area of the adjacent associated structure(s) is the area between the perimeter of the associated structure and a line defined by a distance D away from it. Any part of this area within the collec!ion area of the structure being considered is disregarded (see example 1 in Clause F.6). F 4 2 5 Effective collection area of incoming mains s e ~ c e s . The effective collection areas ... associated with various types of mains services are shown in Table F.1. All incoming and outgoing cables (e.g. to other buildings, lighting towers, remote equipment etc.) are considered separately and the collection areas summated. [+ effective collection area of incoming mains services I collection area of associated structures effective collection area of data lines leaving the earth reference of the building 1 Table F.l Effective collection area of mains services Type of mains service Low voltage overhead cable High voltage underground cable (to on-site transformers) Low voltage underground cable High voltage underground cable (to on-site transformers) NOTE. D is the collection distance in metres (see F.4.2.3). The use of h in place of 0, explained in F.4.2.3, does as not apply. L is the length in metres of power cable with a maximum value of 1000 m. Where the value of L is unknown a value of 1000 m should be used. Effective collection area m2 F.4.2.6 Effective collection area of data lines leaving the earth reference of the building. collection areas associated with various types of data line cables are shown in Table F.2. The If there is more than one data line cable, they should be considered separately and the collection areas sumrnated. In the case of muhicore cables, the cable is considered as a single cable and not as individual circuits. NOTE. Aself powered electroniccircuit housed within an electricallycontinuous, metaldad building and which has data lines that are free of conducting material will not be at risk from lightning. However, a data line containing conductors (not using a fibre optic cable) or a low voltage supply line, connected to the same electronic circuit could dramatically increase the risk of lightning damage. Table F.2 Effective collection area of data lines Type of data line Overhead signal line 1 I Effective collection area m2 10xDxL I I I Underground signal line Fibre optic cable without a conductive metallic shield or core NOTE. l2xDxL ( 0 I 1 D is the collection distance in metres (see F.4.2.3). The use of h in place of D , as explained in F.42.3, does not L is the length in metres of the data line with a maximum value of 1000 m. Where the value of L is unknown a value of 1000 m.should be used. Collection area of surrounding ground Collect ion area of adjacent associated structure L~ighting tower Figure F.6 Collection area of structure and adjacent associated structure F.4.2.7 Assessment of the probable number of strikes per year. The probable number of strikes per year to the defined collection area, P, is as follows: where : Ae = N = , the total effective collection area in square metres; the ground flash density per km2 per year. For Singapore the current estimate given is 12.6 ground flashes per km2per year. F.4.3 Vulnerability Of The System Configuration. The overall risk of a strike to electrical or electronic equipment will depend upon the probability of a strike (P)and each of the following items: (a) (b) (c) Type of structure; Degree of isolation; Type of terrain. In Tables F.3 to F.5 weighting factors F to H are assigned to each of these items to indicate the relative degree of risk in each case. Table F.3 Weighting Factor F (type of construction) I I Type of structure Buildings with lightning protection and equipotential bonding to SS CP 33 Buildings where equipotential bonding for electrical or electronic equipment reference may be difficult (e.g. buildings over 100 m long) I ~alueof~ I I 2.0 I Table F.4 Weighting Factor G (degree of isolation) Degree of isolation Structure located in a large area of structures or trees of the same or greater height, e.g. in a large town or forest Structure located in an area with few other structures or trees of similar height surrounding structures or trees Value of G I I I Structure completely isolated or exceeding at least twice the height of II I 2.0 II I NOTE. Table F.4 has the same weighting factors as Table 2.5 but is repeated here to assist the user. Table F.5 Weighting Factor H (type of terrain) I I I On hill side On hll top Type of terrain Flat land at any level I Value of H 0.3 1.o 1.3 I I II I I NOTE. Table F.5 has the same weighting factors as Table 2.6 but is repeated here to assist the user. F.4.4 Risk Of A Lightning Strike To A Particular System Configuration. The risk of a lightning strike and the vulnerability of the system configuration (weighting factors) can be combined to assess the risk of a lightning strike coupling into electrical or electronic systems through either the incoming/outgoing mains service or incoming/outgoing data lines. The risk of occurrence (R) of a lightning induced transient overvoltage is given by: R = FxGxHxP NOTE. The value of 1/R ind'ites, in years,the average period between lightning induced overvoltage. A is emphasized that such average values are based on data collected over periods of many years. F.5 DECISION TO PROVIDE PROTECTION The decisionto provide protectionshould take into account the consequential effects of damage to important electrical and electronic equipment. Consideration should be given to health and safety hazards due to loss of plant control or essential services. The cost of computer system downtime or plant downtime should be compared with the cost of protection and prevention. A classification of kructures and contents is given in Table F.6. Table F.6 Classification of structures and contents Structure usage and consequential effects of damage to contents Domestic dwellings and structures with electronic equipment of low value and small cost penalty due to loss of operation Commercial or industrial buildings with essential computer data processing where equipment damage and downtime could cause significant disruption Commercial or industrial appiications where loss of data or computer process control could have severe financial costs Highly critical processes where loss of plant control or computer operation may lead to severe environmental or human cost (e.g. nuclear plant, chemical works etc) 3 Consequential loss rating 4 For a particular installation of electronic equipment, the value of R is ascertained (see F.4.4) and the consequential loss rating is established from Table F.6. By using these values in Table F.7 it is possible to determine the exposure level, to which the surge protection devices should be designed. Where the exposure level is negligible protection is not normally necessary (see Clause F.13). Table F.7 Classification of exposure level I Conseauential I Negligible Negligible Low Medium Negligible Low Medium High Ex~osure level -- 1 2 3 4 Low Medium High High Medium High High High NOTE. Exposure level categories in Table F.7 are based on a lightning risk assessment only. If transients of other origin are present, consideration should be given to upgrading protectors e.g. if in an industrial area the risk assessment suggests a surge protection device suitable for a medium exposure level is appropriate, the presence of inductive switching transients may make a high exposure level device appropriate. In these circumstances, specialist/manufacturers advice should be sought. F.6 SAMPLE CALCULATIONS Example 1 A commercial company's computing headquarters, is 15 m high covering an area of 100 m in length by 60 m in width. Located on flat country, the building is largely isolated from other structures of similar height and is protected in accordance with SS CP33. The flash density for the area is 12.6 per km2 per year. The incoming mains supply is a 250 m long LV underground cable and all computer communication lines are on fibre optic cable without metal armouring. An underground cable provides power from the building to a lighting tower, 7 m high, 100 m f r m the building. To determine what protection Is necessary, the risk factors are calculated as follows: (a) Number of flashes per km2 per year = 12.6 Therefore N = 12.6 , (b) Collection area 1) Area of structure 2) Collection area of surrounding ground (see Figure F.6) NOTE. It is assumed that D, the collection distance, is 100 m. 3) Collection area of adjacent associated structures (see Figure F.6) NOTE. For simplicity, the area is assumed to be a semicircle. 4) Cdlection area of mains services (see Table F.1) i) Incoming mains service ii) Mains service to lighting tower = 2 x 1 0 0 ~ 1 0 0= 2oooom2 Total collection area of mains services = 50 000 + 20 000 = 70 000 m2 5) Collection area of data lines leaving the earth reference of the building = 0 NOTE. Collection area is zero due to use of fibre optic cable. The total effective collection area is: 4 c) = 6 M ) O + 6 3 4 1 6 + 1 5 7 0 8 + 7 0 0 0 0 + 0 = 155124m2 Probable number of strikes per year The probable number of strikes per year to the effective collection area is given by: P = A,,xN,xlos = 155 1 2 4 ~ 1 2 . 6 ~ 1 0 ~ = 1.955 d) Risk of occurrence The risk of occurrence of a lightning induced overvoltage is given by: 1) for the whole site area: An R value of 0.587 indicates an occurrence of a lightning induced overvoltage every 1.7 years on average. 2) for the site area associated with the incoming mains supply: i) ii) iii) iv) N = 12.6 , Collection areas = 6 000 + 63 416 + 15 708 + 50 000 = 135 124 m2 Probable number of strikes per year = 135 124 x 12.6 x 10" = 1.703 Risk of occurrence (R) = 1 x 1 x 0.3 x 1.703 = 0.51 1 From Table F.6, the building is considered to have a consequential loss rating of 2. From Table F.7, it is deduced that a surge protection device should be fitted, suitable for a high exposure level environment. 3) for the site area associated with the mains supply to the lighting tower: ii) iii) iv) Collection areas = 6 000 + 63 416 + 15 708 + 20 000 = 105 124 m2 Probable number of strikes per year = 105 124 x 12.6 x 10" = 1.324 Risk of occurrence (R) = 1 x 1 x 0.3 x 1.324 = 0.3967 From Tables F.6 and F.7, it is deduced that a surge protection device should be fitted, suitable for a medium exposure level environment. Example 2 A small water treatment control building has dimensions 6 m x 10 m x 10 m (height x length x width). Located on a hillside, the building is isolated and is protected in accordance with SS CP 33. The mains supply is by a 250 m long LV overhead line and an overhead telephone line of unspecified length provides a telemetry link. The flash density for the area is 12.6 per km2 per year. To determine what protection is necessary, the risk factors are calculated as follows: (a) Number of flashes per km2 per year = 12.6 Therefore N, = 12.6 (b) Collection area 1) Area of structure 2) Collection area of surrounding ground NOTE. It is assumed that D, the collection distance, is 100 m. 3) Collection area of adjacent associated structures = 0 4) Collection area of incoming mains services (see Table F.l) = 10X1OOX250 = 250 000 m2 5) Collection area of data (telephone) lines (see Table F.2) = lOxlo0xlOOO = lo00 000 m2 NOTE. It is assumed that L is 1000 m since the length of the telephone line is unspecified. The total effective collection area of the site is: 4 = 100 + 35416 + 0 + 250000 + 1000000 = 1.2855~10~m~ The effective collection area associated with incoming mains services is: A,,, = 100 + 35 416 + 0 + 250 000 = 285.5 x 1o3 m2 The effective collection area associated with the data (telephone) line is: &= = 100 + 35416 + 0 + 1OOOOOO 1.0355x10~m~ c) Probable number of strikes per year The probable number of strikes per year to the total effective collection area of the site is: The probable number of strikes per year to the effective area associated with incoming mains services is: The probable number of strikes per year to the effective area associated with the data (telephone) line is: P, = A x N x 10" , , d) Risk of occurrence The risk of occurrence of a lightning induced ovewoltage is: 1) for the whole site area: R = FxGxHxP A value of R of 32.394 indicates an occurrence of a lightning induced ovewoltage every 11 days taken on average over a long period. 2) for the site area associated with the incoming mains: R = FxGxHxP, From Table F.6, the site is considered to have a consequential loss rate of 3 since its loss of operation would disrupt the water supply to an entire town. From Table F.7, it is deduced that a surge protection device should be fitted, suitable for a high exposure level environment. 3) for the site area associated with the data (telephone) line: R = FxGxHxP, From Tables F.6 and F.7, it is deduced that a surge protection device should be fitted, suitable for a high exposure level environment. F.7 METHODS OF PROTECTION OF INSTALLATIONS AGAINST LIGHTNING F.7.1 Earthing, Bonding And Potential Equalization. In countries where there are less than 25 thunderstorm days per year, the damage to an electrical installation (e.g. wiring, switches, socket outlets) due to conducted lightning transients from low voltage power supply lines is negligible. In other countries protection against transient overvoltage for the electrical installation may be required if the structure is supplied via an overhead line. General rules of earthing are given in Clauses 3.12 and 3.13. The following recommendations complement them in order to improve earthing with the objective of achieving an equipotential reference plane such that electronic equipment is not exposed to differing earth reference potentials. Incomingservices for structures with extensive communicationsystems e.g. industrial buildings, should be bonded to an equipotential bonding bar which is normally in the form of a metal plate, an internal ring conductor or a partial ring conductor at the inner side of the outer walls or at the periphery of the volume to protect near ground level as appropriate. This equipotential bonding bar is connected to the ring earth electrode of the earthing system. An example is shown in Figure F.7. All external rnetal pipes, electric power and data lines should enter and leave the building at one point so that the armouring, etc., can be bonded to the main earthing terminal at. this single point of entry (see Figure F.3). This minimizes lightning currents crossingthe building internally (see Figure F.8). Where power and data line cables pass between adjacent structures, the earthing system should be interconnected and it is beneficial to have many parallel paths to reduce the currents in individual cables. Meshed earthing systems fulfil this objective. Lightning current effects may be further reduced by enclosingthe cables in metal conduits, trunking, ducts, etc., which should be integrated into the meshed earthing system and bonded to the common cable entry and exit earth point at both ends. Figure F.7 shows a typical example of mesh earthing configuration for a tower and adjacent equipment building. Similar principles to those which apply to the tower also apply to sensors or controls for well equipment (oil, water, etc.), where the bonding should include connections to the steel pipe of the well to reduce the potential difference between the well and the wiring. This bonding should then be interconnected to any other building earth to which the data line cables run. Structures associated with masts should have the extra protection of their own dedicated power supply or be provided with an isdating transformer. F.7.2 Location Of Electronic Equipment And Cables F.7.2.1 Location of electronic equipment within buildings. The choice of location of electronic equipment in a building depends on the building construction. For a building resembling a screened room, i.e. bonded metal clad roof and walls, the location is not critical. In conventional modern rnetal framed buildings the electronic installations should preferably be located in the centre of the building, and preferably should not be on the top floor where it is adjacent to the roof air terminations and roof lightning mesh nor, in preference, should it be near outside walls especially comers of buildings. For a building comprising essentially non-conducting materials but with a lightning protection system the same recommendations apply. In buildings of nonconducting materials which house electronic installations considerable care should be taken in location and specialist assistance should be sought. In particular the electronic installation (including its cabling), should not be located in a building adjacent to a tall structure, e.g. chimney, mast, tower, which could give high local fields when lightning current passes down this single route to ground. F.7.2.2 Location of cables between electronic equipment within the building. Figures F.9 and F.10 illustrate the principal recommendations for internal wiring. As in the case of the computer location the routing and location of wiring within a screened room building is not critical but it is nonetheless good practice to fdlow the recommendations for metal framed buildings. Avoidance of large area loops between the mains supply and the electronic installation wiring is strongly recommended. It is desirable to run mains wiring and electronic equipment cables side by side to minimize loop areas. This can be achieved by a pair of adjacent ducts or a duct containing a metal partition between the cables. In Figure F.10 meshed earthing is employed locally on the floor of the building and star point earthing is used overall. This combination of earthing systems is known as hybrid earthing. Wiring to electronic equipment within the building should not be installed adjacent to possible lightning carrying conductors, e.g. roof conductors, external wall conductors. Wiring should be at floor level and should avoid loops in the vertical plane. Wiring between floors should be as shown in Figure F.lO. The layout in Figure F.10 can also be used for equipment laid out horizontally in several adjacent rooms of a long building, in which case the metal trunking should be re-arranged horizontally to join together separate blocks of equipment. ]I lF\ Transmitter Metal plate or bonded Metallic duct or! reinforced concrete with the metallic reinforcement connected ~abl/e to e a r t h chamber Water, \~arthin~ network o f A the building, 'power cables ( i f necessary in inetallic ducts) NOTE. All cables, ducting etc. enter building at the same point. Figure F.7 Cables entering a building separated from a transmitter mast 144 Interconnect ion for armouring of and bonding of cab trays Instrumentation cables and earth ;\ I I I I I I I I I Internal ring conductor Single point earth feed for electronic equipment To power and instrumentation circuits I I I + I Reinforcing bars \ -------Ring I I I I earth electrode (a) Bonding on entryto a building of computer and power cables to lightning earth I I Multicore armoured cable II armour Metal tube (b) Grounding at entry for pipes, r.f. cables, armoured cables etc in reinforced concrete wall, where maximum protection is needed Figure F.8 Bonding of cables and pipes at entry and exit to buildings Metal Data line \ l nduct ion loop (b) Reduction of induced voltages using a metal clad building (a) Unprotected system Metal conductor, cable duct or cable shield (c) Reduction of inducedvoltages by minimizing areas of loop NOTE. Screens should be bonded at both ends and be electrically continuous. (d) Reduction of inducedvoltages by screeningwith overbraids, conduit, trunking etc. NOTE. Minimal protection is required internally for figure 49(b). Buildings with discrete downconductors (stanchions. tapes etc.) should use a combination of figure 49(c) and figure 49(d) and the hybrid earth principle of figure 50. Figure F.9 Methods of reducing induced voltages I46 y I Metallic cable duct connected o building earth Interconnection between system earth reference /point and cable d u d ,Floor System reference connection of cable screen to the rack is recommended \ Insulation be tween sys tem reference potential plane and the structure NOTE 1. The principle of minimizing loop areas can be applied to equipment laid out laterally. A I interconnections J are in one cable duct to minimize areas of loops as in Figure F.9. NOTE 2. xxxx represents steel reinforcement or other metallic floor constructions. Figure F.10 Hybrid earth system applied to equipment in muli-floor building For buildings constructed of non-conducting materials the wiring layout described in this subclause is essential to minimize damage to equipment or corruption of data. Where the recommendations in this subclause for wiring layout are not practical, the use of surge protection devices is recommended and specialist advice should be sought. F.7.3 Protection Of Building-to-building Data Lines. Where data lines pass between separate buildings, or between separate sections of one building which are not structurally integral (e.g. new wings added to a building joined by brick corridors, etc or sections of a building separated by expansion or settlement gaps which are not bonded across) special care should be taken regarding protection. Where possible, fibre optic links should be used to isolate completely the electronic circuits of one building from the other. This is the optimum method for multi-channel data links for complete freedom from electromagnetic compatibility (EMC) problems of all kinds, not only lightning. However it is recommended that fibre optic cables with metal armouring or draw wires inside should not be used. (f such cables are used, the armouring and draw wire should be bonded directly or indirectly via surge i protective devices to the main cable entry bonding bar at its entry point into each of the buildings. No further bonds to the fibre optic cable armour or draw wire should be made). Where fibre optic links are not an option and conductive data lines are required, e.g., wire pairs, or coaxial cables or LAN's, precautions should be taken to prevent very damaging transients flowing along the line causing multiple damage at both ends. Earthing systems of structures should be interconnected using the armour on cables, braids or metal trunking, raceways, conduits, etc., which are electrically continuous and bonded to the earth systems of the buildings at each end. In industrial installations, the armouring on multiple pair cables should be bonded at both ends to the structure. Where many such cables are available in parallel, very good interlinking of the system exists, resulting in very low induced voltage in the instrumentation loops (for sample calculation see Clause F.lO). In addition earth cables should be installed to provide positive links from structure to structure. Where coaxial cables are installed between buildings, the conductive sheaths of these cables should be bonded to the earthing system of the building at the entry/exit point of the building. In certain types of coaxial or screened systems, it is permissible to bond the cable only once to earth. Where necessary a suitable surge protection device should be used to provide additional bonding. This is the case in certain types of LAN. In most cases, protection is also required for the inner core(s) within a coaxial cable (see Clause F.ll). Where only one or a small number of lines go from building to building as in the case of instrument data or telephone lines, and where fibre optics are not an option, surge suppressors should be Vied which will crowbar (or clamp) the anticipated partial lightning cutrent to earth e.g. with a gas tube or semi-conductor crowbar (or clamp) device and allow only a 'let-through' voltage within the appropriate level for the equipment. A typical system for earthing of surge suppressors is shown in Figure F.11. Recommendationsfor surge protection devices are given in Clause F.14. A combination of the methods discussed in this subclause is possible, e.g. using optic isolator devices for signal or instrumentation lines (including intrinsically safe systems within a potentially explosive atmosphere) in conjunction with bonded arrnouring of cables to keep transients within an acceptable voltage range. Except in the case of long fibre optic links, high impedance isolationdevices are not satisfactory on their own unless they have a withstand voltage greater than 100 kV owing to the large potential difference occurring between unprotected buildings resulting from lightning current flow into the ground from one of them. F.7.4 Protection Of Equipment Having Component Parts On The Outside Of Buildings Or Connected To Towers, Masts Or Process Vessels. Where component parts of equipment are mounted on the outside of buildings (i.e. on the side walls or roof) or are connected to towers, masts or process vessels the following hazards exist: (a) Current injection from direct strike. Current injection from direct strike should be prevented wherever possible by suitably placed air terminations, covers, enclosure of aerials and wiring in order to minimize a chance of direct lightning strike to sensors and wiring (see Figures F.12, F.13 and F.14). Cables attached to masts should be located within the mast to give protection, such as a shroud diverter, from direct injection. Where whip aerials etc. cannot be protected by an air termination from strikes to it, some form of protection, such as a shroud diverter, should be incorporatedat the base of the aerial to limit any surge currents into the down leads. Inductive and resistive voltages. The protection methods described in (a) to prevent strike contact to electronic equipment, wiring and exposed items (sensors, aerials, etc.) will reduce resistive and inductive voltages. Wiring should be installed in metallic conduit or in locations where the structure provides suitable screening e.g. for steel lattice towers inside the corners of the L members or within metal tubular masts (see Figure F.15). In Figure F.15, the relative values of currents to be expected in individual cables in four positions are given. From the braid or sheath resistance and length, the induced voltage can be estimated (see Clause F.12 for a sample calculation). Where the wiring uses a low partial current route and is screened, induced voltages will be minimized. However very short bonding leads at each end of shielding braided tubes etc. should be used to reduce inductive voltages. This technique is satisfactory where the induced voltages can be shown to be less than the immunity level (see Subclause 1.4.31) of the equipment being protected. Where the induced voltage cannot be restricted to less than the immunity level then surge protection devices should be fied. (b) Overall protectionfor a tower and its associated equipment building is shown in Figure F.7 and illustrates many of the points described above including routing, screening, bonding, interconnection of earths, etc. NOTE. This is recommended in CCllT Publication 'The protection of telecommunication lines and equipment against lightning discharges', Chapter 6 'Protective practices for specific parts of telecommunication networks' published in 1978. F.8 CHARACTERISTICS AND EFFECTS OF LIGHTNING F.8.1 Additional Characteristics Of Lightning Relevant To Electronic Equipment. General characteristics are given in Appendix A. Additional characteristics relevant to electronic equipment are as follows: The maximum rate of rise (di/dt) values are: di/dt exceeded by 1 % of strokes, 200 kA/~s; di/dt exceeded by 50 % of strokes, 30 kA/a; di/dt exceeded by 99 % of strokes, 10 kA/~s. A Surge suppressors / ,ELectronic equipment Zero voltage reference (a) Incorrect installation of surge suppressors giving rise to large transients NOTE.Thecm transients can arise from the earth line inductance to the earth reference point B. A' / Surge suppressors + 1 -Electronic equipment A A - - Zero voltage reference (b) Recommended installationof surge suppressors NOTE.Transients minimized by connecting the zero voltage referenceto earth reference ' point 6 by the most direct route. Figure F.ll Earth connection from zero voltage reference of equipment to earth of surge protection devices Cable Top of process vessel (side view) exposed above other plant Exposed cables over the top of the roof (a) Exposed sensor and cable on processvessel (b)Exposed cable on roof Figure F.12 Direct injection into exposed electrical systems Sensor head 0 \ 0 Cable in metallic conduit - Air termination (e-g. handrails etc.) abovesensor head M e t a l cover over cables (a) For sensor and cable on process vessel (b) For cable on roof Figure F.13 Protection from direct injection 1 1 Metallic conduit bonded to Local structure and to T.B. / s = Sensor TB= Local terminal box Bonded a t roof Vesse l - 9 , To control system Figure F.14 Protection of cables located alongside tall vessels and bonding at roof level High current f l o w in unprotected cable \ Low current flow in cable due to protection afforded by cable tray and pipes Medium current f l o w i n cable due to protection afforded by cable tray Cable troy Section A-A Figure F.15 Locations where high, medium or low lightning current can be expected to flow through cables associated with a reaction vessel Other parameters of the lightning pulse are important for other aspects of lightning damage, but peak current and peak di/dt are the principle ones for interference voltage considerations, and the duration of the pulse is significant for energy ratings of suppressors. The representation of a severe negative strike to ground is shown in Figure F.16. Lightning induced transients can have two major effects on electronic equipment. The most serious is equipment damage which might easily be caused by a single stroke. The second major effect, which is computer data and software corruption or disturbance ('upset'), might also be made worse by the multiple pulse aspect of lightning. The combination of the first retum stroke plus numerous restrikes (up to say twenty in a severe case) all occurring in a period of 1 s to 2 s has a capability of causing considerable problems in the correct operation of a computer, unless it uses error checking which can reject 'nonsense' data for periods up to 2 s. F 8 2 Strike Points For Lightning. The positions of strike points to level terrain are very random. .. although tall trees and houses may be slightly more at risk than small trees and houses. However strikes to flat ground between buildings are quite common when the buildings are separated by a distance of more than twice the height of the individual buildings. In process plant installations, strike locations will tend to favour the chimneys, taller fractionating columns, tall lamp posts etc. so minimizing strikes to parts of the plant close to these tall features. However, parts of buildings which are outside the 45 degree cone of protection of the tall buildings are liable to be struck (see Figure F.17). Electronic equipment on towers or tall buildings is especially at risk, not only because of the risk of direct strike, but also because of the relatively high exposure of wiring and other associated equipment. Such systems should be carefully protected. A notional process plant in plan and elevation, showing areas subject to being struck, is shown in Figure F.17. For buildings up to 20 m in height, the 45O cone of protection system is a good working rule in considering lightning protection. However, for tall buildings, towers, etc. (over 20 m tall), the rolling sphere method is better for identifying vulnerable areas, especially as it permits an assessment of the tendency for strikes to occur to the sides of such structures. A rolling sphere radius of 20 m is recommended. In Figure F.17, it is assumed that electronic equipment is located within buildings A and B and that there are connecting cables between. However, it can be seen that strikes are possible to A and to parts of B, as well as to the ground around them both. Thus, in such an example, ground currents from nearby strikes to ground, or to connected or uncorrected buildings, need to be taken into account in assessing the nature and extent of the protection required. F.9 LIGHTNING INDUCED TRANSIENTS AND PROTECTION PRINCIPLES NOTE. This Clause describes the induced voltage coupling mechanisms, magnitudes, waveshapes and provides guidance on establishing safe transient levels by the TCLIETDL principle (see Subclauses 1.4.31, 1.4.32 and F.9.6). Resistively Induced Voltage. When a building is struck by lightning, the current flow into the F91 .. earth develops a large voltage between the building structure, metal work and lightning protection system, considered as a whole, and a remote earth. This is one of the reasons that lightning currents flow in the external conducting parts (e.g. cables) which are bonded to the building and run to remote earths. The voltage so produced is primarily a resistive voltage but on the fast rising part of the lightning waveform, inductive and transmission line effects will also occur at least to a small extent. I di Maximum dope= df time NOTE. The initial strike may be followed by several shorter duration lower arnplbude pulses of current called subsequent strikes or restrikes. Figure F.16 Lightning current characteristics for severe negative strike Strikes possible to ground,outside dashed line and to cross-hatched areas Elevation Figure F.17 Strike points on plant Any current flowing in cable screens and armouring results in the resistive voltages which are injected by way of the wiring into electronic equipment as common mode voltages at both ends of the cable. For that part of the spectrum of the lightning pulse where there is the most energy, say up to 100 kHz, earth resistance and cable screen, armouring, etc. act as resistors so producing resistive voltages similar in waveform to the lightning current pulse. However in certain circumstances (e.g. long underground sheathed cables) considerable elongation of the current pulse will occur (up to perhaps 1000 ~ s and the design of protection equipment should take this into account. ) F.9.2 Inductive Voltage. Lightning current, either flowing in a conductor or in the arc channel produces a time varying magnetic field, which at distances up to 100 m is proportional to the time varying current. This time varying magnetic field produces two effects: (a) (b) A magnetic self inductance L in a cable carrying the current (e.g. for a typical wire (about 2 mm dia) L = 1 ctH/m); A mutual coupling to loops either associated with itself (Transfer lnductance = M or ) , in completely separate loops (Mutual lnductance = M). ,, In each case voltages produced are proportional to di/dt multiplied by L, M or M. (see Figure F.18). The waveforms of these voltages are proportional to the first derivative of the current pulse. Also, for a single conductor carrying the current, the field strength is inversely proportional to distance from the conductor. For a more complex situation, calculations can evaluate L, M or M. For , example lightning current passing down the stanchions of a 15 stanchion building shown in Figure F.5 , give contours of constant M relative to the stanchion. This enables calculations of mutual and transfer inductive voltages to be made as given in the example in Figure F.5. It is also important to take into account the inductance of earth leads to equipment and surge suppressors and 'pigtail' earthing of cable screens. V=L di df V=MTF di dt Mutual inductance to loop-M V=M d i dt Figure F.18 Inductance Current Injection From Direct Strike. Direct lightning strikes to installation wiring or exposed F.9.3 electrical systems such as sensor heads or aerials (see Figure F.12) may inject sufficient current into the wires to cause explosive vaporization. This can cause considerable physical damage to the installation wiring over a considerable length. Enclosuresfor the wiring e.g. plastics or metal conduits or trays and other items very close to the wiring could also be split apart or damaged. Owing to the very high voltages associated with direct injection, damage to other circuits is possible as a result of high voltage breakdown and flashover on the terminal blocks, plugs and sockets, etc. so injecting large currents or voltages into the other circuits and so giving multiple failure in them. This is particularly relevant to situations involving potentially explosive atmospheres. By suitable relocation of wiring, wiring covers and/or the fitting of a suitable air termination such direct contact should be prevented (see F.7.4). F.9.4 Electric Field Coupling. Field strengths have to be taken into account in the whole striking area immediately before the formation of the main discharge when their values are up to the breakdown strength of air (approximately 500 kV/m). At the formation of the main discharge, the field breaks down and field changes of approximately 500 kV/m/~scan occur. The effects of such a field change are not normally a serious problem since protection of the resistive and inductive effects of lightning will also give protection from electric field coupling. Lightning Electromagnetic Pulse Induced Voltage (LEMP). The term LEMP is used to refer F.9.5 to the electric and magnetic fields radiated from either lightning ground flashes or cloud-to-cloudflashes and was coined to correspond with another electromagnetic phenomenon, namely nuclear electromagnetic pulse (NEMP). There are important differences in the spectrum and magnitude of the two effects since NEMP produces much faster rising pulses (a rise time of 10 ns) with very severe amplitude, and the NEMP only interacts with systems as a radiated pulse. By comparison the radiated pulse from lightning is relatively small. Strikes either to the building under consideration or to the ground nearby do not produce true LEMP but principally a near field magnetic coupling which gives inductive voltages (and resistive voltages) as described in F.9.1 and F.9.2. Lightning induced electric field pulses within buildings containing electronic equipment are usually negligible. In exceptional cases external wiring might be at risk, unless it is screened or enclosed (which in any case is necessary for protection against injected currents and induced voltages). In general the worst effects of LEMP are prevented by adopting precautions necessary for protecting against direct lightning strikes. Direct strikes in any case produce much more severe transients than LEMP and direct strike protection is the primary requirement; the protection against LEMP being a secondary advantage and so ensuring that LEMP effects are negligible. F.9.6 Transient Control Level (TCL) EquipmentTransient Design Level (ETDL). For any electronic equipment operating in any transient or other interference environment, protection can be designed and will provide any degree of protection which is economically feasible or necessary on safety or other grounds. Once the pass/fail criteria are decided for equipment, (e.g. no damage to internal components) suitable tests will demonstrate the maximum level of transients, called the ETDL at which the equipment will operate successfully. In the case of lightning this means that up to say N vdts of transient, applied in common or differential mode, the equipment will not suffer component damage. The equipment then has an ETDL of N volts. When installing the equipment it is necessary to ensure that transients in the wiring connected to the equipment are at a level of say P volts, which will be below the ETDL of N volts, (to allow for ageing, safety factors in the calculations, etc.). P is called the TCL and N minus P is called the safety margin. To determine whether a surge protection device can control a transient voltage to within the desired transient control level, its let through voltage, with a voltage waveform of appropriate severity, should be established. Matching the ETDL's of equipment with the transient level of the installation will ensure a safe system, provided attention is paid to earthing and bonding techniques to maintain low transient levels. In particular, attention has to be paid to electronic equipment and surge protection device earthing to prevent significant resistive, and especially inductive voltages, which occur on protector device earth leads, from being added to the basic protector let-through voltage. F.9.7 Protection Principles. F.9.1 and F.9.5 refer to the various coupling mechanisms from lightning. Except in the case of very exposed aerials, equipment protected against the resistive/inductive effects of a lightning strike on or close to a building will be protected from the electric field and the LEMP aspects of it. Direct injection of lightning current into electronic equipment should be prevented owing to the very serious damage caused (see F.7.4). A major factor in the importance of resistive and inductance voltages is that they are both injected with low source impedance hence the resutting energy in the transients is much higher than those available from either LEMP or electric field coupling. Therefore it is the magnitude of resistive and inductive induced voltages and currents which provide the basis for a quantitative assessment of transients and the specifications for protection devices. Lightning protection should therefore protect against the high voltage which might result from a strike, and which can cause large currents to flow into cables and arrestors due to their low source resistance. For protection techniques to be successful, the following conditions should be satisfied : (a) (b) Survivability. Protection devices or techniques should be able to su$ie the full severity of transient overvoltages to be encountered at the device location. Transient control level. Protection should achieve a transient control level below the ETDL of the equipment being protected. Surge protection techniques such as connecting leads and earth cables may add significantly to the transient control level achieved. System compatibility. Any form of protection added should not interfere with the normal operation of the system to be protected. Particular care may be required with high speed communication systems and intrinsically safe circuits. (c) F.10 SAMPLE CALCULATION OF INDUCED VOLTAGE IN INSTRUMENTATION LOOPS The sample calculation of induced vdtage involves the use of armouring as part of the earth interconnection system of many parallel instrument cables spreading out to a plant. Take the case of 100 cables with aluminium armouring, say 65 strands of 1 mm diameter on each cable with a resistivity of 3.0 x 10* am,cables 100 m long. Resistance of each caMe = p.L A 9 - - 3.0 x 104x 100 = 5, 65 x0.0012x z 4 For 100 cables in parallel each taking one hundredth of the total current of say 100 kA flowing from computer room to plant, the current would be 1 kA per cable. Therefore the common mode induced voltage = Resistance x Current = 5 9 i o 4 x 1 x lo3 = 59v. ~ In practice the current distributionamong the cables will not be uniform, but with the assistance of other earth cables in parallel with the instrument cables and also power cables with their armouring bonded the current in each of the Instrument cables is unlikely to exceed 1 kA by a large factor. F. 11 SAMPLE CALCULATION OF PROTECTION OF INNER CORE(S) OF COAXIAL CABLE Consider the case of 20 m of shielded cable with the shield bonded at both ends. 10% of the lightning current flows through the cable and the cable shield has a resistance of 5 nlkm. For a 200 kA strike, the voltage produced is given by: For a 20 kA strike, the voltage produced is given by: These resistive voltages are coupled fully into the internal wires. If the cable were placed on a bonded cable tray, there would be a preference for current to flow in the tray. In typical cases only 10% of current would flow in the coaxial cable. For a 200 kA strike, the voltage produced is given by: For a 20 kA strike, the voltage produced is given by: Depending on the use of the cable, damage may or may not occur, as in the following examples: (a) (b) (c) If the cable was the feed from a thermionic valve radio transmitter to an aerial, then over 2000 V is unlikely to cause damage. If the cable was a part of a robust computer network, then 2000 V would probably cause damage; 200 V and 20 V probably would not. If the cable carried an RS 232 01.24) link then only the last case would be acceptable. In all the other three cases, damage would occur. F.12 SAMPLE CALCULATION OF INDUCED VOLTAGE IN WIRING Figure F.15 shows the relative values of currents in individual cables alongside or inside a process vessel or similar object. As can be seen the currents are a strong function of position relative to the tower and other metal components. Values of induced voltage appropriate to the various positions identlied may be calculated as follows : (a) For a cable protected by a cable tray, assuming a cable screen current of 400 A, a tower of 30 m height and a cable screen resistance of 10 m l m : Total resistance = 30 x 10 x 1o9 = 0.3 n lnduced common mode voltage = I x R = 400 x 0.3 = 120 V (b) For a cable.protected by a cable tray and pipes and assuming a current of 100 A: lnduced common mode voltage = 100 x 0.3 = 30 V (c) F.13 F.13.1 For a cable inside the vessel, or inside a metal cylinder, the induced voltage would be negligible. SURGE PROTECTION DEVICES, LOCATION CATEGORIES AND TESTING Location Categories F.13.1.1 General. As a mainstransient, represented by a 1.2150 ps voltage pulse, propagates through a building, the magnitude of current it can source diminishes (due to inductance of mains cables). This effect is represented by the three location categories C, B and A (see Figure D.4). Category C is on the supply side of an incoming power board, category B represents the mains distribution system and category A represents protection on the load side of a socket outlet. Within a given location category, the severity levels of transients encountered will increase as risk of a transient occurring increases. This can be represented by the system exposure level, which in turn can be derived from the risk assessment. F.13.1.2 Data/signal cables. All datalsignal line surge protection devices fall into location category C irrespective of location as the slower 101700 ILS vdtage pulse used to represent a dataline transient is not attenuated to the same extent by a cable, as a mains transient. F.13.1.3 Mains power F. 13.1.3.1 Location category C. Surge protection devices installed in the following locations fall into category C: (a) Supply side of incoming power distribution boardslswitchgear (i.e. boards that bring power into a building, from the supply authority, HVILV transformer or another building). Load side of outgoing power distribution boardslswitchgear (i.e. boards that take power to other buildings, external lights, pumps etc.). Outside of a building. (b) (c) F. 13.3.1.2 Location category B. Protection devices installed in the following locations fall into category B: (a) (b) (c) On a power distribution system, between the load side of the incoming mains power distribution board/switchgear and supply side of a socket outlet/fused connection unit. Within apparatus that is not fed via a socket outlet/fused connection unit. Load side of socket outlets/fused connection units located less than 20 m cable run from category C. F.13.3.1.3 Location category A. Protection devices installed on the load side of socket outlets/fused connection units and are more than 20 m cable run from category C, fall into category A. NOTE. Category A does not appear in small buildings where socket outlets are all less than 20 m from category C. F.13.2 Magnitude Of RepresentativeWave Forms For Testing Mains Surge Protection Devices. An appropriate test level is selected from Tables F.8 to F.10 for the location category and level of system exposure of the surge protection device under test. Table F.8 Location category A (mains) System exposure Low Medium High Peak voltage kV 2 4 Peak current A 166.7 333.3 500 6 Table F.9 Location category B (mains) System exposure Low Medium High Peak voltage kV 2 4 Peak current kA 1 2 6 3 Table F.10 Location category C (mains) I System exposure Low Medium High Peak voltage kV 6 I Peak current kA 3 10 20 5 10 Testing Mains Surge Protection Devices. The test generator for categories C and B is a F.13.3 combination wave generator, capable of producing 1.2150 ILS voltage and 8/20 f i current waveform (see F.13.7). For category A, a non-inductiveoutput resistor is added to limit current to the appropriate value. The short circuit current waveform will no longer be 8/20 ILS. A test method for surge protection devices is given in section 24 of UL1449. Magnitude Of RepresentativeWaveform For Testing Data Line Barriers. An appropriate F.13.4 test lwei is selected from Table F.11 for the exposure of the surge protection device selected. Table F.11 Location category C (data lines) System exposure High impulse current test k~ H Let-through voltage test Voltage current F.13.5 Testing Data Line Surge Protection Devices F.13.5.1 High current impulse life test. A combination wave generator described in F.13.3 is suitable for these tests. The test method is given in Subclause 4.6 of CClTT IX K12. F.13.5.2 Let-through voltage tests. A suitable test generator is given in Figure 1 of C C l l l IX K17. A test method is given in paragraph 24.3 of UL1449. F.13.6 lnformation To Be Provided By Manufacturers Of Surge Protection Devices F.13.6.1 Information on transient performance. Manufacturers of surge protection devices should be requested to provide the following information on transient performance : , Let-through voltage e.g. 850 V all modes, test 6 kV, 1.2150 1s; 3 kA, 8/20 ps. NOTE 1. This is a test value on the comple& surge protection device, not a theoretical value. NOTE 2 The let-throughvoltage of a surge protection device takes into account the response time o the f device i.e. a slow response time will result in a high let-through voltage. Lightning transients are not particularly fast, response time is more important for faster transients e.g. NEMP etc. The response time of a parallel connected protector will aften be overshadowed by inductive voltage drops on the connecting leads. Modes of protection e.g. line to earth, line to neutral, neutral to earth for mains or line to line or line(s) to earth for data. Maximum surge current e.g. 20 kA, 8/20 ILS. NOTE 1. This is a test value on the complete surge protection device, not a theoretical value. NOTE 2 The energy handling of a protection device is implied by the maximum surge current. The use of energy ratings as an indicator of comparative merit of differing designs can be misleading as the energy deposited in a protector by a transient current source depends on the suppression level (let-through voltage). Therefore a lower energy rating does not necessarily mean a lower capability of survival. (d) System impairment. If the surge protection device impairs the operation of the system after a transient has passed, full details of any effect should be given (see F.13.6.2, item e). A gasdischarge tube used as a surge protection device connected across a mains power supply can short-circuit the supply when it operates. A large mains current will flow through the tube which may cause disruption of the power supply and/or destruction of the tube. F 1 . . Information on passive state performance. Manufacturers of surge protection devices .362 should be requested to provide the following information on passive state performance. (a) (b) (c) (d) (e) Nominal operating voltage; Maximum operating voltage; Leakage current; Current rating; Systems impairments. Any factor that may affect operation of the system should be quoted, e.g. (a) (b) (c) (d) (e) In-line impedance; Shunt capacitance; Bandwidth; Voltage standing wave ratio (VSWR); Reflection co-efficient. APPENDIX G BEHAVIOURAL PRECAUTIONS FOR PERSONAL SAFETY (Informative) G.l GENERAL This appendix provides guidance for personal safety during thunderstorms and mainly applies to behaviour when outdoors. Measures for the protection of persons which should be incorporated in lightning protection systems for buildings and structures are outlined in other sections. 6.2 PERSONALCONDUCT Persons seeking protection from lightning should observe the following precautions: Seek shelter in a substantial building with at least normal headroom or within a totally enclosed, metal-bodied vehicle. Conventional fabric tents offer no protection; small sheds offer uncertain protection. If on open ground, remote from shelter, crouch down, singly, with feet together. Footwear or a layer of any non-absorbing material, such as a plastics sheet, offers some protection against ground currents, should there be a nearby lightning flash. If in an open boat keep a low profile. Additional protection is gained by anchoring under relatively high objects such as jetties and bridges, provided that no direct contact is made with them. Avoid isdated buoys and pylons. Avoid riding horses or bicycles, or riding in any open vehicle such as a tractor or beach buggy, or in any enclosed vehicle with a non-metallic roof. Avoid swimming or wading. Persons in an exposed position during the approach of a thunderstorm are advised to seek shelter. If the time interval between a lightning flash and hearing the thunder becomes less than 15 s, move quickly to a protected location as there is immediate danger of a lightning strike nearby. Avoid high ground and isolated trees. If the vicinity of a tree cannot be avoided, seek a position just beyond the spread of the foliage. Avoid touching or standing close to tall metal structures, wire fences and metal clothes lines. Avoid handling substantial metallic objects, and remove metal objects from the hair or head covering. Limit the use of telephones when a thunderstorm is overhead. Avoid contact with electrical appliances and metal objects, e.g. stoves, refrigerators, meta! Wxkn~ frames, sinks, radios and television sets. If the use of household appliances or the telephone is unavoidable, keep clear of other appliances and metal objects, and keep any such use brief. G.3 EFFECT ON PERSONS AND TREATMENT FOR INJURY BY LIGHTNING The severity of the injuries inflicted on a person by lightning depends on the fraction of the total lightning current that flows through the person's body and the path of the current through or over the body. The worst situation is where the person is struck on the upper part of the body, so all the current must flow through the trunk, where the heart and lungs are the vitally significant organs, or over the skin. A less dangerous situation is where the person is subjected to step or touch potentials, and only a small fraction of the total current passes through the body, although the pathway taken by this fraction is still important. The effects of lightning include bums to the skin, which are usually superficial, damage to various bodily organs and systems, unconsciousness, but, most dangerously, cessation of breathing and cessation of heart beat. Independently of these electrically related effects, temporary or permanent hearing impairment may be experienced as a consequence of the extremely high sound pressure levels associated with a nearby lightning strike. In the first-aid treatment of a patient injured by lightning, it is essential that breathing be restored by artificial respiration and blood circulation be restored by external cardiac massage, if appropriate. These procedures should be continued until breathing and heart beat are restored, or it can be medically confirmed that the patient is dead. It should also be noted that the usual neurological criteria for death may be unreliable in this situation. There is no danger in touching a person who has been struck by lightning. Lightning strike victims are sometimes thrown violently against an object, or are hit by flying fragments of a shattered tree, so firstaid treatment may have to include treatment for traumatic injury. Subsequent treatment of a lightning strike patient is a specialized area with important differences from the treatment of injuries inflicted by electric power current. For example, the nature of the burns, and the extent of damage to underlying muscle tissue tends to be severe with electric power current, but mild with lightning current. Neurological and cardiac injuries also are different, and follow different courses. Standards Referred To: Harddrawn copper and copper-cadmium conductors for over-head power transmission purposes Aluminium conductors and aluminium conductors, steel reinforced, for overhead power transmission Part 1 : 1970 Part 2 : 1970 Aluminium stranded conductors Alumlnium conductors, steel reinforced Stranded steel wire ropes Part 2 : 1987 Specification for ropes for general purposes Wrought steels for mechanical and allied engineering purposes Part 1 : 1991 General inspection and testing procedure and specific requirements for carbon, carbon manganese, alloy and stainless steels Copper alloy ingots and copper and copper alloy and high conductivity copper castings Copper for electrical purposes, high conductivii copper rectangular conductors with drawn or rolled edges Copper for electrical purposes. Rod and bar Steel plate, sheet and strip Part 1 : 1991 Part 2 : 1983 Carbon and carbon manganese plate, sheet and strip Stainless and heat resisting steel plate, sheet and strip Flake graphite cast iron Wrought aluminium and aluminium alloys for general engineering purposes Plate, sheet and strip Wrought aluminium and aluminum alloys for general engineering purposes Drawn tube Wrought aluminum and aluminium alloys for general engineering purposes Rivet, bdt and screw stock - Wrought aluminium and aluminum alloys for general engineering purposes Bars, extruded round tubes and sections Aluminium and aluminum alloy ingots and castings Copper and copper-cadmium stranded conductors for overhead electric traction systems Specification for rolled copper and copper alloys: sheet, strip and foil Copper and copper alloys. Tubes Part 3 : 1972 Tubes for heat exchangers Copper and copper alloys forging stock and forgings Copper and copper alloys - Rods and sections (other than forging stock) Wrought aluminum for electrical purposes. Strip with drawn or rolled edges Wrought aluminium and aluminium alloys for electrical purposes. extruded round tube and sections Steel for cold forged fasteners and similar components Part 1 : 1987 Part 2 : 1987 Carbon and low alloy steel wire Stainless steel Bars, Aluminium allo'j stranded conductors for overhead power transmission Code of Practice for protection of structures against lightning Malleable cast iron High voltage test techniques Part 1 : 1989 IEC 245 : General definitions and test requirements Rubber insulated cables of rated voltages up to and including 4501750 V Part 4 : 1994 Cords and flexible cables IEEE C62.41 SS 254 : - IEEE recommended practice on surge voltages in low-voltageac power circuits Electrical apparatus for explosive gas atmospheres Part 10 : 1982 Classification of hazardous areas Conductors in insulated cables and cords Polyvinyl chloride insulated cables of rated voltages up to and including 4501750 V Part 5 : 1991 Flexible cables (cords) Code of practice for wiring of electrical equipment of buildings Code of practice for earthing ABOUT THE SINGAPORE PRODUCTIVITY AND STANDARDS BOARD The Singapore Productivity and Standards Board (PSB) was established in April 1996. Its mission is to raise productivity so as to enhance Singapore's competitiveness and economic growth for a better quality of life for our people. To achieve this mission, two broad thrusts are adopted -- developing world-class industries and creating a favourable environment for productivity improvement and innovation. The specific areas of focus are: small and medium-sized enterprises (SMEs), productivity and innovation, and standardisation and metrology. SMEs As the lead agency spearheading the upgrading of SMEs, PSB adopts a total approach to develop SMEs into vibrant and resilient enterprises. At the enterprise level, it develops promising SMEs by enhancing their capabilities. At the sector level, it improves the productivity of domestic industries through industry-wide programmes. The broad-based programmes include accelerating the adoption of e-commerce amongst SMEs, developing Singapore as an SME hub and providing SMEs with access to finance. PSB also serves as the first point of contact for SMEs that need information and assistance. Productivity and lnnovation PSB spearheads the national Productivity and lnnovation Movement to cultivate a strong commitment to productivity and innovation and to foster a creative and thinking workforce that is able to translate ideas into action. As the lead agency in charge of workforce development, it works closely with employers, unions and other government agencies to build up the capabilities of the workforce and the training infrastructure in Singapore. Financial incentives are provided to help employers defray the cost of training their employees. Organisations are also encouraged to establish systems that drive continuous improvement and attain business excellence through the Singapore Quality Award and Singapore Quality Class. Standardisation and Metrology As the national standards body and national metrology institute, PSB improves market access for Singapore's exports through its work on standardisation and metrology. Standardisation is also used as a major strategy to raise the productivity of industries. PSB enforces the Weights & Measures Act and Regulations to protect both consumers and traders by ensuring that market transactions based on weights and measurement are accurate, fair and consistent with the relevant standards. As the Safety Authority, it administers the Singapore Consumer Protection (Safety Requirements) Registration Scheme to ensure compliance by industry on the registration of controlled goods. ABOUT THE NATIONAL STANDARDISATION PROGRAMME Under the national standardisation programme, PSB helps companies and industry to meet international standards and conformity requirements by creating awareness of the importance of standardisation to enhance competitiveness and improve productivity, co-ordinating the development and use of Singapore Standards and setting up an information infrastructure to educate companies and industry on the latest developments. PSB is vested with the authority to appoint a Standards Council to advise on the preparation, publication and promulgation of Singapore Standards and Technical References and their implementation. Singapore Standards are in the form of specifications for materials and products, codes of practice, methods of test, nomenclature, services, etc. The respective standards committee will draw up the standards before seeking final approval from the Standards Council. To ensure adequate representation of all viewpoints in the preparation of Singapore Standards, all committees appointed consist of representatives from various interest groups which include government agencies, professional bodies, tertiary institutions and consumer, trade and manufacturing organisations. Technical References are transition documents developed to help meet urgent industry demand for specifications or requirements on a particular product, process or service in an area where there is an absence of reference standards. Unlike Singapore Standards, they are issued for comments over a period of two years before assessment on their suitability for approval as Singapore Standards. All comments are considered when a technical reference is reviewed at the end of two years to determine the feasibility of its transition to a Singapore Standard. Technical References can therefore become Singapore Standards after two years, continue as Technical References for further comments or be withdrawn. In the international arena, PSB represents Singapore in the lnternational Organisation of Standardisation (ISO), the lnternational Electrotechnical Commission (IEC), the Asia-Pacific Economic Co-operation (APEC) Sub-committee for Standards and Conformance (SCSC) and in the ASEAN Consultative Committee for Standards and Quality (ACCSQ).