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UNIVERSITY OF NAIROBIDEPARTMENT OF ELECTRICAL AND INFORMATION ENGINEERING PROJECT INDEX: PRJ 136 PME SYSTEMS BY NAME: BISKY .I. ASYAGO REG. NO: F17/2451/2009 SUPERVISOR: PROF. M. K. MANG'OLI EXAMINER: DR.C.WEKESA Project report submitted in partial fulfillment of the requirement for the award of the Degree of Bachelor of Science in Electrical and Information Engineering of the University of Nairobi Date of submission: 28TH APRIL, 2014 DEDICATION To my Mother, Cyprose Oluya, true example of unconditional love and boundless support, to whom I owe everything. To my Brothers and Sister, Byron, Bell, Brandsar ,Baker and Brenda, my partners in the Quest. ACKNOWLEDGEMENT I would like to thank my supervisor Prof. M. K. Mang'oli for his guidance, useful hints and directions proved to be the strong foundation upon which the project was built. I wish to sincerely express my heart-felt gratitude to my Lecturers in the Department of Electrical and Electronic Engineering for the enlightening experience in the field of Engineering. More thanks to Eng. Osewe for the useful practical knowledge he provided to me with regards to my project. I am also grateful to my family for providing me with the moral support through this project, interest and encouragement To the Almighty God, whom all the glory and honor belongs. ABSTRACT Power systems grounding is probably the most misunderstood element of any Power systems design. This project studies the characteristics of different Power Systems Grounding techniques as currently applied and misapplied within industry today. In practice today for utility distribution network Protective Multiple Earthing (PME) which is a variant of the neutral earthing is the most widely adopted earthing system arrangement. In this project a comparative study of PME and Isolated systems is carried out investigating their fault behavior, safety and requirements. An experiment to demonstrate that there is continuous power dissipation to ground even under normal system operating conditions in the PME system is also performed and based on the findings of the study, a recommendation for the Isolated system where and when possible is made. Keywords : PME analysis, quantification of power losses TABLE OF CONTENTS 24 Figure 3..18 Figure 3.....16 Figure 2....22 Figure 3..17 Figure 2.9: Position of the IMD………………………………………………...29 Figure 3.25 Figure 3.6: TN-S Grounding System…………………………………………………………….5: Neutral-to-ground potentials…………………………………………………….7: TN-C-S (PME) Grounding System…………………………………………………..5: TN-C Grounding System………………………………………………………....7: First fault in IT grounding System………………………………………………….LIST OF FIGURES Figure 1.....0: Direct and Indirect contact …………………………………………………………....30 Figure 3..8: Second Fault in IT grounding systems………………………………………………31 Figure 3.1: Fault-loop in PME systems for single-phase loads………………………………….…………………32 .15 Figure 2.3: Equivalent fault-loop for a short circuit phase-to-PEN in PME……………………..20 Figure 3.4 Figure 2.13 Figure 2..4: TT Grounding System………………………………………………………….1: Interface of earth and electrode………………………………………………………7 Figure 2..26 Figure 3....2: Diagram illustrating the physical sense of earth resistivity ρ…………………...3: IT Grounding System……………………………………………………………….4: Interruption of the PEN conductor in PME…………………………………………....6: Distributed capacitances in IT grounding………………………………………….17 Figure 2.2: Fault-loop for a ground fault on the low-voltage PME distribution system……….... .16: Set up for the estimation of power losses…………………………………………...12: Fall of potential method………………………………………………………….13: Dead Earth Method………………………………………………………………...34 Figure 3.14: Basic clamp-on ground testing methodology………………………………………41 Figure 3.42 ....38 Figure 3.15: Set up for the measurement of the earth electrode resistance…………………….10: ECPs Earthed Individually or in Groups………………………………………….40 Figure 3..11: ECPs Earthed Collectively to a Single Grounding System………………………...42 Figure 3.Figure 3.35 Figure 3. ..43 Table 4.29 Table 3..1: Maximum disconnecting time for AC final circuits not exceeding 32 A………..8 Table 2.2: Experimental result A (Switch ON)………………………………………………...7: Minimum cross-sectional Areas of Earthing Conductors………………………..LIST OF TABLES Table 2..1: Experimental result A (Switch OFF)…………………………………………….8 Table 2...2: Maximum disconnection time in the IT earthing system (second fault)…………..11 Table 3.9 Table 2..3: capacitive values for HF filters built into various devices……………………….....10 Table 2....3: Experimental result B (Switch OFF)………………………………………………43 Table 4.7: Preference in Different load conditions………………………………………….4: Experimental result B (Switch ON)………………………………………………..1: Typical Formulae for Calculating Earth resistance……………………………….6: Performance of the two systems……………………………………………………49 Table 4.43 Table 4.4: Minimum standard cross-sectional areas for PEs………………………………….48 Table 4.9 Table 2.43 Table 4.50 ..39 Table 4.5: The cross-sectional area of MEBs……………………………………………….5: Preference in different types of networks………………………………………….2: Minimum sizes for Steel earth electrodes…………………………………………..3: Minimum sizes for copper earth electrodes……………………………………….32 Table 3. LIST OF SYMBOLS/ NOMENCLATURE Symbol Description VS Step Voltage VT Touch Voltage L Rod length r Rod radius ρ Soil resistivity IG Earth fault current IPH Phase conductor current ZE Earthing Impedance PME Protective Multiple earthing ZPH Phase conductor Impedance RCD Residual current device ECP Exposed conductive parts ZC Faulty circuit loop impedance EXCP Extraneous conductive parts EMC Electromagnetic compatibility EMI Electromagnetic interference ZS Earth fault current loop impedance Ia Current setting of the protective device . e. PEN conductor TN-C-S A neutral grounded electrical supply system where. in part of the installation. i.PEN Protective earthed neutral conductor TN A neutral grounded electrical supply system ZPEN Protective and Neutral conductor Impedance GPR / VG /EPR Ground Potential Rise (Earthing Voltage) VX ∗ The potential at a distance x from the middle of the earth electrode The relative value of that individual potential to the earthing voltage PE Protective earthed conductor /equipment grounding conductor IT Ungrounded electrical supply system with or without a distributed neutral TN-C A neutral grounded electrical supply system where the neutral serves as the protective conductor. a PEN conductor is used and in other parts a separate PE conductor is used TN-S A neutral grounded electrical supply system with separate neutral and protective earthed (PE) conductors TT A neutral grounded electrical supply system where the source neutral and the electrical equipment are grounded . only if it involves the physical earth and in case of a mul-functioning of some part of the system. grounding is broadly classified as equipment grounding and system grounding. the two being related and may refer to the same physical installation in some cases. It may or may not be accessible. As anticipated above. Since that time much research. Second. Grounding or earthing is normally defined as a connection of various exposed conductive parts of equipment together and to a common terminal which is in turn connected by the earthing conductor to an earth electrode. From a physical point of view. First. coupled with experience. In balanced operating conditions these three voltages are phase shifted by 120° and have the value: V √3 V being the phase-to-phase voltage measured between phases. Therefore. is not the same thing. has taken place that is now available to industry. some of the current returns back to the source through the earth [1]. In any medium or low voltage three-phase system there are three single-phase voltages which are measured between each phase and a common point called the "neutral point". grounding is not only limited to equipment but also involves the electrical power system. Grounding should be called earthing. which is used interchangeably with earthing. which is why we refer to the earthing system. the neutral is the common point of three star-connected windings. Equipment grounding. misunderstood concepts and perceptions of the purpose and type of Power Systems grounding to be selected dates back to the 1940's and earlier. referred also as protective grounding is mainly for the prevention from dangerously high shock that may exist when there is a fault current between an energized electrical conductor and the structure that either encloses it or is nearby whereas . the standard definition of grounding according to [2] is the conducting connection whether intentional or accidental between an electrical circuit or conductive equipment part and a common terminal which is in turn connected by a conductor to an earth electrode or to some conducting body of relatively large extent that serves in place of the earth.0 INTRODUCTION In many cases. may or may not be distributed and may or may not be earthed.1. the term grounding. There are two misconceptions in this statement. All the preceding protective measures are preventive. since many components and materials are installed in cabinets. but experience has shown that for various reasons they cannot be regarded as being infallible. Objectives behind the system grounding are to fix the potential at any part of the network with respect to earth and to provide sufficient fault current so that protection equipment can operate [3]. direct contact. In the first case.system grounding is an intentional electrical interconnection between the electrical system conductors and ground. System grounding can be of four different types. The fundamental rule of protection against electric shock is provided by the document IEC 61140 which covers both electrical installations and electrical equipment. assemblies. the neutral is impedance-earthed. Hazardous-live-parts shall not be accessible and accessible conductive parts shall not be hazardous. resistance grounding. control panels and distribution boards. reactance grounding and solid grounding. but has been energized due to failure of the basic i. as the live parts of the electrical installation will be basically protected by:  Insulation. which are ungrounded systems.This protection is reserved only to locations to which skilled or instructed persons only have access. There are two ways of getting a shock from an electrical installation: first by touching a live part of the installation i. Among these reasons may be cited: Lack of .e.This protection consists of an insulation which complies with the relevant standards.  Obstacles. the neutral is said to be isolated or unearthed [4]. in the second case. which distribute the electric power to the widest class of end users. the main concern for design of earthing systems is safety of consumers who use the electric appliances and their protection against electric shocks [1] which is the pathophysiological effect of an electric current through the human body. When there is no intentional connection between the neutral point and earth.e.  Barriers or enclosures. It is obvious that the protection under normal conditions will be quite easy. the neutral is solidly (or directly) earthed and. and secondly by touching a conductive part that is not supposed to be live. indirect contact.This measure is in widespread use. In low-voltage distribution networks. forming part of the operating system. The neutral may be connected to earth either directly or via a resistor or reactor. carelessness. or death by electrocution. and active measures. the chosen protective measures will include a protection against direct and against indirect contact or “protection under normal and under fault conditions. of a normally healthy human being In most cases. The fault current in fig. based on the detection of residual currents to earth are used to disconnect the power supply automatically. But just connecting the ECPs to earth is not enough. when an insulation becomes defective. and certainly for the general public. Automatic disconnection before the voltage can do harm to a person touching the accessible conductive part under fault. for instance flexure and abrasion of connecting leads. Normal (or abnormal) wear and tear of insulation. highly sensitive fast tripping devices. A situation in which insulation is no longer effective In order to protect users in such circumstances. In the protective measures under fault conditions. which will break the current in the faulty circuit. In general. Fig 1. two possibilities exist: passive measures such as supplementary insulation. Imprudence.0 . For the ECPs to be properly protected it is necessary to have a protective earth. Accidental contact.1 creates a potential rise between ECP and the earth and this voltage can be dangerous if it exceeds a certain value for a certain time. etc. 1 will be subjected to the same fault current as the person touching the active part. and with sufficient rapidity to prevent injury to. there will be a rapid evolution to a full fault and therefore the person touching the ECP as in fig.proper maintenance. Immersion in water. in Chapter 5. provide personnel safety. the experimental results and findings of the comparative study in chapter 3 are presented and a detailed analysis is carried out.2 Scope of The Project Chapter 1 gives a brief introduction of the project and measures of protection against direct and indirect contact. Also the role of earthing systems is introduced Chapter 3 gives a comparative study of PME and IT systems and also the design methodology that was adopted in determining and quantifying the power losses in PME system is presented In Chapter 4.From the above discussion it is evident that earthing is important in electrical power systems to minimize voltage and thermal stresses on equipment. 1. TT and TN are discussed. concluding remarks and recommendation of future work to be done are given. 1. and assist in rapid detection and elimination of ground faults.1 Aim of The Project The aim of this project is to analyze the Protective Multiple Earthing(PME) scheme which is the adopted system earthing arrangement for most modern low voltage (LV) power systems including The Kenya Power and Lighting Company (KPLC) with the objective of examining and quantifying the associated power losses and providing a suitable alternative system. . Finally. In Chapter 2. the different system earthing arrangements IT. reduce communications system interference. In as much as the earth is a relatively poor conductor of electricity compared to normal conductors like copper wire its abundance and availability makes it an indispensable component of a properly functioning electrical system. must be employed [6]. one or more ground electrodes. are the fundamental components of the earthing arrangements. Earthing arrangements. which varies with the resistivity of the local soil. Identifying the resistance to ground is mostly dependant on soil resistivity of the area to be grounded.2.  Resistance of the surrounding earth.0 LITERATURE REVIEW 2. 2.2 Earth Electrodes Earth electrodes provides the connection to earth and must provide a reliable link to ground. and bonding conductors. may be used for both functional reasons and safety purposes [6]. protective conductors. 2. Electrodes must be able to carry ground-fault currents and dissipate them to ground. without causing hazards caused by thermal effects and/or electric shock.  Contact resistance between the electrode and the soil adjacent to it.2. but also for the proper functioning of equipment. primarily for safety purposes. in fact. The effectiveness of the earthing system depends upon its ground resistance RG. Once the characteristics of the soil and the minimum acceptable value for safety of RG are known.1 Nature of an earth electrode Resistance to current through an earth electrode actually has three components:  Resistance of the electrode itself and connections to it. even of different nature.1 Fundamental Components of Earthing Systems Earth electrodes. A failure in any of these elements can compromise the electrical safety of the installation as well as its functionality. . 1. The earth shell nearest the electrode naturally has the smallest surface area and so offers the greatest resistance.2.1.2.1 Electrode Resistance Rods. a distance from the electrode will be reached where inclusion of additional earth shells does not add significantly to the resistance of the earth surrounding the electrode [9]. Approximate values of the earth resistance at 50/60 Hz of typical made electrodes may be calculated by using the formulas reported in Table 2. the part below the break is not effective as a part of the earth electrode.1. . These are usually of sufficient size or cross section that their resistance is a negligible part of the total resistance. pipes.2. structures. Finally.2 Electrode-earth contact resistance If the electrode is free from paint or grease. 2.1 Interface of earth and electrode 2. Rust on an iron electrode has little or no effect but if an iron pipe has rusted through.4 Resistance of surrounding earth An electrode driven into earth of uniform resistivity radiates current in all directions as in Fig 2. contact resistance is negligible. and the earth is packed firmly. and other devices are commonly used for earth connections. masses of metal.2. Figure 2.1. The next earth shell is somewhat larger in area and offers less resistance. [6].1. Table2.2 Surface Hot galvanized Stainless Steel Diameter(mm) - Electrode type dip Strip or Round rod for 16 deep earth electrode Round wire for 10 Surface earth Electrode Pipe 25 X-Area(mm2) 90 Thickness(mm) 3 - - - - - 2 .2 are reported in Tables 2. The phenomenon is more pronounced when the ratio of the cathode’s surface to the anode’s surface is large.2 and 2. as per IEC 60364–5-54. common minimum sizes for earth electrodes. the two metals. respectively.3 Corrosion Phenomena Earth electrodes must have a minimum size in order to have adequate mechanical strength and withstand corrosion. Corrosion is an electrochemical process that involves two dissimilar metals electrically connected when embedded in electrolytes.[6] Table 2. When the current leaves the anode. assume the role of cathode and anode of a galvanic cell.1 Type of Electrode Earth Resistance Rod ρ ⁄L Buried Horizontal wire 2ρ⁄L Grid ρ⁄4r 2. In this respect. such as earth. corrosion at the expense of the anode occurs.3. to reclose to the cathode through the electrolyte. Table 2. armors. S (mm2) S ≤ 16 16< S ≤ 35 S > 35 Area of Line Cross-sectional Area of the corresponding PE Conductor S 16 S/2 Protective conductors may not necessarily consist of actual conductors. Minimum standard cross-sectional areas deemed adequate for PEs are shown in Table 2.3 Copper Electrode type Diameter(mm) X-Area(mm2) Bare strip 50 Bare rope 1. Additionally. If a protective conductor is common to more than one circuit. etc. thereby creating a clear path for the fault currents. so that fault currents can promptly activate the protective device and automatically disconnect the supply. Cross-sectional areas of protective conductors must be adequately large.) and metallic conduits can .4 Protective conductor Protective conductors (PEs) provide safety against indirect contact by linking ECPs to the main earthing terminal. or damage.4 Minimum Cross-sectional Conductor. concentric conductors. protective conductors must be able to withstand the flow of the ground-fault current without reaching dangerous temperatures to the surrounding environment or shorten the life of.. it must be selected in correspondence with the phase conductor’s largest cross-sectional area.4. metallic sheaths. but metallic layers of cables (e.8(individual strands) 25 Bare Round wire for 25 surface earth electrode Bare Pipe 20 Tin coated rope 1.Table 2. their insulation.g.when the protective conductor is of the same material as the line conductor.8 for individual 25 strands Zinc coated Strip 50 Thickness(mm) 2 2 2 2. in this case.3. therefore. . Supplementary bonding conductors must comply with the same requirements as the protective conductors as to their minimum size. have negligible resistance and good mechanical strength. Table 2. their minimum standard cross sectional areas can be selected from Table 2. ECPs and EXCPs. main equipotential bonding conductors are employed for connections of EXCPs to the main earthing bus and for supplementary bonding in special locations (e. 2.serve the same protective purpose as long as their equivalent cross-sectional area complies with Table 2. 2. locations containing a bath or shower).. and earthing conductors. in fact. a current divider takes place and the EXCP carries part of the fault current.g. If the earthing conductors are embedded in the soil. they must endure mechanical stress and corrosion without compromising their electrical continuity.2 or 2. To this end. The cross-sectional area of these conductors can be selected or calculated as seen for the protective conductors. for example. This conductor. [6]. its conductance must be at least half that of the PE serving the ECP. The main earthing terminal links together protective conductors.5 Equipotential Bonding Conductors Equipotential bonding conductors cancel dangerous potential differences between metal parts.7 as per IEC 60364–5-54. Also. The cross-sectional area of MEBs can be selected as per IEC 60364–5-54. They must be reliable. is generally at zero potential and. which indicates their minimum standard sizes as provided in table 2. As already discussed.5 Copper(mm2) 6 Aluminum(mm2) 16 Steel(mm2) 50 If the supplementary bonding conductor connects an ECP to an EXCP. equipotential bonding conductors. when ground faults occur. does not require any dielectric insulation.6 Earthing Conductors and Main Earthing Terminal Earthing conductors link the earth electrode(s) to the main earthing bus.5. 2. the PEN conductor may split and originate distinct wires as neutral and PE [14]. Earthing resistance determines the relation between earth voltage VE and the earth current value. 2. the PEN should only be used in a fixed installation and have a cross-sectional area not less than 10 mm2 in copper or 16 mm2 in aluminum. which occurs as a result of current flow in the earth.7 The PEN Conductor PEN conductors provide both functions of neutral and protective conductor. In some point of the installation. To this purpose.7 Minimum cross-sectional Areas of Earthing Conductors Mechanically Protected No Mechanical Protection Copper Iron Copper Iron Protected against 2. These conditions limit the probability of its accidental interruption. The accidental interruption of the PEN. therefore. The potential distribution on the earth surface is an important consideration in assessing the degree of protection against electric shock because it determines the touch and step potentials[8]. its electrical and mechanical continuity must be guaranteed.Table 2.5 10 16 16 corrosion Not protected 25 25 25 25 against corrosion 2.1 Earthing resistance The earthing resistance has two components: .8 Electrical properties of the earthing system The electrical properties of earthing depend essentially on two parameters:  earthing resistance  configuration of the earth electrode. even if healthy. when the line conductors are live. energizes the equipment. The configuration of the earth electrode determines the potential distribution on the earth surface.8. reducing current density. R [8]. The resistance RL is usually much smaller than the dissipation resistance RD. Thus. Where no information is available about the value of ρ it is usually assumed ρ = 100 Ω-m. usually the earthing resistance is estimated to be equal to the dissipation resistance RD. Figure 2. Once the current flows from metal to earth it spreads out.the supply frequency and associated harmonics reactance is usually negligible in comparison to earthing resistance. In order to achieve low values of R the current density flowing from the electrode metal to earth should be low. of a one-metre cube of earth The earth resistivity is expressed in Ω-m. which is the impedance between the earthing system and the reference earth at a given operating frequency. for low frequencies. which is the resistance of the earth between the earth electrode and the reference earth  resistance RL of the metal parts of the earth electrode and of the earthing conductor. measured between two opposite faces. dissipation resistance RD. i. Earth resistivity ρ (specific earth resistance) is the resistance. Low current density extends electrode life. the volume of earth through which the current flows is as large as possible. The earthing resistance R of an earth electrode depends on the earth resistivity ρ as well as the electrode geometry. In AC circuits one must consider essentially the impedance of an earthing ZE. At low frequencies . it is assumed that the earthing impedance ZE is equal the dissipation resistance RD.2 . The reactance of the earthing system is the reactance of the earthing conductor and of metal parts of the earth electrode.e.Diagram illustrating the physical sense of . Thus. which is in turn assumed to be approximately equal to the earthing resistance. but must be taken into account for high frequencies such as lightning transients. The calculation of earthing resistance is usually performed under the assumptions that the ground is boundless and of uniform structure with a given value of resistivity. and are driven or buried to a depth greater than 1 m and usually from 3 m to 30 m or more. 2. 2.8.1 = Eq.2 and its relative value. This is because the moisture content is higher and more stable for deeper ground layers than for shallow layers. as well as distribution of the earth surface potential during the current flow in the earthing system. 2. are important parameters for protection against electric shock. can be formulated with the following equation: = Eq. either as a single ended strip or a ring  meshed electrodes. One can distinguish several types of earth electrodes including:  simple surface earth electrodes in the form of horizontally placed strip or wire.2 Earthing voltage and earth surface potential distribution Earthing voltage. The earthing potential can be described as follows: . in which earth current IE flows.  rod electrodes which can consist of a pipe. Layers near the surface are influenced more by seasonal and short-term weather variations and are subject to freezing. etc.earth resistivity ρ The earth resistance depends significantly on how deep the electrode is sunk in the ground. ∗ Where VG is the earthing voltage. The potential of any point located at distance x from the middle of earth electrode. which is equal to the earthing potential (assuming that the potential of the reference earth is equal zero). rod. constructed as a grid placed horizontally at shallow depth  cable with exposed metal sheath or armor which behaves similarly to a strip-type earth electrode  foundation earth electrodes formed from conductive structural parts embedded in concrete foundation providing a large area contact with the earth. Where [1]: . 2. IT and TN. illustrates the step potential ∆VS. TT. earth surface potential existing between two feet.7 2.9 Types of System Earthing Arrangements(SEA) The international standards IEC 60364 distinguishes three families of system earthing arrangement (SEA) using the two letter codes namely. the voltage between a palm and a foot of a person who is just touching the earth electrode or metal parts connected to it [8]: = − Eq. where aS is assumed to be equal to 1 metre. Particularly for x = r and a = a T = 1m the formula (2. ∗ = Eq. The first letter indicates the connection between the power supply equipment (generator/ Transformer) and earth.e.6 and its relative value ∗ = Eq. 2. 2. A similar relationship can be described for any other distances x and a.4) enables the calculation of the touch voltage.3 The potential difference between two points on the earth surface: one at distance x and other at distance x+ aS.4 and its relative value.e. when a person stands at that position on the earth surface: = − Eq. i.= = Eq. 2. i. 2.5 where x ≥r. installation earth conductors are not at risk should any . It finds applications in laboratory rooms.9. and other environments that are supplied via engine-generators where there is an increased risk of insulation faults. mobile electrical installations. medical facilities. where: "T".1 IT Grounding System In " IT" (Isolation Terre) earthing arrangement the electrical distribution system is isolated from earth and therefore the supply does not provide an earthing connection for the consumers. construction sites. often use an IT earthing arrangement supplied from isolation transformers [3].Connection to earth is via the neutral of the supply 2.2 TT Grounding System In "TT" (Terre-Terre.9.Again denotes presence of a direct connection to earth "N".3 2. or earth-earth) earthing arrangement the earth connection of the consumer is provided by a local connection to earth independent of the supply side connection to earth.3 Fig 2. repair workshops."T".Denotes isolation from earth except perhaps a connection to earth via a high impedance path The second letter indicates the connection between earth and the electrical installation being supplied.Denotes direct connection to earth ("T" derived from the French word "Terre" meaning earth) "I". Thus the consumers have to have local dedicated earth rods. The big advantage of the TT earthing system is that it is clear of high and low frequency noises that come through the neutral wire from connected equipment also in locations where power is distributed overhead and TT is used. An illustration of a IT network is shown in figure 2. a fallen tree or branch [3].5 . Both the supply and consumer have direct connections to earth as shown in figure 2. Fig 2. TN-C.3. TN-S and TN-C-S. TN-C networks save the cost of an additional conductor needed for separate N and PE connections as is shown below.overhead distribution conductor be fractured by.4 2. say. This scheme has three different variations namely. 2.9.3 TN Grounding System In " TN" earthing arrangement also known as neutral earthing. the star point of the supply side is directly connected to earth and the consumer earth terminal connected to earth via the neutral of the supply side.4 Fig 2. That is one conductor functions as the protective earth as well as the neutral of the power system.1 TN-C "TN-C " earthing scheme has a combined protective earth and neutral conductor (PEN).9. usually between the substation and the entry point to the installation beyond which it splits up into a separate protective earth and a neutral conductor. This scheme has the advantages of both the TN-C and TN-S schemes.2 TN-S In" TN-S" earthing scheme the protective earth conductor and the neutral conductor of the system are two separate conductors and are connected together only near the power supply equipment. This arrangement is shown below.3. protective Multiple earthing scheme(PME). It is this multiple earthing that gives this scheme its name i. This is of particular importance with some types of telecommunication and measurement equipment[3] [6].6 2.e. the consumer has a low-noise connection to earth. The combined PEN is usually earthed at different convenient places along its run to reduce the risk of broken neutrals and by doing so ensures the reliability of the earth neutral path. Fig 2.3. In TN-S systems.3 TN-C-S/Protective Multiple Earthing "TN-C-S" earthing scheme Is a hybrid of the TN-C and TN-S earthing schemes where part of the system uses the combined protective earth and neutral. The protective earth and neutral conductor should never be joined within the installation [6]. An Illustration of the PME scheme is shown in figure 2.7 .2.9.9. which does not suffer from the voltage that appears on the N conductor as a result of the return currents and the impedance of that conductor. 7 Concerning the safety of persons . and thus trips the over.10 Importance of Earthing The purpose of system grounding in an electrical system is therefore:  Provides a low resistance path to ground for any surges or lightning strikes that may occur on the electrical system.  Provides a stable reference point for the voltages in each phase of a circuit. and most importantly. the three systems are equivalent if all requirements are fully met. 2.Fig 2. A solid ground point prevents the phase-to-ground and the phase-to-phase voltages from fluctuating. limitation of electromagnetic disturbances .  Electromagnetic compatibility (EMC) i.current protective device (circuit breaker or fuse) quickly when a fault or short circuit occurs. protection of buildings and installations against lightning.e.  Provides a low resistance path to ground for fault currents. 1 Protective Multiple Earthing (PME/TN-C-S Grounding System) In PME systems the utility supply neutral conductor is solidly grounded at the source and at each fourth span along its distribution. .0 METHODOLOGY/DESIGN 3. In these conditions. will basically return to the supply through the distributor’s neutral conductor.3. arising at the user’s installation. The purpose of the system ground is to allow the operating voltage to-earth to remain stable and to limit overvoltages in fault conditions. the earth will be partially involved as a return path to the source because of the connection of the EXCPs to the main grounding bus.1 It is assumed that in fault conditions. Here the fault-loop does not comprise the actual earth. At the terminal pole. the utility must earth the center of its transformer’s secondary star point. the PEN. IPh phase N IG1 PEN PE RN REXCP IG2 Fig 3. the above earthing arrangement may cause part of the neutral current to return to the source through the earth. the ground-fault currents. however. Moreover. As already substantiated. this bond is essential to guarantee equipotentiality between simultaneously accessible metal parts. in the absence of ground faults. the PEN is separated into the neutral and protective conductors. The bonding connection is of utmost importance. which increases with the distance of the fault’s location from the supply source. as their return path. instead. since.1. the total impedance of the fault-loop |ZLoop| may be low enough to allow users to use overcurrent devices for automatic disconnection. as a result of the fall to earth of overhead cables or of a contact of the line with an EXCP not connected to a protective conductor. Ze. which must ensure public safety. at the service entrance of the dwelling unit. If utilities cannot “certify” the neutral potential as harmless to persons.1. the ground fault current might be so low and there would be no automatic disconnection and thus the installation of RCDs in dwelling houses. In fault conditions. Therefore. although redundant in the case of low value of ZLoop. ground-fault currents arising at the user’s location have large magnitude.3 Ground Fault on the Low-Voltage Utility Distribution System The PEN conductor may become live due to a ground fault occurring along its distribution system. the utility PEN. If the total fault-loop impedance ZLoop = Ze + Zuser is excessive. indeed.2 Energization of the PEN Conductor in PME Systems PME systems imply a considerable responsibility of the local utility. can. An illustration of this condition is as . Such neutral-to-ground voltage can be transferred as a shock potential to the users’ ECPs and EXCPs. users are exposed to electric shock hazards. 3. is parallel to earth. the building would become a TT system.1 Fault-Loop Impedance in PME Systems In PME the fault-loop impedance includes the impedance Ze of the utility low-voltage distribution system. as is substantiated later on. guarantee safety when ZLoop is too high. with respect to the earth. may also change in time without the user knowing it because of modifications in the utility distribution system. 3. a TT system should be employed. which are not strictly necessary in PME. wherein in the absence of RCDs. therefore essential to periodically inspect and maintain such connection. Upon loss of this bond.In this systems. the distributor provides the users with an earth connection. together with the electric energy. This renders the presence of RCDs in PME systems not a mandatory safety requirement. may assume a voltage. even in PME systems. between the PEN and the PE. It is.1. 3. the PEN. which is usually unknown to the customer. for example. although multiple grounded. To identify safe values for the PEN potential and the maximum earth resistance RN: V =V × R R +R +Z +Z ≅V × R R +R ≤ 50V Eq. therefore. the user’s ECPs. 3.1 Where both the phase conductor impedance Zph and the internal impedance of the source Zi ignored because they are generally negligible with respect to RN and RE. exposing persons to the risk of electric shock.shown in figure 3... at the distribution poles) but also at the customers’ dwelling units by means of their ground electrodes (e.g. The contact resistance with earth also limits this ground current and in some cases can be very high (e. by circulating through RN and RE energizes the PEN conductor and. The earth current. The distributor’s overcurrent devices.2 where RN represents the ground resistance of the utility’s earth electrode system: the neutral conductor is earthed not only at intervals along its run (e.2 VN RN RE is the minimum earth resistance of EXCPs not connected to an equipotential system. through which a fault may occur.. ZPh Zi IG V Ph RE Fig. 3. cold water pipe). therefore.g. As a safety criterion. line in contact with snow or sand). may not be able to clear the fault within the maximum permissible times. we can assume as “safe” the PEN conductor if its potential VN does not exceed the safety limit of .g. 5 Faults Phase-to-PEN in Low-Voltage PME Networks Another cause of Energization of the PEN conductor may be the accidental contact with the phase conductor in low-voltage distribution networks The resulting short circuit causes a circulation of current back to the source through the PEN conductor. in order to ensure that the neutral potential due to primary faults is not dangerous. At the occurrence of a ground fault at the transformer primary. are exposed to the whole earth potential VN during the utility fault-clearing time. Such current is greatly limited by the EXCPs’ resistance-to-ground. RN reaches the potential VN and so does the PEN conductor. 3. the fault current by circulating through the earth and reclosing toward the upstream source of the supply network energizes the system ground. being an ECP. Consequently. the distributor must alternatively separate the service PEN conductor from the transformer enclosure’s earth by creating two distinct grounds. We assume to neglect the fault current derived by the EXCPs at the user’s location. (3. When it is impossible to decrease RN. which is much larger than the impedance of the PEN conductor. needs to be earthed and therefore may be connected to the system ground RN. must accordingly lower the neutral resistance RN.1) we derive the condition RN must comply with to keep the PEN potential rise below 50 V: R 50 ≤ R V − 50 Eq. Low. persons in contact with low-voltage ECPs. In PME. remotely supplied by the “live” PEN. share the same earth terminal where the PEN conductor originates. connected for equipotential reasons to the PEN. 3.4 Ground Fault on the Medium-Voltage Utility Distribution System The enclosure of the utility transformer.and medium-voltage systems.1.2 3. utilities.50 V.1. then. The equivalent fault-loop circuit is as shown below . From Eq. PME have a much large geographical extension and therefore the risk of the interruption of the PEN conductor and of the Energization of the ECPs of more than one customer is higher.3 Taking the cross-sectional area of the PEN as half of the phase conductor (common situation). even if healthy. . the PEN conductor is usually placed below the Phase conductors to minimize the probability of its break . Hence.zi ZPH V PH VST ZPEN Fig 3.1. the user’s ECPs will reach the following prospective touch voltage: = × + = × 2 2 + = 2 × 3 3.6 Interruption of the PEN Conductor in PME The accidental interruption of the PEN conductor causes all the ECPs supplied downstream of the interruption to be energized at the line-to-line potential. where it enters the user’s premises. As they depend on the supplied loads. The reason being that supply cables employed to power up the customers. be energized above ground. although earthed at the substation. in fact. have finite impedance. The absence of the PEN as a return path causes a voltage divider between the two users’ single-phase loads.4 PE N User A PE N User B The loss of the PEN conductor also triggers overvoltages. they are likely to escalate over time.L1 L2 L3 PEN Fig 3. composed of phase and PEN conductors. may.4. From Fig 3. This may cause the supply to each load to exceed the nominal value. 3. thereby exposing persons to touch voltages 3. Stray currents may produce interferences among electrical systems by transferring potential rises to healthy systems.1. which may cause voltage drops along their runs .7 Stray Currents Unavoidable stray currents continuously circulate through the actual earth. with great risk of overheating of the equipment and therefore of initiating fire.8 Stray Voltages The neutral conductor. which are now supplied by the line-to-line voltage.1. where two users are supplied by two different phases and the same PEN. a low-resistance object might bridge the gap between them. The neutral-to-ground voltage. 0. Use of RCDs on TN-C-S systems means that the protective conductor and the neutral conductor must be separated upstream of the RCD. VNG at User B is greater than at User A. also exists between the neutral and the protective conductors. which may reach several volts. In order to ensure adequate protection.5 VNG N PE User A N PE User B The voltage drop along the PEN increases with its length and the contributions from the customers. therefore. In the case of a defect in the insulation between these two conductors.L1 L2 L3 IN PEN Fig 3. with the possible result of setting on fire any flammable material eventually present.9 Automatic disconnection for PME systems Principle The automatic disconnection is achieved by overcurrent protective devices or RCD’s. the earth-fault current: = Where.8 × must be higher or equal to = earth fault current loop impedance = the faulty-circuit loop impedance .1. Any insulation fault to earth results in a phase to neutral shortcircuit resulting in high fault currents which allow for use of overcurrent protection but can give rise to touch voltages exceeding 50% of the phase to neutral voltage at the fault position during the short disconnection time. 3. .1 Phase Voltage (VPH) Disconnecting Time (Ts) 50 < ≤ 120 0. The instantaneous trip unit of a circuit-breaker will eliminate a short-circuit to earth in less than 0. since all types of trip unit.= current required to operate the protective device in the time specified 3. must always be taken into consideration.1  For all other circuits. magnetic or electronic.1 3.4 240 < ≤ 400 0. This limit enables discrimination between protective devices installed on distribution circuits Table 3. it is sufficient to verify that the fault current will always exceed the current-setting level of the instantaneous or short-time delay tripping unit (Im). The maximum tolerance authorized by the relevant standard. It is sufficient therefore that the fault current determined by calculation be greater than the instantaneous trip-setting current.1.1. to be sure of tripping within the permitted time limit. are suitable: Ia = Im. the maximum disconnecting time is fixed to 5s.10 Specified maximum disconnection time The IEC 60364-4-41 specifies the maximum operating time of protective devices used for the protection against indirect contact:  For all final circuits with a rated current not exceeding 32 A.1 second.11 Protection by means of circuit-breaker If the protection is to be provided by a circuit breaker. automatic disconnection within the maximum allowable time will always be assured. or than the very short-time tripping threshold level.2 < 400 0. instantaneous or slightly retarded. the maximum disconnecting time will not exceed the values indicated in Table 3. In consequence.8 120 < ≤ 240 0. however. 3. The condition to observe therefore is that: < 3. This is due to the voltage drop in the phase conductor and the lower impedance of the PEN conductor compared with the consumer earthing in TT systems.  A fault in the LV network may cause touch voltages at other LV customers.e.1. The value of current which assures the correct operation of a fuse can be ascertained from a current/time performance graph for the fuse concerned.  PME earthing system could work with simple over current protection.1. protection cannot be achieved if the loop impedance Zs or Zc exceeds a certain value.12 Protection by means of fuses Ia can be determined from the fuse performance curve. fault current is large enough to activate the over current protection devices). This ensures a distributed grounding and reduces the risk of a customer not having a safe grounding. The grounding conductors at the transformer and at all customers are interconnected.1.3. The fault current as determined above must largely exceed that to ensure positive necessary to ensure positive operation of the fuse.14 Disadvantages of the PME Earthing System  Faults in the electrical network at a higher voltage level (MV) may migrate into the LV grid grounding causing touch voltages at LV customers.  Potential rise of exposed conductive parts with the neutral conductor in the event of a break of the neutral network conductor as well as for LV network phase to neutral and phase to ground faults and MV to LV faults.  No overvoltage stress on equipment insulation. In any case. the fault voltages (touch voltages) are generally smaller than in TT earthing systems.  High reliability of disconnection of a fault by over current devices (i.  Lower earthing resistance of the PEN conductor.  Has the advantage that in case of an insulation fault.13 Advantages of the PME Earthing System  The earthing system always provides a return path for faults in the LV grid. . is not solidly connected to earth. therefore. must be grounded. By this way. cables and earth can be seen as armatures of a capacitor. typically in high-/medium-voltage systems. to distribute the neutral conductor in order to facilitate its insulation from ground. whose dielectric is the surrounding air. constitutes another leaking path in parallel to the system net capacitance. The utility is not only responsible for a proper grounding but also for the safety of customers during disturbances in the power grid  Protection to be fitted in case of network modification (increase of fault loop impedance). However electrical systems cannot be completely isolated from ground. though. even in the absence of any intentional connection to the earth. At the system frequency.2 IT Grounding System In IT (Isolation Terre) systems as mentioned in chapter 2. system fixes the neutral point and the voltages are not floating. . may be considered as an open circuit in parallel to the system capacitance Figure 3. indeed. is defined as ungrounded. the resistance offered by the cable insulation to ground. although not forbidden by technical standards. in groups or collectively. The insulation of secondary sides of supply transformers from the earth may be obtained through a high. the system is still coupled to ground through the distributed capacitances. individually. Enclosures of ECPs. the power source. 3. which has the magnitude of a few mega ohms per kilometer. and therefore.resistance grounding resistor. The leakage resistance is. In addition. It is not advisable. very high and.6. 2. as previously anticipated. The two sets of vector quantities identical in normal conditions will now differ. V2N. V1N. in fact.2. The earth electrode resistance RG. with = R I ≤ 50V.andV3G) with respect to the systems voltages (i. If V . appears between the point of neutral N at the source and the ground G at the faulty ECP. to an unbalanced capacitive-resistive load. where I is the first- . is in parallel to the capacitance of the faulty phase. persons are exposed to the risk of electrocution..3.e. a potential difference VNG = VNVG. with no neutral wire.1 First Fault in the IT earthing system The occurrence of a ground fault causes the system capacitance to become unbalanced. Figure 3. Upon the first fault the system evolves from a balanced three-phase capacitive load..V2G . Because of this unbalance. also referred to as neutral potential rise.1. 3. Cf increases with the cables’ extension and so does the ground-fault current flowing through the person. and V3N).e.7 To avoid this hazard. The current circulating after the first fault. The presence of the intentional ground lowers the prospective touch voltage to the potential drop across evident benefit for the safety against indirect contact. is mainly a small capacitive and will always go undetected by overcurrent devices.1 Overvoltages Due to Faults in IT Systems The main issue in IT systems is the possibility of overvoltages induced by ground faults.V1G. The presence of VNG changes the voltage between the line conductors and the ground(i. the ECPs must be earthed. In the case of indirect contact with a faulty ECP. However it is essential to know that there is a fault and need to track and eliminate it promptly. including the neutral.2. the automatic disconnection of supply is not necessary. Figure 3. the phase-to-phase voltage drives the fault current. When the neutral is not distributed (three-phase three-wire distribution). the following condition must be satisfied: ≤ 0.866 When the neutral is distributed (three-phase four-wire distribution and single phase distribution). as the ground fault does not cause any hazard to persons and may persist within the ECP. and an actual short circuit occurs.8 In this situation. To meet this need the fault information is provided by an Insulation Monitoring Device (IMD) monitoring all live conductors. On the other hand.5 3.1. In this case.2 Second fault in the IT earthing system During an unresolved first fault to ground. the second fault exposes persons to risk of . before a second fault occurs. the following condition must be satisfied: ≤ 0. a second fault involving a different phase might take place. at least one of the protective devices safeguarding the circuits will trip and disconnect the supply.fault current to ground. e. an audible/visual alarm will be initiated. With reference to Fig. VST1 on ECP 1 = = √3 + √3 + × The fault-loop has a driving potential which is not the phase voltage (240 V). and an a.10. To prevent these hazards.9 The IMD continuously monitors the impedance to ground (i. prospective touch voltage is calculated as. 3.2 Insulation Monitoring Device The IMD supervises the insulation reactance and/or resistance between the power lines and the earth. and as already anticipated. persons in contact with ECP 1 would be exposed to nearly the whole line-to-line voltage.electrocution in the time frame the overcurrent devices take to trip. resistance and capacitive reactance) by injecting both a d. the first fault should be resolved in the shortest possible time by the maintenance team.2.. current through the neutral point of the system. but the line to line voltage (415 V).1 If such impedance decreases below a predetermined value. Such alarm will alert the maintenance crew and will stay .c. It is clear that if RG2 were low when compared to RG1.c. due to a first fault to ground. Figure 3. 3. 2. because it flows entirely back through its toroid via the system distributed impedance. prevents their proper operation and renders their installation ineffective. individually or collectively). generally in correspondence of a ground current of at least five times their residual operating currents.. therefore. 3.1 ECPs Earthed Individually or in Groups If ECPs are earthed individually. because of the accidental connection of the faulty phase to earth. 3. the IT system is no longer ungrounded. The RCD does not sense any unbalance and. is not effective in IT systems. in the event of a second fault. Once the faulty circuit has been located and fixed. the system becomes TT and we are in the case in Fig. Protection against indirect contact is achieved if the following condition. RCDs can clear the fault within the safe time required.2. or in groups. 3. normally provided by RCDs in PME. cannot activate the RCD. in fact.4 Protection Against Indirect Contact in the Event of a Second Fault to Ground After the occurrence of the first fault to earth. The fault current.10.2.on for the entire duration of the fault. The fault current circulating through the earth. In the event of a second fault involving a different phase. . due to the second fault may in fact be too low to operate promptly the overcurrent devices. in fact. is fulfilled: ≤ 50 The optimum protection against indirect contact in TT systems is carried out by RCDs. applied to the generic ith ECP. 3. operators will manually switch it off. the IT system “evolves” into TT or TN according to the earthing arrangement of ECPs (i.e.4. cannot intervene as the nature of the fault-loop.3 Protection Against Direct and Indirect Contact by Using RCDs in IT Systems The additional protection. This possibility renders extremely challenging the prediction of the total fault-loop impedance in IT systems “evolved” into a TN. which. drives the fault current.2. equal to 1. the system becomes TN. and due to the installation of individual ground electrodes. 3. Thus. The second fault may randomly occur in a different circuit. connected to one single grounding system. In this case. for example. supplies an ECP remotely located with respect to the location of the first fault. .732 times the voltage between line conductor and neutral. the voltage between the line conductors. involving a different live conductor.L1 L2 L3 ECP 2 ECP 1 Figure 3.10 IG The additional costs due to the necessity of RCDs. that is. If the neutral is not distributed.4. the fault impedance may be due to the contributions of line conductors and protective conductors of different cross-sectional areas.2 ECPs Earthed Collectively to a Single Grounding System The ECPs may be earthed collectively. at the occurrence of a second fault. usually induce designers to collectively earth the ECPs to a single ground electrode. effective under second fault conditions in the previous arrangement. 5 400 0.2 Maximum disconnection time in the IT earthing system (second fault).0 240 0. the protection against indirect contact of persons touching one faulty enclosure is effective if the following condition is fulfilled: √3 2 ≥ If the neutral conductor is distributed to loads.06 0. a first or second fault may involve this conductor. Breaking time (sec) No Neutral With Neutral No Neutral With Neutral 120 0. the voltage Vph between the faulty line and neutral conductor will drive the fault current. Breaking time (sec) VL=25V.8 0.2 0.4 0. Phase Voltage (V) VL=50V.4 1.2 .4 0.11 To take into account the second fault in a different circuit. Condition for safe automatic disconnection of supply is: 2 ′ ≥ Table 3.2 0.L1 L2 L3 IG ECP 1 ECP 2 Figure 3.8 5 0. In this case.  Less downtime on control and monitoring circuits.This corresponds to an average electrical power availability that is 91 times better in IT than in TN or TT. Electricity is often the cause of fire. Even if safety of persons with respect to the electrical hazard is guaranteed by the various earthing systems. The main advantage of using the IT earthing system for network operation is without doubt the continuity of supply it offers.  Increased safety against the fire hazard. set the threshold for this risk at 500mA on an insulation.  Increased availability . This value can be considerably exceeded. It is because the current of the first fault is particularly low that the IT earthing system has been chosen for use in certain establishments at risk from fire and explosion.6 Disadvantages of IT Earthing Arrangement The restrictions for using the IT system are linked to loads and networks. More care must therefore be taken when wiring such circuits in the TT and TN than in the IT system. thereby guarding against electrical and mechanical hazards. airport take-off runways. plants with continuous manufacturing processes. cold storage units. Consequently. Standards.2. electrical power plants. as the latter warns the operator of the incident (first insulation fault).  Limits linked to loads With a high earth capacitive coupling (presence of filters). IT is the only earthing system that monitors insulation of the neutral conductor. vessels. 3. A number of devices fitted with capacitive filters offer the same disadvantage. laboratories. or by use of Safety by Extra Low Voltage (SELV).5 Advantages of IT Earthing Arrangement  Continuity of supply. as very long networks when the IT system is used.3. due to their number.2. These capacitive leakages . as there is no need to trip on the first fault Another of this system’s strong points is guaranteed safety against the fire. preference is frequently given to the IT earthing system for use in: hospitals. particularly with stray currents that flow through building structures when faults occur in the TN system. safety of persons with respect to mechanical hazards may not be guaranteed in certain cases. e. Office computer equipment: micro-computers. for example for electrical power distribution in a number of buildings at a distance from one another. irrespective of the voltage source in operation. Device Network/ Earth capacitance Micro computer 20nF-40nF UPS 40nF Variable speed controllers 70nF Fluorescent tubes 20nF  Limits due to the physical characteristics of networks. . i. This current is referred to as the stray current.  Case of networks with replacement power supply. When an insulation fault occurs. The fact that a network can be supplied by several sources makes it necessary to detect the first fault and to trip on the second fault. 3.have a particularity. containing long feeders. is an example of this. To demonstrate and quantify this phenomenon an experiment was performed. they can also cause flow of residual currents likely to generate nuisance tripping “by sympathy” of the RCDs placed on very long or highly capacitive feeders thus use of the IT system is thus advised against for very long networks. Typical values are provided below. with respect to distributed capacity mainly due to network cables.3 capacitive values for HF filters built into various devices.3 Estimation of Power losses in PME Because the neutral conductor is connected to earth. concentrated on the same single-phase feeder. it forms a deliberate or intentional short circuit and because the earth is a conductive mass though poor as it may be some current flows through the this link to ground. For the quantification of the power losses the knowledge of the earth resistance was vital and so a measurement of the earth resistance had to be done. Table 3. they can be unbalanced. monitors and printers. The driven reference rod C should be placed as far from the earth electrode as practical.3.1Fall of Potential (wenner)Method This three-terminal test is the method that can be carried out with three or four terminal earth testers. Reference probe P is then moved to a location 40% of the distance to C.  Dead Earth method (two-point test). With a four-terminal tester.1 Simplified analysis for fall-of-potential test This reading with P at 50% of the distance from the earth electrode to C is noted as R1.3.1.1. connect X to the earth electrode. or the geography of the surroundings.1 Conventional Test Method for Earth Resistance There are three basic test methods as noted below.3. The reading at this point . P1 and C1 terminals on the instrument are ‘jumpered’ and connected to the earth electrode under test.13 Fall of potential method With a three-terminal instrument. this distance may be limited by the length of extension wire available.  Clamp-on test method 3.  Fall-of-potential method.1.3. to eliminate mutual inductance. the use of either three of four terminals is largely optional for testing the resistance of an installed electrode Figure 3. Potential reference rod P is then driven in at a number of points roughly on a straight line between the earth electrode and C. or three-terminal test. Leads should be separated and not run close and parallel to each other. Although four terminals are necessary for resistivity measurements. 3. water pipe) system to be outside its sphere of influence.3. however important to note the limitations to this test method as follows:  The dead earth metal (e.2 Dead earth method When using a four-terminal instrument. R2 and R3 is calculated as RA.  The dead earth metal (e. A third reading. P1 and C1 terminals connect to the earth electrode under test. probe C has to be placed farther away and the tests repeated.is noted as R2.g. You can then take the instrument reading as being the resistance of the electrode under test. The average of R1.14 It is. water pipe) system must be metallic throughout  The earth electrode under test must be far enough away from the dead earth (e.3. connect X to the earth electrode.3 The clamp-on ground tester Unfortunately.g.1. The water system could be any suitable metal structure or similar. water pipe) system must be extensive enough to have a negligible resistance. 3. If the water system is extensive (covering a large area). P2 and C2 terminals connect to an all-metallic metal system (e. Figure 3. see Figure 3. With a three-terminal instrument. P and C to the pipe system. RA can be used as the test result.2 times this percentage is less than the desired test accuracy.3. If 1.1. is made with P at a 60% distance. R3. the Fall of Potential method also comes with several drawbacks: . The maximum deviation from the average is then determined by finding the greatest difference between individual readings and the average. 3. a water pipe system).g. its resistance should be low and is usually much less than one ohm. If the result is not within the required accuracy.g. The method employs the use of a simple electronic circuit as shown below to measure the earth resistance. however they use two clamps. Four pole earth testers also perform a stake less test.  Individual ground electrodes must be disconnected from the system to be measured. where the ground resistance is large compared to the effective return loop resistance. The method benefits from low return path resistance. a voltage clamp (V clamp) and a current clamp (I clamp). Fig 3. measures resultant current flow and calculates the loop resistance of the circuit. All elements of the loop are measured in series.  There are situations where disconnection is not possible.3. For a six rod system the resistance-to-ground measure at R6 is: Rloop = Vloop/Iloop = R6 + (1/(1/R1 + 1/R2 + 1/R3 + 1/R4 + 1/R5)) 3.15 Basic clamp-on ground testing methodology All elements of the loop are measured in series.2 Proposed Method In this project a new method for the estimation of the earth electrode resistance was formulated. The method benefits from low return path resistance. The clamp-on ground tester performs a ‘stake less’ test which is a ground resistance test performed without disconnecting the ground. the stake less test induces a known voltage in a loop circuit that includes ground.e. Based on Ohm’s Law (R=V/I). It is extremely time consuming and labor intensive. and keep the clamps separate to prevent interaction between the two. i. . 50Hz = 10Ω. 1 4 Figure 3..5 With switch on: The voltage across the resistor is measured as and the current flowing to earth calculated as. to calculate the electrode resistance the equations representing the earth fault current as shown below ∆ = From which = ∆ × . ∆ = − This change in voltage is due to the earth resistance .16 Set up for the measurement of the earth electrode resistance With switch off: = 20 × 10 = 0. = Then change in voltage can be worked as. = .With known the dissipated power can be estimated as follows. 0120 81.14 0.124 2.10 0.40 0 1.95 0.4.3 1.885 0.2 Switch ON (V) (V) (V) (V) ∆ (V) (A) (Ω) 2.136 1.36 0.5 1.0146 85.67 + 76.10 0.0131 85.1 Analysis The average earth resistance was obtained as = 85.43 + 85.20 0.25 0 1.89 + 86. The following results were obtained from the set up to measure the earth electrode resistance Table 4.2 1.09 0.19 0.136 2.980 0.05 0.4 1.99 0.7 1.42 2.42 + 86.110 2.0125 86.115 0.00 2.96 = 83.210 0.0 EXPERIMENTAL RESULTS A.131 1.00 + 81.31 1.05 0 1.1 1.59 + 85.125 1.4 1.15 1.21 1.99 0.10 0 1.43 2.254 0.099 Table 4.0136 85.129 2.26 1.05 0.67 2.96 4.120 0.164 0.35 0 1.0115 76.36 1.5 1.075 0.04 0.00 0 0.29 0.20 0 1.0 1.3 1.15 0.105 2.7 1.140 1.59 2.24 0.119 0.89 2.6 1.115 2.30 0 1.146 1.1 Switch OFF (V) (V) = (V) (V) 2.99 ≈ 84Ω 7 .120 2.22 0.6 1.0140 86.1 1.2 1. 9 + 10.5 0.9 + 11.1085 10.2 0.9 + 11.9 2.8 + 10.9 2.1097 11.4 0.1087 10.1091 10.8 2.1095 11.7 0.9 2.0 = 10.9 2.1081 10. = =( sin ) = (0.9% 7 The instantaneous ac electrical power that is dissipated to earth due the system grounding can be estimated as.3 ∗ ∗ 2.0 2.1092 10. ∗ = at different source voltages and tabulated as shown below Table 4.9 + 10.3 0.The relative value of earth current to the current that flows in the circuit when isolated from earth was calculated as.1 0.0 + 10.11 sin ) .0 % Giving the average earth current as a percentage of the isolated circuit current as ∗ (%) = 10.6 0. 4 x f(x) f'(x) 0 40 80 120 160 200 240 0 0.1 from which table 4.0054 Table 4.4336 0.0839 1.0054 0.6503 0.0054 0.0054 0. Figure 4.2168 0.1 Table 4.The curve of the earth current as a function of the source voltage was plotted and showed a linear relationship as shown in figure 4.3006 0 0.5 Comparison of the Performance of the two systems Characteristics TN-C-S IT 1 IT 2 f''(x) Comments 0 0 0 0 0 0 0 .8671 1.0054 0.4 was deduced.0054 0. the touch voltage is very low if system is equipotential. otherwise it is high PROTECTION Indirect contact + + + All SEAs are equal if properly implemented Persons with emergency sets - + - Systems where protection is ensured by RCDs are not sensitive to a change in the internal impedance of the source + + + Continuous overvoltage + + + Transient overvoltage - + - Protection against fire (with an All SEAs in which RCDs can be used are equivalent. but is considerable for the second Touch voltage - + - In the TT system. RCD) OVERVOLTAGES Systems with high fault currents may cause transient overvoltages Overvoltage if transformer + + + + + + - + + breakdown (primary/secondary) ELECTROMAGNETIC COMPATIILITY Immunity to nearby lightening strikes Continuous emissions of an EM field Connection of the PEN to the metal structures of the building is conducive to the continuous generation of electromagnetic fields - + - The PE is no longer equipotential if there is a high fault current Interruption for first fault - + + Only the IT system avoids tripping for the first insulation fault Voltage dip during insulation - + - Both TN-C-S and IT (2nd fault) systems generate high fault Transient non-equipotentiality of the PE CONTINUITY OF SERVICE fault currents which may cause phase voltage dips INSTALLATION Special devices + - - The IT system requires the use of IMDs .ELECTRICAL Fault Current -- + -- Only the IT system offers virtually negligible first-fault currents Fault voltage - + - In the IT system. the touch voltage is very low for the first fault. a reduction in the number of cables MAINTENANCE Cost of repairs -- - -- The cost of repairs depends on the damage caused by the amplitude of the fault currents Installation damage - ++ - Systems causing high fault currents require a check on the installation after clearing the fault .Number of earth electrodes + -/+ -/+ The IT system offers a choice between one or two earth electrodes Number of cables + - - Only the TN-C system offers. in certain cases. It is by combining all requirements in terms of regulations.0 CONCLUSION In terms of the protection of persons. . selection does not depend on safety criteria. and that both systems guarantee automatic disconnection of the supply. the three system earthing arrangements (SEA) are equivalent if all installation and operating rules are correctly followed. operating conditions and the types of network and loads that it is possible to determine the best system. Given that the requirements to install the two system earthing arrangements is the almost the same. except for high basic insulation levels in IT.5. Also evident is that the isolated systems portray superior qualities to the PME systems except for the likelihood of high overvoltages in the occurrence of a second fault or simultaneous faults. From the findings of the experiment it is true that there is electrical energy dissipated to ground and is dependent on the magnitude of the current drawn by the load as well as the earth resistance. continuity of service. Consequently. the IT system earthing arrangement provides a better solution. Elsevier Ltd.J Hardy ACGI CEng FIET Transmission and Distribution Electrical Engineering Third Edition. USA. [3] IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems .K. 142. 178 The IT earthing system (unearthed neutral) in LV [12] Dr. Bayliss CEng FIET and B.IEEE Std. Electrical installation of buildings.Chapter 8 .5. 2007 [4] Schneider Electric Industrial electrical network design guide T & D 6 883 427/AE [5] Dr.Cahier Technique Merlin Gerin n° 173 Earthing systems worldwide and evolutions [11] Cahier technique no. Massimo A. Mehta & Rohit Mehta Principles of Power System Chapter 26 pg 587-599 [7] Prof Henryk Markiewicz & Dr Antoni Klajin Power Quality Application Guide Earthing & EMC [8] Megger Getting Down to Earth A practical guide to earth resistance testing pg 7-33 [9] Schneider Electric .Electrical installation guide 2008-Protection against electric shocks [10] Bernard Lacroix and Roland Calvas .2 REFERENCES [1] Schneider Electric-Electrical Installation Guide 2008-LV Distribution [2] IEC 60364-1:2005(E). C. 2007.R.143-145 [6] V.G. Part 1: General principles. Mitolo-Electrical safety of Low voltage systems pp.
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