Applying Grounding and Shielding for instrumentation.pdf

Instrumentation Trainee Task Module 12309NATIONAL CENTER FOR CONSTRUCTION EDUCATION AND RESEARCH Objectives Upon completion of this module, the trainee will be able to: 1. 2. 3. 4. Identify the minimum Identify the minimum Properly terminate an Properly terminate an requirements for grounding in an installation. requirements for shielding in an installation. equipment ground per drawing specifications. equipment shield per drawing specifications. Prerequisites Successful completion of the following Task Module(s) is required before beginning study of this Task Module: Instrumentation Level 3, Task Modules 12307 and 12316. Required Trainee Materials 1. Trainee Module 2. Required Safety Equipment Copyright © 1993 National Center for Construction Education and Research, Gainesville, FL 32614-1104. All rights reserved. No part of this work may be reproduced in any form or by any means, including photocopying, without written permission of the publisher. TABLE OF CONTENTS Section 1.0.0 2.0.0 2.1.0 2.2.0 2.3.0 2.4.0 2.5.0 3.0.0 3.1.0 3.2.0 3.3.0 3.4.0 3.5.0 3.6.0 3.7.0 3.8.0 3.9.0 3.10.0 3.11.0 4.0.0 4.1.0 4.2.0 4.3.0 5.0.0 6.0.0 7.0.0 7.1.0 7.2.0 7.3.0 7.4.0 7.5.0 7.6.0 7.7.0 8.0.0 8.1.0 8.2.0 8.3.0 9.0.0 9.1.0 9.2.0 9.3.0 9.4.0 Introduction Grounding Grounding for Grounding for Grounding for Grounding for Grounding for Page 5 5 6 Fire Prevention 6 Electrical Shock Avoidance Equipment Ground Fault Protection .. 7 7 Lightning Protection 8 Electrical Noise Control 9 9 Safety Grounds 10 Signal Grounds Single-Point Ground Systems 12 15 Multipoint Ground Systems 15 Hybrid Grounds 15 Practical Low-Frequency Grounding Hardware Grounds 16 19 Single-Ground Reference for a Circuit 21 23 Grounding of Cable Shields 27 Ground Loops Noise 29 29 Capacitive-Coupled Noise 32 Inductive-Coupled Noise 33 Directly-Coupled Noise 34 Instrumentation Shielding 34 Electrical Signal Noise 36 36 The Effectiveness of Shielding 36 Field Characteristics and Shielding Material 36 Shield Geometry 36 Noise Reduction Signal Cable Installation 38 Shield Termination 39 39 Use of Multiple Shields 40 Signal Cable Types 40 Foil Shields 40 40 Coaxial Cable 41 Practical Instrument Shielding 41 Amplifier Shield 43 Signal Entrances to a Shield Enclosure 44 Shield-Drain Direction 44 APPLY PROPER GROUNDING AND/OR SHIELDING OF INSTRUMENT WIRING — MODULE 12309 Trade Terms Introduced In This Module Absorption: The ability of shielding to absorb magnetic fields. Attenuates: To decrease the level of an electrical signal. Bonded: The permanent joining of metallic parts to form an electrical conductive path. Chokes: A term used for a coil. Common Mode Voltages: A voltage of the same polarity on both terminals. Electromagnetic shield: Iron used to shield electromagnetic fields. Electrostatic shield: A braided copper shield that surrounds the insulated signal lead. Ferromagnetic: A term used to describe permeability. Filter capacitors: A capacitor used as part of a filter network in a circuit. Ground (NEC): A conducting connection, whether intentional or accidental, between an electrical circuit or equipment and the earth, or to some conducting body that serves in place of the earth. Grounded (NEC): Connected to earth or to some conducting body that serves in place of the earth. Grounded Conductor (NEC): A system or circuit conductor that is intentionally grounded. Grounding Conductor (NEC): A conductor used to connect equipment or the grounded circuit of a wiring system to a grounding electrode or electrodes. Grounding Conductor, Equipment (NEC): The conductor used to connect the noncurrentcarrying metal parts of equipment, raceways, and other enclosures to the system grounded conductor, the grounding electrode conductor, or both, at the service equipment or at the source of a separately derived system. Grounded, Effectively (NEC): Intentionally connected to earth through a ground connection or connections of sufficiently low impedance and having sufficient currentcarrying capacity to prevent the buildup of voltages that may result in undue hazards to connected equipment or to persons. Grounding Electrode Conductor (NEC): The conductor used to connect the grounding electrode to the equipment grounding conductor, to the grounded conductor, or to both, of the circuit at the service equipment or at the source of a separately derived system. Ground-Fault Circuit-Interrupter (NEC): A device intended for the protection of personnel that functions to de-energize a circuit or portion thereof within an established period of time when a current to ground exceeds some predetermined value that is less than that required to operate the overcurrent protective device of the supply circuit. Ground-Fault Protection of Equipment (NEC): A system intended to provide protection of equipment from damaging line-to-ground fault currents by operating to cause a disconnecting means to open all ungrounded conductors of the faulted circuit. This protection is provided at current levels less than those required to protect conductors from damage through the operation of a supply circuit overcurrent device. Kilohertz: A thousand cycles Normal Mode Voltages: A voltage induced across the input terminals. Optical couplers: A device that couples a signal between two circuits using fiber optics. Reactance: The opposition, either inductive or capacitive, to a current in an AC circuit. Reflection: The ability of shielding to reflect electric fields. Shunt: A term used to indicate parallel. Grounding and shielding is an important part of any instrumentation installation. Proper grounding and shielding procedures must be followed to ensure an effective and safe electrical environment. This course covers the minimum requirements that must be met when installing or working on instrumentation. Grounding means a connection to earth. The connection can be via structural steel, metallic piping, electrical equipment, raceways, and grounding conductors (wires). Grounding practices are a requirement for a safe and secure facility. Most facilities have many conductors connected to earth such as building steel, utility conduit, and reinforcing bars. The conductors that carry power current can be earthed only in very specific ways. The other earthed conductors form a grid that must eventually connect to the earthed power conductors. APPLY PROPER GROUNDING AND/OR SHIELDING OF INSTRUMENT WIRING — MODULE 12309 Specifically. The housing is unsafe until the overcurrent detector opens the circuit. This heat could become a fire hazard. If the housing is not a low impedance back to the overcurrent protection. only its electrical safety. 3. This may take cycles.1Ω generates 1 kW. At this moment the housing is at the potential of the power conductor. 2. 2. This code does not address the issues of noise control or reduction. The housing is momentarily unsafe.1. 2. Fire protection Electrical shock avoidance Equipment ground fault protection Lightning protection Electrical noise control Limiting of high voltage. Proper grounding is a requirement of the National Electrical Code (NEC: ANSI/ NFPA-70). A shock hazard exists if a power conductor faults to its housing. Heat can be generated in defective equipment or in equipment improperly operated. 5. 4. or in the earth. any fire that results is not apt to spread. The systems designer must find a way to meet code requirements and still provide a noise-free system. This can be accomplished by the use of insulating jackets and further by locating all power conductors in properly grounded metal housings. Heat is simply PR: 100 A flowing in 0. equipment housings. 6. the housing stays unsafe. Grounding schemes can be built that meet all of these requirements or a limited subset. Fences and other forms of mechanical guards are also used to keep people away from hazardous areas. These needs are somewhat interrelated and must not be treated as separate issues by designers.All of these conductors form a grid that is an integral part of a grounding system. it is not involved with the performance of equipment. The deliberate earthing of the power system provides: 1.0 GROUNDING FOR FIRE PREVENTION Heat can be generated by current flow in poor connections. This heat can ignite any nearby combustible material.0 GROUNDING FOR ELECTRICAL SHOCK AVOIDANCE The simplest way to avoid shock is to insulate all conductors carrying a voltage. Connections between conductors are apt to be a weak spot in a conductive path. If the circuits are located in metal housings.2. INSTRUMENT TRAINEE TASK MODULE 12309 . all metal surfaces that may come into contact with a power conductor are bonded together and connected back to the service entrance ground and earth via a low-impedance path. Items with a lot of use wear out. but the controlling document is the National Lightning Protection Code (ANSI/NFPA-78). APPLY PROPER GROUNDING AND/OR SHIELDING OF INSTRUMENT WIRING — MODULE 12309 . If there is a fault. the equipment housing may be electrically "hot. Currents of this magnitude can destroy electrical equipment. The NEC covers some aspects of this requirement. damage structures. If the body of the drill comes in contact with a power conductor and the user is standing in water.000 A. or even minutes depending on the magnitude of the fault current. particularly where sensitive or critical electronics are operated. It is clear that some form of lightning protection should be placed in most facilities. the excess current flow that results can damage the equipment. Grounding the housing in a proper manner forces the repair of the equipment so that it is not further damaged and it cannot become a fire hazard. This method of grounding makes sure that there will never be a lethal potential difference between any of the earthed conductors in a facility. and electrocute humans and animals. The chassis assumes a potential of one-half the power voltage or about 60 V." If an overcurrent detector is not tripped.3. 2. 2. Anyone touching the housing and another grounded conductor can be electrocuted. Excessive heat causes insulation to become brittle and crack apart. The third wire or equipment grounding conductor should not be defeated. Consider an equipment housing that is earthed but not grounded by a separate conductor. Insulation can be used to reduce shock hazard. he may be electrocuted. Under no circumstances should these metal conductors carry any load current. For example. A frayed cable can be a lethal object. Many deaths result each year from faulty equipment grounding.0 GROUNDING FOR LIGHTNING PROTECTION Lightning pulses can carry currents in excess of 100.0 GROUNDING FOR EQUIPMENT GROUND FAULT PROTECTION Equipment faults should not be allowed to persist.4. This path should be deliberately designed and installed. Another good example of a shock hazard occurs when filter capacitors are placed from the power conductors to a metal chassis that is not grounded by an equipment grounding conductor.seconds. a dangerous situation can occur when the safety conductor in a hand drill is not connected. A person touching a grounded conductor and the chassis will receive a shock. The best protection consists of providing a convenient and direct path for lightning current to flow to earth. To avoid this possibility. Lightning need not strike a facility directly to cause damage to electronics. Attempts to provide lightning protection often falls short.000 V. INSTRUMENT TRAINEE TASK MODULE 12309 . how noise-free systems can be built within this framework. There is little chance of testing for lightning safety. Even with good protection. One of these conductors can be a ground or the earth. If signal or power wiring is not correctly handled then energy can enter a facility on these conductors and damage equipment. or utility conduits. Grounds include power conductors. Both issues need to be well understood. this type of fire is not apt to spread. Facilities that appear safe may fail. These conductors make many connections to the earth. 2. If the lightning pulse should enter a grounding grid. In general. Ground potential differences in the vicinity of a strike can exceed 10.0 GROUNDING FOR ELECTRICAL NOISE CONTROL Every pair of conductors can support the transport of electrical energy. If the lightning current should ignite insulation within the electrical system and it is enclosed in a metal housing. this is no reason to avoid lightning protection issues in building construction. Fortunately there are techniques for handling all noise problems that need not be in conflict with power safety. Good protection requires an understanding of bonding and low-inductance wiring. building steel.The current need not flow in a circuit to do damage. safety conductors. A high impedance results when there is a sharp bend or loop in the current path. which can crack or damage the structure. This rapidly changing field can induce large voltages into sensitive circuits. If lightning currents enter a facility on the power conductors. The magnetic field near the path of current flow is very intense.5. This is particularly true at frequencies above a few kilohertz. these potential differences cannot be shorted out by adding conductors. moisture in this path can turn to steam. This multiplicity of grounds causes many of the noise problems encountered in electronics. If the path is through steel encased in concrete. Lightning-related injuries are rather rare. However. lightning paths are often unpredictable and damage can result. Currents flowing in these grounds implies that there must be potential differences between ground points. or concrete. what constitutes good safe power engineering and second. The resulting explosion can start a fire. a relatively high-impedance circuit may cause the current to "side flash" or follow a path through air. Designers not familiar with sound instrumentation processes may seek solutions that create a hazard. First. wood. the impedance should be low enough to avoid any lethal potential differences. Grounding is one of the primary ways of minimizing unwanted noise and pickup.1. a safety ground wire (green) must be connected to all equipment enclosures and hardware. If the chassis is grounded. APPLY PROPER GROUNDING AND/OR SHIELDING OF INSTRUMENT WIRING — MODULE 12309 . can solve a large percentage of all noise problems. and returns through the neutral wire (white). The right-hand diagram of Figure 1 shows a second and far more dangerous situation: a fused AC line entering an enclosure. The only time the green wire carries current is during a fault. Since no load current flows in the safety ground. In many cases. AC power distribution and wiring standards are contained in the NEC. since its potential is determined by the relative values of the stray impedances over which there is very little control. which is fused. Load current flows through the hot wire (black). Safety grounds are usually at earth potential. Anyone coming in contact with the chassis and ground would be connected directly across the AC power line. a safety ground is required at a point that is unsuitable for a signal ground. One advantage of a well-designed ground system is that it can provide protection against unwanted interference and emission. the chassis would then be capable of delivering the full current capacity of the fused circuit. it may be called an earth ground. and Z 2 is the stray impedance between the chassis and ground. A good ground system must be designed. In comparison. Proper use of grounding and cabling. in combination. The chassis could be a relatively high potential and be a shock hazard. If there should be an insulation breakdown such that the AC line comes in contact with the chassis. Why this is so can be seen in Figure 1. In the left-hand diagram Z1 is the stray impedance between a point at potential V 1 and the chassis. an improperly designed ground system may be a primary source of interference and emission. thus removing the voltage from the chassis. If the chassis is grounded. In the United States. and this may complicate the noise problem. such an insulation breakdown will draw a large current from the AC line and cause the fuse to blow. One requirement of this code specifies that 115-V AC power distribution in homes and buildings must be a three-wire system. and then only momentarily until the fuse or breaker opens the circuit. 3. however. its potential is zero since Z2 becomes zero. whereas signal grounds may or may not be at earth potential. The potential of the chassis is determined by impedances Z1 and Z2 acting as a voltage divider. Grounds fall into two categories: (1) safety grounds and (2) signal grounds. as shown in Figure 2.0 SAFETY GROUNDS Safety considerations require the chassis or enclosure for electric equipment to be grounded. however. In addition. If the ground is connected to the earth through a low impedance path. It implies that since current is flowing through some finite impedance. An important consideration in determining this area is the ground path taken by the current in returning to the source. To do otherwise would allow some of the neutral current to return on the ground conductor. If the load requires only 230 V. however.it has no IR drop.0 SIGNAL GROUNDS A ground is normally defined as a point that serves as a reference potential for a circuit or system. To understand the limitations and problems of "real world" ground systems. a better definition for a signal ground is a low-impedance path for current to return to the source. This "current concept" of a ground emphasizes the importance of current flow. Often this is not the path intended. The actual path taken by the ground current is important in determining the magnetic coupling between circuits. and this point shall be at the main service entrance. A combination 115/230-V system is similar. except an additional hot wire (red) is added. 3. But what is the loop area of a system containing multiple ground paths? The area is the total area enclosed by the actual current flow. there will be a difference in potential between two physically separated points. Therefore. the neutral (white) wire shown in Figure 3 is not required. The reference point concept defines what a ground ideally should be.2. The magnetic or inductive coupling is proportional to loop area. is not representative of practical ground systems because it does not emphasize the importance of the actual path taken by the current in returning to the source. and the enclosures connected to it are always at ground potential. INSTRUMENT TRAINEE TASK MODULE 12309 . as shown in Figure 3. It is important to know the actual current path to determine the radiated emission or the susceptibility of a circuit. The NEC specifies that the neutral and safety ground shall be connected together at only one point. This definition. it would be better to use a definition more representative of the actual situation. whereas the current concept defines what a ground actually is. APPLY PROPER GROUNDING AND/OR SHIELDING OF INSTRUMENT WIRING — MODULE 12309 . and 3. (2) multipoint grounds. The resistances shown represent the impedance of the ground conductors. The series connection is also called a common ground or daisy chain. the effect of this voltage drop on the performance of the other circuits connected to the ground must be considered. Two physically separated ground points are seldom at the same potential.2. the frequency of operation. Signal grounds usually fall into one of three categories: (1) single-point grounds.In designing a ground it is important to ask: How does the current flow? The path taken by the ground current must be determined. Single-point and multipoint grounds are shown in Figures 4 and 5. This is a series connection of all the individual circuit grounds. The voltage measured between two points on the power ground is typically hundreds of millivolts. and I3 are the ground currents of circuits 1. two key points should be kept in mind: 1. the size of the system (self-contained or distributed). and I1 I2. since any conductor-carrying current will have a voltage drop. In general. This is excessive for low-level signal circuits. it is desirable to distribute power in a manner that parallels the ground structure.0 SINGLE-POINT GROUND SYSTEMS With regard to noise. and in some cases. the most undesirable single-point ground system is the common ground system shown in Figure 6. At 11 kHz. generally consisting of both resistance and inductance. In the following discussion of grounding techniques. a straight length of 22-gauge wire one inch above a ground plane has more inductive reactance than resistance. and (3) hybrid grounds. such as safety. No one ground system is appropriate for all applications. and then the power is distributed in a similar manner. The AC power ground is of little practical value as a signal ground. All conductors have a finite impedance. A hybrid ground is shown in Figure 6. A single-point connection to the power ground is usually required for safety. Point A is not at zero potential but is at a potential of and point C is at a potential of INSTRUMENT TRAINEE TASK MODULE 12309 . and other constraints.3. however. The proper signal ground system is determined by the type of circuitry. 3. Usually the ground system is designed first. respectively. and the parallel connection is called a separate ground system. many volts. respectively. Then. There are two subclasses of single-point grounds: those with series connections and those with parallel connections. 2. APPLY PROPER GROUNDING AND/OR SHIELDING OF INSTRUMENT WIRING — MODULE 12309 . At high frequencies there is no such thing as a single-point ground. but they will also act as antennas and radiate noise. INSTRUMENT TRAINEE TASK MODULE 12309 . however. the most critical stage should be the one nearest the primary ground point. in turn. Note that point A in Figure 6 is at a lower potential than point B or C. since the high-level stages produce large ground currents which. Ground leads should always be kept shorter than one-twentieth of a wavelength to prevent radiation and to maintain a low impedance. At still higher frequencies the impedance of the ground wires can be very high if the length coincides with odd multiples of a quarter-wavelength. where the inductances of the ground conductors increase the ground impedance. The potentials at points A and C.Although this circuit is the least desirable single-point grounding system. since in a large system an unreasonable amount of wire is necessary. for example. This system should not be used between circuits operating at widely different power levels. adversely affect the low-level stage. For non-critical circuits it may be perfectly satisfactory. it is probably the most widely used because of its simplicity. A limitation of the single-point ground system occurs at high frequencies. This system is mechanically cumbersome. Not only will these grounds have large impedance. When this system is used. The separate ground system (parallel connection) shown in Figure 7 is the most desirable at low frequencies. are as follows: The ground potential of a circuit is now a function of the ground current and impedance of that circuit only. That is because there is no cross coupling between ground currents from different circuits. The three separate ground return circuits should be connected together at only one point. The low ground impedance is due primarily to the lower inductance of the ground plane. usually the chassis. APPLY PROPER GROUNDING AND/OR SHIELDING OF INSTRUMENT WIRING — MODULE 12309 . racks. A practical application of this principle is the cable-grounding scheme. A third "hardware" ground should be used for mechanical enclosures. since current flows only on the surface due to skin effect. 3. An illustration of how these grounding principles might be applied to a nine-track digital tape recorder is shown in Figure 9. Multipoint grounds should be avoided at low frequencies since ground currents from all circuits flow through a common ground impedance—the ground plane. the length of these ground leads must be kept to a small fraction of an inch. The connections between each circuit and the ground plane should be kept as short as possible to minimize their impedance. the power ground (green wire) should be connected to the hardware ground. The signal ground used for low-level electronic circuits should be separated from the "noisy" ground used for circuits such as relays and motors. 3. The key to balancing these factors successfully is to group ground leads selectively. Most systems require a minimum of three separate ground returns. Such a combination is a compromise between the need to meet the electrical noise criteria and the goal of avoiding more wiring complexity than necessary. so that circuits of widely varying power and noise levels do not share the same ground return wire.5.3. At low frequencies.0 PRACTICAL LOW-FREQUENCY GROUNDING Most practical grounding systems at low frequencies are a combination of the series and parallel single-point ground. while other high-level circuits share a different ground return conductor. At high frequencies. Use of this basic grounding configuration in all equipment would greatly minimize grounding problems. the common impedance of the ground plane can be reduced by silver plating the surface.6. the cable shield is single-point grounded. If AC power is distributed throughout the system.0 MULTIPOINT GROUND SYSTEMS The multipoint ground system is used at high frequencies and in digital circuitry to minimize the ground impedance. and so on. chassis.4. Thus. In this system circuits are connected to the nearest available lowimpedance ground plane. several low-level circuits may share a common ground return. and at high frequencies it is multipoint grounded. Increasing the thickness of the ground plane has no effect on its high frequency impedance. as shown in Figure 8.0 HYBRID GROUNDS A hybrid ground is one in which the system-grounding configuration appears differently at different frequencies. In very high frequency circuits. the relays. the capstan motor control circuit is the most sensitive.0 HARDWARE GROUNDS Electronic circuits for any large system are usually mounted in relay racks or cabinets.There are three signal grounds. and four are connected to the other. Of these elements. it is properly connected closest to the primary ground point. These racks and cabinets must be grounded for safety. 3. a block diagram similar to Figure 9 can be very useful in determining the proper interconnection of the various circuit grounds. The signal grounds. The nine write amplifiers. the nine read amplifiers. The most sensitive circuits. which operate at a much higher level than the read amplifiers. are grounded by using two separate ground returns. and it may have fairly high resistance due to joints and seams in the rack or in pull-out drawers. In some systems such as electromechanical telephone offices. When designing the grounding system for a piece of equipment. and the interface and control logic are connected to a third ground return. and hardware ground should be connected together only at the source of primary power. The rack ground is often very noisy. noisy ground. INSTRUMENT TRAINEE TASK MODULE 12309 . the power supply.7. that is. and one hardware ground. The three DC motors and their control circuits. Five amplifiers are connected to one. one noisy ground. The hardware ground provides the ground for the enclosure and housing. and the solenoids are connected to the noisy ground. the racks serve as the return conductor for relay switching circuits. Rack number 1. and there is a ground loop between points 1. noise currents on the rack cannot return to ground through the electronics ground. At high frequencies some of the rack noise current can return on the electronics ground due to capacitive coupling between the rack and electronics. on the right. on the left. shows an incorrect installation in which the circuit ground is connected to the rack ground. shows correct grounding.Figure 10 shows a typical system consisting of sets of electronics mounted on panels which are then mounted on two relay racks. 3. 2. and the racks are strapped together and tied to ground at the primary power source. The panel is strapped to the rack to provide a good ground. 4. This capacitance should therefore be kept as small as possible. Noise currents on the rack can now return on the electronics ground. In this way. Rack 2. The electronics circuit ground does not make contact with the panel or rack. and 1. APPLY PROPER GROUNDING AND/OR SHIELDING OF INSTRUMENT WIRING — MODULE 12309 . The Kelvin Bridge is a portable instrument designed to accurately measure resistance. The instrument includes a built-in solid state null detector. and so on.If the installation does not provide a good ground connection to the rack or panel. One piece of equipment used to check ground connections is the Kelvin Bridge. or be sure that there is no ground at all. bridge and detector batteries. INSTRUMENT TRAINEE TASK MODULE 12309 . to provide a reliable ground connection. depending on whether or not the ground is made. and the necessary switches and terminals for operation as a self-contained unit. hinges.0001 to 11. When the ground is of a questionable nature. and then provide a definite ground by some other means. Do not depend on sliding drawers.0 ohms. it is best to eliminate the questionable ground. performance may vary from system to system or time to time. The high sensitivity of the unit permits measuring resistances of 0. careful attention must be paid to the electrical properties of seams. For example. This condition is illustrated in Figure 11. 3. care must be taken to prevent galvanic corrosion and to ensure that galvanic voltages are not troublesome. Improperly made ground connections may perform perfectly well on new equipment but may be the source of mysterious trouble later. APPLY PROPER GROUNDING AND/OR SHIELDING OF INSTRUMENT WIRING — MODULE 12309 . brazing. If chassis are to be used as ground planes.0 SINGLE-GROUND REFERENCE FOR A CIRCUIT Since two ground points are seldom at the same potential. or soldering. such as welding. a signal source is grounded at point A and an amplifier is grounded at point B. When joining dis-similar metals for grounding.8. and openings. When electrical connections are to be made to a metallic surface. joints. such as a chassis. finish aluminum with a conductive alodine or chromate finish instead of the non-conductive anodized finish. are better than those made by screws and bolts. the metal should be protected from corrosion with a conductive coating.Hardware grounds produced by intimate contact. the difference in ground potential will couple into a circuit if it is grounded at more than one point. two different ground symbols are used to emphasize t h a t two physically separated grounds are not usually at the same potential. one of the ground connections must be removed. Thus. almost all of the 100-mV ground differential voltage is coupled into the amplifier. and RL = 10kΩ. however. RC1 = RC2 = 1Ω. as shown in Figure 12.Note that in this discussion an amplifier is generally mentioned as the load. Both the source and one side of the amplifier input are grounded. the noise voltage VN at the amplifier terminals is equal to Consider the case where the ground potential in Figure 12 is equal to 100 mV. It is usually easier. a value equivalent to 10 A of ground current flowing through a ground resistance of 0. to eliminate ground connection A at the source. then the noise voltage at the amplifier terminals is 95 mV. If Rs = 500Ω. Elimination of the ground connection at B means the amplifier must operate from an ungrounded power supply. Voltage V G represents the difference in ground potential between points A and B. however. Resistors R C 1 and R C2 represent the resistance of the conductors connecting the source to the amplifier. To eliminate the noise. The amplifier is simply a convenient example. In Figure 11 the input voltage to the amplifier is equal to V s + VG. The effect of isolating the source from ground can be determined by considering a low-level transducer connected to an amplifier.01Ω. In Figure 11 and subsequent illustrations. and the grounding methods apply to any type of load. For the case where RC2 < Rs + R C1 + RL. INSTRUMENT TRAINEE TASK MODULE 12309 . the noise voltage VN at the amplifier terminals is Most of the noise reduction obtained by isolating the source is due to ZSG.9. From the equivalent circuit. If the impedance Z SG from source to ground is 1 MΩ and all other values are the same as in the previous example. 3. For the case where RC2 < R s + R C1 + R L and Z S G > RC2 + RG. Figure 14 shows the parasitic capacitance that exists between the amplifier and the shield. This is a reduction of 120 dB from the previous case where the source was grounded. it can be seen that the stray capacitances C 3S and C 1S provide a feedback path from output to input. Ideally.0 AMPLIFIER SHIELDS High-gain amplifiers are often enclosed in a metallic shield to provide protection from electric fields. The question then arises as to where the shield should be grounded. the amplifier may oscillate. it has some large finite value. as shown in Figure 13. If this feedback is not eliminated.The source can be isolated from ground by adding the impedance Z SG. The only shield connection that will eliminate the unwanted feedback path is the one shown at the bottom of Figure 14 where the shield is connected to the amplifier common terminal. there is no noise voltage coupled into the amplifier.095 uV. but due to leakage resistance and capacitance. If Z SG is infinite. APPLY PROPER GROUNDING AND/OR SHIELDING OF INSTRUMENT WIRING — MODULE 12309 . the impedance Z SG would be infinite. the noise voltage at the amplifier terminals is now only 0. INSTRUMENT TRAINEE TASK MODULE 12309 . and D. it is the capacitance between the input leads and the shield that provides the noise coupling. a voltage is generated across the amplifier input terminals due to the generators VG2 and VG1 and the capacitive voltage divider formed by C1 and C2. This noise current flowing through the impedance of the signal lead produces a noise voltage in series with the signal. and D. B. B. The three lower drawings in Figure 15 are equivalent circuits for grounding connections B. and the feedback is eliminated. and D. In the case of a shielded twisted pair. The input shield may be grounded at any one of four possible points through the dotted connections labeled A. As can be seen. 3. The only connection that precludes a noise voltage V12 is connection C. the shield current generates a noise voltage by causing an IR drop in the shield resistance. But if the shield is to be grounded at only one point. since it allows shield noise current to flow in one of the signal leads. even if this point is not at earth ground. and D. is unsatisfactory. C. Equivalent circuits are shown at the bottom of Figure 16 for shield connections A. where should that point be? The top drawing in Figure 15 shows an amplifier and the input signal leads with an un-grounded source. This shield connection should be made even if the common is not at earth ground. With ground connection D.0 GROUNDING OF CABLE SHIELDS Shields on cables used for low-frequency signals should be grounded at only one point when the signal circuit has a single-point ground. This connection. Generator VG1 represents the potential of the amplifier common terminal above earth ground. for a circuit with an ungrounded source and a grounded amplifier. Any extraneous voltage generated between the amplifier input terminals (points 1 and 2) is a noise voltage. Connection C is obviously not desirable since it allows shield noise currents to flow in one of the signal conductors to reach ground. The case of an ungrounded amplifier connected to a grounded source is shown in Figure 16.By connecting the shield to the amplifier common. capacitance C2S is short-circuited. regardless of the value of generators VG1 or VG2. For ground connection C. In the case of coaxial cable. If the shield is grounded at more than one point. Thus. Generator VG1 represents the potential of the source common terminal above the actual ground at its location. a voltage is generated across the amplifier input terminals due to generator VG1 and the capacitive voltage divider C1 and C2. noise current will flow. there is no voltage V12. the shield currents may inductively couple unequal voltages into the signal cable and be a source of noise. C. and generator VG2 represents the difference in ground potential between the two ground points. too. only connection A produces no noise voltage between the amplifier input APPLY PROPER GROUNDING AND/OR SHIELDING OF INSTRUMENT WIRING — MODULE 12309 . With grounding arrangement B. Connection A is obviously not desirable. The four possible connections for the input cable shield are again shown as the dashed lines labeled A. C. Since the shield has only one ground.10. B. the input shield should always be connected to the amplifier common terminal. even if this point is not at earth ground. Preferred low-frequency shield grounding schemes for both shielded twisted pair and coaxial cable are shown in Figure 17. but not at both ends. for the case of a grounded source and ungrounded amplifier. Therefore.terminals. the input shield should be connected to the source common terminal. Circuits A through D are grounded at the amplifier or the source. INSTRUMENT TRAINEE TASK MODULE 12309 . APPLY PROPER GROUNDING AND/OR SHIELDING OF INSTRUMENT WIRING — MODULE 12309 . INSTRUMENT TRAINEE TASK MODULE 12309 . they do have disadvantages. the shield of the coaxial cable is grounded at both ends to force some ground-loop current to flow through the lower-impedance shield. rather than the center conductor. the ground loop must be broken. the amount of noise reduction possible is limited by the difference in ground potential and the susceptibility of the ground loop to magnetic fields. In the case of circuit E. They are large. the shielded twisted pair is also grounded at both ends to shunt some of the ground-loop current from the signal conductors. this can cause an unwanted noise voltage in the circuit. In these cases. if multiple signals are connected between the circuits. or a differential amplifier. or when low-level analog circuits are used. have limited frequency response. it is necessary to provide some form of discrimination or isolation against the ground-path noise. The common-mode noise voltage now appears across the windings of the choke and not at the input to the circuit. The magnitude of the noise voltage compared to the signal level in the circuit is important: if the signal-to-noise ratio is such that circuit operation is affected. (2) common-mode chokes. The noise coupling is primarily a function of the parasitic capacitance between the transformer windings and can be reduced by placing a shield between the windings. the ground loop can be avoided by removing one of the grounds. (3) optical couplers. Figure 18 shows a system grounded at two different points with a potential difference between the grounds. steps must be taken to remedy the situation. Second. Isolation can be achieved by (1) trans-formers. This can be done by using transformers. (4) balanced circuitry. Figure 19 shows two circuits isolated with a transformer. In addition. In circuit F. multiple signal leads can be wound on the same core without crosstalk. or (5) frequency selective grounding (hybrid grounds). In Figure 20 the two circuits are isolated with a transformer connected as a common-mode choke that will transmit DC and differential-mode signals while rejecting common-mode AC signals. provide no DC continuity.When the signal circuit is grounded at both ends.11. As shown in the figure. as shown in Figure 18.0 GROUND LOOPS Ground loops at times can be a source of noise. Two things can be done. and are costly. optical couplers. Since the common-mode choke has no effect on the differential signals being transmitted. the effect of the multiple ground can be eliminated or at least minimized by isolating the two circuits. The operation of the common-mode choke is described in the next section. This is especially true when the multiple ground points are separated by a large distance and are connected to the AC power ground. The preferred shield ground configurations for cases where the signal circuit is grounded at both ends are shown in circuits E and F of Figure 17. thus converting the system to a singlepoint ground. First. 3. The ground noise voltage now appears between the transformer windings and not at the input to the circuit. Although transformers can give excellent results. If additional noise immunity is required. APPLY PROPER GROUNDING AND/OR SHIELDING OF INSTRUMENT WIRING — MODULE 12309 . multiple transformers are required. INSTRUMENT TRAINEE TASK MODULE 12309 . These are inherently fragile and susceptible to damage and/or malfunction from electrical noise. lightning. Its potential to cause damage or malfunction is increasing today as electronic circuits become more and more complex. the operation of SCR-based power circuits such as phase controllers. depending on the resistance of the loop and the type of signal being used. The power lead is at an AC potential of 115 VAC 60 Hz.0 CAPACITIVE-COUPLED NOISE Capacitive-coupling occurs when AC power lines are run parallel to signal leads. switched-mode power supplies. At the same time. Since the power line voltage is constantly changing. an AC signal is coupled into the signal lead. A capacitor. emitted radio frequencies. Today's computers and microprocessor-based systems operate at higher speeds and provide more features with reduced size and weight through the use of complex solid state components. in its various forms. APPLY PROPER GROUNDING AND/OR SHIELDING OF INSTRUMENT WIRING — MODULE 12309 .Optical coupling (optical isolators or fiber optics). The signal lead is at a DC potential between 0 VDC and 90 VDC. 4. and a host of others. This difference in potential forms an electrostatic field. When the wires are run together. is nothing more than two parallel conductors or plates separated by an insulating material called a dielectric. as well as being very susceptible to it. the strength of the undesirable signal is inversely proportional to the capacitive reactance of the parallel lines. employed for their greater efficiency and smaller size. Electrical noise. and the value of the load resistance RL. can adversely affect any product using electronic circuitry. a capacitor is formed. The magnitude of the undesirable signal is proportional to the difference in potential between the lines. The capacitor formed by the two parallel wires attempts to charge to the difference between the potentials on each wire. The current trend toward more performance in smaller size has contributed to the noise problem. The power leads and signal leads are a conductive material.1. both analog and digital. if you recall. It has led to the use of digital circuits with high frequencies that can be both a source of electrical noise. and circuit breakers. It is most useful when there are very large differences in voltage between the two grounds. even thousands of volts. contactors. usually copper. The undesired common-mode noise voltage appears across the optical coupler and not across the input to the circuit. also utilize high frequencies and may contribute additional noise. is a very effective method of eliminating common-mode noise since it completely breaks the metallic path between the two grounds. the physical distance between the lines. The conductive leads are separated by a non-conductive or insulating material. There are also the more conventional sources of noise such as the opening and closing of relays. It is similar to resistance in a DC circuit. the noise would be present on only one of the signal leads. as shown in Figure 22. If only one of the signal leads was capacitively-coupled to the AC power lead. the capacitive reactance is relatively high.Capacitive reactance is the opposition to current flow by a capacitor. Capacitive reactance is frequency dependent. for example 60 Hz. it could measure across the input terminals of the recorder and should. As a result. then the voltage from each signal lead to ground will be equal. This reactance drops a portion of the AC potential difference between the signal lead and power lead." • The definition of Common Mode Voltage is "a voltage of the same polarity on both terminals" with respect to ground. through earth ground to the grounded AC source. and back through the AC power line. The charging path for C1 and C2 is completed by capacitor C3. be Normal Mode Noise. Therefore. Figure 21 illustrates capacitive-coupling of common mode noise from an AC power lead into a pair of measurement signal leads. Amplifier voltage inputs are classified as either Normal Mode Voltages or Common Mode Voltages: • The definition of Normal Mode Voltage is "a voltage induced across the input terminals. If capacitors C1 and C2 have equal values of capacitance. So. therefore. at low frequencies. and C2 represents the capacitor formed by the power lead and the negative signal lead. C1 represents the capacitor formed by the power lead and the positive signal lead. capacitive reactance Xc decreases. INSTRUMENT TRAINEE TASK MODULE 12309 . as frequency increases. only a portion of the potential difference between these lines is actually coupled into the signal lead by interlead capacitance. The capacitors can charge through the recorder to case ground. the capacitor formed between components within the recorder and case ground. the smaller the magnitude of the induced voltage becomes.Figure 23 shows an equivalent circuit in which a resistor represents the capacitive reactance. the capacitive reactances and the load resistance form a voltage divider. Xc. As you can see. or the smaller RL becomes. APPLY PROPER GROUNDING AND/OR SHIELDING OF INSTRUMENT WIRING — MODULE 12309 . The amount of induced voltage developed across the load resistance. depends on its size with respect to X c l and X c 2. the larger X c l or Xc2 become. of C1 and C2. RL. Therefore. Using the left hand. To generate a voltage or electrical potential. To understand the mechanism for inductive-coupling. Current passing through the AC power line is continuously expanding and collapsing magnetic field is formed around the power line. the magnetic field is moving relative to the conductor. This is the mechanism for inductive noise coupling. a magnetic field. there is relative motion between a conductor and a magnetic field. If the current is continuously increasing. In this case. When a conductor moves through a stationary magnetic field.0 INDUCTIVE-COUPLED NOISE Inductive-coupling occurs when signal lines are run parallel to AC power leads or when signal leads pass in the proximity of electric motors or generators. An EMF can also be produced when a conductor is in the proximity of an expanding and collapsing magnetic field. one must be familiar with some fundamentals of magnetism and generators. as shown in the figure. If the wire is held in the left hand. The energy of the magnetic field causes electrons to move. an EMF is induced into the conductor. one can determine the direction of the lines of force. a magnetic field is formed around the conductor. the remaining four fingers indicate the direction of the magnetic lines of force. If the ends of the conductor are connected outside the magnetic field to form a closed circuit. there must be a conductor. When a measurement channel signal lead is run parallel to the power lead. and relative motion between the conductor and field.4.2. an EMF will be induced into the signal INSTRUMENT TRAINEE TASK MODULE 12309 . Recall that when current passes through a conductor. such that the thumb points in the direction of current flow. decreasing. current flows in the circuit. then the magnetic lines will continuously build and collapse in one direction and then build and collapse in the opposite direction. and reversing direction as with AC current. therefore. An expanding and collapsing and magnetic field can be used to generate an electrical potential. as illustrated in Figure 24. the user of the instrument must be prepared to either: (a) evaluate the extent of noise. Another directly-coupled noise source is leakage currents. During maintenance. This is because the manufacturer is usually uncertain of the type of environment in which the instrument will be placed. Leakage currents are a result of poor insulation that allows current to pass from one lead into another or from a signal lead to ground. capacitor. this undesirable current is alternating at the same frequency as the power line current that induced it. When there is leakage between source and signal leads. a noise signal is introduced into the signal circuit similar to those introduced through capacitive and inductive-coupling. Ground loops can exist whenever interconnected. 4.lead. or (b) eliminate the causes of unacceptable noise. or other circuit component to touch the instrument case or adjacent components. Since the signal lead is a part of a complete electrical circuit. The instrument can be designed with filter circuits to attenuate noises that might originate from within the instrument.3. Interference potentials can result from thermoelectric potentials developed by the joining of dissimilar metals with a temperature gradient. The finite resistance present in the ground plane or in earth ground causes a potential to be developed. non-isolated instruments are grounded at more than one location. so. then leakage current path can be introduced into the measurement signal circuit. The interference potential that causes current to flow through the ground loop may be due to faults in electrical equipment that cause leakage currents through ground. if a technician causes a resistor. As such. Further-more. noise will be induced into the signal lines by the same means. Large magnetic fields exist around AC motors and generators. but any attempt by the manufacturer to add filter circuits to attenuate noises is based on an "assumed" amount and type of noise. If an interference potential exists between the ground points of the input circuit and output circuit. a current will result from the induced voltage. Non-isolated simply means that there is no isolation between the input circuit of the instrument and its output circuit. if signal lines are run in the vicinity of these machines. which may result in a determination that the existing noise is not significant. Noise cannot be totally accounted for by the manufacturer. APPLY PROPER GROUNDING AND/OR SHIELDING OF INSTRUMENT WIRING — MODULE 12309 . called a galvanic potential. This potential. The interface potential could also be produced in the same manner as the potential in a battery. Another source of leakage currents is through improperly spaced components within instruments. is developed when two dissimilar metals come into contact in an electrolytic solution.0 DIRECTLY-COUPLED NOISE The ground loop is probably the most difficult circuit noise source to locate. an undesired current begins to flow. The input circuit of the instrument is connected to the output circuit by a measurable resistance. or (c) prevent the unacceptable noise from interfering with the instrument. Other methods used in the industry to reduce noise are: The use of filters (usually capacitors) to prevent noise from entering instrument amplifiers. The complexity of modern industrial processes often necessitates the monitoring and control of the plant from one control room. or at least its reduction to a tolerable level is necessary for proper process control.Noise can be a major source of inaccuracy in measurement channels. signal leads could be relocated away from power leads or electrical machinery. power leads are also twisted as a means of reducing interference. Often. Periodic insulation checks of signal cables to detect paths for leakage currents. Many factors must be considered when designing these transmission systems to ensure that reliable and accurate indication and control of the process is achieved. c. is minimizing the amount of noise present on the input leads. Detection and removal of ground loops. To provide this central control. As previously discussed. Obviously. Elimination of this undesirable voltage or current. b. the problem of noise removal can be attacked at two levels. Noise or interference may take various forms. One is using circuit design that reduce of the effects of noise. The other. noise reduction. Several such methods are employed in instrumentation systems. the best way to reduce unwanted signals within an instrument loop would be to eliminate the source of the noise. This concentrates on methods external to the instrument's electronic circuits that are used to reduce the magnitude of the noise induced into signal leads. One is noise elimination. The first method is the capacitive coupling of electrical energy from electrostatic fields into the signal lead. though. For example. keeping any noise on the input leads from reaching the amplifier. The use of shielding and shielded cables can be very effective in reducing the magnitude of noise induced in signal leads. a. usually adjacent wiring or equipment. In most cases. Therefore. process information must be transmitted over long distances. d. it is impractical to eliminate the noise or the adverse effects caused by noise. Noise is an undesirable voltage or current induced in measurement signal leads by an external source. The use of twisted pair cable for signal transmission is also an effective way to limiting interference. or it may be direct from alarm circuits. Proper grounding of instrumentation loops. The second method is the inductive coupling of electrical energy from INSTRUMENT TRAINEE TASK MODULE 12309 . there are three methods by which noise is introduced into a signal lead. It may be alternating current or voltage of high and low frequencies from utility service. Current transmission systems are less susceptible to induced noise than voltage transmission because current-controlled devices have low input and output impedances. it would have to induce a sizable amount of current. this improves instrument linearity over the entire span of operation. Shielding is a must in voltage transmissions that extend over long distance. For the noise to develop a significant amount of voltage drop across the low impedance. The output signal span must be large enough to provide satisfactory resolution and accuracy while minimizing the maximum signal level to allow the use of smaller. a distinct difference exists between a minimum signal and a missing signal. to obtain satisfactory results. For process instruments that require voltage inputs. APPLY PROPER GROUNDING AND/OR SHIELDING OF INSTRUMENT WIRING — MODULE 12309 . two or greater. DC voltage transmission systems require circuits of higher quality than current systems.electromagnetic fields into the signal lead. The third method involves the direct coupling of current into the signal leads through ground loops or leakage currents. precautions should still be taken to minimize noise by shielding signal cables and by locating signal cables away from power leads and heavy electrical machinery. especially if the system uses low voltage signal levels. For electronic instrumentation. Contrast this to the characteristically high impedances of voltage-controlled devices. DC current signal transmission has found the greatest acceptance in electronic process control systems with the ranges of 4-20 maDC and 10-50 maDC being most commonly used. This provides an immediate indication of a failure and makes locating the cause easier. Current transmission systems are more susceptible than voltage transmission systems to interference introduced by leakage currents and ground loop currents. The signal-to-noise ratio must be relatively large. A signal range with other than 0 maDC or 0 VDC as the minimum signal level was selected because when a "live" zero is used. a range of 4-20 maDC was chosen. a much smaller amount of induced current will cause a significant change in measured voltage. In the process instrumentation industry. there is an effort to standardize signal ranges so that instruments made by one manufacturer are compatible with those made by another manufacturer. In addition. Generally. These signal ranges are sufficiently high to eliminate the need for special signal cable and yet are low enough to allow the use of small gauge wire. an elevated zero will bias active electronic components into their linear range of operation. Although it is widely accepted by both users and manufacturers of process instruments. signal ranges used in the process industry have an elevated zero range. other nonstandard signal ranges are still widely used. a voltage signal can easily be derived from the current signal by inserting a resistor in series with the signal leads and measuring the voltage developed across the resistor. lighter components within the instrument and to reduce the power requirements of the instrument power supply. However. FIELD CHARACTERISTICS AND SHIELDING MATERIAL When a time-varying electromagnetic field impinges on a shield. Electrical conduit serves the same purpose as copper braid shielding. both ends. must be considered. This phenomenon increases with frequency. conductivity and permeability of the shielding material. 7.Shielding is the use of a conducting and/or ferromagnetic (permeable) barrier between a potentially disturbing noise source and sensitive circuitry. or signal lead bundle. Careful planning is needed in determining the number of grounding connections to be made along the entire shield.2. Skin effect is the tendency of high frequency AC current to concentrate on the conductor surface. but it is not as effective. increasing the AC resistance of the conductor. while magnetic fields are attenuated by absorption. it induces currents which tend to neutralize the magnetic field that created them.0 strength. This is due to the fact that inductance is lower on the surface of the conductor. 7. INSTRUMENT TRAINEE TASK MODULE 12309 . Shielding attenuates noise signals by two methods: absorption and reflection.0 SHIELD GEOMETRY In practice. enclosures. In determining the effectivity of a particular material in shielding against noise at high frequencies. Electrostatic shielding is usually a braided copper shield that surrounds the insulated signal lead.3. Shields are used to protect cables (data and power) and electronic circuits. They may be in the form of metal barriers. The magnitude of these currents depends on the conductivity and thickness of the shield material. signal lead. 7. physical geometry of the shield such as thickness and number of openings. stray capacitances between the shield and ground form resonant circuits with the impedance of the shield at high frequencies. A plastic or rubber insulator surrounds the shield to protect it. grounding of the shield: at one end. or at multiple points. electric fields are reflected. In general. or skin effect.0 NOISE REDUCTION There are two types of shielding that can be used: electrostatic shielding and electromagnetic shielding. and frequency of the time-varying magnetic field. a property known as skin depth. or wrappings around source circuits and receiving circuits.1.4. angle of incidence.0 THE EFFECTIVENESS OF SHIELDING The effectiveness of shielding is dependent on the following factors: • • • • The The The The 7. Because these voltages are. on the other hand. they cancel each other at the line termination. The electrostatic field developed by the power lead is also terminated on the grounded shield. is not a good electromagnetic shield because of its low permeability. such as the power leads. electromagnetic shielding is not a commonly used method for reducing signal noise. prevents this energy from influencing the signal carried by a signal lead. Furthermore. Electromagnetic shielding consists of iron that has high permeability. With the shield surrounding the signal. The use of twisted pair cable for signal transmission as a method of noise reduction offers many advantages. First.Figure 25 is an illustration of a signal lead surrounded by a shield. although made of steel. has minimum influence on the signal carried by the signal lead. and thus. identical voltages are induced in each lead. the signal lead would then be influenced by electrostatic fields in these areas. High permeability iron. The continuous twisting of the leads and their closeness together exposes each individual lead in a cable to the same electrostatic and electromagnetic fields. Therefore. of the same polarity in both the positive and negative lead. Electrical conduit. These induced APPLY PROPER GROUNDING AND/OR SHIELDING OF INSTRUMENT WIRING — MODULE 12309 . twisted pair cable is inexpensive and easy to install. and noise would be coupled into the signal lead. This property of a material provides a short circuit path for electromagnetic energy. If the shielding were damaged such that there were sections where shielding had been removed. is usually very expensive. at any instant. the potential on conductors outside the shield. the potential of the signal lead cannot influence the signal on any other conductor because the electrostatic field at the shield is at ground potential. So. Permeability is the ability of a material to conduct or carry magnetic lines of force. we will limit our discussion of signal transmission lines to those that carry low level DC signals. Normally. the electrostatic and electromagnetic fields surrounding each of the leads cancel one another. For this reason. In large process plants with central control rooms. 7. 500. but it does make the induced voltage on each lead equal. The cable shield reduces the magnitude of the field present. Other instruments such as magnetic flowmeters. or 1. or be run through rigid conduit installed overhead or buried in trenches. and the twisting of the signal leads causes the remaining field to induce common mode noise which is easily eliminated. care should be taken that instrument power lines and signal lines are separated to minimize noise INSTRUMENT TRAINEE TASK MODULE 12309 . Other multiconductor cables are available.5. the signal is usually converted to a low level DC current or voltage. It should be pointed out that a twisted pair of signal leads does not reduce the induced voltage as does a shielded cable. ultra-sonic level detectors. that have 2 to 100 individual conductors. Each wire in these cables is either color-coded or numbered at approximately one-foot intervals. When running signal cables.0 SIGNAL CABLE INSTALLATION The majority of instruments used in the process industry produce low level DC current or voltage signals. Signal cable is normally available in spools of 100.000 feet. The signal lead wire size ranges from 16 AWG to 24 AWG depending on the signal range used in the loop. either shielded or unshielded. Most AC power leads are twisted because this is an effective way to reduce signal interference. and radioactive sensing devices may produce signals that are AC currents or voltages or high voltage DC. Twisted pair cable can be purchased with 2 to 100 conductor pairs. the voltage induced in the lead would be a normal mode signal that would add to or subtract from the desired process signal. This action greatly reduces the noise available to be induced into signal leads. but before the process information contained in these signals is transmitted to other instruments in the loop. Multiconductor cable is normally used for electronic signal transmission. the signal leads are terminated at terminal strips where individual panel mounted instruments are connected. The use of twisted pairs and shielding provides the largest reduction of undesirable signals. Signal transmission lines can be run in overhead cable trays or wireways. particularly those induced by electrostatic fields. If only a single lead were passed through an electrostatic or electromagnetic field. At the control room panels. again shielded or unshielded. When AC power supply and return leads are spaced closely together and twisted. one wire in each pair is either color-coded or numbered to allow easy identification of each pair. Large multiconductor cables carry the signals between the local junction boxes and the central control room. the signal leads to and from individual plant instruments are run to junction boxes located in the process area.voltages are a common mode signal. Ensure that the plated metal surface is protected against oxidation. The signal wires should be run as far as possible from electrical motors. DC motors starting have caused inductive voltages high enough to activate alarm circuits when the wires are in the same conduit. Where backshell connectors are not available. but it provides the best possible protection of signal leads. and other electrical noise producing equipment. such as alarm or communication by a minimum of six inches. Precautions should also be taken to ensure that signal cable is protected from damage due to mechanical vibration. transformers. specifically to walls. Rigid conduit is expensive. and 2 feet from power utility distribution lines. When terminating shields. Multi-loop high frequency shield grounds reduces the loop areas that can couple to external fields. Never install utility AC or DC power lines in the same tray or conduit as signal or alarm lines. the high frequency shield is grounded to the source ground and terminating bulkheads. and rough handling. While the low frequency shield is grounded where the signal grounds. The ideal mounting surface would be a plated metal surface. APPLY PROPER GROUNDING AND/OR SHIELDING OF INSTRUMENT WIRING — MODULE 12309 . Another termination consideration is surface condition. corrosive atmosphere. a one-foot tray may be used with the signal lines separated from other low voltage lines.introduction. To protect analog instrumentation against high frequency noise.0 SHIELD TERMINATION Shield quality is usually compromised at the termination point.7. a second external shield must be used.0 USE OF MULTIPLE SHIELDS The use of guard shields in analog instrumentation does not provide high frequency noise shielding. If the current flowing on the outside surface of a shield is pinched down to a connecting wire. Backshell connectors will provide a continuous shield around the entire cable. A rule of thumb to follow that will minimize noise pickup when installing instrument lines. the field associated with this current can easily enter the inside of the cable. Signal cables can be run together in conduit but without other type wires. 7. 7. If installed in a tray. This high frequency shield may be grounded at more than one point.6. the best way to minimize noise is by the use of backshell connectors. then straight connections from the braid to the wall will provide the best available form of noise protection. At the point of poor terminations. Terminations should never be mounted on painted surfaces. generators. common mode voltage is generated. is that all instrument lines be twisted shielded pairs separated by a minimum of 6 inches from alarm or other on-off DC or communication lines. low frequency signals will generate a voltage gradient due to the current flow through the shield.0 COAXIAL CABLE Coax is used for the transport of high-frequency signals. This is because aluminum foil has poor high frequency attenuation characteristics. To prevent the voltage gradient from developing. 8.Two common types of material frequently used as shield material are foil shields and braided cable. 8. This external return path implies that there is a field outside of the sheath. The grounding of coax relates only to how the signal is generated and how it is terminated.0 BRAIDED CABLE Braided copper cable is the most commonly used sheath for shielded cables. Braided cable disadvantages include sheath gaps and developed voltage gradients. single braid cable is effective for most high frequency applications. This has nothing to do with termination or grounding at either end.2. This is because the foil itself is an excellent low frequency electrostatic shield. INSTRUMENT TRAINEE TASK MODULE 12309 . Foil wrap cables are not intended for the transport of high frequency energy. the latter being presented because it is common to find throughout process control systems. This section will present these two types and coaxial cabling. The fields used in transmission are fully contained inside the cable. Gaps in the sheath promote electrostatic coupling of external fields to the conductors internal to the shield.1. 8. When the signal return current uses a conductor outside of the coaxial sheath then the cable is not used as coax. Although double braid is superior to single braid. If the cable is not terminated correctly then energy is reflected. The difficulty with using foil shield is that they tear easily and cannot be soldered. To minimize the noise coupled to the conductor inside of the shield. The braiding provides flexibility and reasonable cost. To ensure proper foil shield termination. one end of the shield must be floated. the drain wire should be located external to the foil shield. a conductor known as a drain wire is used with the shield.0 FOIL SHIELDS Aluminum foil is frequently used as shield material on shielded cables.3. Braiding is most effective as a very fine weave single braided cable. but it is still inside the coax. When braided cable is grounded at both ends. should be connected to the zero signal reference potential of any circuitry contained within the shield. Conductor 3 is called the zero signal reference conductor as it is common to both the input and the outputs. This potential difference will be amplified and appear across conductors 2 and 3. This circuit. should be connected to the zero signal reference potential of any circuitry contained within the shield.1. to be effective.Shields that terminate on one end and that do not carry signal current are used as electrostatic shields (also called guard shields). Notice the significant mutual capacitances for an element of gain in Figure 26. Single point shield grounding for each signal is the domain of analog instrumentation. instrument shielding is necessary to prevent interference or noise from affecting signal conductors contained within the shielding material. is completely shielded from external electrostatic influences. The shield conductor should be connected to the zero signal reference potential at the signal-earth connection. At low frequencies a shield grounded at both ends assumes a voltage gradient that is the same on the outside and inside surfaces of the shield. Coax and multiple grounding are the domain of high-frequency energy transport. effective instrument shielding can be implemented. This solution assumes no ground potential differences in the system. shown in Figure 26. However. The number of separate shields required in a system is equal to the number of independent signals being processed plus one for each power entrance. The mutual capacitances form a feedback structure around the gain element and cannot be avoided. Restated: an electrostatic shield enclosure. Shields are often connected together and grounded to a single point. As previously discussed. Further assume that the device is self-powered and no circuit conductors enter or exit the box. If the signal is grounded then this single point is that ground. The symbology indicates that a potential difference will exist between conductors 1 and 3. the feedback process can be eliminated by tying the shield enclosure to conductor 3. The resultant equivalent circuit is shown in Figure 27. 9. This follows the first rule for shielding. to be effective. But how do we effectively accomplish instrument shielding? To properly shield instrument conductors. three basic rules must be followed: Rule 1 Rule 2 Rule 3 An electrostatic shield enclosure. These shields are connected to the zero potential reference point for the signal.0 AMPLIFIER SHIELD Consider an electrical device completely contained within a metal box. By following these rules. APPLY PROPER GROUNDING AND/OR SHIELDING OF INSTRUMENT WIRING — MODULE 12309 . INSTRUMENT TRAINEE TASK MODULE 12309 . This is shown in Figure 29. In practice. If two conductors are within the shield it is called two-conductor shielded wire. A shield enclosure is effective when Rule 1 is applied.0 SIGNAL ENTRANCES TO A SHIELD ENCLOSURE The gain element in Figure 26 is impractical without input and output connections. APPLY PROPER GROUNDING AND/OR SHIELDING OF INSTRUMENT WIRING — MODULE 12309 . Conductors that carry the signal to and from any amplifier are called signal conductors. in racks. This shielded wire is used to transport the signal from its source to the amplifier and can be thought of as an extension of the electrostatic enclosure of Figure 26. These neighboring conductors (grounds) are usually at differing potentials. the electrostatic enclosures shown in Figure 28 often parallel several external conductors. or along floors. long runs of shielded wires are contained in raceways. For example. the shield is automatically defined at this external reference potential. When the shield is tied to this same earth potential Rule 1 is applied and the system is correct. in conduit. In particular. Figure 28 shows a gain element and its shield enclosure. conductors 1 and 3 are signal conductors. these potentials are not the zero-signal reference potential of the shield enclosure. These neighboring potentials will cause currents to flow in the mutual capacitances between conductors. The input signal zero is ohmically connected to an earth point. The input and output connections are two-wire shielded conductors. This is the key to connecting signal conductors to a gain element. Since the shield must be at zero-signal reference potential.9. and since the signal is often derived from some reference point in the external environment.2. For example. in floor wells. in parallel with other wires. and this cable is called shielded wire. Signal conductors are usually enclosed in a braided metallic sheath or shield. This rule places no restriction on the shield potential relative to the external environment. is usually very low.SEGMENTS By Rule 1.9. INSTRUMENT TRAINEE TASK MODULE 12309 . If the shield segments are individually treated the difficulties can be expected. as a percentage effect. No statement is included as to where this connection should be made.0 SHIELD CONNECTIONS . the electrostatic enclosure should be at zero-signal reference potential.4. The pickup here. If the shield is split in sections Rule 2 places a constraint on the treatment of these segments. The connection is correctly made in Figure 28. Shield-drain processes in input conductors should be closely watched as the pickup here is subject to amplification.3. 9. This procedure ensures that parasitic currents will flow in the shield only and not flow in the signal conductors. It is usually not too difficult to follow Rule 2 everywhere to avoid this and other difficulties that can result. The rule requires that the shields be tied in tandem as one conductor and then connected to zerosignal reference potential at the signal-earth point.0 SHIELD-DRAIN DIRECTION Rule 1 requires that the shield be connected to zero-signal reference potential. Shield connections that permit current to flow in an output or high-signal-level conductor are often ignored. The shield can be thought of as a drain path to carry unwanted current back to an earth point. Fewer problems result when output cables are coaxial but crosstalk problems can still exist. At high frequencies and in digital circuitry. Third Edition. and correct procedures must be followed to ensure compliance with NEC requirements. a noise voltage is usually coupled to the signal. The inner conductor is a signal conductor. and the outer shield functions as a drain for unwanted noise current flow. Triaxial cable is also a shielded cable type. NFPA APPLY PROPER GROUNDING AND/OR SHIELDING OF INSTRUMENT WIRING — MODULE 12309 . Wiley 1990 by Ralph Morrison National Electrical Code. a multipoint ground system should be used. A low-frequency system should have a minimum of three separate ground returns. Grounding schemes are necessary for proper and safe operation of instrumentation circuits. References For advanced study of topics covered in this Task Module. These should be: signal ground.Rule 2 can be followed when a two-conductor (twin-axial) shielded cable is used. Proper grounding and shielding procedures must be followed to ensure an effective and safe electrical environment. SUMMARY Grouding and shielding is an important part of any instrumentation installation. If noise current flows in the outer conductor. The first shield functions as a signal conductor and as a coaxial return. The basic objectives of a good ground system are to minimize the noise voltage from two ground currents flowing through a common impedance. Wiley 1986 by Ralph Morrison Grounding and Shielding in Facilities. It is assumed that the two shields are insulated from each other. This course covered the minimum requirements that must be met when installing or working on instrumentation. noisy ground. the following works are suggested: Grounding and Shielding in Instrumentation. Some of the points that were brought up in this course are stated below At low frequencies a single-point ground system should be used. and hardware ground. Single shielded wire (coaxial cable) obviously forces the outer conductor to be both a shield and a signal conductor.