Applying Grounding and Shielding for instrumentation.pdf

Instrumentation Trainee Task Module 12309

NATIONAL CENTER FOR

CONSTRUCTION EDUCATION AND RESEARCH

Objectives Upon completion of this module, the trainee will be able to: 1.

Identify the minimum requirements for for grounding in an installa tion.

2. 3. 4.

Identify the minimum requirements for for shielding in an installat ion. Prop erly ter min ate an equipment ground per drawing specifications. specifications. Pro perl y ter min ate an equipment shield per drawing specifications specifications..

Prerequisites Successful completion of the following Task Module(s) is required before beginning study of  thi s Task Module: Ins tru men tat ion Lev Level el 3, Task Modules Modules 12307 12307 and 12316.

Required Trainee Materials 1. 2.

Tr ai nee Module Module Requi red Safety Safety Equ ipm ent

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 Page

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

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

Introduction Grounding Grounding for Grounding for Grounding for Grounding for Grounding for

APPLY PROP PROPER ER GROUNDIN G AND/OR SHIELDING OF INSTRUMENT WIRING — MODULE 12309

TABLE OF CONTENTS Page

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

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

Introduction Grounding Grounding for Grounding for Grounding for Grounding for Grounding for

APPLY PROP PROPER ER GROUNDIN G 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, Equip ment (NEC): (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 capaci capacity ty to preven t the buildup of voltages that may r esu lt in undue ha zard s 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  perso nnel th a t functions to de-energize a circuit or portion there of wi th in an estab lished period of time when a current to ground exceeds some predetermined value that is less than th at required to operate t he overcurrent protective protective devic devicee of the supp ly circuit. Ground-Fault Prot ect ion of Equip ment (NEC): (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 shielding is an importa nt par t of any instr ume ntati on installa tion. Proper ground ing and shielding proc edu res mu st be foll followe owed d to ensur e an effective effective and safe electrical environment. This course covers covers the minimu m requirements th at mus t be met when installing or working on instrumentation.

Ground ing me ans a connection to earth. The connectio connection n can can be via str uc tu ra l steel, metallic piping, electrical equipment, raceways, and grounding conductors (wires). Grounding practices ar e a re qu ire me nt for a safe safe and secure faci facilit lity. y. Most facilities hav e many conductors connected to earth such as building steel, utility conduit, and reinforcing bars. The conductors th at carr y power cur rent can be ear the d only only in very specific specific ways. The other earthed conductors form a grid that must eventually connect to the earthed power conductors.

APPLY PROPER PROPER GROUNDING AND/OR SHIELDING OF INSTRUMENT WIRING — MODULE 1 2309

 All of these conductors form a grid that is an integral part of a gr ou nd in g system. The deliberate earthing of the power system provides: 1. Fire protection 2. Electrical shock avoidance 3. Equipment ground  fault protection 4. Ligh tning protection 5. Electrical noise control 6. Limiting of high voltage. These needs are somewhat interrelated and must not be treated as separate issues by  designers. Grounding schemes can be built tha t meet all of these requ ireme nts or a limited subset. Proper grounding is a requirement of the National Electrical Code (NEC: ANSI/ NFPA-70). This code does not a ddress the iss ues of noise control or reduction. Specifically, it is not involved wit h th e performance of equipment, only it s electrical safety. Th e sys tems designer  must find a way to meet code requirements and still provide a noise-free system. 2.1.0

GROUNDING FOR FIRE PREVENTION

Heat can be generated by current flow in poor connections. Heat is simply PR : 100 A flowing in 0.1Ω gener ates 1 kW. This he at could become a fire hazard. Connections between conductors are apt to be a wea k spot in a conductive pat h. Heat can  be generated in defective equipment or in equipment improperly operat ed. This heat can ignite any nearb y combustible mat eri al. If the circuits are located in me ta l housings, any  fire that results is not apt to spread. 2.2.0 GROUNDING FOR ELECTRICAL SHOCK AVOIDANCE

The simplest way to avoid shock is to insula te all conductors carrying a voltage. This can  be accomplished by the use of insula ti ng jackets an d further by locating all power conductors in properly  grounded  metal housings, equipment housings, or in the earth. Fences and other  forms of mechanical guards are also used to keep people away from hazardous areas.  A shock haz ar d exists if a power conductor faults to its housing. At th is mo me nt th e housing is at the potential of the power conductor. The housing is momentarily unsafe. If the housing is not a low impedance back to the overcurrent protection, the housing stays unsafe. The housing is unsafe until the overcurrent detector opens the circuit. This may take cycles,

INSTRUMENT TRAINEE TASK MODULE 12309

seconds, or even min ute s depending on th e magn itu de of th e fault curre nt. Anyone touching the housing and another grounded conductor can be electrocuted. To avoid th is possibility, 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. Und er no circumstances should thes e meta l conductors carry any load cur ren t. This method of grounding makes sure that there will never be a lethal potential difference between any of the earthed conductors in a facility. Insul ation can be used to reduce shock haz ard . Ite ms with a lot of use wear out. Excessive he at causes ins ula tio n to become br it tl e and crack ap ar t. A frayed cable can be a let hal object. For example, a dangerous situation can occur when the safety conductor in a hand drill is not connected. If th e body of the drill comes in contact with a power conductor and the user is standing in water , he may be electrocuted. The third wire or equ ipm ent grounding conductor should not be defeated. Many dea ths r esul t each year from faulty equipment grounding.

2.3.0

GROUNDING FOR EQUIPMENT GROUND FAULT PROTECTION

Equipm ent faults should not be allowed to persist . Consider an equipment hous ing tha t is eart hed but not grounded by a sepa rat e conductor. If ther e is a fault, the equ ipm ent housing ma y be electrically "hot." If an overcu rren t detector is not tripped, the excess c urre nt flow th at results can damage the equipment. Grounding the housin g in a prope r ma nn er forces the repair of the equipment so that it is not further damaged and it cannot become a fire hazard. 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. The chassis assumes a potent ial of one-half th e power voltage or about 60 V. A person touching a grounded conductor and the chassis will receive a shock.

2.4.0

GROUNDING FOR LIGHTNING PROTECTION

Lightning pulses can carry curr ents in excess of 100,000 A. Curr ents of th is mag nitu de can destroy electrical equipment, damage str uct ure s, and electrocute hum ans an d animal s. It is clear that some form of lightning protection should be placed in most facilities, particularly where sensitive or critical electronics are oper ated. The best protection consists of providing a convenient and direct path for lig htni ng curr ent to flow to earth. This p at h should be deliberately designed and installed. The NEC covers some aspects of thi s requi rem ent , but the controlling document is the National Lightning Protection Code (ANSI/NFPA-78).

APPLY PROPER GROUNDING AND/OR SHIELDING OF INSTRUMENT WIRING — MODULE 12309

The current need not flow in a circuit to do dama ge. The magneti c field ne ar the path of  cur ren t flow is very inte nse. This rapidly changi ng field can induce larg e voltages into sensitive circuits. If th e lightning pulse should enter a grounding grid, th e impedance should be low enough to avoid any lethal potential differences. If lightning currents enter a facility on the power conductors, a relatively high-impedance circuit may cause the current to "side flash" or follow a path through air, wood, or concrete. A hig h impedance res ul ts when ther e is a shar p bend or loop in the curr ent pa th . If the path is through steel encased in concrete, moisture in this path can turn to steam, which can crack or damage th e structu re. The res ult ing explosion can st ar t a fire. If th e light ning current should ignite insulation within the electrical system and it is enclosed in a metal housing, this type of fire is not apt to spread. Light ning need not str ike a facility directly to cause damage to electronics. Ground potent ial differences in th e vicinity of a strike can exceed 10,000 V. If signal or power wiring is not correctly handled th en energy can ente r a facility on these conductors and damage equipme nt. Lightning-r elated injuries are ra th er rar e. However, this is no reas on to avoid light ning protection issues in building construction. Atte mpts to provide ligh tnin g protection often falls short. Even wit h good protection, ligh tnin g pat hs are often unpre dicta ble an d damage can result. Ther e is little chance of testing for light ning safety. Facilities tha t appea r safe may fail. Good protection re qu ir es an unde rsta ndin g of bonding and low-inductance wiring.

2.5.0

GROUNDING FOR ELECTRICAL NOISE CONTROL

Every pair of conductors can support the tra nsp or t of electrical energy. One of these conductors can be a ground or the ear th . Grounds include power conductors, safety conductors, building steel, or utility conduits. These conductors make many connections to the e art h. Curr ents flowing in these grounds implies th at there mus t be potentia l differences between ground poi nts. This multiplicity of grounds causes many of the noise problems encountered in electronics. In general, these potential differences cannot be shorted out by add ing conductors. This is particu larl y tr ue at frequencies above a few kilohertz. Fortunately there are techniques for handling all noise problems that need not be in conflict with power safety. Designers not familiar wit h sound ins tru men tat ion processes may seek  solutions tha t create a hazar d. Both issues need to be well understood. Fir st, wh at constitutes good safe power engineering and second, how noise-free systems can be built within this framework.

INSTRUMENT TRAINEE TASK MODULE 12309

Grounding is one of the primary ways of minimizing unwanted noise and pickup. Proper use of grounding and cabling, in combination, can solve a large percentage of all noise problems. A good ground system mu st be designed. One advantage of a well-designed ground system is that it can provide protection against unw ante d interference and emission. In comparison, an improperly designed grou nd system may be a primary source of interference and emission. Grounds fall into two categories: (1) safety grounds and (2) signal grounds. If th e ground is connected to the earth through a low impedance path, it may be called an earth ground. Safety grounds are usually at earth potential, whereas signal grounds may or may not be at eart h poten tial. In many cases, a safety ground is requir ed at a point th at is unsuitable for a signal ground, and this may complicate the noise problem.

3.1.0

SAFETY GROUNDS

Safety considerations require the chassis or enclosure for electric equipment to be grounded. Why this is so can be seen in Figure 1. In the left-hand diagram Z 1 is the stray impedance between a point at potential V1 and the chassis, and Z2 is the stray impedance between the chassis and ground. The potential of th e chassis is determine d by imped ances Z 1 and Z2 acting as a voltage divider.

The chassis could be a relatively high potential and be a shock hazard, since its potential is determined by the relative values of the stray impedances over which there is very little control. If the chassis is grounded, however, its poten tial is zero since Z2 becomes zero. The right-hand diagram of Figure 1 shows a second and far more danger ous situatio n: a fused AC line enter ing an enclosure. If the re should be an insulati on bre akdo wn such th at the AC line comes in contact with the chassis, the chassis would then be capable of delivering th e full curre nt capacity of the fused circuit. Anyone coming in contact with the chassis and ground would be connected directly across th e AC power line . If the chassis is grounded, however, such an insulation breakdown will draw a large current from the AC line and cause the fuse to blow, thus removing the voltage from the chassis. In the United States, AC power distribution and wiring standards are contained in the NEC. One req uire ment of this code specifies that 115-V AC power distribution in homes and buildings must be a three-wire system, as shown in Figure 2. Load current flows through the hot wire (black), which is fused, and re tu rn s throu gh the neut ral wire (white). In addition, a safety ground wire (green) must be connected to all equipment enclosures and hardware. The only time the green wire carries current is during a fault, and then only momentarily un til the fuse or bre ake r opens the circu it. Since no load cu rre nt flows in th e safety ground, APPLY PROPER GROUNDING AND/OR SHIELDING OF INSTRUMENT WIRING — MODULE 12309

it has no IR drop, and the enclosures connected to it ar e always at ground pot enti al. The NEC specifies that the neutral and safety ground shall be connected together at only one point, and this point shall be at the mai n service entran ce. To do otherwise would allow some of th e neutra l curr ent to retu rn on the ground conductor. A combination 115/230-V system is similar, except an additional hot wire (red) is added, as shown in Figure 3. If the load requires only 230 V, the neutral (white) wire shown in Figure 3 is not required.

3.2.0

SIGNAL GROUNDS

A ground is normally defined as a point that serves as a reference potential for a circuit or system. This definition, however, is not repr esen tati ve of practical ground system s because it does not emphasize th e importance of the actual p ath tak en by the c urrent in re tur nin g to the source. It is imp ort ant to know the actu al curren t pat h to determine the radi ate d emission or the susceptibility of a circuit. To unde rst and the limitat ions and problems of  "real world" ground systems, it would be better to use a definition more representative of  the a ctu al situation. Therefore, a bett er definition for a signal ground is a low-impedance pa th for curren t to re tu rn to the source. This "cu rrent concept" of a ground emph asize s the impor tance of curr ent flow. It implies th at since curr ent is flowing thro ugh some finite impedance, there will be a difference in potential between two physically separated points. The reference point concept defines what a ground ideally should be, whereas the current concept defines what a ground actually is. The actual path taken by the ground current is important in determining the magnetic coupling between circuit s. The magnetic or inductive coupling is proportional to loop area. But wha t is the loop are a of a system contai ning multiple ground paths ? The ar ea is the total ar ea enclosed by the actual curr ent flow. An imp ort ant consideration in deter min ing thi s area is the ground pat h take n by the cur ren t in ret urn ing to the source. Often this is not the path intended. INSTRUMENT TRAINEE TASK MODULE 12309

APPLY PROPER GROUNDING AND/OR SHIELDING OF INSTRUMENT WIRING — MODULE 12309

In designing a groun d it is impor tan t to ask: How does the curre nt flow? The pat h tak en by th e ground curr en t mu st be determine d. Then, since any conductor-carrying curr ent will have a voltage drop, the effect of this voltage drop on the performance of the other circuits connected to the ground must be considered. The proper signal ground system is determined by the type of circuitry, the frequency of  operation, th e size of the syste m (self-contained or distri buted) , and oth er constr aints, such as safety. No one ground system is appro priat e for all applications. Signal grou nds usua lly fall into one of th ree categor ies: (1) single-point grounds, (2) multipo int gro und s, and (3) hybri d grounds. Single-point and multip oint grounds are shown in Figures 4 an d 5, respectively. A hybrid groun d is shown in Figure 6. There are two subclasses of single-point ground s: those with series connections and those with parall el connections. The series connect ion is also called a common ground or daisy chain, and the parallel connection is called a separate ground system. In general, it is desirable to distribute power in a man ne r that parallels the ground structure . Usually the ground system is designed first, and then the power is distributed in a similar manner. In the following discussion of grounding techniques, two key points should be kept in mind: 1. All conductors have a finite impedance, general ly consisting of both resistanc e and inducta nce. At 11 kHz, a strai ght len gt h of 22-gauge wire one inch above a ground plane has more inductive reactance than resistance. 2. Two physically sep ara ted ground point s are seldom at the sam e potent ial. The AC power ground is of little practi cal value as a signal ground. The voltage measu red between two poin ts on the power ground is typically hun dr eds of millivolts, and in some cases, many volts. This is excessive for low-level signal circui ts. A single-point connection to the power ground is usually required for safety, however.

3.3.0

SINGLE-POINT GROUND SYSTEMS

With regard to noise, the most undesirable single-point ground system is the common ground system shown in Figure 6. This is a series connection of all the individual circuit grounds. The resistances shown represent the impedance of the ground conductors, and I 1 I2, and I3 are the ground cu rre nts of circuits 1,2, and 3, respectively. Point A is not at zero potenti al but is at a potential of 

and point C is at a potential of 

INSTRUMENT TRAINEE TASK MODULE 12309

APPLY PROPER GROUNDING AND/OR SHIELDING OF INSTRUMENT WIRING — MODULE 12309

Although this circuit is the least desirable single-point grounding system, it is probably the most widely used beca use of its simplicity. For non-critical circuits it may be perfectly satisfactory. This s yst em should not be used between circuits oper ating at widely different power levels, since the high-level stages produce large ground currents which, in turn, adversely affect the low-level stage. When thi s system is used, t he most critical sta ge should be the one nea re st the primar y ground point. Note th at point A in Figure 6  is at a lower potential than point B or C. The separate ground system (parallel connection) shown in Figure 7  is the most desirable at low frequencies. Th at is because ther e is no cross coupling betwe en ground cur ren ts from different circuits. The potentials at poin ts A and C, for exam ple, are as follows:

The ground potential of a circuit is now a function of the ground current and impedance of tha t circuit only. Thi s system is mechanically cumbersome, however, since in a large system an unreasonable amount of wire is necessary. A limitation of the single-point ground system occurs at high frequencies, where the inductances of th e ground conductors increase the ground 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 -wavel ength. Not only will these grou nds have large impedance, but they will also act as ant enna s and rad iat e noise. Ground lead s should always be kept shorter than one-twentieth of a wavelength to prevent radiation and to maintain a low impedance. At high frequencies the re is no such thin g as a single-point ground.

INSTRUMENT TRAINEE TASK MODULE 12309

3.4.0

MULTIPOINT GROUND SYSTEMS

The multipoint ground system is used at high frequencies and in digital circuitry to minimize th e ground impedan ce. In thi s system circuits are connected to the nearest available lowimpedance ground pla ne, usual ly th e chassis. The low gro und impedance is due primarily to the lower induct ance of the grou nd plane. The connections between each circuit and the ground plane should be kept as short as possible to minimize thei r impedance. In very high frequency circuits, the length of these ground leads must be kept to a small fraction of an inch . Multipoint grounds should be avoided at low frequencies since ground curr ents from all circuits flow thr ou gh a common ground impedance—th e ground plan e. At high frequencies, the common impedance of the ground plane can be reduced by silver plating th e surface. Incr easing th e thickne ss of th e ground plane has no effect on its hig h frequency impedance, since current flows only on the surface due to skin effect.

3.5.0

HYBRID GROUNDS

A hybrid ground is one in which the system-grounding configuration appears differently at different frequencies. A practical application of this principle is the cable-grounding scheme. At low frequencies, the cable shield is single-point grounded, and at high frequencies it is multipoint grounded.

3.6.0

PRACTICAL LOW-FREQUENCY GROUNDING

Most practical grounding systems at low frequencies are a combination of the series and paralle l single-point gro und . 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. The key to balancing these factors successfully is to group ground leads selectively, so that circuits of widely var yin g power and noise levels do not sh ar e t he same grou nd re tu rn wire. Thus, several low-level circuits may share a common ground return, while other high-level circuits share a different ground return conductor. Most systems require a minimum of thr ee separ ate ground re tu rn s, as shown in Figure 8. The signal ground used for low-level electronic circuits should be separated from the "noisy" ground used for circuits such as relays and motors. A th ir d "hardware" grou nd should be used for mechanical enclosures, chassis, rack s, and so on. If AC power is distribut ed throughout the system, the power ground (green wire) should be connected to the hardware ground. The three se pa rat e ground re tu rn circuits should be connected toget her at only one point. Use of thi s basic grounding configuration in all equi pme nt would greatl y minimize grounding problems. An illustration of how these grounding principles might be applied to a nine-track digital tape recorder is shown in Figure 9.

APPLY PROPER GROUNDING AND/OR SHIELDING OF INSTRUMENT WIRING — MODULE 12309

There are three signal gro unds , one noisy ground, and one ha rd wa re ground. The most sensitive circuits, the nine read amplifiers, are grounded by using two separate ground ret ur ns . Five amplifiers are connected to one, and four are connected to the other. The nine write amplifiers, which operate at a much higher level than the read amplifiers, and the interface and control logic are connected to a thi rd ground re tu rn . The three DC motors and their control circuits, the relays, and the solenoids are connected to the noisy ground. Of these elements, the capstan motor control circuit is the most sensitive; it is properly connected closest to the prim ary ground point. The hardw are groun d provides the grou nd for th e enclosure and housin g. The signal grounds, noisy ground, a nd hard ware ground should be connected together only at the source of primary power, that is, the power supply.

When designing the grounding system for a piece of equipment, a block diagram similar to Figure 9 can be very useful in determining the proper interconnection of the various circuit grounds. 3.7.0

HARDWARE GROUNDS

Electronic circuits for any large system a re usually mount ed in relay rack s or cabinets. These racks an d cabinets must be grounded for safety. In some systems such as electromechanical teleph one offices, the racks serve as the ret ur n conductor for rela y switching circuits. The rack ground is often very noisy, and it may have fairly high resistance due to joints and seams in the rack or in pull-out drawers. INSTRUMENT TRAINEE TASK MODULE 12309

Figure 10 shows a typical system consisting of sets of electronics mounted on panels which are then mounted on two relay racks.

Rack number 1, on the left, shows correct ground ing. The pane l is strapped to th e rack to provide a good ground, and the racks are strapped together and tied to ground at the primary power source. The electronics circuit ground does not mak e contact with the p ane l or rack. In this way, noise currents on the rack cannot return to ground through the electronics ground. At high frequencies some of the rack noise cur rent can ret ur n on the electronics ground due to capacitive coupling between the rac k and electronics. This capacitance should therefore be kept as small as possible. Rack 2, o n th e right, shows an incorrect inst alla tion in which the circuit grou nd is connected to th e rack ground. Noise curren ts on the rac k can now return on the electronics ground, and there is a ground loop between points 1, 2, 3, 4, and 1.

APPLY PROPER GROUNDING AND/OR SHIELDING OF INSTRUMENT WIRING — MODULE 12309

If the installation does not provide a good ground connection to the rack or panel, it is best to eliminate the questionable ground, and then provide a definite ground by some other means , or be sure th at there is no ground at all. Do not depend on sliding drawers, hing es, and so on, to provide a reliab le grou nd connection. When the groun d is of a questiona ble nature, performance may vary from system to system or time to time, depending on whether or not the ground is made. One piece of equipment used to check ground connections is the Kelvin Bridge. The Kelvin Bridge is a portable in str um en t designed to accurately measur e resistance. The high sensitivity of the unit permits measuring resistances of 0.0001 to 11.0 ohms. The instrument includes a built-in solid state null detector, bridge and detector batteries, and the necessary switches and terminals for operation as a self-contained unit.

INSTRUMENT TRAINEE TASK MODULE 12309

Hardware grounds produced by intimate contact, such as welding, brazing, or soldering, are bette r than those ma de by screws and bolts. When joining dis-similar met als for groundin g, care must be taken to prevent galvanic corrosion and to ensure that galvanic voltages are not troubles ome. Improp erly mad e ground connections may perform perfectly well on new equipment but may be the source of mysterious trouble later. When electrical connections are to be made to a metallic surface, such as a chassis, the metal should be protected from corrosion wit h a conductive coating. For example , finish alum in um with a conductive alodine or ch rom ate finish instea d of the non-conductive anodized finish. If chassis are to be used as ground planes, careful attention must be paid to the electrical properties of seams, joints, and openings.

3.8.0

SINGLE-GROUND REFERENCE FOR A CIRCUIT

Since two ground points are seldom at the same potential, the difference in ground potential will couple into a circuit if it is grounded at more tha n one point. This condition is illus tra ted in Figure 11; a signal source is grounded at point A and an amplifier is grounded at point B.

APPLY PROPER GROUNDING AND/OR SHIELDING OF INSTRUMENT WIRING — MODULE 12309

Not e th at in thi s discussion an amplifier is generally mentioned as the load. The amplifier  is simply a convenient example, however, and the grounding methods apply to any type of  load. Voltage V G repre sents th e difference in ground potential between poin ts A and B. In Figure 11 and subsequent illustrations, two different ground symbols are used to emphasize th at two physically separ ated grounds a re not usuall y at the same potential. Resistors R C1 and R C2 represent the resistance of the conductors connecting the source to the amplifier. In Figure 11 the input voltage to the amplifier is equal to V s + V G. To eli minat e th e noise, one of the ground connections must be removed. Elimination of the ground connection at B means th e amplifier must operate from an ungrounded power supply. It is usua ll y easier, however, to eliminate ground connection A at the source. The effect of isolating the source from ground can be determined by considering a low-level transducer connected to an amplifier, as shown in Figure 12. Both the source and one side of the amplifier input are grounded.

For the case where R C2 < R s + R C1 + R L , the noise voltage V N at the amplifier terminals is equal to

Consider the case where the ground potential in Figure 12  is equal to 100 mV, a value equivalent to 10 A of ground curr ent flowing through a ground resistance of 0.01Ω. If R s = 500Ω, R C1 = R C2 = 1Ω, and R L  = 10kΩ, then the noise voltage at the amplifier terminals is 95 mV. Thus, almost all of th e 100-mV ground differential voltage is coupled into th e amplifier. INSTRUMENT TRAINEE T ASK MODULE 12309

The source can be isolated from ground by adding the impedance Z SG, as shown in Figure 13. Ideally, the impedance Z SG would be infinite, but due to leakage resistance and capacitance, it ha s some large finite val ue. For the case where R C2 < R s + R C1 + R L  and Z SG > R C2 + R G, the noise voltage V N at the amplifier terminals is

Most of the noise reduction obtained by isolating the source is due to Z SG . If ZSG is infinite, th er e is no noise voltage coupled into the amplifier. If the impedance Z SG from source to ground is 1 MΩ and all other values are the same as in the previous example, the noise  voltage at the amplifier terminals is now only 0.095 uV. This is a reduction of 120 dB from the previous case where the source was grounded. 3.9.0

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 th e shield should be grounded. Figure 14  shows the parasitic capacitance th at exists between the amplifier and the shield. From the equivalent circuit, it can be seen that the stray capacitances C 3S and C1S provide a feedback  path from output to input. If thi s feedback is not eliminated, the amplifier may oscillate. 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.  APPLY PROPER GROUNDING AND/OR SHIELDING OF INSTRUMENT WIRING — MODULE 12309

INSTRUMENT TRAINEE TASK MODULE 12309

By connecting the shield to the amplifier common, capacitance C2S is short-circuited, and the feedback is elimin ated. This shield connection should be made even if the common is not at earth ground.

3.10.0

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. If the shield is grounded at more than one point, noise cur re nt will flow. In the case of a shielded twisted pair, t he shield curren ts may inductively couple unequal voltages into th e signal cable and be a source of noise. In the case of coaxial cable, the shield current generates a noise voltage by causing an IR drop in the shield resist ance. But if th e shield is to be ground ed at only one point, where should th at point be? The top drawing in Figure 15 shows an amplifier and the input signal leads with an un-gr ounde d source. Gene rato r VG1 repr esent s th e potent ial of the amplifier common terminal above earth ground, and generator VG2 represents the difference in ground potential between the two ground points. Since the shield has only one ground, it is the capacitance between the input leads and the shield t ha t provides the noise coupling. The in pu t shield may be grounded at any one of  four possible points through the dott ed connections labeled A, B, C, and D. Connection A is obviously not desirable, since it allows shield noise current to flow in one of the signal leads. Th is noise curr ent flowing thr oug h the impedance of the si gnal lead produces a noise voltage in series with the signal. The three lower drawings in Figure 15 are equivalent circuits for grounding connections B, C, and D. Any extraneous voltage generate d between the amplifier inpu t termi nals (points 1 and 2) is a noise voltage. With grounding arr ang eme nt B, a voltage is generated across the amplifier input terminals due to the generators V G2 and VG1 and the capacitive voltage divider formed by C1 and C2. Thi s connection, too, is unsatisfactory. For ground connection C, there is no voltage V12, regardless of the value of generators VG1 or VG2. With grou nd connection D, a voltage is generated across the amplifier input terminals due to generator VG1 and the capacitive voltage divider C 1 and C2. The only connection th at precludes a noise voltage V12 is connection C. Thus, for a circuit wit h an ungroun ded source and a groun ded amplifier, the input shield should always be connected to the amplifier common terminal, even if this point is not at earth ground. The case of an ungrounded amplifier connected to a grounded source is shown in Figure 16. Generator VG1 represents the potential of the source common terminal above the actual ground at it s location. The four possible connections for the inpu t cable shield are aga in shown as th e dashed lines labeled A, B, C, and D. Connection C is obviously not desirable since it allows shield noise currents to flow in one of the signal conductors to reach ground. Equivalent circuits are shown at the bottom of Figure 16  for shield connections A, B, and D. As can be seen, only connection A produces no noise voltage between the amplifier inp ut APPLY PROPER GROUNDING AND/OR SHIELDING OF INSTRUMENT WIRING — MODULE 12309

termina ls. Theref ore, for the case of a grounded source and ungrounded amplifier, the input shield should be connected to the source 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. Circuits A through D are grounded at the amplifier or the source, but not at both ends.

INSTRUMENT TRAINEE TASK MODULE 12309

APPLY PROPER GROUNDING AND/OR SHIELDING OF INSTRUMENT WIRING — MODULE 12309

INSTRUMENT TRAINEE TASK MODULE 12309

When the signal circuit is grounded at both ends, the amount of noise reduction possible is limited by the difference in ground potential and the susceptibility of the ground loop to magnetic fields. 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. In circuit F, 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. In the case of circuit E, the shielded twisted pair is also grounded at both ends to shunt some of the ground-loop current from the signal conductors. If additional noise immunity is required, the ground loop must be broken. This can be done by using transformers, optical couplers, or a differential amplifier.

3.11.0

GROUND LOOPS

Ground loops at times can be a source of noise. This is especially true when the multiple ground points are separated by a large distance and are connected to the AC power ground, or when low-level analog circuits are used. In these cases, it is necessary to provide some form of discrimination or isolation against the ground-path noise. Figure 18  shows a sy stem groun ded at two different poi nts wi th a potentia l difference between the grounds.

As shown in the figure, this can cause an unwanted noise voltage in the circuit. The mag nit ude of th e noise voltage compared to th e signa l level in the circuit is impo rta nt: if  the signal-to-noise ratio is such that circuit operation is affected, steps must be taken to remedy the situation. Two things can be done, as shown in Figure 18. First, the ground loop can be avoided by removing one of the grounds, thus converting the system to a singlepoint ground. Second, the effect of th e multiple gro und can be elimi nate d or at least minimized by iso lati ng the two circuits. Isolation can be achieved by (1) tran s-fo rmer s, (2) common-mode chokes, (3) optical couplers, (4) balanced circuitry, or (5) frequency selective grounding (hybrid grounds). Figure 19 shows two circuits isolated with a transformer. The ground noise voltage now appears between the transformer windings and not at the input to the circuit. 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. Although transformers can give excellent results, they do have disadvantages. They are large, have limited frequency response, provide no DC continuity, and are costly. In addition, if multiple signals are connected between the circuits, multiple transformers are required.

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 signal s. The common-mode noise voltage now appe ars a cross th e windings of th e choke and not at th e inpu t to the circuit. Since th e common-mode choke has no effect on th e differential signals being transmitted, multiple signal leads can be wound on the same core without crosstalk. The operation of the common-mode choke is described in the next section. APPLY PROPER GROUNDING AND/OR SHIELDING OF INSTRUMENT WIRING — MODULE 12309

INSTRUMENT TRAINEE TASK MODULE 12309

Optical coupling (optical isolators or fiber optics), is a very effective method of eliminating common-mode noise since it completely breaks the metallic path between the two grounds. It is most useful when there are very large differences in voltage between the two grounds, even thou sand s of volts. The unde sired common-mode noise voltage app ear s across th e optical coupler and not across the input to the circuit.

Electrical noise, in its various forms, can adversely affect any product using electronic circuitry. Its potenti al to cause damage or malfunction is increas ing today as electronic circuits become more and more complex. Today's computers and microprocessor-based systems operate at higher speeds and provide more features with reduced size and weight thr oug h the use of complex solid sta te components, both analog and digital. These are inherently fragile and susceptible to damage and/or malfunction from electrical noise. The current trend toward more performance in smaller size has contributed to the noise problem. It ha s led to th e use of digital circuits with high frequencies tha t can be both a source of electrical noise, as well as being very susceptible to it; switched-mode power supplies, employed for their greater efficiency and smaller size, also utilize high frequencies an d may contribute addit ional noise. There ar e also the more conventional sources of noise such as the opening and closing of relays, contacto rs, and circuit break ers , th e operation of  SCR-based power circuits such as phase controllers, emitted radio frequencies, lightning, and a host of others.

4.1.0

CAPACITIVE-COUPLED NOISE

Capacitive-coupling occurs when AC power lines are run parallel to signal leads. The power leads and signal leads are a conductive material, usually copper. The conductive leads are separated by a non-conductive or insulatin g mat eri al. When th e wires are ru n together, a capacitor is formed. A capacitor, if you recall, is no thin g more th an two paral lel conductors or plates separated by an insulating material called a dielectric. The signal lead is at a DC potential between 0 VDC and 90 VDC, 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. This difference in potential forms an electrostatic field. The capacitor formed by the two parallel wires attempts to charge to the difference between the potentials on each wire. Since the power line voltage is constantly changing, an AC signal is coupled into th e signal lead. The magn itud e of th e undes ira ble signal is proportio nal to the difference in potential between the lines, the physical distance between the lines, and the value of the load resistance RL. At the same time, the s tre ngt h of the un desirab le signal is inversely proportional to the  capacitive reactance of th e par allel lin es.

APPLY PROPER GROUNDING AND/OR SHIELDING OF INSTRUMENT WIRING — MODULE 12309

Capacitive reactance is th e opposition to cur ren t flow by a capacitor. It is si mil ar to resi stan ce in a DC circuit. Capacitive reactance is frequency dependent; as frequency increases, capacitive reactance X c decreases. So, at low frequencies, for example 60 Hz, the capacitive reactance is relatively high. This reactance drops a portion of the AC potential difference between the signal lead and power lead. Therefore, only a portion of the potential difference between these lines is actually coupled into the signal lead by interlead capacitance. Amplifier voltage inputs are classified as either Normal Mode Voltages or Common Mode Voltages: • The definition of Norma l Mode Voltage is "a voltage induced across t he i npu t termin als. " • The definition of Common Mode Voltage is "a voltage of the same polarity on both terminals" with respect to ground. Figure 21 illustrates capacitive-coupling of common mode noise from an AC power lead into a pair of measurement signal leads.

C1 represents the capacitor formed by the power lead and the positive signal lead, and C2 rep rese nts th e capacitor formed by the power lead and the negative signa l lead. The charging path for C1 and C2 is completed by capacitor C3; the capacitor formed between components within the recorder and case grou nd. The capacitors can charge thr oug h th e recorder to case ground, through earth ground to the grounded AC source, and back through the AC power line. If capacitors C1 and C2 hav e equal values of capacitance, th en t he voltage from each signal lead to ground will be equal. If only one of the signal lead s was capacitively-coupled to the AC power lea d, as shown in Figure 22, the noise would be present on only one of the signal leads. As a result, it could measure across the input terminals of the recorder and should, therefore, be Normal Mode Noise. INSTRUMENT TRAINEE TASK MODULE 12309

Figure 23 shows an equivalent circuit in which a resistor represents the capacitive reactance, Xc, of C1 and C2.

As you can see, the capaci tive reactan ces and the load resistance form a voltage divider. The amount of induced voltage developed across the load resistance, RL, depends on its size with respect to X c l and X c 2. Therefore, the larger X c l or Xc 2 become, or the smaller RL becomes, the smaller the magnitude of the induced voltage becomes.

APPLY PROPER GROUNDING AND/OR SHIELDING OF INSTRUMENT WIRING — MODULE 12309

4.2.0

INDUCTIVE-COUPLED NOISE

Inductive-coupling occurs when signal line s ar e run paral lel to AC power lead s or whe n signal lea ds pass in the proximity of electric motors or generators. To underst and the mechanism for inductive-coupling, one must be familiar with some fundamentals of magnetism and generators. Recall that when current passes through a conductor, a magnetic field is formed around the conductor. Using th e left hand, as ill ust rat ed in Figure 24, one can determine the direction of th e line s of force. If th e wire is held in the left hand, as shown in the figure, such th at the thumb points in the direction of current flow, the remaining four fingers indicate the direction of the mag netic lines of force. If th e cur rent is continuously increasing , decr easing, and reversing direction as with AC current, then the magnetic lines will continuously build and collapse in one direction and then build and collapse in the opposite direction.

An expanding and collapsing and magnetic field can be used to generate an electrical pote ntia l. To generat e a voltage or electrical potential, ther e must be a conductor, a magnetic field, and relativ e motion between the conductor and field. When a conductor moves th rou gh a stat ion ary magnet ic field, an EMF is induced into the conductor. The energy of the magn etic field causes electrons to move. If the ends of the conductor are connected outside the magnetic field to form a closed circuit, cur ren t flows in the circuit. An EMF can also be produced wh en a conductor is in th e proximity of an expandin g an d collapsing mag net ic field. In thi s case , the magnetic field is moving rel ati ve to the conductor. This is the mechan ism for induct ive noise coupling. Curr ent passin g through the AC power line is continuously exp and ing and collapsing magnetic field is formed around the power line. When a measurement channel signal lead is run parallel to the power lead, there is relative motion between a conductor and a magnetic field; therefore, an EMF will be induced into the signal INSTRUMENT TRAINEE TASK MODULE 12309

lead. Since the sig nal lead is a par t of a complete electrical circuit, a cur re nt will result from the induced voltage. Furth er-mo re, this undesirable current is alte rna ti ng at the same frequency as the power line c urr en t th at induced it. Larg e magnetic fields exist around AC motors and generators, so, if signal lines are run in the vicinity of these machines, noise will be induced into the signal lines by the same means.

4.3.0

DIRECTLY-COUPLED NOISE

The gro und loop is probab ly the most difficult circuit noise source to locate. Gro und loops can exist whenever interconnected, non-isolated instru men ts are grounded at more th an one location. Non-isolated simply me an s th at there is no isolation between th e inp ut circuit of  the ins trum ent and its ou tput circuit. The input circuit of the inst rum ent is connected to th e outpu t circuit by a mea sur abl e resistance. If an interference pote ntia l exists between the ground points of the input circuit and output circuit, an undesired current begins to flow. The interference potential that causes current to flow through the ground loop may be due to faults in electrical equi pmen t th at cause leakage curre nts through grou nd. The finite resistance present in the ground plane or in earth ground causes a potential to be developed. The interface potential could also be produced in the same manner as the potential in a bat tery . This potential , called a galvani c potential , is developed when two dissim ila r met als come into contact in an electrolytic solution. Interference potentials c an resu lt from thermoelectric potentials developed by the joining of dissimilar metals with a temperature gradient. Another directly-coupled noise source is leakage curr ent s. Leakage cur ren ts ar e a resu lt of  poor insulation that allows current to pass from one lead into another or from a signal lead to ground. When ther e is leak age between source and signal lead s, a noise signal is introduced into the signal circuit similar to those introduced through capacitive and inductive-coupling. Another source of leakage curre nt s is throu gh imp roper ly spaced components within ins tru men ts. During maintenance, if a technician causes a resistor, capacitor, or other circuit component to touch the instrument case or adjacent components, then leakage current path can be introduced into the measurement signal circuit. Noise cannot be totall y accounted for by th e manuf acture r. The in st ru me nt can be designed with filter circuits to attenuate noises that might originate from within the instrument, but any attempt by the manufacturer to add filter circuits to attenuate noises is based on an "assumed" amount and type of noise. This is because th e manufacturer is usua ll y unce rtai n of the type of enviro nmen t in which the inst rum ent will be placed. As such, th e user of the ins tru me nt mus t be prepare d to either: (a) evaluat e the extent of noise, which may resu lt in a determination that the existing noise is not significant, or (b) eliminate the causes of  unacceptable noise, or (c) prevent the unacceptable noise from interfering with the instrument.

APPLY PROPER GROUNDING AND/OR SHIELDING OF INSTRUMENT WIRING — MODULE 12309

Noise can be a major source of inaccu racy in me asu rem en t channels. Eli min ati on of this undesirable voltage or current, or at least its reduction to a tolerable level is necessary for pro per process control. Obviously, the best way to redu ce unwanted signals with in an in st ru me nt loop would be to elimi nate the source of th e noise. For example, s ign al leads could be relocated away from power leads or electrical machinery. Often, tho ug h, it is impr acti cal to eliminate th e noise or the ad verse effects caus ed by noise. One is us in g circuit desi gn tha t reduce of the effects of noise. This concentr ates on methods ex te rn al to the instrument's electronic circuits that are used to reduce the magnitude of the noise induced into signal leads. Several such methods are employed in instru menta tion syste ms. The use of shielding and shielded cables can be very effective in reducing the magnitude of noise ind uce d in signal lead s. The use of twis ted pai r cable for signal tra nsm iss ion is also an effective way to limiting interference . In most cases, power leads are also twisted as a mean s of reducing interference. Other methods used in the industry to reduce noise are: a. b. c. d.

The use of filters (usually capacitors) to pre ven t noise from ent eri ng in st ru  ment amplifiers. Periodic insu lati on checks of signal cables to detect path s for lea kag e cur ren ts. Detection and removal of ground loops. Proper grounding of ins trum ent atio n loops.

Therefore, th e problem of noise removal can be atta cke d at two levels. On e is noise elimi natio n; keeping any noise on the inp ut leads from reach ing the amplifier. The other, noise reduction, is minimizing the amount of noise present on the input leads.

The complexity of modern industrial processes often necessitates the monitoring and control of th e pla nt from one control room. To provide this cen tral control, process infor matio n must be tran smi tted over long distances. Many factors mus t be considered when design ing these transmission systems to ensure that reliable and accurate indication and control of the process is achieved.

Noise is an undesirable voltage or current induced in measurement signal leads by an ext ern al source, usua lly adjacent wiring or equipme nt. Noise or interferen ce may take var iou s forms. It may be alt ern at ing cur ren t or voltage of high and low frequencies from utility service, or it may be direct from alarm circuits. As previously discussed, there are three methods by which noise is introduced into a signal lead . The first method is the capacitive coupling of electrical energy from elect rosta tic fields int o the signal lead. The second method is the inductive coupling of electrical ener gy from

INSTRUMENT TRAINEE TASK MODULE 12309

electromagnetic fields into the signal lead. The th ird method involves th e direct coupling of current into the signal leads through ground loops or leakage currents. 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 manuf acture r. For electronic inst rum enta tion , a ran ge of 4-20 maDC was chosen. Although it is widely accepted by both users and manufacturers of process instruments, other non standard signal ranges are still widely used. Generally, signal ran ge s used in the process in du st ry have an elevated zero ra ng e. 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 distinct difference exists between a minimum signal and a missi ng signal. This provides an immediate ind icatio n of a failure an d mak es locating the cause easier. In addition, an elevated zero will bia s active electronic components into thei r linear range of operation; this improves instrument linearity over the entire span of operation. 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, lighter components within the in strum ent and to reduce the power requiremen ts of the instr ume nt power supply. DC cu rre nt signal tran smi ssio n has found th e gr ea tes t acceptance in electronic process control syst ems with the ran ge s of 4-20 maDC a nd 10-50 maDC being most commonly use d. 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 . Cur ren t trans miss ion syste ms are less susceptible to induced noise than voltage transmission because current-controlled devices hav e low input and ou tpu t impedances. For th e noise to develop a significant amo unt of  voltage drop across the low impedance, it would have to induce a sizable amount of current. Con tra st this to th e characteristically high impe danc es of voltage-controlled devices; a much sma ller a mount of induce d current will cau se a significant change in mea sur ed voltage. However, precautions should still be taken to minimize noise by shielding signal cables and by locating signal cables away from power lea ds an d heavy electrical machinery . Cur ren t transmission systems are more susceptible than voltage transmission systems to interference introduced by leakage currents and ground loop currents. For process instruments that require voltage inputs, 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. DC voltage transmission systems require circuits of higher quality than current systems, especially if the sy stem uses low voltage sign al levels. The signal-to-noise ratio m ust be relati vely large, two or greate r, to obtain satisfacto ry results . Shielding is a mu st in voltage transmissions that extend over long distance.

APPLY PROPER GROUNDING AND/OR SHIELDING OF INSTRUMENT WIRING — MODULE 12309

Shielding is the use of a conducting and/or ferromagnetic (permeable) barrier between a potenti ally disturbing noise source and sensitive circuitr y. Shields are used to protect cables (data an d power) and electronic circuits. They ma y be in the form of met al barr ier s, enclosures, or wrappings around source circuits and receiving circuits. Shielding attenuates noise signals by two methods: absorption and reflection. In general, electric fields are reflected, while magnetic fields are attenuated by absorption.

7.1.0

THE EFFECTIVENESS OF SHIELDING

The effectiveness of shielding is dependent on the following factors: •

The stre ngt h, angle of incidence, and frequency of the time-varying magnet ic field.

• • •

The conductivity and permeability of the shield ing materia l. The physical geometry of the shield such as thi ckne ss and numb er of openings. The grounding of th e shield: at one end, bot h ends, or at multiple points.

7.2.0

FIELD CHARACTERISTICS AND SHIELDING MATERIAL

When a time-varying electromagnetic field impinges on a shield, it induces currents which tend to neutra lize the magnet ic field tha t created the m. The magni tude of these cur ren ts depends on the conductivity and thickness of th e shield material . In determi ning th e effectivity of a particular material in shielding against noise at high frequencies, a property known as ski n depth, or skin effect, must be consider ed. Skin effect is th e tendency of hig h frequency AC current to concentrate on the conductor surface. This is due to the fact t hat inducta nce is lower on th e surface of the conductor. This phenomenon increases w ith frequency, increasing the AC resistance of the conductor.

7.3.0

SHIELD GEOMETRY

In practice, stray capacitances between the shield and ground form resonant circuits with the impedance of the shield at high frequencies. Careful planning is needed in dete rmin ing the number of grounding connections to be made along the entire shield.

7.4.0

NOISE REDUCTION

There are two types of shielding that can be used: electrostatic shielding and electromagnetic shielding. Electrostatic shielding is usually a bra ide d copper shield th at surro unds th e insu late d signal lead, signal lead, or signal lead bundle . A plastic or rub ber ins ulat or sur rou nds the shield to protect it. Electrical conduit serves the same purpose as copper bra id shielding, but it is not as effective.

INSTRUMENT TRAINEE TASK MODULE 12309

Figure 25 is an ill ust rat ion of a signal lead sur rou nde d by a shield. With the shield surrounding the signal, 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.

Furthermore, the potential on conductors outside the shield, such as the power leads, has minimum influence on th e signal carri ed by the signal lead. The elect rostatic field developed by the power lead is also termi nat ed on the grounded shield. If the shielding were damaged such that there were sections where shielding had been removed, the signal lead would then be influenced by electrostatic fields in these areas, and noise would be coupled into the signal lead. Electromagnetic shielding consists of iron th at has hig h permeability. Permeability is the ability of a ma te ri al to conduct or carry magnetic line s of force. This property of a mater ial provides a short circuit path for electromagnetic energy, and thus, prevents this energy from influencing th e signal carried by a signal lead. Electric al conduit, al thou gh made of steel, is not a good electromagnetic shield because of it s low permeability. High permeability iron, on the other hand , is usu all y very expensive. So, electromagnetic shielding is not a commonly used method for reducing signal noise. The use of twisted pair cable for signal transmission as a method of noise reduction offers many adva ntag es. Fir st, twiste d pair cable is inexpen sive and easy to inst all . The continuous twisting of the leads and their closeness together exposes each individual lead in a cable to the same electr ostatic and electromagnetic fields. Therefore, ident ical voltages are induced in each lead. Because the se voltages are, at any in st an t, of the same polarity in both the positive and negat ive lead, they cancel each other at the line termin atio n. These induced

APPLY PROPER GROUN DING AND/OR SHIELDING OF INSTRUMENT WIRING — MODULE 12309

voltages are a common mode signal. It should be pointed out tha t a twi ste d pair of signal leads does not reduce the induced voltage as does a shielded cable, but it does make the induced voltage on each lead equal. If only a single lead were passed through an electrostatic or electromagnetic field, the voltage induced in the lead would be a normal mode signal that would add to or subtract from the desired process signal. The use of twisted pairs and shielding provides the largest reduction of undesirable signals, particul arly those indu ced by electrostatic fields. The cable shield reduc es the magnitude of the field pre sen t, an d the twis ting of th e signal lea ds c auses the re ma ini ng field to induce common mode noise which is easily eliminated. Most AC power leads are twisted because this is an effective way to reduce signal interference. When AC power supply and return leads are spaced closely together and twisted, the electrostatic and electromagnetic fields surrounding each of the leads cancel one another. This action greatly reduces the noise available to be induced into signal leads.

7.5.0

SIGNAL CABLE INSTALLATION

The majority of instruments used in the process industry produce low level DC current or voltage signals. Ot he r ins tru men ts such as magn etic flowmeters, ultra-son ic level detectors, and radioactive sen sin g devices may produce signals th at a re AC cu rrents or voltages or high voltage DC, but before the process information contained in these signals is transmitted to other instruments in the loop, the signal is usually converted to a low level DC current or voltage. For this rea son , we will limit our discussion of signal tran smis sion lines to those that carry low level DC signals. Multiconductor cable is normally used for electronic signal transmis sion. The signal lead wire size ranges from 16 AWG to 24 AWG depending on the signal range used in the loop. Twisted pair cable can be purchased with 2 to 100 conductor pairs, either shielded or unshielded. Normal ly, one wire in each pair is eith er color-coded or nu mb er ed to allow easy identification of each pair. Othe r multiconductor cables are available, ag ain shielded or unshielded, tha t ha ve 2 to 100 individual conductors. Eac h wire in th es e cables is either color-coded or nu mb er ed at approx imately one-foot intervals . Signal cable is normally available in spools of 100, 500, or 1,000 feet. In large process plants with central control rooms, the signal leads to and from individual plan t ins tru men ts a re ru n to junction boxes located in the process area. Lar ge multiconductor cables carry the s ign als between the local juncti on boxes and the ce ntral control room. At the control room panels, the signal leads are terminated at terminal strips where individual panel mounted instruments are connected. Signal transmission lines can be run in overhead cable trays or wireways, or be run through rigid conduit ins tal le d overhead or burie d in tren ches . When runnin g sig nal cables, care should be taken that instrument power lines and signal lines are separated to minimize noise INSTRUMENT TRAINEE TASK MODULE 12309

introdu ction. A rul e of thum b to follow th at will minimi ze noise pickup when installi ng instrument lines, 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, and 2 feet from power utilit y dist rib uti on lines. Signal cables can be ru n toge ther in conduit bu t without other type wires. If inst alled in a tra y, a one-foot tra y ma y be used with th e signal lines separated from other low voltage lines, such as alarm or communication by a minimum of  six inches. Never in sta ll utility AC or DC power lines in th e same tr ay or conduit as signal or alar m lines. DC motor s start ing have caused inductive voltages high enoug h to activate ala rm circuits whe n th e wires are in the same conduit. Th e signal wires should be ru n as far as possible from electrical motors, generators, transformers, and other electrical noise producing equip ment. Precaut ions should also be ta ke n to ensure th at signal cable is protected from damage due to mechanical vibration, corrosive atmosphere, and rough handl ing. Rigid conduit is expensive, but it provides the be st possible protection of signal leads. 7.6.0

SHIELD TERMINATION

Shield quality is usu all y compromised at the termin atio n point. If the cu rr ent flowing on the outside surface of a shield is pinched down to a connecting wire, the field associated with this current can easily enter the inside of the cable. At the point of poor term inat ions, common mode voltage is generate d. When te rmi nat ing shields, specifically to walls, the best way to minimize noise is by the use of  backshell  connectors. Backs hell connectors will provide a continuous shield around the enti re cable. Where backshell connectors are not available, then straight connections from the braid to the wall will provide the best available form of noise protection. Another ter min ati on consideration is surface condition. Termi nations should never be mounted on pain ted surfaces. The ideal mounting surface would be a plat ed meta l surface. Ensure that the plated metal surface is protected against oxidation. 7.7.0

USE OF MULTIPLE SHIELDS

The use of guard shields in analog instrumentation does not provide high frequency noise shielding. To protect analog inst rum enta tio n against high frequency noise, a second externa l shield must be used. While the low frequency shield is groun ded where the signal grounds, the high frequency shield is grounded to the source ground and terminating bulkheads. This high frequency shield may be grounded at more th an one point. Multi-loop high frequency shield grounds reduces the loop areas that can couple to external fields.

APPLY PROPER GROUNDING AND/OR SHIELDING OF INSTRUMENT WIRING — MODULE 12309

Two common types of material frequently used as shield material are foil shields and braided cable. This section will pre sen t these two types and coaxial cabling; the la tte r being presented because it is common to find throughout process control systems.

8.1.0

FOIL SHIELDS

Aluminu m foil is frequentl y used as shield ma ter ial on shielded cables . This is because the foil itself is an excellent low frequency electrostatic shield. Foil wrap cables are not intended for the tra ns po rt of hig h frequency energy. This is because alu mi nu m foil has poor high frequency attenuation characteristics. The difficulty with usi ng foil shield is th at the y tea r easily and cannot be soldered. To en sure proper foil shield termination, a conductor known as a drain wire is used with the shield. To minimize the noise coupled to the conductor inside of the shield, the drain wire should be located external to the foil shield.

8.2.0

BRAIDED CABLE

Braided copper cable is the most commonly used she ath for shielded cables. The braid ing provides flexibility an d reaso nab le cost. Braidi ng is most effective as a very fine weave single braide d cable. Althou gh double braid is supe rior to single braid, single br aid cable is effective for most high frequency applications. Braided cable disadvantages include sheath gaps and developed voltage gradients. Gaps in the sheath promote electrostatic coupling of external fields to the conductors internal to the shield. When brai ded cable is grounded at both ends, low frequency signals will gene rate a voltage grad ien t due to th e curre nt flow throu gh the shield. To prev en t th e voltage gradi ent from developing, one end of the shield must be floated.

8.3.0

COAXIAL CABLE

Coax is used for th e tra ns po rt of high-frequency signals. The fields used in transm issio n are fully contained insi de the cable. This has nothin g to do with term ina tio n or grounding at either end. If th e cable is not term ina ted correctly then energy is reflected, but it is still inside the coax. The gro und ing of coax rel ate s only to how the si gna l is gene rate d and how it is terminated. When the signal re tu rn cu rr en t uses a conductor outside of the coaxial she at h the n th e cable is not used as coax. This extern al retu rn pa th implies that ther e is a field outside of the sheath.

INSTRUMENT TRAINEE TASK MODULE 12309

Shields that terminate on one end and that do not carry signal current are used as electrostatic shields (also called gua rd shields). These shields are connected to the zero poten tial reference poi nt for the signal. If the signal is grou nded then th is single point is that ground. Shields are often connected together an d grounded to a single point. This solution assumes no ground potentia l differences in the system. Single point shield groundi ng for each signal is the domain of analo g instrum entati on. Coax and multiple grounding are the domain of  high-frequency energy tra nspo rt. At low frequencies a shield grounded at both ends assum es a voltage gradient that is the same on the outside and inside surfaces of the shield.

As previously discussed, instrument shielding is necessary to prevent interference or noise from affecting signa l conductors contained within the shield ing mate ria l. But how do we effectively accomplish ins tru men t shielding? To properly shield inst ru me nt conductors, th ree basic rules must be followed: Rule 1 Rule 2 Rule 3

An ele ctrostati c shield enclosure, to be effective, should be connected to th e zero signal reference potential of any circuitry contained within the shield. The shield conductor should be connected to the zero signal reference pot ential at the signal-earth connection. The nu mb er of separat e shields required in a syst em is equal to the num ber of  independent signals being processed plus one for each power entrance.

By following these rules, effective instrument shielding can be implemented.

9.1.0

AMPLIFIER SHIELD

Consider an electrical device completely contained within a met al box. Fu rt he r assume th at the device is self-powered and no circuit conductors ente r or exit the box. Thi s circuit, shown in Figure 26, is completely shielded from external elect rostat ic influences. The symbology indicates th at a pot ent ial difference will exist between conductors 1 and 3. This potent ial difference will be amplified and appea r across conductors 2 an d 3. Conductor 3 is called the zero signal reference conductor as it is common to both the input and the outputs. Notice the significant mutual capacitances for an element of gain in Figure 26. The mutual capacitances form a feedback structure around the gain element and cannot be avoided. However, the feedback process can be eliminated by tying the shield enclosure to conductor 3. The res ult ant equi val ent circuit is shown in Figure 27. This follows the first rule for shielding. Res tat ed: an electrostatic shield enclosure, to be effective, should be connected to the zero signal reference potential of any circuitry contained within the shield.

APPLY PROPER GROUNDING AND/OR SHIELDING OF INSTRUMENT WIRING — MODULE 12309

INSTRUMENT TRAINEE TASK MODULE 12309

9.2.0

SIGNAL ENTRANCES TO A SHIELD ENCLOSURE

The gain element in Figure 26  is impractical without input and output connections. Conductors tha t carry the si gnal to and from any amplifier are called signal conductors. For examp le, conductors 1 and 3 ar e signal conductors. Signal conductors are usually enclosed in a braid ed metallic she ath or shield, an d thi s cable is called shielded wire. If two conductors are wit hin the shield it is called two-conductor shielded wire. This shielded wire is used to transport the signal from its source to the amplifier and can be thought of as an extension of th e electrostatic enclosure of  Figure 26. A shield enclosure is effective when Rule 1 is applied. This rule plac es no restriction on the shield potent ial relative to th e extern al environment. This is the key to connecting signa l conductors to a gain element. Since the shield must be at zero-signal reference potential , and since the signal is often derived from some reference point in the external environment, the shield is automatically defined at this external reference potential. Figure 28  shows a gain elemen t and its shield enclosure. The in pu t and output connections are two-wire shielded conductors. The inpu t signal zero is ohmically connected to an ea rt h point. When the shield is tied to this same ea rt h potenti al Rule 1 is applied and the syste m is correct.

In practice, the electrostatic enclosures shown in Figure 28  often parallel several external conductors. This is shown in Figure 29. For example, long runs of shielded wires are contained in raceways, in conduit, in floor wells, in parallel with other wires, in racks, or along floors. These neighbor ing conductors (grounds) are usually at differing potenti als. In parti cular , these poten tials are n ot the zero-signal reference potenti al of the shield enclosure. These neighboring potentials will cause currents to flow in the mutual capacitances between conductors.

APPLY PROPER GROUNDING AND/OR SHIELDING OF INSTRUMENT WIRING — MODULE 12309

9.3.0

SHIELD-DRAIN DIRECTION

Rule 1 requir es tha t the shield be connected to zero-signal reference potent ial. No state men t is included as to where this connecti on should be made. The connection is correctly made in Figure 28. This procedure ensures that parasitic currents will flow in the shield only and not flow in the signal conductors. The shield can be thought of as a drain path to carry unwanted current back to an earth point. 9.4.0

SHIELD CONNECTIONS - SEGMENTS

By Rule 1, the electrostatic enclosure should be at zero-signal reference potent ial. If th e shield is split in sections Rule 2 places a constr aint on the tre atm ent of thes e segments. The rule requires that the shields be tied in tandem as one conductor and then connected to zerosignal reference potential at the si gnal -ear th point. If the shield segment s are individually treated the difficulties can be expected. Shield connections that permit current to flow in an output or high-signal-level conductor are often ignored. The pickup here, as a percentage effect, is usually very low. Shield-drain processes in input conductors should be closely watched as the pickup here is subject to amplification. It is usually not too difficult to follow Rule 2 everywhere to avoid this and other difficulties that can result. INSTRUMENT TRAINEE TASK MODULE 12309