applying grounding and shielding for instrumentation.pdf

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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 grounding in an installa tion.

2. Identify the minimum requirements for shielding in an installat ion.

3. Properly terminate an equipment ground per drawing specifications.

4. Properly terminate an equipment shield per drawing specifications.

Prerequisites

Successful completion of the following Task Module(s) is required before beginning study of

this Task Module: Ins trumentat ion 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 workmay be reproduced in any form or by any means, including photocopying, without written permission of the publisher.



TABLE OF CONTENTS

Section

1.0.02.0.0

2.1.0

2.2.0

2.3.02.4.0

2.5.03.0.03.1.0

3.2.03.3.03.4.0

3.5.03.6.0

3.7.03.8.0

3.9.03.10.0

3.11.0

4.0.04.1.04.2.0

4.3.05.0.06.0.07.0.07.1.0

7.2.07.3.07.4.0

7.5.07.6.0

7.7.0

8.0.08.1.0

8.2.08.3.0

9.0.0

9.1.0

9.2.09.3.0

9.4.0

IntroductionGroundingGrounding for Fire Prevention

Grounding for Electrical Shock Avoidance

Grounding for Equipment Ground Fault Protection ..Grounding for Lightning ProtectionGrounding for Electrical Noise Control

Safety GroundsSignal GroundsSingle-Point Ground Systems

Multipoint Ground Systems

Hybrid GroundsPractical Low-Frequency GroundingHardware Grounds

Single-Ground Reference for a Circuit

Grounding of Cable ShieldsGround LoopsNoise

Capacitive-Coupled NoiseInductive-Coupled Noise

Directly-Coupled NoiseInstrumentation ShieldingElectrical Signal Noise

The Effectiveness of Shielding

Field Characteristics and Shielding MaterialShield Geometry

Noise ReductionSignal Cable InstallationShield Termination

Use of Multiple Shields

Signal Cable TypesFoil Shields

Coaxial CablePractical Instrument Shielding

Amplifier Shield

Signal Entrances to a Shield EnclosureShield-Drain Direction

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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 o

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): The conductor used to connect the noncurrent

carrying 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 current

carrying capacity to prevent the buildup of voltages that may resu lt 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, o

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 wi thin an established

period of time when a current to ground exceeds some predetermined value that is less than

that required to operate t he overcurrent protective device of the supply circuit.

Ground-Fault Prot ect ion of Equipment (NEC): A system intended to provide protection

of equipment from damaging line-to-ground fault currents by operating to cause adisconnecting 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 par t of any instrumentation installa tion. Proper

grounding and shielding procedures must be followed to ensure an effective and safe electrical

environment. This course covers the minimum requirements that mus t be met when

installing or working on instrumentation.

Grounding means a connection to earth. The connection can be via struc tu ra l 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 ear thed only in very specific ways. The other

earthed conductors form a grid that must eventually connect to the earthed power conductors.




All of these conductors form a grid that is an integral part of a grounding system. The

deliberate earthing of the power system provides:

1. Fire protection

2. Electrical shock avoidance

3. Equipment ground fault protection

4. Lightning 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 requirements or a limited

subset.

Proper grounding is a requirement of the National Electrical Code (NEC: ANSI/ NFPA-70).

This code does not address the issues of noise control or reduction. Specifically, it is not

involved with the performance of equipment, only its electrical safety. The systems 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 generates 1 kW. This he at could become a fire hazard.

Connections between conductors are apt to be a weak spot in a conductive pat h. Heat can

be generated in defective equipment or in equipment improperly operated. This heat can

ignite any nearby combustible material. If the circuits are located in meta 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 insulate all conductors carrying a voltage. This can

be accomplished by the use of insulating jackets and 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 hazard exists if a power conductor faults to its housing. At this moment the housingis 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 minutes depending on the magnitude of the fault current. 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.

Under no circumstances should these 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.

Insulation can be used to reduce shock hazard . Items with a lot of use wear out. Excessive

heat causes insulation to become brit tle and crack apart. A frayed cable can be a lethal object.

For example, a dangerous situation can occur when the safety conductor in a hand drill is

not connected. If the 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 equipment grounding

conductor should not be defeated. Many dea ths resul t each year from faulty equipment

grounding.

2.3.0 GROUNDING FOR EQUIPMENT GROUND FAULT PROTECTION

Equipment faults should not be allowed to persist . Consider an equipment housing tha t is

earthed but not grounded by a separate conductor. If there is a fault, the equipment housing

may be electrically "hot." If an overcurrent detector is not tripped, the excess current flow

th at 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.

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 potential of one-half the 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 currents in excess of 100,000 A. Currents of this magnitude can

destroy electrical equipment, damage structures, and electrocute humans and animals. It

is clear that some form of lightning protection should be placed in most facilities, particularly

where sensitive or critical electronics are operated. The best protection consists of providinga convenient and direct path for lightning current to flow to earth. This pat h should be

deliberately designed and installed. The NEC covers some aspects of this requirement, but

the controlling document is the National Lightning Protection Code (ANSI/NFPA-78).




The current need not flow in a circuit to do damage. The magnetic field near the path of

cur ren t flow is very intense. This rapidly changing field can induce large voltages into

sensitive circuits. If the lightning pulse should enter a grounding grid, the 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 high impedance results when there is a sharp bend or loop in the current path . If the

path is through steel encased in concrete, moisture in this path can turn to steam, which

can crack or damage the structure. The result ing explosion can st ar t a fire. If the lightning

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.

Lightning need not str ike a facility directly to cause damage to electronics. Ground potential

differences in the vicinity of a strike can exceed 10,000 V. If signal or power wiring is not

correctly handled then energy can enter a facility on these conductors and damage equipment.

Lightning-related injuries are rather rare. However, this is no reason to avoid lightning

protection issues in building construction. Attempts to provide lightning protection often

falls short. Even with good protection, lightning paths are often unpredictable and damage

can result.

There is little chance of testing for lightning safety. Facilities tha t appear safe may fail.

Good protection requires an understanding of bonding and low-inductance wiring.

2.5.0 GROUNDING FOR ELECTRICAL NOISE CONTROL

Every pair of conductors can support the transport of electrical energy. One of these

conductors can be a ground or the earth. Grounds include power conductors, safety

conductors, building steel, or utility conduits. These conductors make many connections to

the earth. Currents flowing in these grounds implies that there mus t be potential differences

between ground points. This multiplicity of grounds causes many of the noise problems

encountered in electronics. In general, these potential differences cannot be shorted out by

adding conductors. This is particularly true 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 with sound instrumentat ion processes may seek

solutions tha t create a hazard. Both issues need to be well understood. First, what constitutes

good safe power engineering and second, how noise-free systems can be built within this

framework.




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 must be designed.

One advantage of a well-designed ground system is that it can provide protection againstunwanted interference and emission. In comparison, an improperly designed ground system

may be a primary source of interference and emission.

Grounds fall into two categories: (1) safety grounds and (2) signal grounds. If the 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 earth potential. In many cases, a safety ground is required 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 the chassis is determined by impedances Z 1 and Z2

acting as a voltage divider.

The chassis could be a relatively high potential and be a shock hazard, since its potentialis determined by the relative values of the stray impedances over which there is very little

control. If the chassis is grounded, however, its potential is zero since Z2 becomes zero.

The right-hand diagram of Figure 1 shows a second and far more dangerous situation: a

fused AC line enter ing an enclosure. If the re should be an insulation breakdown such that

the AC line comes in contact with the chassis, the chassis would then be capable of delivering

the full current capacity of the fused circuit. Anyone coming in contact with the chassis and

ground would be connected directly across the 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 requirement 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 rns through 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

until the fuse or breaker opens the circuit. Since no load current flows in the safety ground,




it has no IR drop, and the enclosures connected to it are always at ground potential. TheNEC specifies that the neutral and safety ground shall be connected together at only one

point, and this point shall be at the main service entrance. To do otherwise would allow

some of the neutral current to return 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 represen tative of practical ground systems becauseit does not emphasize the importance of the actual path taken by the current in re turning

to the source. It is important to know the actual current path to determine the radiated

emission or the susceptibility of a circuit. To underst and the limitat ions and problems of

"real world" ground systems, it would be better to use a definition more representative of

the actual situation. Therefore, a better definition for a signal ground is a low-impedance

path for current to retu rn to the source. This "current concept" of a ground emphasizes the

importance of current flow. It implies that since current is flowing through 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 circuits. The magnetic or inductive coupling is proportional to loop area.

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. An important consideration in determining

this area is the ground path taken by the cur ren t in return ing to the source. Often this

is not the path intended.




In designing a ground it is impor tant to ask: How does the current flow? The path taken

by the ground current must be determined. Then, since any conductor-carrying current 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, the size of the system (self-contained or distributed), and other constraints, suchas safety. No one ground system is appropriate for all applications.

Signal grounds usua lly fall into one of three categories: (1) single-point grounds, (2)

multipoint grounds, and (3) hybrid grounds. Single-point and multipoint grounds are shown

in Figures 4 and 5, respectively. A hybrid ground is shown in Figure 6. There are two

subclasses of single-point grounds: those with series connections and those with parallel

connections. The series connection 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 manner 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 resistanceand inductance. At 11 kHz, a straight length of 22-gauge wire one inch abovea ground plane has more inductive reactance than resistance.

2. Two physically separa ted ground points are seldom at the same potential.

The AC power ground is of little practical value as a signal ground. The voltage measuredbetween two points on the power ground is typically hundreds 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 currents of circuits 1,2, and 3, respectively. Point A is not at zero potential

but is at a potential of

and point C is at a potential of




Although this circuit is the least desirable single-point grounding system, it is probably the

most widely used because of its simplicity. For non-critical circuits it may be perfectly

satisfactory. This system should not be used between circuits operating at widely different

power levels, since the high-level stages produce large ground currents which, in turn,

adversely affect the low-level stage. When this system is used, the most critical stage should

be the one nea rest the primary ground point. Note that 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. That is because there is no cross coupling between ground currents from

different circuits. The potentials at points A and C, for example, are as follows:

The ground potential of a circuit is now a function of the ground current and impedanceof tha t circuit only. This 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 the 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-wavelength. Not only will these grounds have large impedance,

but they will also act as antennas and radiate noise. Ground leads 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 thing as a single-point ground.




3.4.0 MULTIPOINT GROUND SYSTEMS

The multipoint ground system is used at high frequencies and in digital circuitry to minimize

th e ground impedance. In this system circuits are connected to the nearest available low-

impedance ground plane, usually the chassis. The low ground impedance is due primarily

to the lower inductance of the ground plane. The connections between each circuit and the

ground plane should be kept as short as possible to minimize thei r impedance. In very highfrequency 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 currents from

all circuits flow through a common ground impedance—the ground plane. At high

frequencies, the common impedance of the ground plane can be reduced by silver plating

the surface. Increasing the thickness of the ground plane has no effect on its high 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 atdifferent 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 ground. 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 thatcircuits of widely varying power and noise levels do not share the same ground re turn 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 three separate ground re turns, 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 ird "hardware" ground should be

used for mechanical enclosures, chassis, racks, and so on. If AC power is distributed

throughout the system, the power ground (green wire) should be connected to the hardware

ground. The three separate ground return circuits should be connected together at only one

point. Use of this basic grounding configuration in all equipment would greatly 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.




There are three signal grounds, one noisy ground, and one hardware ground. The most

sensitive circuits, the nine read amplifiers, are grounded by using two separate ground

returns . 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 return. 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 properlyconnected closest to the primary ground point. The hardware ground provides the ground

for the enclosure and housing. The signal grounds, noisy ground, and hardware 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 are usually mounted in relay racks or cabinets. These

racks and cabinets must be grounded for safety. In some systems such as electromechanical

telephone offices, the racks serve as the return conductor for relay 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.




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 grounding. The panel is strapped to the 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 make contact with the panel 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 current can return on the electronics

ground due to capacitive coupling between the rack and electronics. This capacitance should

therefore be kept as small as possible. Rack 2, on the right, shows an incorrect installa tion

in which the circuit ground is connected to the rack ground. Noise currents on the rack can

now return on the electronics ground, and there is a ground loop between points 1, 2, 3, 4,

and 1.




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 that there is no ground at all. Do not depend on sliding drawers, hinges,

and so on, to provide a reliable ground connection. When the ground is of a questionable

nature, performance may vary from system to system or time to time, depending on whetheror not the ground is made.

One piece of equipment used to check ground connections is the Kelvin Bridge. The Kelvin

Bridge is a portable instr ument designed to accurately measure 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.




Hardware grounds produced by intimate contact, such as welding, brazing, or soldering, are

better than those made by screws and bolts. When joining dis-similar metals for grounding

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.

When electrical connections are to be made to a metallic surface, such as a chassis, the metal

should be protected from corrosion with a conductive coating. For example , finish aluminumwith a conductive alodine or chromate finish instead 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 than one point. This condition is illus trated

in Figure 11; a signal source is grounded at point A and an amplifier is grounded at point B




Note that in this discussion an amplifier is generally mentioned as the load. The amplifie

is simply a convenient example, however, and the grounding methods apply to any type o

load. Voltage V G

represents the difference in ground potential between points A and B. I

Figure 11 and subsequent illustrations, two different ground symbols are used to emphasiz

th at two physically separated grounds are not usually at the same potential. Resistors R C

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 eliminate the noise

one of the ground connections must be removed. Elimination of the ground connection a

B means the amplifier must operate from an ungrounded power supply. It is usually easie

however, to eliminate ground connection A at the source.

The effect of isolating the source from ground can be determined by considering a low-leve

transducer connected to an amplifier, as shown in Figure 12. Both the source and one sid

of the amplifier input are grounded.

For the case where R C2

< R s

+ R C1

+ R L , the noise voltage V

Nat the amplifier terminals i

equal to

Consider the case where the ground potential in Figure 12 is equal to 100 mV, a valu

equivalent to 10 A of ground current flowing through a ground resistance of 0.01Ω. If R

= 500Ω, R C1

= R C2

= 1Ω, and R L

= 10kΩ, then the noise voltage at the amplifier terminal

is 95 mV. Thus, almost all of the 100-mV ground differential voltage is coupled into th

amplifier.




The source can be isolated from ground by adding the impedance ZSG,

as shown in Figure

13. Ideally, the impedance ZSG

would be infinite, but due to leakage resistance and

capacitance, it has some large finite value. For the case where R C2

< R s

+ R C1

+ R L

and

ZSG

> R C2

+ R G, the noise voltage V

Nat the amplifier terminals is

Most of the noise reduction obtained by isolating the source is due to ZSG

. If ZSG

is infinite,

there is no noise voltage coupled into the amplifier. If the impedance ZSG

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 the shield should be grounded. Figure 14 shows

the parasitic capacitance that exists between the amplifier and the shield. From the

equivalent circuit, it can be seen that the stray capacitances C3S

and C1S

provide a feedback

path from output to input. If this 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.



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By connecting the shield to the amplifier common, capacitance C2S is short-circuited, and the

feedback is eliminated. 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 whenthe signal circuit has a single-point ground. If the shield is grounded at more than one point,

noise current will flow. In the case of a shielded twisted pair, the shield currents may

inductively couple unequal voltages into the 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 resistance. But if the shield is to be grounded at only one point, where should

that point be? The top drawing in Figure 15 shows an amplifier and the input signal leads

with an un-grounded source. Generator VG1 represents the potential 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 that provides the noise coupling. The input shield may be grounded at any one of

four possible points through the dotted 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. This noise current flowing through the impedance of the signal 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 generated between the amplifier input terminals (points

1 and 2) is a noise voltage. With grounding arrangement B, 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 connection, too, is unsatisfactory. For ground connection

C, there is no voltage V12, regardless of the value of generators VG1 or VG2. With ground

connection D, a voltage is generated across the amplifier input terminals due to generator

VG1 and the capacitive voltage divider C1 and C2. The only connection that precludes a noise

voltage V12 is connection C. Thus, for a circuit with an ungrounded source and a grounded

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 input cable shield are aga in

shown as the 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 input




terminals. Therefore, 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.




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 currentfrom 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 system grounded at two different points with a potential difference between

the grounds.

As shown in the figure, this can cause an unwanted noise voltage in the circuit. The

magnitude of th e 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, 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 single-

point ground. Second, the effect of the multiple ground can be eliminated or at least

minimized by isolating the two circuits. Isolation can be achieved by (1) trans-formers, (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

signals. The common-mode noise voltage now appears across the windings of the choke and

not at the input to the circuit. Since the common-mode choke has no effect on the 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.




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 thousands of volts. The undesired common-mode noise voltage appears across the 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 potential to cause damage or malfunction is increasing 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

through 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 has led to the use of digital circuits with high frequencies tha t can be both asource 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

and may contribute addit ional noise. There are also the more conventional sources of noise

such as the opening and closing of relays, contactors, and circuit breakers , the 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 insulating material. When the wires are run together, a

capacitor is formed. A capacitor, if you recall, is nothing more than 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 the signal lead. Themagnitude of the undesirable signal is proportional 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 trength of the undesirable signal is inversely proportional to the

capacitive reactance of the parallel lines.




Capacitive reactance is the opposition to curren t flow by a capacitor. It is similar to resistance

in a DC circuit. Capacitive reactance is frequency dependent; as frequency increases,

capacitive reactance Xc 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 Normal Mode Voltage is "a voltage induced across the inpu t terminals."

• 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

represents the 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 componentswithin the recorder and case ground. The capacitors can charge through 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 have equal values of capacitance, then the voltage from each

signal lead to ground will be equal.

If only one of the signal leads was capacitively-coupled to the AC power lead, 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.




Figure 23 shows an equivalent circuit in which a resistor represents the capacitive reactance,

Xc, of C1 and C2.

As you can see, the capacitive reactances 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 Xcl and Xc2. Therefore, the larger Xc l or Xc2 become, or the smaller RL becomes,

the smaller the magnitude of the induced voltage becomes.




4.2.0 INDUCTIVE-COUPLED NOISE

Inductive-coupling occurs when signal lines are run paral lel to AC power leads or when signal

leads pass in the proximity of electric motors or generators. To understand 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 the left hand, as illust rated in Figure 24, one can determine the direction

of the lines of force. If the wire is held in the left hand, as shown in the figure, such that

the thumb points in the direction of current flow, the remaining four fingers indicate the

direction of the magnetic lines of force. If the current is continuously increasing, decreasing,

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

potential. To generate a voltage or electrical potential, there must be a conductor, a magnetic

field, and relative motion between the conductor and field. When a conductor moves through

a stat ionary magnetic field, an EMF is induced into the conductor. The energy of the magnetic

field causes electrons to move. If the ends of the conductor are connected outside the magnetic

field to form a closed circuit, current flows in the circuit. An EMF can also be produced

when a conductor is in the proximity of an expanding and collapsing magnetic field. In this

case, the magnetic field is moving relative to the conductor. This is the mechanism for

inductive noise coupling. Current passing through the AC power line is continuously

expanding 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




lead. Since the signal lead is a par t of a complete electrical circuit, a current will result

from the induced voltage. Further-more, this undesirable current is alte rna ting at the same

frequency as the power line current th at induced it. Large 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 ground loop is probably the most difficult circuit noise source to locate. Ground loops

can exist whenever interconnected, non-isolated instru men ts are grounded at more than one

location. Non-isolated simply means that 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 rument is connected to

the output circuit by a measurable resistance. If an interference potentia 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 equipment th at cause leakage currents through ground. 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

battery . This potential , called a galvanic potential , is developed when two dissimilar metals

come into contact in an electrolytic solution. Interference potentials can result from

thermoelectric potentials developed by the joining of dissimilar metals with a temperature

gradient.

Another directly-coupled noise source is leakage currents. Leakage cur rents are a resu lt of

poor insulation that allows current to pass from one lead into another or from a signal leadto ground. 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. Another source of leakage currents is through improper ly spaced

components within ins truments. 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 totally accounted for by the manufacturer. The instrument 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 the manufacturer is usua lly uncertain

of the type of environment in which the instrument will be placed. As such, the user of the

ins trument must be prepared to either: (a) evaluate 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.




Noise can be a major source of inaccuracy in measurement channels. Elimination of this

undesirable voltage or current, or at least its reduction to a tolerable level is necessary for

proper process control. Obviously, the best way to reduce unwanted signals with in an

inst rume nt loop would be to eliminate the source of the noise. For example, s ignal leads

could be relocated away from power leads or electrical machinery. Often, though, it is

impractical to eliminate the noise or the adverse effects caused by noise. One is using circuit

design tha t reduce of the effects of noise. This concentrates on methods ex ternal to theinstrument's electronic circuits that are used to reduce the magnitude of the noise induced

into signal leads. Several such methods are employed in instrumenta tion systems. The use

of shielding and shielded cables can be very effective in reducing the magnitude of noise

induced in signal leads. The use of twisted pai r cable for signal transmission is also an

effective way to limiting interference. In most cases, power leads are also twisted as a means

of reducing interference.

Other methods used in the industry to reduce noise are:

a. The use of filters (usually capacitors) to preven t noise from entering inst rument amplifiers.

b. Periodic insulation checks of signal cables to detect paths for leakage curren ts.

c. Detection and removal of ground loops.

d. Proper grounding of ins trumentation loops.

Therefore, the problem of noise removal can be attacked at two levels. One is noise

elimination; keeping any noise on the input leads from reaching 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 the plant from one control room. To provide this central control, process information must

be transmi tted over long distances. Many factors mus t be considered when designing 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 anexternal source, usually adjacent wiring or equipment. Noise or interference may take

var ious forms. It may be alt ernat 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 electrosta tic fields

into the signal lead. The second method is the inductive coupling of electrical energy from




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.

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. For electronic inst rumenta tion, a range 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 ranges used in the process industry have an elevated zero range. 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

missing signal. This provides an immediate indication of a failure and makes locating the

cause easier. In addition, an elevated zero will bias active electronic components into their

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 instrument and to reduce the power requirements of the instrument power supply.

DC current signal transmission has found the greates t acceptance in electronic process control

systems with the ranges of 4-20 maDC and 10-50 maDC being most commonly used. 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 . Current transmission systems are less

susceptible to induced noise than voltage transmission because current-controlled devices

have low input and output impedances. For the noise to develop a significant amount of

voltage drop across the low impedance, it would have to induce a sizable amount of current.

Contrast this to the characteristically high impedances of voltage-controlled devices; a much

smaller amount of induced current will cause a significant change in measured voltage.

However, 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. Current

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 system uses low voltage signal levels. The signal-to-noise ratio must be

relatively large, two or greater, to obtain satisfactory results . Shielding is a must in voltage

transmissions that extend over long distance.




Shielding is the use of a conducting and/or ferromagnetic (permeable) barrier between a

potentially disturbing noise source and sensitive circuitry. Shields are used to protect cables

(data and power) and electronic circuits. They may be in the form of metal barrier 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 strength, angle of incidence, and frequency of the time-varying magnetic field.

• The conductivity and permeability of the shield ing material.

• The physical geometry of the shield such as thickness and number of openings.• The grounding of the shield: at one end, both 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 neutralize the magnetic field tha t created them. The magnitude of these curren ts

depends on the conductivity and thickness of th e shield material . In determining the

effectivity of a particular material in shielding against noise at high frequencies, a property

known as skin depth, or skin effect, must be considered. Skin effect is the tendency of high

frequency AC current to concentrate on the conductor surface. This is due to the fact thatinductance is lower on the surface of the conductor. This phenomenon increases with

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 determining

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 ided copper shield th at surrounds the

insulated signal lead, signal lead, or signal lead bundle . A plastic or rubber insulator

surrounds the shield to protect it. Electrical conduit serves the same purpose as copper bra id

shielding, but it is not as effective.




Figure 25 is an illustrat ion of a signal lead surrounded 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 the signal carried by the signal lead. The electrostatic field developed

by the power lead is also terminated 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 signallead.

Electromagnetic shielding consists of iron that has high permeability. Permeability is the

ability of a material to conduct or carry magnetic lines of force. This property of a mater ial

provides a short circuit path for electromagnetic energy, and thus, prevents this energy from

influencing the signal carried by a signal lead. Electrical conduit, al though made of steel,

is not a good electromagnetic shield because of its low permeability. High permeability iron,

on the other hand, is usually 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 advantages. First, twisted pair cable is inexpensive and easy to install . 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, identical voltages are induced

in each lead. Because these voltages are, at any instant, of the same polarity in both the

positive and negat ive lead, they cancel each other at the line termination. These induced




voltages are a common mode signal. It should be pointed out tha t a twisted 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,particularly those induced by electrostatic fields. The cable shield reduces the magnitude

of the field present, and the twisting of the signal leads causes the remaining 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. Other ins truments such as magnetic flowmeters, ultra-sonic level detectors,

and radioactive sensing devices may produce signals th at are AC currents 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 reason, we will limit our discussion of signal transmission lines to those

that carry low level DC signals.

Multiconductor cable is normally used for electronic signal transmission. The signal leadwire 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. Normally, one wire in each pair is either color-coded or numbered to allow easy

identification of each pair. Other multiconductor cables are available, again shielded or

unshielded, tha t have 2 to 100 individual conductors. Each wire in these cables is either

color-coded or numbered at approximately 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

plant ins truments are run to junction boxes located in the process area. Large multiconductorcables carry the s ignals between the local junction boxes and the central 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 led overhead or buried in trenches . When running signal cables, care

should be taken that instrument power lines and signal lines are separated to minimize noise




introduction. A rule of thumb to follow that will minimize noise pickup when installing

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 utility dist ribution lines. Signal cables can be run together in conduit but without

other type wires. If installed in a tray, a one-foot tray may be used with the signal lines

separated from other low voltage lines, such as alarm or communication by a minimum of

six inches. Never insta ll utility AC or DC power lines in th e same tray or conduit as signal

or alarm lines. DC motors start ing have caused inductive voltages high enough to activate

ala rm circuits when the wires are in the same conduit. The signal wires should be ru n as

far as possible from electrical motors, generators, transformers, and other electrical noise

producing equipment. Precautions should also be taken to ensure that signal cable is

protected from damage due to mechanical vibration, corrosive atmosphere, and rough

handling. Rigid conduit is expensive, but it provides the best possible protection of signal

leads.

7.6.0 SHIELD TERMINATION

Shield quality is usually compromised at the termination point. If the current 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 terminations, common mode voltage is generated. When terminating

shields, specifically to walls, the best way to minimize noise is by the use of backshell

connectors. Backshell connectors will provide a continuous shield around the entire 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 termination consideration is surface condition. Terminations should never be

mounted on pain ted surfaces. The ideal mounting surface would be a plated metal 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 rumentation against high frequency noise, a second external

shield must be used. While the low frequency shield is grounded 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.




Two common types of material frequently used as shield material are foil shields and braided

cable. This section will present these two types and coaxial cabling; the latte r being

presented because it is common to find throughout process control systems.

8.1.0 FOIL SHIELDS

Aluminum foil is frequently used as shield mater 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 transport of high frequency energy. This is because aluminum foil has poor high

frequency attenuation characteristics.

The difficulty with using foil shield is that they tear easily and cannot be soldered. To ensure

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 sheath for shielded cables. The braiding

provides flexibility and reasonable cost. Braiding is most effective as a very fine weave single

braided cable. Although double braid is superior to single braid, single braid 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 tothe shield. When braided cable is grounded at both ends, low frequency signals will generate

a voltage gradient due to the current flow through the shield. To prevent the voltage gradient

from developing, one end of the shield must be floated.

8.3.0 COAXIAL CABLE

Coax is used for the transpo rt of high-frequency signals. The fields used in transmission

are fully contained inside the cable. This has nothing to do with termina tion or grounding

at either end. If the cable is not terminated correctly then energy is reflected, but it is still

inside the coax. The grounding of coax rel ates only to how the signal is generated and howit is terminated.

When the signal re tu rn cu rrent uses a conductor outside of the coaxial sheath then the cable

is not used as coax. This external retu rn pa th implies that there is a field outside of the

sheath.




Shields that terminate on one end and that do not carry signal current are used as

electrostatic shields (also called guard shields). These shields are connected to the zero

potential reference point for the signal. If the signal is grounded then th is single point is

that ground.

Shields are often connected together and grounded to a single point. This solution assumes

no ground potentia l differences in the system. Single point shield grounding for each signal

is the domain of analog instrumentation. Coax and multiple grounding are the domain of

high-frequency energy transport. 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.

As previously discussed, instrument shielding is necessary to prevent interference or noise

from affecting signal conductors contained within the shielding materia l. But how do we

effectively accomplish ins trument shielding? To properly shield inst rument conductors, th ree

basic rules must be followed:

Rule 1 An electrostatic shield enclosure, to be effective, should be connected to the zero

signal reference potential of any circuitry contained within the shield.

Rule 2 The shield conductor should be connected to the zero signal reference potential at

the signal-earth connection.

Rule 3 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.

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

9.1.0 AMPLIFIER SHIELD

Consider an electrical device completely contained within a metal box. Fu rthe r assume that

the device is self-powered and no circuit conductors enter or exit the box. This circuit, shown

in Figure 26, is completely shielded from external electrostat ic influences. The symbology

indicates that a potent ial difference will exist between conductors 1 and 3. This potential

difference will be amplified and appear across conductors 2 and 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 mutualcapacitances 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 result ant equivalent circuit is shown in Figure 27. This follows the first rule for

shielding. Restated: an electrostatic shield enclosure, to be effective, should be connected

to the zero signal reference potential of any circuitry contained within the shield.




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 signal to and from any amplifier are called signal conductors. For

example, conductors 1 and 3 are signal conductors. Signal conductors are usually enclosed

in a braided metallic sheath or shield, and this cable is called shielded wire. If two conductors

are within the shield it is called two-conductor shielded wire. This shielded wire is usedto transport the signal from its source to the amplifier and can be thought of as an extension

of the electrostatic enclosure of Figure 26.

A shield enclosure is effective when Rule 1 is applied. This rule places no restriction on

the shield potential relative to the external environment. This is the key to connecting signal

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 element and its shield enclosure. The input and output connectionsare two-wire shielded conductors. The inpu t signal zero is ohmically connected to an earth

point. When the shield is tied to this same earth potential Rule 1 is applied and the system

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 neighboring conductors (grounds) are usually at differing potentials. In

particular , 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.




9.3.0 SHIELD-DRAIN DIRECTION

Rule 1 requires tha t the shield be connected to zero-signal reference potential. No statement

is included as to where this connection 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 potential. If the

shield is split in sections Rule 2 places a constraint on the treatment of these segments. The

rule requires that the shields be tied in tandem as one conductor and then connected to zero-

signal reference potential at the signal -ear th point. If the shield segments are individuallytreated 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.




Rule 2 can be followed when a two-conductor (twin-axial) shielded cable is used. Single

shielded wire (coaxial cable) obviously forces the outer conductor to be both a shield and a

signal conductor. If noise current flows in the outer conductor, a noise voltage is usually

coupled to the signal. Fewer problems result when output cables are coaxial bu t crosstalk

problems can still exist.

Triaxial cable is also a shielded cable type. The inner conductor is a signal conductor. Thefirst shield functions as a signal conductor and as a coaxial return, and the outer shield

functions as a dra in for unwanted noise current flow. It is assumed th at the two shields

are insulated from each other.

SUMMARY

Grouding and shielding is an important part of any instrumenta tion installation. Propergrounding and shielding procedures must be followed to ensure an effective and safe electrical

environment. This course covered the minimum requirements tha t mus t be met when

installing or working on 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.

At high frequencies and in digital circuitry, a multipoint ground system should be used.

A low-frequency system should have a minimum of three separate ground re tu rns. Theseshould be: signal ground, noisy ground, and hardware ground.

The basic objectives of a good ground system are to minimize the noise voltage from two

ground currents flowing through a common impedance.

Grounding schemes are necessary for proper and safe operation of ins trumentation circuits,

and correct procedures must be followed to ensure compliance with NEC requirements.

References

For advanced study of topics covered in this Task Module, the following works are suggested:

Grounding and Shielding in Instrumentation, Third Edition, Wiley 1986 by Ralph Morrison

applying grounding and shielding for instrumentation.pdf

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