8 10 - University of the Witwatersrand

All techniques for measuring earth resistance are made by way of

circulating a test current between the earth system under study and r

remote electrode, measuring at the same time the change of voltage of the

earth system relative to a potential reference electrode.

The most commonly used method for the measurement of earth-electrode

resistance and also one which has been found to be most reliable is the

so-called "fal1-of-potential" method. This method, Figure 8.10, requires

two reference earth rods, refer' J to as potential electrode P and

auxiliary current electrode C. A. alternating test current is circulated

between the electrode system E and C. The potential drop V is then

measured (usually using a null-balanced technique of potential

measurement), between E and P. By repeating these measurements for a

variety of distances 'd' of the electrode P, away from the earth system

E, it is possible tc derive a resistance curve of the form shown in Figure

8 .10 .

The above method encounters difficulties when the resistance of a large

area earth-electrode system, consisting of a number of rods, trenches,

etc., all connected in parallel, is required to be measured. A major

problem has been the need for large distances (about 1 km) between the

auxiliary current electrode C and the earth system E so as to validity

assumptions. This introduces such problems as finding a clear run for

the required length and also long leads which possess reactances and which

can cause difficulties vith measuring equipment. To solve these problems,

Tngg^10^ has devised the "intersection Method" which eliminates the use

of very long leads. It is concluded that a more accurate measurement of

resistance is achieved by placing the electrode P at a distance which is

62% of the total distance D between the earth system E and the auxiliary

current electrode C, as illustrated in Figure 8.10.

An additional guide to accurate measurement is the minimum requirement

that the distance D be five time the diagonal across the earth-electrode

system being measured. Furthermore, it has been sugge^ ed that tests in

iwo directions at right angles be made, both as a check and to enable the

electrical centre of the earth system to be determined more accurately.

However, both the above points may be difficult to meet in practice.

Th* Earth Electrode System251

FIGURE B. 10 FALL-OF-POTENTIAL EARTH

RESISTANCE MEASUREMENT

The Earth Electrode System 252

Field testing of large earthing systems, which include extended earths,

by the usual earth resistance meter (earth meggar) may fail to give

realistic values because of the inductive component in the earthing

impedance or the increase of ac resistance over dc resistance (normally

this happens when the earth resistance is less than 0.5 Q), and due to

inductive interference in the leads used for the measurement. In these

cases, it has been suggested^111 ̂that the heavy current injection method

be applied, at or close to, the power frequency. For a more complete

treatment of this method the reader is referred to this paper by Seljeseth

et al^11^ . Other methods which are variations of this method include

the 'synchronous power-frequency method', 'the beat-frequency method' and

'the interference compensation method', which are all described in the

same paper.

An interesting method described in a paper by Sarmiento et al^187 ̂ is the

frequency scanning method which falls in the class of low current

injection methods. This method uses digital techniques and allows for a

complete plot of impedance and phase angle over a wide frequency range.

Figure 8.11 illustrates the general lay-out for the frequency method.

The signal source consists of a pseudorandom noise generator, noise

conditioning filter and a power amplifier of about 1.5 kW. The bandwidth

of the noise source is set from 0 Hz to 400 Hz. An external filter is

used to limit the noise signal to a range of 10 to 400 Hz in order to

protect the power amplifier output transformer and the power transistors

from the effects of low frequency saturation. The digital signal analyser

has to be a dual channel instrument. With the instruments set up as shown

in the figure, the current and voltage signals are first filtered, then

digitised and transformed in the frequency domain through a FTT (fast

fourier transform) routine. Using the transfer function of the spectrum

analyser, the display (either CRT or X-Y plotter) will show the impedance

magnitude and phase over the selected frequency range.


Figure 8.11 Frequency Scanning Method

8.8 CORROSION AND BONDING CONSIDERATION

The problems of bonding and corrosion of earth systems require particular

consideration due to the harsh nature of the industrial plant environment.

To ensure the integrity of the earth system as well as the connections

made to it, such factors as electrode material, soil resistivity and

environment conditions should bo taken into account. The subject of

corrosion and corrosion prevention is a science unto itself and is beyond

the scope of this study. Salient points are noted below and should be

considered when engineering an earth system.

The causes of corrosion of both electrodes and bonds are manifold and

include the following^3 :


1. Differential aeraticn of the soil - arising during the backfilling

process and through uneven distribution of moisture in the vicinity

of the electrode.

2. The acidity and chemical content of the soil as well as the presence

of foreign materials including cinders, »crap metal, or organic

material.

3. The presence of stray electric currents - particularly dc - perhaps

originating from nearby traction systems.

4. The interconnection of dissimilar metals in the soil or in the open,

where moisture is present.

Three basic techniques can be used to lessen the corrosion rate of buried

metals. The obvious method is to insulate the metals from the soil. This

method is however not a acceptable corrosion preventive for earth

electrodes.

The '.e^ond technique, and one which should be considered at the design

stage after a site survey, is avoiding the use of dissimilar metals to

reduce galvanic corrosion. For example, if all metals in contact with

the soil are of one type (such as iron or lead or copper), galvanic

corrosion is minimised. Copper is a desirable material for the earth

electrode system; apart from its high conductivity, the oxidation

potential of copper is such that it is relatively corrosion resistant.

Since copper is cathodic relative to the more common structural metals,

its corrosion resistance is at the expense of other metals, such ac iron

and aluminium. As an alternative to copper, steel rods may be used,

especially if the earth electrode system is installed in an area where

there are known to be many buried galvanised steel pipes, buried

structural steel, etc.

The third technique, for combating corrosion caused by strong direct

currents and dissimilar metal unions is commonly called cathodic

protection. Cathodic protection may be implemented through the use cf

sacrificial anodes or the use of an external current supply to counteract

The Earth Elertrode System255

the voltage developed by oxidation. Discussion of cathodic protection

is beyond the scope of this study. For detailed information refer to the

(1891

relevant texts and the paper by Nelson et al

Another approach which offers advantages in corrosive soils is the use

of concrete encased galvanised steel electrode systems. In addition to

all of this, surface connections should also be protected from moisture

and made accessible for inspection. Bonds are best brazed or welded and

a number of specialised techniques are available for this purpose (See

Chapter 9 on bonding).

8.9 DESIGN AND INSTALLATION GUIDELINES

1. Establish the design objectives in terms of required value of earth

system resistance. As discussed in Section 8.1 the earth electrode

system for a facility should exhibit a resistance to earth of 5 ohms

or less. However, it should be noted that the final value obtained

weald be dictated by such factors as soil resistivity, moisture

content, rock formation and landscape features, as well as r cost

analysis which would take account of the costs of various alternative

design approaches.

2. Considering the relative advantages and disadvantages given in Table

8.6, select and configure a type of earth system most appropriate for

meeting the functional requirements of the facility at the site under

construction. In addition, cognizance should also be taken of such

factors as the final leakage resistance, the geometric disposition

which influences surge impedance, the resistivity depth profile and

the personnel safety considerations as well as the need to minimise

surface potential gradients, particularly during lightning

conditions.


Advantages Disadvantages

Vertical Rods

Horizontal Grid

Plates

Horizontal Wires

(Radials)

Incidental

Electrodes

(Utility pipes,

building founda

tions, burled

tanks)

TABLE 8.6 OF THE

Straightforward design.

Easiest to instaH (parti

cularly around an existing

facility.) Hardware read

ily available. Can be ex

tended to reach water

table.

Minimum surface potential

gradient. Straightforward

Installation if done before

construction. Can achieve

low resistance contact in

areas where rock formations

prevent use of vertical rods.

Can be combined with verti

cal rods to stabilize resis

tance fluctuations.

Can achieve low resistance

contact in limited area.

Can achieve low resistance

where rock formations pre

vent use of vertical rods.

Low impulse impedance.

Good RF counterpoise when

laid in star pattern.

Can exhibit very low resis

tance (if electrically con

tinuous. Generally lowest

initial cost (borne by

others).

RELATIVE ADVANTAGES AND PRINCIPLE TYPES OF EARTH

(SOURCE: DENNY*3))

High impulse impedance.

Not useful where large

rock formations are

near surface. Step

voltage on earth sur

face can be excessive

under high fault cur

rents or during direct

lightning strike.

Subject to resistance

fluctuation with soil

drying if vertical rods

not used.

Most difficult to

install.

Subject to resistance

fluctuations with soil

drying.

Little or no control

over future alterations.

DISADVANTAGES ELECTRODES


3. The general electrode arrangement should broadly conform to the

perimeter of associated structures on the site - as shown in Figure

8.12. The minimum spacings between rods should exceed twice the

individual rod lengths. The rods should preferably be copper-clad

steel ("Copperweld") or stainless steel. For horizontal conductor

(trench earth), preferred material is electrical grade copper and the

minimum cross-sectional area should not be less than 50 mm1 or

diameter not less than 9.5 mm. Rectangular conductors should

preferably be about 30mm x 3mm or 50mm x 3mm.

A. Where two or more structures or facilities are located in the sane

general area and are electrically interconnected with signal,

control, and monitor cables, a common earth electrode system may be

provided, or in the case of extended inter-connections, separate

earth electrode systems should be bonded with low-impedance buried

horizontal trench earth systems. Examples of the two approaches are

shown in Figure 8.13. Note, however, that the screens of data, signal

or control cables should not be used as the only interconnecting

conductors between two structures, at least one dedicated

interconnecting conductor should be connected in parallel between the

sa*ne points.

5. Access to the -«arth electrode system for the purpose of periodic

measurements should be provided at certain strategically chosen

positions in the system.

6. All joints in the earth electrode system should preferably be brazed

or welded and proprietary methods are available for this purpose

("Cadweld", for example). Bolted connections should be used where

these are accessible for regular inspection and testing and where such

connections will not be subjected to a corrosive environment.


The Earth Electrode System

259

\ r — r----BDNDED IF

-I I2SJ ]------•

AUXILIARY STRUCTURE- NECESSARY

RADIALS —---- DRIVEN ROD

LOOP TRENCH EmRTH

a) ADJACENT STRUCTURES

\\

A\

\

b) INTERCONNECTED SYSTEMS

FIGURE 8.13 ELECTRODE CONFIGURATIONS FOR ADJACENT AND INTERCONNECTED STRUCTURES


CHAPTER 9

Chapter 9

261

9.0 GROUNDING OF CONTROL SYSTEMS

9.1 INTRODUCTION

This chapter is essentially concerned with the aspects of implementing

the concepts, tachniques and guidelines developed in earlier chapters.

To ease understanding, electronic systems are classified as either

isolated or clustered systems. The grounding of typical process '•.ontrol

system configurations - i.e. centralised, decentralised and distributed

systems * are analysed in terms of these two systems. It is shown that

most system grounding requirements can be viewed as an exercise in the

grounding of a majot clustered system. This leads to the presentation

of grounding guidelines for clustered system, which include signal

grounding networks, computer grounding, I/O rack grounding and facility

grounding. Bonding is then examined and practical guidelines suggested.

9.2 SYSTEM GROUNDING REQUIREMENTS

The term system is wadely used to mean many things and generally

encompasses what the user wishes. It could, for example, represent

electrical circuitry totally within the confines of a case, cabinet, or

rack, or it could refer to an extended collection of equipment racks or

consoles distributed over a wide geographical area. In the process

control sense the term system would represent the full collection of items

named in Chapter 2, and would include sensors, actuators, signal

conditioners, process control units and data processing units. These

items can be configured in many different ways. It can be centralised,

decentralised or distributed (hierarchical), as illustrated in Figure

9.1. These configurations refer to both functions and to location.

Grounding of Control Systems 262

Grounding of Control Systems

263

FIG

URE

9.1

- DIF

FERENT

SYSTEM

ST

RU

CT

UR

ES

In any process area the sensing and actuation is always done locally.

The ellipsoids in Figure 9.1 refer to the different control system

functions which can be functionally and/or geographically distributed.

Of importance is the aspect of geographical distribution since this would

determine the earthing and grounding requirements. Jn general, these

requirements will depend upon the type of system configuration.

One way of distinguishing between different types of systems is to examine

the manner by which power is obtained and how the equipment elements are

interconnected with each other and with other systems. These two

considerations form the basis for the rationalisation of the earthing end

grounding requirements of the different types of systems. To ease

analysis of systems, two distinct types of systems are identified; namely

isolated and clustered systems. These two systems form the building

blocks upon which rationalisations of the earthing and grounding

requirements of complex system configurations can be accomplished. The

properties of these two systems and the associated grounding issues are

presented below.

9.2.1 ISOLATED SYSTEMS

An isolated s y s t e m ^ is one in which all functions are accomplished with

one equipment enclosure. Typical examples are handheld calculators,

desktop computers (off line), radios and television receivers. A

characteristic feature is that only a single power supply housed within

the enclosure is associated with such systems. For personnel and/or

lightning protection, only one ground connection is required which could

be structure or earth ground. As the name suggests, no outside

interconnection (power or signal) with other equipment or devices, which

are grounded, are present or needed.

The grounding requirements for isolated systems are as illustrated in

Figure 9.2. This typically includes a three wire input where the third

wire (green or earth) provides the ground, if single-phasc j,owered.


Figure 9.2 Grounding of AC Powered Isolated System

Refer to Figures 7.2 and 7.3 of Chapter 7 for the required earthing and

grounding configurations for single-phase and three-phase systems,

respectively. To prevent flashover or arcing, good practice dictates that

a safe distance between isolated systems and lightning-down conductors

should be maintained (See Chapter 6). Also, adequate screening and

separation must be provided in a very high RF field strength environment.

9 .2 .2 CLUSTERED SYSTEMS

A clustered system*' ̂ is characterised as having multiple elements, such

as equipment racks and consoles, in a localised area. Figure 9.3

illustrates a typical clustered system.


U j

_Jo

z

□u

3:u

f-

</)

>-

(/)

n

0-

a:

□o

a

u

_ IA © j <

/ “

sW

E»

S2C/3

aEdOS

w

E-CO

D

i—J

o

CO

05w

OS

tD

U

»—<

fcl


266

Examples of clustered systems Include minicomputers, medium scale data

processors, and multi-terminal processing systems.

A distinguishing feature of a clustered system is that each element is

powered from one common power source, i.e. a generator or a single ac

power connection. Multiple interconnecting cables (signal, power and

control) exist between the different units and equipment but not with any

other system, i.e. another system in another building. Thus, a clustered

system is a self-contained system that requires no interaction with any

other system other than between the members of the system itself.

Grounding for clustered ayeterns requires that only one connection be made

to facility (structure) ground to realise personnel safety and lightning

protection requirements. Again, the earthing and grounding arrangements

illustrated in Figures 7.2 and 7.3 for single-phase and three-phase

networks apply. The safety ground conductor connected to the equipment

case would provide the necessary structural connections, as illustrated

in Figure 9.4.

The signal ground referencing scheme us«d ' tween the elements of the

system would reflect the particular signal characteristics (frequency,

amplitude, etc) of the various pieces of equipment (See Chapters 3 and

5). This could be single-point, multiple-point or hybrid grounding. A

more detailed discussion of signal grounding of clustered systems is

presented from Section 9.6 onwards.

9.3 GROUNDING OF CENTRALISED SYSTEMS

A centralised system, as the name suggests, is functionally and/or

geographically centralised where all process measurements are biought

into a central-informat ion processing centre. This system is clearly

different from an isolated or clustered system in that integral elements

of the system such as transmitters, sensors and actuators extend out from

the central area, usually at long physical and electrical distances.

Grounding oi Control Systems 267


268

FIGURE 9.4 POWER GROUNDING OF CLUSTERED SYSTEMS

The important point to note about this system is that it is powered by a

single power source and connections are not made any where except at the

main element. In addition, extended elements obtain their power from the

central element.

The grounding of a centralised system should be viewed as an exercise in

the grounding of a complex central or primary element and as such could

be thought of as a major clustered system. Hence, the grounding

philosophy set forth in Chapter 7 and the interference minimisation

techniques presented in Chapters 4 ana 5 should be applied. In addition,

the guidelines presented in Section 9.6 should be observed.

Depending upon the operating frequency ranges and signal levels of the

extension elements and the characteristics of the EM environment, either

a single-point (starpoint ground) configuration or a multiple-point

configuration may be required. Since >.he operating frequency range of

transducers and sensing elements are generally narrowband and

low-frequency, a single-point grounding system is desirable. However,

depending on the EM environment, multiple-point grounding may be

required. Though not good practice in general, such multiple-point

grounding can be effective in some high energy cases. Thus each situation

should be considered on its own merit and the best configuration

determined experimentally.

Sxnc« the central element forms the reference base, all the extended

elements should be floated or balanced. Also field wiring should be

twisted pair or balanced lines. Shield integrity should not be

compromised. The good practice techniques suggested in Chapters 4 and 5

should be applied. In more critical circuits, isolation, shielding,

filtering and mode conversion must be considered.

Grounding of Control Systems 2b9

9.4 GROUNDING OF DECENTRALISED SYSTEMS

In a functionally decentralised system, the information processing is

also done locally as the sensing and actuating for each control loop or

group of loops. Thus several local systems are contained and operating

in the same general process area. Typically, decentralised systems share

the primary power sources. The grounding guidelines presented for

centralised systems would apply to each localised system. However, these

systems may be located in an inherently noisy environment and thus run a

high risk of interfering with each other and being susceptible to

interference i,z& cility noise and the external environment. Thus, in

addition to having grounding requirements like those discussed for

distributed systems (See in Section 9.5), additional isolation, shielding

and filtering ars necessary to minimise system interference.

9.5 GROUNDING OF DISTRIBUTED SYSTEMS - HIERARCHICAL

Distributed systems, as already noted in Chapter 2, are systems which are

functir 'ally and geographically distributed on site. Typ'-.ally th-*s

means physical separation of major elements in a way that requires

equipment or cluster of equipment to be variously fed from different pow«.r

outlets, branch circuits, different phases of the line or even different

transformer banks. This could also mean distribution ol equipment in a

plant close to the process.

Distributed systems are characterised by multiple conductor (signal, data

and control) paths which typically leave a particular system element and

extend beyond the earth line area of such a system to connect to another

system element with its own earth system. In such cases conductor lengths

are likely to be greater than X/15 at frequencies where an interference

threat exists. Other examples besides the distributed process control

system include environmental monitoring and control, communications

switching and computer nets.


Effective grounding of distributed systems to achieve the required safety

and lightning protection for equipment and personnel, while minimising

noise and EMI, requires cereful application of the principles set forth

in previous Chapters, particularly Chapters 4, 5 and 6. Due to the

variety and multiplicity of configurations available on site, it is not

considered prudent to describe a stereotyped network. However, some

general guidelines for proceeding are presented.

In any new installation planned for distributed control careful

consideration should be given to how control is tc be distributed. This

is essential because this will not only help resolve the earthing and

grounding issues but also result in cost savings through proper

installation, which is the real cost. Thus, it is recommended that I/O

clusters be assembled together to minimise wiring to sensors and actuators

while also maintaining closely interdependent I/O within the cluster.

Also one or more controllers should be specified at the centre of each

cluster for local control at that location. This then constitutes a major

element where the different types of equipment is essentially located at

a particular location. Major elements in the plant can then bo

interconnected via a network (serial hiway) to provide for interdependent

I/O between the elements. The earthing and grounding of a major element

should then proceed along the following lines.

Consider each major element as a clustered system where the grounding

practice should be applied along the lines discussed in Sections 9.2.2

and 9.6. In addition, each major element could be considered as a

centralized system and signal I/O grounding and shielding should observe

the guidelines suggested in Chapter 5.

In the case of inter-element I/O, each and every signal port on a

clustered subsystem that must interface with other portions of the total

system, i.e. other clustered subsystems, should be viewed as having tc

interface with a noisy "outside” world. As such, the techniques discussed

in Chapters 4 and 5 for separation, Isolation, and control of unwanted

coupling of radiated and conducted interference into the signal paths must

be fully employed. In addition, all cables are required to be shielded

and adequate protection applied to limit lightning induced surges into


equipment (Refer to Chapter 6). Figure 9.5 illustrates protection of

low-level analog signals, which extend beyond the facility earth area,

against indirect lightning induced transients. The cable shield is

grounded directly at the signal termination point (equipment area) and

over a gas arrestor at the transducer end (to prevent hum during normal

operation). Obviously, discretion will be necessary and thus each signal

port should be considered on its own merit.

A particular case in point is *hat of clusters of equipment separated by

short distances, e.g. 200m. and concentrated in a localised general area,

as illustrated in Figure 8.14. The earthing requirements should follow

the general recommendations given in Chapter 8. Furthermore, where

practicable, the structures should share a common earth ground system to

minimise voltage differentials between structures. Grounding, bonding,

and protection (lightning) considerations for cables between structures

should folk.w the recommendations given in Chapter 6.

Of particular concern in a distributed system is the need to maintain

electrical integrity of the network in an inherently noisy industrial

environment. The choice of a network media should primarily be dictated

by consideration of the level to which it is immune to electrical noise.

Twisted pair is the least noise-immune; it relies on the network

receivers' ability to reject noise that appears equally on both

conductors. Unfortunately, the conductors do not always pick up noise

equally, causing unbalanced signal distortion before the signal reaches

the network receiver. Adding a shield to the twisted pair provides

another reference for noist rejection (earth ground), and another

"antenna"; it still is susceptible to electrical noise generated by

starters, welders, relays, and similar equipment. As a result, twisted

pair is only recommended for the less-noisy office environment, where the

implications of a network failure are not as drastic as they are for

control.

A acceptable choice for a network medium is coaxial cable which has

significantly better noise rejection capability than twisted pair.

However, < ’imitation of coax is that it is single-ended, and requires

transmitter impedance matching to avoid signal reflection.

Grounding of Cowtrol Systems272

sr.*

Grounding of Control Systems273

FIG

URE

9.5

ANALO

G

SIGNAL

PROTECTIO

N

OUTSID

L

TH

EEARTHIN

G

AR

EA

Furthermore, since the shield is part of the signal path, noise currerts

may flow in the shield in a strong EM environment. The alternative is

to consider twinax cables which, however, are more costly and thus less

preferred.

The grounding of coax cables should conform to a single-point arrangement

as outlined in Chapter 5. However, a prime concern is that of protecting

the coax against lightning surges. Since the network cable must span the

entire plant area, making it vulnerable to lightning damage (which is

intolerable in a distributed control system), additional protection is

required. In particular, where the voltage between core and sheath of

the coaxial cable is likely to exceed the acceptable levels for the

connected equipment a voltage limiting device is required. The choice

of device should depend on the frequency, i.e. devices with appreciable

capacitance are not suitable at high frequencies. Thus low capacitance

devices, typically 3 - 100 pF, sho*.Id be applied for protection against

lightning surges. For protection details refer to Chapter 6.

Fiber optic cable as a network medium for harsh environments is the most

ideal since noise rejection is not an issue. Hence, baring such problems

as handling, termination and cost, fiber optic cables in highly

recommended.

9.6 GROUNDING OF CLUSTERED SYSTEMS

The following discussion make specific reference to process control

computer systems which is considered to make up the clustered system.

This is purely intended to clarify the earthing and grounding issues

relating to such systems which is the subject oi discussion in this study.

Of prime importance in any process control computer system is the power

source. Since the clustered system forms the "nerve centre" of the plant,

the electric power serving the process control computer system must be

as "clean", stable, and reliable as> possible. This means the system must


be protected against power outages, voltage fluctuations, distortions,

transients, surges, and spikes. These problems can occur in a process

or manufacturing plant no matter if power is supplied by the utility or

by an on-site generator. Various methods exists to "clean" up line power,

eliminate noise and to provide backup power in emergencies. These include

isolation transformers, voltage regulators, line conditioners, motor

generators, and uninterruptible power sources. Figure 9.6 illustrates a

typical configuration for providing clean, reliable p^wer for a process

control computer system.

The power system shown in the figure has two separate feeders, an

uninterruptable power source (UPS) and an isolation transformer. To

protect tilt system from high-speed transients and high-frequency

common-mod* and differential-mode noise, a isolation transformer is

required. Such a transformer has physically separated primary and

secondary windings and an electrostatic (Faraday) shield between the

windings. Most modern shielding methods can achieve a common-mode

reject n ratio of 10 000 000:1 or 140 dB. Typical specification figures

when selecting isolation transformers include^7^^:

o Input-to-output, capacitance of 0.005 pF or less;

o Leakage resistance of 1000 MB or greater between the primary and

secondary windings or between either windings and ground.

In addition, the following rules should be followed when setting up a

power source for process control computer systems

1. Primary power should come from the most reliable source available.

2. A second power feed, completely separate as to source and

distribution, should be provided for backup.

3. The power line feeder to the computer and instrumen ition system

should originate at the plant substation or main service entrance.


w

H

sCO

02

fcu

E-

CL,S

o o

CO

CO

w

u

o05CL

05 O

k-

o

:z;I—I

C l5S

o05O

w

u

05D

O

CO

05

o a. u

<

I

CD05w

055o

>— i


276

4. The computer system should be the only equipment that the feeder services.

5. Do not route the feeder close to other lines that supply noise-generating equipment.

6. The feeder should have enough capacity to accommodate future load increases.

The grounding of the clustered system, i.e. the cluster of process contra'1 computer equipment, should be in accordance with the recommendations given in Chapter 7, i.e. an isolated single point ground system is required. This means that the system's ground connects to earth ft only one point as illustrated in Figure 7.6 and Figure 9.6. This point may be facility earth ground or a dedicated earth rod.

As discussed in Chapter 7, two ground systems are desirable, namely, safety/equipment ground and signal ground. The signal ground referencing scheme used between the elements within the systems should reflect the particular signal characteristics (frequency, amplitude, etc) of the various pieces of equipment. This could be single-point or multiple-point grounding. However, since low frequency circuits and systems are particulaily susceptible to power line related noise and other low frequency interference, a single-point ground system should be provided.

9.6.1 LOW FREQUENCY SIGNAL GROUND NETWORK

The low frequency (below 1 MHz) grounding network for a facility should be configured as an isolated tree, like that shown in Figure 9.7. As shown, such a systec has grounding branches that serve various parts of the computer and instrumentation system.


“ z z y u □X 2; »-HH- £ I- □ h C

ZD Oo a a( - Ul _J

<rorZDy- o o z ~Z> ZDa: □t— O'</) LD

a.

*cn£w2

ZD001o

3z0I—I1/5>“■u2 Ed t3 Of Ed OS

Et*

1sEhOCJ

E-<3Ed

ffiUCfi

I'-(~Ed

OE


Major branches connect to the system ground plate, while minor branches connect to low frequency analog, digital or rack ground buses. Each

ground branch must be connected at one end only: the far ends of each ground branch must be disconnected from ground.

The system ground plate should be an isolated, 6.35mm (1/4 - in.) copper plate that is centrally located to provide a single point, and a low resistance interconnection between the signal ground network, the facility ground system, and the power system ground. The dimensions should be such that the plate will have enough tapped holes to accommodate all the ground cables or buses that need to be connected. An important point to note is that conduit, cable raceways, or build~.ig steel should not be used to distribute the single point ground from point to point in the system. The distribution should be through well insulated, dedicated cable of appropriate size - at least 25 mm*’ or more. Figure 9.8 shows the grounding of equipment racks in a single-point scheme with some typical mistakes. The routing of the conductor should be well planned to minimise conductor length as well as resistance. Long runs in parallel with unshielded power cables should be avoided and routed as far as possible from lightning down conductors.

After installation, but before interconnections with equipments t»re made,the isolation between the separate grounds networks should be checked.This is done by disconnecting one of the networks from the system groundplate end testing for continuity. At least 1 Mft should be registered

(3)between networks . The tests will detect and help eliminate unwanted ground loops. For further information on tests and measurement refer to Chapter 10.

9 .6 .2 GROUNDING COMPUTER COMPONENTS

Due to the fast rise time characteristics and high-frequency energies associated with computers, special care should be taken in creating a "interference-clean" signal/logic ground


Author Ambelal Dependra

Name of thesis Earthing And Grounding For The Control Of Emi In Industrial Instrumentation And Control Systems. 1986

PUBLISHER: University of the Witwatersrand, Johannesburg

©2013

LEGAL NOTICES:

Copyright Notice: All materials on the Un i ve r s i t y o f the Wi twa te r s rand , Johannesbu rg L ib ra ry website are protected by South African copyright law and may not be distributed, transmitted, displayed, or otherwise published in any format, without the prior written permission of the copyright owner.

Disclaimer and Terms of Use: Provided that you maintain all copyright and other notices contained therein, you may download material (one machine readable copy and one print copy per page) for your personal and/or educational non-commercial use only.

The University of the Witwatersrand, Johannesburg, is not responsible for any errors or omissions and excludes any and all liability for any errors in or omissions from the information on the Library website.

Date post:	21-Oct-2021
Category:	Documents
Upload:	others
View:	6 times
Download:	0 times

8 10 - University of the Witwatersrand

Documents