CONTENTS

Sr. No. T O P I C 1.0 Introduction. 2.0 Terminology. 3.0 General Requirements. 4.0 System Grounding.

4.1 Floating / Ungrounded Neutral 4.2 Grounded Neutral 4.3 Advantages of Grounded Neutral over Ungrounded system 4.4 System Characteristics with various grounding methods 4.5 How to Ground the System 4.6 Calculation of Ground Fault Currents

5.0 Equipment Grounding 5.1 Earthing of Miscellaneous Equipment

6.0 Earth Electrodes 6.1 Electrode Material 6.2 Current Loading of Earth Electrodes 6.3 Location & Number of Earth Electrodes 6.4 Salt Treatment 6.5 Voltage Gradient around Earth Electrodes 6.6 Types of Earth Electrodes 6.7 Maintenance of Earth Electrodes 6.8 Measurement of Earth Electrode Resistance

7.0 Design Considerations 7.1 No of Connections 7.2 System Resistance 7.3 Effect of Temperature on Earth Resistance 7.4 Soil Selection 7.5 Effect of Moisture content on Earth Resistivity 7.6 Artificial Treatment of Soil 7.7 Representative Values of Soil Resistivity in various parts of India 7.8 Potential Gradients 7.9 Effect of Corrosion

7.10 Earth Bus and Earth Wires 7.11 Size of Earth Bus and Earth Wire

8.0 Grounding System Calculations 8.1 Design Basis 8.2 Determination of Size of Earthing Conductor for Sub-station Grid 8.3 Determination of Size of Earthing Conductor for Plant Grid 8.4 Calculation of Earth Resistance

9.0 Questionnaire for Validation References

INTRODUCTION

The word “Earthing / Grounding” is very familiar to power system work to cover both “System” and “Equipment” grounding also the users of electrical equipment in the domestic and industrial front.

The basic definition of Earthing / Grounding is intentional low resistance

connection to earth / ground which is assumed to be at zero potential. To avoid confusion or possible misunderstanding, this training manual is devoted

exclusively to the subjects of system and equipment earthing. This training manual also covers various aspects like - Terminology used, General

requirements as per Indian Electricity Rules : 1956, Various types of system grounding, Advantages of grounded system over ungrounded system, System characteristics with various grounding methods, How to ground the system, Objectives of equipment grounding, Calculation of ground fault currents, How do ground faults occur & how to sense them, Design considerations like - Number of connections, system resistance, effect of temperature on earth resistance, soil selection, effect of moisture content on earth resistivity, artificial treatment of soil, potential gradients & effect of corrosion, Earth electrodes - types, material, current loading, location & number of electrodes, maintenance of electrodes, Size of earth bus & earth wires, Earthing practices for industrial, domestic appliances, electro medical apparatus, transformers, overhead lines, sub-stations, Measurement of earth electrode resistance / loop impedance.

Apart from all the above aspects, an example on grounding system is given which

shows how to determine the size of earthing conductors for sub-station and plant grids in this manual.

Since the distribution of LT power, by and large, is done through the 3 Phase, 4 Wire

system, the earthing practice associated with it needs much attention. It has been observed that in several installations earthing of the neutral has not been effective. The end result of this is a shift in neutral potential beyond the permissible limits in the event of any unbalanced power supply or during variations in load impedance. This shift in neutral potential reflects on all the equipment connected and may cause an electrical hazard.

1.0

TERMINOLOGY

As per IS:3043 - 1966, the various definitions of terms applicable to earthing are alphabetically listed below :

• Bond - To connect together electrically two or more conductors or metal parts.

• Earth - A connection to the general mass of earth by means of an earth electrode.

• Earth Electrode - A metal plate, pipe or other conductor or an array of conductors electrically connected to the general mass of the earth.

• Earth Fault - Live portion of a system getting accidentally connected to earth. • Earthing Terminal - A terminal provided on a piece of apparatus for the purpose

of making a connection to earth. • Earthed system - A system in which the neutral or any one conductor is

deliberately connected to earth directly or through impedance. • Earthing Ring - A ring or bus formed by connecting earth electrodes. • Fault Current - A current flowing from one conductor to earth, or to another

conductor, owing to failure of the insulation between line and earth or line and line.

• Leakage Current - A fault current of relatively small value, as distinguished from

that due to a short circuit, due to the leakage through the insulation. • Neutral Point - of a System - The point which has the same potential as the point of junction of a

group of equal resistance, connected at their free ends to the appropriate main terminals or lines of the system. The number of such resistances is 2 for single-phase, 4 for two-phase (applicable to 4-wire systems only) and 3 for three phase systems.

of a Symmetrical System - The point with respect to which the potential of the

conductors is symmetrical. It is usually connected to earth. of a Single-phase System - The term neutral is used in relation to single-phase

systems to denote that conductor which is connected with earth at one or more points.

• Step Potential - The maximum value of the potential difference possible of being

shunted by a human body between two accessible points on the ground separated by the distance of one pace which may be assumed to be one metre.

2.0

• Touch Potential - The maximum value of the potential difference between a

point on the ground and a point on an object likely to carry fault current such that the points which can be touched by a person, which may be assumed to be one metre.

• Mesh Potential - It is the potential difference in volts from grid conductor to

ground surface at the centre of mesh grid.

GENERAL REQUIREMENTS

• Earthing shall generally be carried out in accordance with requirements of Indian Electricity Rules, 1956 as amended from time to time and the relevant regulations of the Electricity Supply Authority concerned.

• All medium voltage equipment shall be earthed by two separate and distinct

connections with earth through an earth electrode. In the case of high and extra high voltages the neutral points shall be earthed by not less than two separate and distinct connections with earth each having its own electrode at the generating station or substation and may be earthed at any other point provided no interference is caused by earthing. If necessary, the neutral may be earthed through a suitable impedance.

• In cases where direct earthing may prove harmful rather than provide safety,

for example, high frequency and mains frequency coreless induction furnaces, special precautions are necessary. The metal of the furnace charge is earthed by electrodes connected at the bottom of the charge, and the furnace coils are connected to the mains supply but are unearthed. A relay is connected by a detection circuit which itself is earthed to the coils. The object is to prevent dangerous break through of hot metal through the furnace lining, the earth detection circuit giving a continuous review of the conditions for the furnace lining. When leakage current attains a certain set maximum it becomes necessary to take the furnace out of service and to re-line.

• Earth electrodes shall be provided at generating stations, sub-stations and

consumer premises in accordance with the requirement.

• As far as possible all earth connections shall be visible for inspection.

• All connections shall be carefully made; if they are poorly made or inadequate for the purpose for which they are intended, loss of life or serious personal injury may result.

• Each earth system shall be so devised that the testing of individual earth

electrode is possible. The earth resistance shall not be more than 5 ohms.

3.0

SYSTEM GROUNDING

As defined by the National Electrical Code, a system ground is a connection to ground from one of the current-carrying conductors of a distribution system or of an interior wiring system.

This is also known as neutral earthing. A set of conductors in which at least one conductor or point (usually middle point or neutral point of a distribution system or transformer or generator) is intentionally connected to earth either solidly or through an impedance in series is known as system / neutral grounding. This practice of earthing / grounding is there ever since the beginning of electrical power system.

This is provided to obtain sufficiently low neutral to ground resistance, to limit system over voltage and to help in the operation of the protective relays.

Most of our present mode of power distribution is 3 Phase system and particularly 3 Phase, 4 Wire system. The 4 Wire system further has the choice of operating with either a “Floating Neutral” (unconnected to earth) or with a “Grounded Neutral”.

4.1 Floating / Ungrounded Neutral :

As per AIEE (Association of International Electrical Engineers) standard no.32, ungrounded means without an intentional connection to ground except through potential indicating or measuring devices.

In any practical system where there is no grounding, a capacitive coupling between the system conductors and the ground will take place as shown in the Figure-4.1a. Consequently so-called ungrounded system in reality becomes a “Capacitive ground system” by virtue of the distributed capacitance from the system conductors to the ground.

R B Y Ib Iy Ir Figure-4.1a

4.0

Earth Potential

A line to ground fault on this system causes a very small ground fault current to flow through the cables, transformers and other electrical equipment on the system. This current may have a magnitude of less than 5 Amps in system voltage up to 415 Volts and may reach up to 25 Amps on larger systems. This fault current in general being quiet low remains unidentified without the conventional overcurrent protective equipment in the system reacting to the fault. Hence uninterrupted power supply continues on the other two healthy lines. The advocates of the free neutral system thus claimed its superiority on the basis of this sustained continuity of supply on the two healthy lines while an earth fault occurred on the third line. This argument for continuity of supply seemed true and valid particularly for an overhead system where a fault of one phase to earth is unlikely to develop into a fault of two or more lines.

But actually when the neutral of a system is not grounded, destructive transient over

voltages of several times normal (6 to 8 times) can appear from the line to ground during normal switching of a circuit having line to ground fault. Over voltages by repeated restrikes of the arc during interruption of a line to ground fault also occur particularly in low voltage systems. These over voltages may cause insulation failure at other locations on the system than the point of fault as indicated in Figure-4.1b.

Figure-4.1b

A ground fault of one phase causes full line to line voltage to appear throughout the system between ground and two unfaulted phases. This voltage is 73% greater than normal and is indicated in Figure-4.1c. If this excess voltage persists for too long, insulation failures compounded by age, severe working condition and contamination due to moisture, dust, etc. would take place. Particularly since distribution by underground cable system is more widely adopted, overheating & burning of insulation is likely, later culminating in a fault between two lines. Thus a line to ground fault on one circuit may result in damage to equipment and interruption of service on the other circuits. Further situations get worse if a second line to ground fault occurred in addition to the existing ground fault. By and large double faults are not uncommon simply because the first fault is left on hoping to be cleared while the second fault occurs in all probability on the same circuit. Although several devices

Ungrounded Power Source

Healthy Circuit

Transient Over- voltage may cause fault here

Earth Potential

Single line to earth fault

C . B

C . B

C . B

could be made available for detecting the location of the fault, it does not prevent the occurrence of the fault and if at all the fault could be corrected it would be at the cost of high downtime and considerable loss of production. Thus eventually a short circuit of two lines is inevitable, causing major damages and interruptions in power supply.

Line-Earth Voltage Line-Earth Voltage During Fault = Line-

Line

Earth Potential

Figure-4.1c

4.1.1 Arcing Grounds :

The phenomena of arcing grounds is commonly experienced with ungrounded systems. A temporary fault caused by falling on a branch, lightning surge, etc. creates an arc between an overhead line and ground. The arc extinguishes and restrikes in a repeated, regular manner. This phenomena is called ‘arcing grounding’. A simple explanation is illustrated below.

Phenomena of Arcing Grounds (Distributed Capacitance - Line to Ground - gets discharged through the earth fault. Figure-4.1.1.

From the Figure-4.1.1, each line has an inherent distributed capacitance with respect to earth. Consider an earth fault on line B. The distributed capacitance discharges

Voltage

Neutral floats at Earth Potential

B

Y

R

ER

EY EB

+-

- +

-+

Ground

Earth Fault

F

through the fault when the gap between F and ground breaks down. The capacitance, again gets charged and again discharged. Such repeated charging and discharging of line to ground capacitance resulting in repeated arcs between line and ground is called arcing grounds. These arcing grounds produce severe voltage oscillations reaching three to four times normal voltage. Secondly, a temporary fault grows into a permanent fault due to arcing grounds. The problem of arcing grounds can be solved by earthing the neutral through a coil called Peterson Coil or Arc suppression coil connected between neutral and earth. Thereby the arc is extinguished.

4.2 Grounded Neutral :

A grounded system is a system of conductors in which at least one conductor or point (usually the middle wire or neutral point of a transformer or generator windings) is intentionally grounded, either solidly or through a current-limiting device.

Most of the problems which are mentioned in Floating Neutral could be eliminated

by earthing the neutral. In the earthed neutral system, a return path for the fault current is made available at the neutral point. This can be utilised to bring about discriminative operation of the protection equipment such as the earth fault relays or the Residual Current Circuit Breakers / Earth Leakage Circuit Breakers (RCCB / ELCB) depending upon the level of protection required and the protected entity. Thus the fault gets localised and isolated from the healthy parts of the system which in turn continues to power the loads. Service reliability is high with the faults located quickly and corrected with ease.

Further chances of multiple earth faults are rare in grounded systems. Earthed neutral system limits the voltage on the healthy parts of the system to its line

to neutral voltage. In the event of an earth fault in an effectively earthed system the voltage to earth of the healthy lines does not exceed 80% of the line to line voltage. Further earth faults in control wiring can cause 58% of line-line voltage on contactor connected between two lines.

In addition it suppresses the transient overvoltages occurring due to earth fault arcs

which can be damaging to the system insulation as a whole.

Further, static charges induced are conducted to earth without any disturbance. The transient overvoltages and static charges are detrimental to installations related to electronic, communication equipment and computers which are ever on the rise. Hence it is important to protect these kinds of installations.

Neutral grounding also results in improved protection against lightening surge on equipment and noise interference in the instrumentation system.

Sometimes it is necessary to determine how solidly the system is grounded. this could be determined by comparing the magnitude of earth fault current with the system three phase fault currents.

4.2.1 Solid Grounding :

Solidly grounded means grounded through an adequate ground connection in which no impedance has been inserted intentionally. This is also known as directly grounded.

In the case of all solidly grounded systems as in Figure-4.2.1 it is necessary that the

ground fault current be in the range of 25 - 100% of the three phase fault current to prevent the occurrence of high transient over-voltages. For 50 kA fault levels the symmetrical RMS current shall be at least 12.5 kA. In the case of solidly grounded systems the protection equipment should be set to avoid damage at the fault point because of the higher fault levels.

Solid Earthing Resistance Earthing

Figure-4.2.1 Figure-4.2.2a

4.2.2 Resistance Grounding :

Resistance grounding means grounding by series connected resistance as shown in Figure-4.2.2a and Figure-4.2.2b limits the fault current to a lower value just enough to activate the over current devices.

Single Line-Earth Fault

Neutral Potential

Line-Line Earthed by Resistor Voltage Voltage Drop in faulted phase due to Earth Current

Figure-4.2.2b

4.2.3 Reactance Grounding :

Earth Potential Unearthed Neutral

Reactance grounding means grounding by series connected reactance as shown in Figure-4.2.3 is usually used for protection of generator windings where direct grounding may result in fault currents exceeding the short circuit current.

Reactance Earthing ( Figure-4.2.3 ) Earth Fault Neutralizer ( Figure-4.2.4 )

4.2.4 Resonant Grounding :

Resonant grounding means reactance grounded through such values of reactance that, during a fault between one of the conductors and earth, the rated-frequency current flowing in the grounding reactances and the rated-frequency capacitance current flowing between the unfaulted conductors and earth shall be substantially equal. In the fault, these two components of the fault current will be substantially 1800 out of phase.

Sometimes, a tuned inductance as indicated in Figure-4.2.4 is connected in the

neutral circuit to neutralise the capacitive currents of the system. This is also called as a ground fault neutralizer.

4.3 Advantages of Grounded neutral over Ungrounded system :

Based on the experience of operators who have used both grounded and ungrounded neutral systems that the failure rate is substantially lower and the time the system is out of service is less on the grounded system. This results from the fact that the transient overvoltages are greatly reduced on a grounded neutral system. As the grounding reduces these overvoltages, the life of electric insulation will be increased and service interruptions will be minimised. Even though the overvoltages of an ungrounded neutral system may not be high enough to cause multiple failures, every time a ground fault occurs, the repeated application of these overvoltages will weaken the insulation and cause a higher failure rate than in a grounded neutral system. A summary of advantages of the grounded neutral over ungrounded neutral system of various voltage levels are :

• 440 Volt System (Low Voltage System) :

CRITERIA Grounded-neutral system Ungrounded system Safety

Safest - Only 254 V to ground at any time (assume good ground and 440 V maximum line to line). Safest - Voltage on system limited to about 254 V when primary to secondary failure occurs in transformer supplying system. Safest - Ground fault in control wiring can put 58% line voltage on line-to-line connected contactor closing coils.

Normally 254 V to ground when no ground on system. 440 V to ground on two conductors when one phase is grounded. Voltage on secondary system may be as high as primary voltage for breakdown between primary and secondary transformer windings. Control circuit ground fault likely to put full voltage on contactor closing coils.

Service reliability

Highest - Ground faults are readily located and repaired; system need not be taken out to find ground faults. Highest - Ground faults are localised and trip off immediately. Highest - Minimizes transient over-voltages on the system. Highest - Floating grounds are very unlikely.

Part or all of system must be taken out of service to find ground faults subject to service transient overvoltages. Ground faults if not removed may upon occurrence of a second ground fault cause two circuits to go out at once, thus causing a loss of twice as much production equipment. Floating or arcing grounds likely.

Maintenance cost

Lowest - Ground faults are easily located. Time must be spent hunting ground faults.

Capital cost About same as delta connected sub-station and ground detector.

High voltage fluorescent lighting

Provides 254 V for direct operation of fluorescent lights, resulting in a cost saving by the elimination of lighting transformers and a reduction in copper.

Must use step-down transformers from 440 V to 254 V or lower.

• 2.4 to 15 KV System (Medium Voltage System) :

CRITERIA Grounded-neutral system Ungrounded system Safety Safest - Single line-to-line faults are

tripped off immediately. Subject to severe transient overvoltages.

Service reliability

Highest - Ground faults are readily located and repaired. Highest - Limited fault current causes a minimum of damage to equipment (with conventional resistance grounding). Highest - Minimizes transient over-voltages on the system.

Part or all of system must be taken out of service to find faults. Ground faults, if not removed, may upon occurrence of a second ground fault cause two circuits to go out at once, thus causing a loss of twice as much production equipment. High fault current associated with two line-to-ground faults may result in more damage to equipment.

Maintenance cost

Lowest - Ground faults are easily located. Time must be spent hunting ground faults.

Capital cost About same; Adds cost of resistor and neutral relaying.

Requires ground detector and fault locator equipment to be comparable.

4.4 System Characteristics with various Grounding methods :

Ungrounded Essentially grounded Reactance _ grounding

Ground-fault Neutralizer

Resistance Grounding

Solid Low value Reactor High value reactor Current for phase-to-ground fault in % of three-phase fault current.

Less than 1%

Varies, may be 100% or more

Usually designed

to produce 25 to 100%

5 to 25%

Nearly zero fault

current

5 to 20%

Transient overvoltages

Very High Not Excessive Not Excessive Very High Not Excessive Not Excessive

Automatic segregation of faulty zone

No

Yes

Yes

Yes

No

Yes

Ungrounded neutral type

Grounded neutral type

Grounded neutral type if current is

60% or more

Ungrounded neutral type

Ungrounded neutral type

Ungrounded neutral type

Remarks Not recommended due to over-voltages and segregation of faults

Generally used on systems are : (1) 600 V and below

(2) Over 15 KV

Not used due to excessive

overvoltages

Best suited for high voltage

overhead lines where faults may be self-

healing

Generally used on industrial

systems of 2.4 to 15 KV

4.5 How to Ground the system :

Depending upon the conditions, various types of grounding systems which are in practice are illustrated with respect to the Low and Medium Voltage systems.

• Low Voltage Systems - Typical Voltages : 208, 240, 480, 600 Volts.

Condition Grounding Practice Diagram Remarks 1.If Y- connected

generators on syst-em.

Ground generator neutral through low value reactance.

1.Total capacity of generators should be adequate for grounding (refer Table-4. 5).

2.Grounding reactance should

pass ground currents equal to at least 25% of three phase value.

2.If low voltage

system is supplied by transformer having Y-connect-ed secondaries.

Ground transformer neutrals solidly.

1.Total capacity of transformers should be adequate for grounding (refer Table-4. 5).

3.No Y-connected generators or trans-former secondaries on system.

Use grounding trans-former solidly groun-ded.

1.Grounding transformer should pass ground fault currents equal to at least 25% of three phase value. Check adequacy of this fault current for tripping circuit breakers and any fuses on system.

Table-4. 5 :

Minimum ratings of Generators & Power Transformer banks for Grounding :

Max. System Short-circuit KVA

Min. Rating KVA

1,000,000 7500 500,000 3750 250,000 1750 150,000 1000 100,000 750 50,000 375 25,000 187

• Medium Voltage Systems - Typical Voltages : 2.4, 4.16, 4.8, 6.9, 11, 13.8 KV.

Condition Grounding Practice Diagram Remarks 4.If Y- connected

generators on syst-em.

Use resistance ground-ing. Do not ground directly.

1.Total capacity of generators should be adequate for grounding (refer Table-4. 5).

2.When severe lightning

exposure is present, generator may be grounded through low vakue reactance to permit use of grounded neutral type lightning arresters.

3.Small systems, where the

resulting ground fault current would not be excessive, may be reactance grounded, if desired, in the interest of economy.

5.If Y-connected

transformers on system (use trans-formers which supply power to the system, avoid transformers which are loads on the system).

Use resistance ground-ing.

1.Total capacity of transfor-mers should be adequate for grounding (refer Table-4. 5).

2.Small systems, where the resulting ground fault current would not be excessive, may be solidly grounded, If desired, in the interest of economy.

6.If no Y-connected generators or trans-former on system.

Use one or more grounding transfor-mers with resistors.

1.In small systems, where the resulting ground fault current would not be excessive, the grounding transformer may be solidly grounded in the interest of economy.

7.Solidly ground neutrals of all systems above 15 KV (no rotating equipment assumed operating directly at these voltages).

4.6 Calculation Of Ground Fault Currents :

The value of the current in the event of an earth fault on a solidly grounded system is quantified by the impedance of the grounded apparatus plus the impedance of the lines and cables leading to the fault and the impedance of the ground return path. For interconnected systems the calculations may be rather complicated. In the case of a single line to earth fault for resistance or solid grounding the fault current may be computed from the simplified formula given below :

Igf = VL/(1.732 x R) where VL is line to line voltage and R is the resistance of the soil, cables, joints plus grounding resistance if any.

In case of a neutral earthed through a reactor the fault current Igf can be determined by the

formula as given below : Igf = 3 x E [X1 + X2 + Xo + 3 (Xn + Xg)] Where, X1 = System +ve sequence reactance.

X2 = System -ve sequence reactance.

Xo = system zero sequence reactance (Zero for solidly grounded system).

Xn = Reactance of neutral reactor.

Xg = Reactance of earth return path.

E = Line to Neutral voltage

4.6.1 How do ground faults occur ?

Usually the cause is a failure for whatever reason, in the insulation system. The problems are created by damaged conductors, loose connections, moisture and dust. In lower voltages fortunately the character voltages fortunately the characteristics of the voltage drop across the arc and low levels of fault current self extinguishes the arc, but on higher voltages and larger systems the arc may not be self extinguishing the considerable damages to equipment, property and personnel are not ruled out.

EQUIPMENT GROUNDING

An equipment grounding refers to the permanent and continuous connecting together of all non-current carrying metal parts of equipment enclosures, such as conduit, boxes, cabinets, housings, frames of motors and lighting fixtures to a system grounding electrode located at the service equipment or on the source side. This is also known as Protective Grounding.

The basic objectives of equipment earthing are :

• To ensure freedom from dangerous electric shock voltages exposure to persons in the area.

• To provide current carrying capability, both in magnitude and duration, adequate to accept the ground fault current permitted by the overcurrent protective system without creating a fire or explosive hazard to building or contents.

• To contribute a better performance of the electrical system.

5.1 Earthing of Miscellaneous Equipment :

5.1.1 Electrically Driven Machine Tools :

Irrespective of the size or type of the machine tool, the bed plate of the machine should be earthed by means of a strip or conductor not less than 6.5mm2 cross-sectional area if of copper, 10mm2 if of aluminium and 16mm2 if of galvanized iron or steel. The strip conductor should be securely fastened to the bed plate by means of bolting.

5.1.2 Industrial Electronic Equipment :

Any industrial electronic equipment which derives its supply from two pin plugs incorporates small capacitors connected between the supply and the metal case of the instrument to cut down interference. This capacitor must be properly earthed.

In case of an Oscilloscope, which is used for examining the wave-form of a high frequency

source, the Oscilloscope should be earthed by a conductor entirely separate from that used by the source of high frequency power. In case when an Oscilloscope is used on a circuit where the negative is above earth potential and also connected to its metallic case, proper care should be taken as earthing is not possible.

In case of high frequency induction heaters, earthing should be provided by means of separate

earth wire by as direct a route as possible. In cases where direct earthing proves harmful than safety, e.g. Coreless Induction Furnaces, the metal of the furnace charge should be earthed by electrodes connected at the bottom of the charge, and the furnace coils are connected to the mains supply but are unearthed.

5.1.3 Electric Arc Welding Equipment :

5.0

All components should be effectively bonded and connected to earth. The transformers and separate regulators forming multi-operator sets and capacitors for power factor correction, if used, should be included in the bonding. In case of double wound welding transformer sets, an ‘Earth & Work’ terminal should be provided. In case of single-phase, this terminal should be connected to one end of the secondary winding (referFfigure-5.1.3) and in case of three-phase sets, this should be connected to the neutral point of the secondary winding.

Main Switch Welding Transformer Supply Primary Secondary Uninsulated Terminal B Mild Steel Tank A Earth & Work Connection for Welding

Figure-5.1.3

5.1.4 Electro Medical Apparatus :

Since most of the medical apparatus like electro-cardiographs (ECG), electro-encephalographs (EEG), etc. are connected to a patient’s body by means of electrodes having a very low contact resistance, special care should be taken in design and installation of these apparatus. They should be connected to a good low resistance earth. In case of X-Ray apparatus, the metallic shield in which the tube is enclosed should be connected to earth to avoid potential of the shield rising and thus protect both the operator and the patient from possible contact with voltages of the order of 100 KV.

5.1.5 Elevators / Lifts :

Frames of motor, winding machine, control panel, and cases and covers of tappet switch and similar electrical apparatus which normally carry the current should be properly earthed. The sizes of wires and earth-continuity conductors should be not less than 1.5 mm2 for copper and 2.5 mm2 for aluminium and need not be greater than 70 mm2 for copper and 120 mm2 for aluminium.

5.1.6 Domestic Appliances :

Note : If the return cable Ais not connected to the work piece, welding current will return via cable B.

Earthing of domestic appliances arises in case they have only functional insulation. Appliances having reinforced / double insulation need not be earthed. In case of plugs & sockets, three pin type should be used, one of the pins being connected to earth. In case of lighting fittings, if the bracket type holders are of metallic construction, they should be earthed properly. In case of fans & regulators, cooking ranges, electric water heaters, washing machines, refrigerators, electric irons, air conditioners, coolers, etc. may be earthed by use of three pin plugs.

5.1.7 Transformers :

The earthing for various types of transformers can be provided in the following manner : Generator transformers - The neutral points of these transformers should be directly earthed. Sub-station transformers - Resistance earthing may be used in order to limit the fault current. as

the fault current is expected to be too high. Distribution Transformers - Neutral point should be directly connected to the earth. Transformers with Delta Windings - Earthing transformer may be used. This provides a star point

which may be either directly or through a resistance, if desired. Instrument Transformers - The secondary windings and as well as the cases & frames of Current &

Potential transformers should be earthed.

5.1.8 Lightning Arresters :

The bases of lightning arresters should be directly connected to the earth by conductors as short as straight as possible to ensure minimum impedance. In addition, there should be as direct connection as possible from the earth side of the lightning arresters to the frame of the apparatus being protected. However, surge counters may be inserted in the circuit, provide lightning arrester is mounted on an insulated base.

In case of station type lightning arrester, individual earth electrode should be provided and where

as for distribution type, one electrode for a set of lightning arresters may be provided. If the lightning arresters are mounted near transformers, earthing conductors should be located bit far from the tank and coolers in order to avoid possible oil leakage caused by arcing. The earth connection should not pass through iron pipes as it would increase the impedance of the connection.

5.1.9 Generators :

Three-phase Generators - The neutral points of three-phase high-voltage generators should be earthed either by Direct earthing / Resistance earthing / Reactance earthing.

Single Generator - The earthing may be done without any impedance in the circuit. If a resistance

is inserted between neutral and earth, quick acting protective devices should be used so that on the occurrence of a fault, the generator and its field shall automatically be disconnected. If a

reactance earthing is used, the fault current should be considered to a maximum of three-phase short-circuit current.

Generators in Parallel - When more than one generator are operating in parallel, all the generator

neutrals should be earthed through an isolating switch. In order to limit the earth fault current and also to avoid circulating currents between generators, at any time, only one earth isolating switch should be kept ‘on’ if more than one generator are in service.

Generators connected to Overhead lines - If the generators are directly connected to overhead lines,

they get subjected to the effects of travelling waves or impulses due to lightning. In such a case if the neutral is earthed through a reactance, positive reflection of the waves can take place and may cause damage to the winding insulation. To prevent this, the reactor should be shunted by a non-linear resistance which can limit the surge voltage to such a value which the machine can withstand. The same can also be achieved by connecting surge arresters to the machine terminals.

5.1.10 Overhead Power Lines :

While designing, following points should be taken care :

- Avoiding danger from a broken line conductor or from leakage due to insulation breakdown, and to ensure that in such circumstances the protective gear will operate effectively.

- Should ensure that the current in an earth wire or to support metal work due to lightning stroke shall be conveyed to earth without causing back flashover.

- Minimizing inductive interference with communication circuits.

High-Voltage Lines - If the metal work bonded and earthed at each support, it provides protection against the danger of pole top fires from leakage, provided the resistance to earth support is sufficiently low to permit protective gear to operate in the event of contact between a line conductor and earthed metal work. The same can be adopted in case of wooden poles also.

If metal work bonded and connected to an aerial earth wire, the earth wire should be connected to

neutral of the transformer or generator and should be earthed at least on four towers in every 1.6 KM. This method also provides protection against lightning if the earth wire is run above the line conductors.

Low-Voltage Lines - In case of low-voltage power lines, an earth wire may be run either above or

below the phase conductors with suitable cradles or safety device from pole to pole so that in the event of breakage of any one of the phase conductors, it will make contact with the earth wire.

If wooden poles are used, a bonding wire should be connected to all the metal work on the pole

including the supporting metal work of all insulators. All the stay wires other than those which are connected with earth by means of a continuous earth

wire should be insulated at a height of not less than 3 M from the ground to prevent danger from leakage.

5.1.11 Sub-stations :

Sub-stations giving no External Low-Voltage Supply - Common earth electrodes should be used for both system and equipment earthing.

Sub-stations giving an External Low-Voltage Supply - Neutral should be connected to the station

earth system. Sub-stations giving an External High-Voltage Supply - It is recommended to have common earth-

bus for both high & low-voltage systems, where as the manual operating lever handles should be connected to the system earth electrode.

Carrier-Current Equipment - A separate earth electrode (Rod or Pipe) should be provided

immediately adjacent to the structure supporting the coupling capacitors. Cables - Metal pipes or conduits in which the cables have been installed should be properly

bonded or earthed. At specified points on the route where the presence of stray currents is suspected, the joints, the metal sheath armour, if any, of the cables should be bonded to the earthing system and connected to one or more earth electrodes.

Disconnecting Switches - In the case of isolated-phase systems, disconnecting switches tied to the

main bus-bar should have their bases earthed with connections equal in cross-section to the earth bus.

Power Circuit Breakers - The earth connection is limited to the amount of current that can be

passed through the frames of the breakers to the point where suitable earth connection can be made. The size of earthing conductor is determined by considering voltage-drop & temperature rise. Under fault conditions the voltage drop between two normally earthed parts with which any one is likely to be in simultaneous contact should not exceed 32 Volts. The temperature rise in case of swatted and riveted joints is limited to 2500 C and for brazed joints to 4500 C.

Rods / Handles of Outdoor Gang-Operated Isolators - The operating rods or handles of all

outdoor gang-operated isolators should be connected to earth either directly or through steel mounted structure.

Casings of Instruments, Meters and Relays (Operating Voltage < 650 Volts) - If the current

carrying parts are less than or up to 650 Volts, and these are not situated on switchboards, and accessible to other than qualified persons, their casings and other exposed metal parts should be earthed. If they are situated on switchboards having no live parts on the front of the panels, their casings should not be earthed. If they are situated on switchboards having exposed live parts on the front of panels, their casings should not be earthed but rubber mats or other suitable insulation should be provided for the operator.

Casings of Instruments, Meters and Relays (Operating Voltage > 650 Volts) - If the current

carrying parts are over 650 Volts, they should be isolated and be elevated or protected by suitable barriers, such as earthed metal or insulating covers or guards. Their cases should not be earthed, except in electrostatic earth detectors where the internal earth segments of the instrument are connected to the instrument casing and earthed.

A typical Earthing arrangement for an Outdoor Sub-station (Figure-5.1.11)

EARTH ELECTRODES

The earth electrodes should have low resistance, depending on the system voltage and fault current envisaged under all climatic conditions. The rise potential between the earth system and the general body of the earth should be kept as low as possible. The earth electrodes should be capable of carrying such currents as may arise in normal operation and during fault and surge conditions without increase in resistance.

6.1 Electrode Material :

Though the electrode material does not affect initial earth resistance, proper care should be taken to select a material, which is resistant to corrosion. Normally the material is of Copper, Iron or Mild Steel. If the soil is of excessive corrosive in nature, Copper electrode or Copper clad electrode or Galvaniszed Iron electrode (Zinc coated) should be used. In case of DC system, only Copper electrodes should be used. The electrodes should be kept free from grease, paint & enamel, etc. It is always better to use similar material for earth conductors & earth electrodes.

6.2 Current Loading of Earth Electrodes :

An earth electrode should be designed to have an adequate loading capacity for the system for which it forms a part, should be capable of dissipating without failure under any circumstances of the operation of the system. The two conditions of operation require - Long duration overloading as with normal system operation and Short time overloading as under fault conditions in directly earthed system. The time taken by an earth electrode to fail on short time overloading is inversely proportional to the specific loading.

The maximum permissible current density, I = 7.57 x 10

3 A/m

2 √ρt Where, t = Duration of earth fault in seconds. ρ = Resistivity of the soil in Ohm metre. 6.3 Location & Number of Earth Electrodes :

The earth electrodes should be so placed that all lightning protective earths may be brought to the earth electrode by as short and straight a path possible to minimise the surge impedance. As far as possible earth electrodes for generating stations and indoor sub-stations should be within and adjacent to the perimeter fence. The approximate number of earth electrodes required for a given area can be seen from the Figure-6.3.

6.0

Approximate Number of Rod Earth Electrodes Required in a Given Area

Figure-6.3 6.4 Salt Treatment :

In the case of pipe electrodes it is recommended to perforate these and to treat the soil by pouring salt solution down them. If the electrodes are installed in a trench, use salt as a top dressing left to percolate through the soil with the surface moisture. Although substantial reduction of earth resistance can be achieved by the use of coke around the earth electrode for a distance of 2.5cm, this method is not recommended, as it results in rapid corrosion not only of the electrode but also of cable sheaths, water pipes or steel frame work, etc., to which it is bonded.

6.5 Voltage Gradient Around Earth Electrodes :

Voltage gradients around earth electrodes under fault conditions may reach such values as to damage telephone and pilot cables in the vicinity. The area over which such injurious voltages occur is dependent on whether the feeders are overhead lines or buried cables. It is recommended that steps should be taken to protect any pilot or light current carrying lines within the area over which a dangerous voltage may be expected to exist, either by applying suitable sheath insulation to such lines where they are in cable trench or by running them overhead within the danger area.

6.6 Types of Earth Electrodes :

1. Rod & Pipe Electrodes. 2. Strip or Conductor Electrodes. 3. Plate Electrodes. 4. Cable Sheaths.

6.6.1 Rod & Pipe Electrodes :

Where the fault current is envisaged to be low and the soil resistivity is high, it is recommended

to make use of rod electrodes. Dimensionally they should be at least 16mm dia in case of steel or galvanized iron and 12.5mm in case of Copper. Refer Figure-6.6.1.

Pipe electrodes should be dimensionally at least 38mm internal dia in case of galvanized iron or steel and 100mm in case of cast iron.

Length of Rod and Pipe electrodes should not be less than 2.5m.

Where the soil resistivity decreases with depth, deeply driven pipes and rods are effective.

To reduce the depth of burial of an electrode without increasing the resistance, a number of rods or pipes should be connected together in parallel. The distance between two electrodes should be preferably not less than twice the length of the electrode.

If necessary, rod electrodes shall have a galvanized iron pipe buried in the ground adjacent and parallel to the electrode itself. It’s one end shall be at least 5cm above the surface of the ground and not be more than 10cm . The difference between the lengths of the electrode and pipe under the earth’s surface shall not be more than 30cm and in no case the length of the pipe exceed that of the electrode.

The resistance may be calculated from the following formula : R = 100ρ loge 4l Ohms. 2πl d Where, ρ = Soil resistivity in ohm-mtr. l = Length of rod or pipe in cm. d = Diameter of rod or pipe in cm.

Pipe Earth Electrode (Figure-6.6.1 )

6.6.2 Strip or Conductor Electrodes :

Where the fault current is envisaged to be high and the soil resistivity is high, it is recommended to use earthing mat or bare strip or round conductor. Dimensionally, the strip electrode should not be smaller than 25 x 1.60mm in case of copper and 25mm x 4mm in case of galvanized steel.

If round conductors are used, their cross-sectional area shall not be less than 3.0mm2 in case of

copper and 6.0mm2 in case of iron or steel. The length of buried conductor shall not be less than 1.5m. and shall be buried in trenches not

less than 0.5m deep. If it is necessary to use more than one strip, they shall be laid either in parallel trenches or in

radial trenches. The resistance may be calculated from the following formula : R = 100ρ loge 2l2 Ohms. 2πl wt Where, ρ = Soil resistivity in ohm-mtr. l = Length of strip in cm. w = Depth of burial of electrode in cm. t = Width (in case of strip) or twice the dia (in case of round conductor) in cm.

6.6.3 Plate Electrodes : Plate type of electrode is used where the current carrying capacity is the prime consideration.

E.g. Generating stations and sub-stations. Dimensionally, it should not be less than 6.30mm in thickness in case of steel or galvanized iron

and 3.15mm in case of copper. The size should be at least 60cm x 60cm. Refer Figure-6.6.3. These plate electrodes shall be buried such that its top edge is at a depth not less than 1.5m from

the surface of the ground. If the resistance of one plate electrode is higher than the required value, two or more plates

should be used in parallel and should be separated from each other by not less than 8.0m. Preferably, the plates should be set vertically. If necessary, plate electrodes shall have a galvanized iron pipe buried vertically and adjacent to

the electrode. One end of the pipe should be at least 5cm above the surface of the ground and not more than 10cm. The internal dia of the pipe should be at least 5cm and not more than 10cm. The length should be such that it should be able to reach the centre of the plate, however, it should not be more than the depth of the bottom edge of the plate.

The resistance may be calculated from the following formula : R = ρ π Ohms. 4 √ A Where, ρ = Soil resistivity in ohm-mtr. and A = Area of both sides of plate in m2.

Plate Earth Electrode (Figure-6.6.3 )

6.6.4 Cable Sheaths :

Where an extensive underground cable system is available, lead sheathed and steel armoured cables may be used as earth electrodes provided the bond across the joints is at least of the same conductivity as of the sheath. The resistance of such an electrode is generally less than 1Ohm.

6.7 Maintenance of Earth Electrodes :

Periodical check tests of earth electrodes should be carried out and values should be recorded. If Earth-Leakage Circuit Breakers are installed, it should be tested by operating the ‘test’ device periodically. The surrounding soil to the earth electrode should be kept moist, where necessary, by pouring water through a pipe where fitted along with it.

6.8 Measurement of Earth Electrode Resistance :

In order to measure the resistance of earth electrode, two auxiliary earth electrodes and one test electrode are placed at suitable distances as shown in the Figure-6.8. A measured current is passed between the electrode A to be tested and an auxiliary current electrode C and the potential difference between the electrode A and the auxiliary potential electrode B (value should be less than 20,000 Ohms )is measured. This method is known as Fall of Potential method. The resistance of the test electrode A is :

R = V / I Where,

across the joints is at least of the same

R = Resistance of the test electrode in Ohms. V = Voltmeter reading in Volts. I = Ammeter reading in Amps. Current Source Ammeter Voltmeter A B C 1 m Test Potential Current Electrode Electrode Electrode

Measurement of Earth Electrode Resistance

Figure-6.8

At the time of testing, the test electrode should be separated from the earthing system. The auxiliary electrodes usually consist of 12.5mm dia mild steel rod up to 1m into the ground. All the test and current electrodes should be so placed that they are independent of the resistance area of each other.

If the test electrode is in the form of rod, pipe or plate, the auxiliary current electrode C should be

placed at least 30m away from it and the auxiliary potential electrode B should be placed midway between them.

If three consecutive readings of test electrode resistance with different spacings of electrodes do

not match, the test should be repeated by increasing the distance between A and C up to 50m and each time placing the electrode B midway between them.

t t t

DESIGN CONSIDERATIONS

As mentioned in General Requirements, the system, which shall be earthed should be carried out

in accordance with the Indian Electricity Rules, 1956. Earthing may not give protection against faults which are not essentially earth faults.

7.1 No of Connections :

Every medium, high and extra high voltage equipment shall be earthed by not less than two separate and distinct connections with earth is designed primarily to preserve the security of the system by ensuring that the voltage on each live conductor is restricted to such a value wrt the potential of the general mass of earth as is consistent with level of insulation applied.

7.2 System Resistance :

The earth system resistance should be such that when any fault occurs against which earthing is designed to give protection, the protective gear will operate to make the faulty portion harmless.

The resistance to earth of an electrode of given dimensions is dependent on the electrical resistivity of the soil in which it is installed.

7.3 Effect of Temperature on Earth Resistance :

The soil temperature also has some effect on soil resistivity, the fundamental nature and properties of a soil in a given area cannot be changed, can be made of purely local conditions in choosing suitable electrode sites and of methods of preparing the site selected, to secure optimum resistivity. These measures may be summarised as :

1. Wet marshy ground and grounds containing refuse, such as ashes, cinders and brine waste.

2. Clayey soil or loam mixed with small quantities of sand.

3. Clay & loam mixed with varying proportions of sand, gravel and stone.

4. Damp and Wet sand pit.

The temperature coefficient of resistivity for soil is negative, but is negligible for temperatures above freezing point. At about 20oC, the resistivity change is about 9% per oC. Below 00C the water in the soil begins to freeze and introduces a tremendous increase in the temperature coefficient, so that as the temperature becomes lower, the resistivity rises enormously. It is therefore, recommended that in areas where the temperature is expected to be quite low, the earth electrodes should be installed well below the frost line.

7.4 Soil Selection :

7.0

A site should be chosen which is normally not well drained. A water logged situation, however, is not essential unless the soil be sand or gravel as in general no advantage results from an increase in moisture content above 15% to 25%. Perennial wells may also be used as sites for earth electrodes with advantage where the bottom of the earth is rocky.

Electrodes should preferably situated in a soil which has a fine texture and is packed by watering and ramming as tightly as possible. Where practicable, the soil should be shifted and lumps should be broken up and stones removed in the immediate vicinity of the electrodes.

In places where the soil conditions appear to be extensively corrosive, the soil may be chemically examined before deciding the material of the earth electrode.

7.5 Effect of Moisture content on Earth Resistivity :

Moisture content is one of the controlling factors in earth resistivity. Figure-7.5 shows the variation of resistivity of red clay soil with percentage of moisture, expressed in percent by weight of the dry soil. Dry soil weighs about 1440 Kg./M3 and thus 10% moisture content is equivalent to 144 Kg of water/M3 of dry soil. From the figure, above 20% moisture the resistivity is very little affected, below 20% the resistivity increases very abruptly with decrease in moisture content. The normal moisture content of soils ranges from 10% in dry seasons to 35% in wet seasons, and an approximate average may be perhaps 16 to 18%.

Variation of Soil Resistivity with Moisture Content Figure-7.5

7.6 Artificial Treatment of Soil :

Multiple rods even in large number may sometime fail to produce an adequately low resistance to earth. This condition arises in installations involving high resistivity. The alternative is to reduce the resistivity of the soil surrounding the earth electrode. This can be achieved by adding the substances like Sodium Chloride (NaCl), Calcium Chloride (CaCl2), Sodium Carbonate (Na2CO3), Copper Sulphate (CuSO4), Salt and Soft coke and Charcoal in suitable proportions. The curve in Figure-7.6 shows the reduction in soil resistivity effected by salt. The salt content is expressed in % by weight of the contained moisture. The effect of salt is different for different soils. Decreasing the soil resistivity causes a corresponding decrease in the resistance of a driven electrode.

Variation of Soil Resistivity with Salt (NaCl) Content, Clay-soil having 3% Moisture

Figure-7.6

7.7 Representative Values of Soil Resistivity in Various Parts of India :

The type of soil largely determines its resistivity and representative values for soils found in India are tabulated in Table-7.7. Earth conductivity is, however, essentially electrolytic in nature and is affected therefore by moisture content of the soil and its chemical composition and concentration of salts dissolved in the contained water. Grain size and distribution and closeness of packing are also contributory factors since they control the manner in which the moisture is held in soil. Many of these factors vary locally and some seasonally and, therefore, the values given in Table-7.7 should be taken only as a general guide. Local values should be verified by actual measurement and this is specially important where the soil is stratified, as owing to disposition of earth current, the effective resistivity depends not only on the surface layers but also on the underlying geological formation. • Table-7.7 :

S.No Locality Type of Soil Resistivity (ΩΩ-metre)

1 Kakarapar, Surat(Dist.), Gujrat Clayey black soil 6 - 23 2 Taptee Valley Alluvium 6 - 24 3 Narmada Valley Alluvium 4 - 11 4 Purna Valley (Deogaon) Agricultural 3 - 6 5 Dhond, Bombay Alluvium 6 - 40 6 Bijapur dist., Karnataka a) Black Cotton Soil

b) Moorm 2 - 10

10 - 50

7 Garimenapenta, Nellore dist., Andhra Pradesh

Alluvium (Highly Clayey) 2

8 Kartee a) Alluvium b) Alluvium

3 - 5 9 - 21

9 Cossipur, Calcutta Alluvium 25 approx. 10 Delhi

a) Najafgarh

a) Alluvium(dry sandy soil) b) Loamy to Clayey soil c) Alluvium (saline)

75 - 170 38 - 50 1.5 - 9

b) Chhatarpur Dry Soil 36 - 109 11 Korba, Madhya Pradesh a) Moist Clay

b) Alluvium Soil 2 - 3

10 - 20

12 Trivendrum dist., Kerala Lateritic Clay 2 - 5 13 Bhagalpur, Bihar a) Alluvium

b) Top Soil 9 - 14

24 - 46 14 Bharatpur Sandy loam (saline) 6 - 14 15 Kalyadi, Mysore Alluvium 60 - 150 16 Kolar Gold Fields Sandy surface 45 - 185 17 Wajrakarur, Andhra Pradesh Alluvium 50 - 150 18 Koyna, Satara dist.,

Maharashtra Lateritic 800 - 1200

(Dry) 19 Kutch-Kandla (Amjar area) a) Alluvium (Clayey)

b) Alluvium (Sandy) 40 - 50 60 - 200

20 Villupuram, Madras Clayey sands 11 21 Ambaji, Banaskantha (Gujrat) Alluvium 170 22 Ramanathapuram dist., Madras a) Alluvium

b) Lateritic soil 2 - 5

300 approx.

7.8 Potential Gradients :

In case of large electrical installations, it is necessary to ensure that when a person walking on the ground or touching an earthed object, in or around the premises shall not have large dangerous potential differences impressed across his body in case of a fault within or outside the premises. Such danger may arise if step potential gradients exist within the premises or between boundary of the premises and an accessible point outside. For this, step & touch potential should be investigated and kept within safe limits. The step & touch potentials can be lowered to any value by reducing the mesh interval of the grid.

7.9 Effect of Corrosion :

The resistivity of the soil varies at different places, degree of corrosiveness affects the resistivity

and the size of the conductor. However, the practical results have shown that the average corrosion maximum penetration for steel can be taken as 61 mils for 12 years, for next 8 years can be taken at half of the rate. i.e. for a period of 20 years, it will be 81 mils ~ 2mm.

7.10 Earth Bus and Earth Wires :

7.10.1 Earth Bus :

These are either stranded or solid bars or flat rectangular strips of copper, galvanised iron or steel or aluminium, may be bare, provided due care is taken to avoid corrosion and mechanical damage to it. The interconnections should be reliable and good electrical connections are permanently ensured. Welding, bolting & clamping are permissible.

7.10.2 Earth Wires :

All earth wires should be of copper, galvanised iron or steel or aluminium. They should be protected against mechanical damage and possibility of corrosion particularly at the point of connection to earth electrode or earth continuity conductor. The minimum allowable size of the wire is determined principally by mechanical consideration for they are more liable to mechanical injury and should therefore be strong enough to resist any strain that is likely to be put upon them. The path of the earth wire shall, as far as possible, be out of reach of any person.

If the metal sheath or armour are used as earth electrode, the armour should be bonded to the

metal sheath and the connection between earth wires and earthing electrode should be made to the metal sheath. In no case, neutral conductor should be used as earth wire.

7.11 Size of Earth Bus and Earth Wire :

The minimum sizes of earth bus (continuity conductor) and earth wires for buildings, industrial locations, miscellaneous electrical installations, generating stations and sub-stations shall be done based on various factors like,

7.11.1 Earth-continuity conductors & Earth wires not contained in the cables :

The size of the earth-continuity conductors should be co-related with the size of the current carrying conductors, that is, the sizes of earth-continuity conductors should not be less than half of the largest current carrying conductors, provided the minimum size of earth-continuity conductors is not less than 1.5 mm2 for copper and 2.5 mm2 for aluminium and need not be greater than 70 mm2 for copper and 120 mm2 for aluminium. As regards the sizes of galvanized iron and steel, they may be equal to the size of the current-carrying conductors with which they are used. In case of aluminium current-carrying conductors should be calculated on the basis of equivalent size of copper current-carrying conductors.

7.11.2 Earth-continuity conductor & Earth wires containing in the cables :

For flexible cables, the size of the earth-continuity conductors should be equal to the size of the current carrying conductors.

7.11.3 Voltage drop and Temperature rise :

While determining the size of the conductor, one should consider the voltage drop and

temperature rise also. Under fault conditions, the voltage drop between two normally earthed parts with which any one is likely to be in simultaneous contact should not exceed 32 Volts.

The thermal rating should be based on the short-time current rating of the associated switchgear

and a maximum temperature which will not cause damage to the earth connections or to apparatus with which they may be in contact. This can be determined by the amount of current flow and its duration, based on a maximum allowable temperature rise, which in the case of swatted and riveted joints is limited to 2500 C and for brazed joints to 4500 C.

GROUNDING SYSTEM CALCULATIONS

The intent of this topic is to determine the number of ground electrodes, size of main earthing

conductor in order to have safe and reliable grounding system for a project. 8.1 Design Basis :

• For a substation grounding calculations following parameters are considered -- a) Fault Level : 50 KA b) Duration : 1 Sec. c) Max. Soil Resistivity : 100 Ohm.mtr. d) Material of ground electrode : Hard-drawn copper

• For plant area grounding calculations following parameters are considered -- a) Largest drive : 110 KW. b) Cable size : 4 Core x 240 mm2. c) Length of cable : 175 mtr. d) Duration of fault : 0.5 Sec.[ considering that instantaneous protection

device (MCCB or Fuse) is provided in MCC for shot circuit protection which trips within max. 40 m-sec (i.e. 0.04 sec).

8.2 Determination of size of earthing conductor for sub-station grid :

The size of earth grid conductor is decided on mechanical, thermal & electrical considerations. As per IEEE - 80, 1976,

V log10 Tm - Ta +1

I = A 234 + Ta

33 S

8.0

Where, I = Max. Fault current in Amps = 50000 A = Cross-sectional Area in Circular Mils S = Duration of fault in Sec = 1 Sec. Tm = Max. allowable temp. in oC for bolted joints = 250 oC (As per IEEE 80) Ta = Max. Ambient Temp. = 50o C Substituting all these values, we get A = 596933.75 Cm

= 596933.75 x 0.0005067 mm2

= 302.5 mm2

Therefore size of earthing conductor for substation is 300 mm2 [Min. As per ITB (International

Testing Bureau) is 70 mm2 ]. 8.3 Determination of size of earthing conductor for plant grid : As considered earlier, Largest drive rating : 110 KW Cable Size : 40 x 240 Sq. mm. Cable length : 175 Mr. Fault current for fault at motor terminals,

If 0.8 x V x Aph x T r x L x (1 + m) where , r = Resistivity at normal operating temp. = 22.5 x 10 3 Ohm - mm 2 / Mtr. V = Phase to Earth Voltage = 440/ \/3 = 254 Volts.

Aph = Phase conductor size = 240 mm2 L = Length = 175 Mtr. APE = Earth Conductor Size m = APH / APE T = No. of runs of cables = 1

Therefore, If = 6192 Amps. In above formula considering m = O i.e. assuming that return path for fault current is through

earth grid & not 4th core of cable, we get

If = 12385 Amps. Using the above value of fault current (12385 Amps) in following formula,

V log10 Tm - Ta +1

I = A 234 + Ta

33 S Taking S = 0.5, Tm = 2500C and Ta = 500C, We get,

A = 109322.84 Cm (Cir. Mils)

= 109322.84 x 0.0005067 mm2

= 55.39 mm2

Therefore, Size Selected is 70 mm2 which is in line with Minimum size indicated in ITB. 8.4 Calculation of Earth Resistance :

As per IEEE 142, earth resistance of pipe earth electrode is given as,

R = ρ

Ln 4L _ 1 2πL a Where,

ρ = Soil resistivity = 100 ohm - Mtr.

L = Length of Rod = 3 Mtr. = 300 Cm.

a = Dia of Rod = 25 mm = 2.5 Cm.

Therefore, R = 27.45 Ohms. In order to get station earth resistance < 1 Ohm min 28 rods are required.

No.of Rods used = 30 Nos. & Effective Resistance, R= 27.45 / 30 = 0.91 Ohms. (Min0.5 as per ITB)

QUESTIONNAIRE FOR VALIDATION

1.0 Introduction :

1. Define “Earthing”. 2.0 Terminology :

1. Define “Step Potential” 2. Define “Touch Potential”

3.0 General Requirements :

1. Earthing shall be carried out in accordance with ______________ Rules. 2. What is the maximum permissible value of earth resistance ?

4.0 System Earthing :

1. What is “System Earthing” ? 2. What is the necessity of system earthing ? 3. What do you mean by “Floating Neutral” ? 4. What is the effect of ground fault in ungrounded system ? 5. What is “Arcing Ground” ? 6. What is the effect of arcing ground in electrical power system ? 7. Solid grounding is used in .

5.0 Equipment Grounding :

1. Equipment grounding is done for . 2. In case of generator transformer the neutral points should be directly earthed : TRUE / FALSE 3. The secondary windings, casings & frames of current and potential transformers should not be

earthed - TRUE / FALSE. 4. In case of generators in parallel, only one earth isolating switch should be kept ‘on‘ if more

than one generator are in service. Why ?

6.0 Earth Electrodes :

1. What are the various types of earthing electrodes ? 2. How do you measure the earth electrode resistance ?

7.0 Design Considerations :

1. What is the procedure for carrying out artificial treatment of soil ? 2. What are the factors affecting earth wire and earth bus size ?

9.0

REFERENCES

1. Code of Practice for Earthing, IS : 3043 - 1966.

2. Industrial Power System Hand Book, By : Donald Beeman.

3. National Electrical Code (NEC)

<<<>>><<<>>>

ANSWERS FOR THE QUESTIONNAIRE

1.0 Introduction :

1. Define “Earthing”.

The basic definition of Earthing / Grounding is intentional low resistance connection to earth / ground which is assumed to be at zero potential.

2.0 Terminology :

1. Define “Step Potential”

Step Potential - The maximum value of the potential difference possible of being shunted by a human body between two accessible points on the ground separated by the distance of one pace which may be assumed to be one metre.

2. Define “Touch Potential”

Touch Potential - The maximum value of the potential difference between a point on the ground and a point on an object likely to carry fault current such that the points which can be touched by a person, which may be assumed to be one metre.

3.0 General Requirements :

1. Earthing shall be carried out in accordance with ______________ Rules.

Indian Electricity Rules. 2. What is the maximum permissible value of earth resistance ?

The earth resistance shall not be more than 5 Ohms.

4.0 System Earthing :

1. What is “System Earthing” ? 2. 3. What is the necessity of system earthing ? 4. What do you mean by “Floating Neutral” ? 5. What is the effect of ground fault in ungrounded system ? 6. What is “Arcing Ground” ? 7. What is the effect of arcing ground in electrical power system ? 8. Solid grounding is used in .

5.0 Equipment Grounding :

1. Equipment grounding is done for . 2. In case of generator transformer the neutral points should be directly earthed : TRUE / FALSE 3. The secondary windings, casings & frames of current and potential transformers should not be

earthed - TRUE / FALSE.

4. In case of generators in parallel, only one earth isolating switch should be kept ‘on‘ if more

than one generator are in service. Why ?

6.0 Earth Electrodes :

1. What are the various types of earthing electrodes ? 2. How do you measure the earth electrode resistance ?

7.0 Design Considerations :

1. What is the procedure for carrying out artificial treatment of soil ? 2. What are the factors affecting earth wire and earth bus size ?