Handbook on Electrical Earthing

CAMTECH/E/10-11/El-Earthing/1.0

Handbook on Electrical Earthing December, 2010

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December, 2010 Handbook on Electrical Earthing

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FOREWORD

Earthing of electrical installations/ equipments plays very important role in safe functioning of system as well as safety of personnel. It provides low impedance path to fault currents and ensures prompt and consistent operation of protective devices during ground faults.

CAMTECH has prepared this handbook on electrical earthing system for general services to disseminate knowledge to working personnel.

The handbook contains construction of earthing, earthing arrangement at substation, maintenance schedules, maintenance free earthing etc.

I hope this handbook will prove to be useful to maintenance personnel working in general services department.

CAMTECH, Gwalior S.C. Singhal Date: 27.01. 2011 Executive Director



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6

PREFACE

The proper upkeep and maintenance of earthing system is necessary to ensure good and reliable protection system for electrical equipment and to avoid shock to the operator. This handbook on Electrical Earthing has been prepared by CAMTECH with the objective of making our maintenance personnel aware of earthing systems to be adopted in field.

It is clarified that this handbook does not supersede any existing provisions of IS code of earthing (IS: 3043), IE Rules and other existing provisions laid down by RDSO or Railway Board. This handbook is for guidance only and it is not a statutory document.

I am sincerely thankful to all field personnel who helped us in preparing this handbook.

Technological up-gradation & learning is a continuous process. Please feel free to write to us for any addition/ modification in this handbook. We shall highly appreciate your contribution in this direction.

CAMTECH, Gwalior Peeyoosh Gupta Date:31.12.2010 Jt.DirectorElectrical e - mailid:[email protected]



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CONTENTS

Sr.No. Description Page No.

Foreword iv Preface vi Contents viii Correction Slip xii

1.0 INTRODUCTION 01 1.1 ADVANTAGES OF EARTHING 02 1.2 TERMINOLOGY 03 1.3 EARTH AS CONDUCTOR 05 1.4 IMPORTANT INDIAN ELECTRICITY

RULE RELATED TO EARTHING 05 1.5 GENERAL REQUIREMENT

FOR EARTHING 07 1.6 DIFFERENCE BETWEEN

GROUNDING AND EARTHING 10 1.7 HUMAN ELEMENT & ELECTRIC

SHOCK 11 1.8 FACTORS WHICH DETERMINE

RESISTIVITY OF SOIL 13 1.9 LOCATION OF EARTH ELECTRODE 19

2.0 DESIGN, SIZE AND TYPES OF EARTH ELECTRODE 20 2.1 ELECTRODE RESISTANCE TO EARTH 20 2.2 INFLUENCING FACTORS FOR

ELECTRODE RESISTANCE 21 2.3 ELECTRODE SIZE 22 2.4 DESIGN OF EARTH ELECTRODES 22 2.5 EARTH ELECTRODE 24



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Sr.No. Description Page No.

2.6 TYPES OF ELECTRODES 26 2.7 TYPES OF EARTHING 32 2.8 EARTHING LEAD 35

3.0 EARTHING SYSTEM 35 3.1 CLASSIFICATION OF EARTHING 35 3.2 EARTHING SYSTEM IN SUB STATION 41 3.3 EARTHING OF VARIOUS EQUIPMENT IN

THE SUB-STATIONS 43 3.4 DISTRIBUTION TRANSFORMER

STRUCTURE EATHING 47 3.5 EARTHING OF DISTRIBUTION LIENE

STRUCTURES 51 3.6 EARTHING AT CONSUMERS PREMISES 53 3.7 EARTHING IN INDUSTRIAL PREMISES 56 3.8 DANGERS OF IMPERFECT EARTHING 57 3.9 PRECAUTIONS 58

4.0 TESTING & MAINTENANCE 59 4.1 TESTING OF EARTHING SYSTEM 59 4.2 MAINTENANCE SCHEDULES 62

5.0 MAINTENANCE FREE EARTHING 65 5.1 EARTH RESISTANCE 66 5.2 APPLICATIONS 66 5.3 MAINTENANCE FREE EARTHING SYSTEM 66

6.0 DOS & DONTS 77 6.1 DOS 77 6.2 DONT 78 REFRENCES 81



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ISSUE OF CORRECTION SLIP

The correction slips to be issued in future for this handbook will be numbered as follows:

CAMTECH/E/10-11/El-Earthing/1.0/ C.S. # XX date---

Where XX is the serial number of the concerned correction slip (starting from 01 onwards).

CORRECTION SLIPS ISSUED

Sr. No. Date of issue

Page no. and Item no. modified

Remarks



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1.0 LrkoukLrkoukLrkoukLrkouk INTRODUCTION

Earthing is a connection done through a metal link between the body of any electrical appliance, or neutral point, as the case may be, to the deeper ground through these metal links, normally of MS flat, CI flat, GI wire penetrated to the earth grid. Object of earthing is that all parts of apparatus other than live parts shall be at earth potential.

Earthing eliminates the possibility of any dangerous potential rise on the body of electrical equipment. It drains away the charge on the equipment through an earth connection. When an earth fault is occurres such as winding insulation failure etc. causes a heavy current flow into the general mass of the earth. This causes blowing out of fuse or operation/ tripping of protective devices. The potential under and around of the object shall be uniform nearly to zero w.r.t. earth.

Apart from this it is to ensure that operators or working personnel shall be at earth potential at all times, so that there will be no potential difference to cause shock or injury to a person, whenever any short circuit takes place.

The primary requirements of a good earthing system are: a. It stabilizes circuit potential with respect to

ground potential and limits the potential rise. b. It protects men & materials from injury or

damage due to over voltage or touching.



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c. It provides low impedance path to fault currents to ensure prompt & consistent operation of protective devices during earth fault.

d. It keeps the maximum voltage gradient along the surface inside & around the substation within safe limits during earth fault.

e. It protects underground cables from overall ground potential rise & voltage gradient during ground fault in the system.

1.1 vfFkZax ds ykHkvfFkZax ds ykHkvfFkZax ds ykHkvfFkZax ds ykHk ADVANTAGES OF EARTHING

For efficient/effective operation of any power system, it is essential to connect the neutral to suitable earth connection. The following are the few advantages: Reduced operation & maintenance cost Reduction in magnitude of transient over voltages. Improved lightning protection. Simplification of ground fault location. Improved system and equipment fault protection. Improved service reliability Greater safety for personnel & equipment Prompt and consistent operation of protective

devices during earth fault.



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1.2 ikfjHkkf"kd 'kCnkoyhikfjHkkf"kd 'kCnkoyhikfjHkkf"kd 'kCnkoyhikfjHkkf"kd 'kCnkoyh TERMINOLOGY The following terms are commonly used in

earthing systems:

1.2.1 vFkZ vFkZ vFkZ vFkZ Earth The conductive mass of the earth, whose

electrical potential at any point is conventionally taken as zero.

1.2.2 vFkZ bySDVksMvFkZ bySDVksMvFkZ bySDVksMvFkZ bySDVksM Earth electrode A Galvanized Iron (GI) pipe in intimate contact

with and providing an electrical connections to earth.

1.2.3 vfFkZavfFkZavfFkZavfFkZax fxzMx fxzMx fxzMx fxzM Earthing grid A system of a number of interconnected,

horizontal bare conductors buried in the earth, providing a common ground for electrical devices and metallic structures, usually in one specific location.

1.2.4 midj.kmidj.kmidj.kmidj.k vfFkZax vfFkZax vfFkZax vfFkZax Equipment Earthing It comprises earthing of all metal work of

electrical equipment other than parts which are normally live or current carrying. This is done to ensure effective operation of the protective gear in the event of leakage through such metal work, the potential of which with respect to neighboring objects may attain a value which would cause danger to life or risk of fire.

1.2.5 .kkyh .kkyh .kkyh .kkyh vfFkZaxvfFkZaxvfFkZaxvfFkZax System Earthing Earthing done to limit the potential of live

conductors with respect to earth to values which the insulation of the system is designed to withstand and to ensure the security of the system.



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1.2.6 Vp oksYVstVp oksYVstVp oksYVstVp oksYVst Touch Voltage (E Touch)

The potential difference between a ground metallic structure and a point on the earths surface separated by a distance equal to the normal maximum horizontal reach of a person, approximately one meter as shown in figure-1

1.2.7 LVsi oksYVstLVsi oksYVstLVsi oksYVstLVsi oksYVst Step Voltage (E Step)

The potential difference between two points on the earth's surface separated by distance of one pace that will be assumed to be one meter in the direction of maximum potential gradient as shown in figure -2

Figure-1

Figure-2



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1.2.8 eSk oksYVsteSk oksYVsteSk oksYVsteSk oksYVst Mesh Voltage (E mesh)

The maximum touch voltage to be found within a mesh of an earthing grid.

1.3 vFkZ pkyd vFkZ pkyd vFkZ pkyd vFkZ pkyd EARTH AS CONDUCTOR Resistivity () of earth is 100-M. Resistivity () of copper is 1.7x 10 -8 -M. Resistivity () of G. I. is 1.7x 10 -7 -M.

Take as reference, 25x 4 mm copper strip. To obtain the same resistance, the size of G.I. will be 65mm x10mm. The corresponding figure for earth is 800mtrs x 800mtrs (158 acres.)Hence, it shows metallic conductor is a preferred alternative conductor to earth to bring the fault current back to source.

1.4 vvvvffffFkZFkZFkZFkZ axaxaxax ls lEcfU/kr egkRoiw.kZ Hkls lEcfU/kr egkRoiw.kZ Hkls lEcfU/kr egkRoiw.kZ Hkls lEcfU/kr egkRoiw.kZ Hkkkkkjrh; fo|qr fu;ejrh; fo|qr fu;ejrh; fo|qr fu;ejrh; fo|qr fu;e IMPORTANT INDIAN ELECTRICITY RULE RELATED TO EARTHING

fu;e 33 fu;e 33 fu;e 33 fu;e 33 RULE No: 33 Earth terminals on consumers premises.

fu;e 61 fu;e 61 fu;e 61 fu;e 61 RULE No: 61. (A) Max: permissible resistance of earthing system.

Large power station : 0.5 ohms. Major sub-station : 1.0 ohms. Small sub-station : 2.0 ohms. In all other cases : 8.0 ohms. The earth continuity

inside an installation : 1.0 ohms.



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(B) Connection with Earth: Earthing of neutral conductor of a 3-phase,

4-wire system.

Earthing of all metal casing / covering of electric supply lines or apparatus.

Testing of such earth resistance not less than once in every two years during a dry day of a dry season shall be conducted and recorded.

Test results should be recorded and shall be made available to the EIG or Assisting officer to EIG, when required.

fu;e 67 fu;e 67 fu;e 67 fu;e 67 RULE No: 67. Connection to earth

All equipments associated with HV/EHV installation shall be earthed by not less than two distinct and separate connections with the earth having its own electrode, except an earth mat.

Testing of such earth resistance not less than once in every year during a dry day of a dry season shall be conducted & recorded

fu;e 90 fu;e 90 fu;e 90 fu;e 90 RULE No: 90. Earthing In distribution system, all metal supports and all

reinforced/ pre-stressed cement concrete supports of overhead line and metallic fittings attached shall be permanently and effectively earthed.

Each stay wire shall be similarly earthed, unless insulators have been provided in it at a height not less than three mtrs from the ground.



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Every 5th pole as a minimum shall be grounded, if the foundations are not cements concrete blocks.

fu;e fu;e fu;e fu;e 91 91 91 91 RULE No: 91. Safety and protective device Every overhead line erected over any part of street

or public place shall be protected with a device, approved by the EIG, for rendering the line electrically harmless in case it brakes.

The owner of every high and extra high overhead line shall be protected to the satisfaction of the EIG, to prevent unauthorized persons from ascending any of the supports of such overhead lines.

fu;e 92 fu;e 92 fu;e 92 fu;e 92 RULE No: 92. Protection against lightening The owner of every overhead line which is so

exposed, as may be liable to injury from lightening, shall adopted efficient means for diverting to earth, any electrical surge during lightening.

The earthing lead for any lightening arrester shall not pass through any iron or steel pipe but shall be taken as directed as possible from the lightening arrester to a separate earthing electrode/ mat.

1.5 vfFkZax vfFkZax vfFkZax vfFkZax ds fy, lkekU; vko;drk;sa ds fy, lkekU; vko;drk;sa ds fy, lkekU; vko;drk;sa ds fy, lkekU; vko;drk;sa GENERAL REQUIREMENT FOR EARTHING Earthing shall generally be carried out in

accordance with the requirement of I.E. rules, 1956, as amended from time to time and the relevant regulation of the electricity supply. Codes /Standard given below may also be referred : i) IS:3043 - Code of practice for earthing

(latest)



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ii) National Electricity Code - 1985 of BIS iii) IEEE guide for safety in AC substation

grounding no. ANSI/IEEE standard, 80-1986.

In cases where direct earthing may prove harmful rather than provide safety, relaxation may be obtained from the competent authority.

Earth electrodes shall be provided at generating stations, substations and consumer premises in accordance with the requirements.

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

All connections shall be carefully made. If they are not properly made or are inadequate for the purpose for which they are intended, loss of life or serious personnel injury may result.

Each earth system shall be so devised that the testing of individual earth electrode is possible. It is recommended that the value of any earth system resistance shall not be more than 5 ohms unless otherwise specified.

The minimum size of earthing lead used on any installations shall have a nominal cross-section at areas of not less than 3.0 mm2 if of copper and 6.0 mm2 if of galvanized iron or steel. The actual size will depend on the maximum fault current which the earthing lead will be required to carry safely.

It is recommended that a drawing showing the main earth connection and earth electrode be prepared for each installation.



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No addition to the existing load whether temporary or permanent shall be made, which may exceed the assessed earth fault or its duration until it is ascertained that the existing arrangement of earthing is capable of carrying the new value of earth fault current resulting due to such addition.

All materials, fittings etc. used in earthing shall confirm to Indian Standard specification wherever these exist. In the case of material for which Indian standard specifications does not exists, the material shall be approved by the competent authority.

An earthing electrode shall not be situated with in a distance of 1.5 meter from the building whose installation system is being earthed.

The earthing electrode shall always be placed in vertical position inside the earth or pit so that it may not be in contact with all the different earth layers.

The sensitivity of the protective equipment, system voltage and the maximum fault current directly relate to permissible value of earth resistance. In case the earth exceeds the permissible value, then in the event of earth fault, the fault current may not reach a sufficient value to operate the protective equipment (such as fuses or relays) and dangerous condition may arise.

The earth wire and earth electrode will be of same material. The earth wire shall be taken through G.I. pipe of 13 mm diameter for at least 30 cm length above and below ground surface to



10

the earth electrode to protect it against mechanical damage.

All the earth wires run along the various sub-circuits shall be terminated and looped firmly at the main board and from main board the main earth shall be taken to earth electrode. The loop earth wires used shall not be either less than 2.9 mm2 (14 SWG) or half of the size of the sub circuit conductor.

1.6 xzkf.Max ,oa vfFkZax esa vUrjxzkf.Max ,oa vfFkZax esa vUrjxzkf.Max ,oa vfFkZax esa vUrjxzkf.Max ,oa vfFkZax esa vUrj DIFFERENCE BETWEEN GROUNDING AND EARTHING

1.6.1 xzkfUxzkfUxzkfUxzkfUMax Max Max Max Grounding

Grounding implies connection of current carrying parts to ground. It is mostly either generator or transformer neutral. Hence it is generally called neutral grounding. Grounding is for equipment safety.

There are three requirements for grounding: a. Shall provide a low impedance path for the

return of fault current, so that an over current protection device can act quickly to clear the circuit.

b. Shall maintain a low potential difference between exposed metal parts to avoid personnel hazards.

c. Shall control over voltage.



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1.6.2 vfFkZaxvfFkZaxvfFkZaxvfFkZax Earthing

Earthing implies connection of non current carrying parts to ground like metallic enclosures. Earthing is for human safety.

Under balanced operating conditions of power systems, earthing system does not play any role. But under any ground fault condition, it enables the ground fault current to return back to the source without endangering human safety as shown in figure -.3

1.7 fo|qr dk >Vdk ,oa ekuo vo;ofo|qr dk >Vdk ,oa ekuo vo;ofo|qr dk >Vdk ,oa ekuo vo;ofo|qr dk >Vdk ,oa ekuo vo;o HUMAN ELEMENT & ELECTRIC SHOCK

Electric shock is possible only when the human body bridges two points of unequal potential as shown in fgure-4.

Maximum tolerable current for human body is 160 mA for one second. If this limit exceeds, it will result in death due to ventricular fibrillation (heart attack).

EARTHING

GENERATOR TRANSFORMER

NEUTRAL GROUNDINGNEUTRAL GROUNDING

Figure-3



12

Allowable body current IB (Amperes) for two body weights, as per IEEE STD:-80 are given:

IB = 0.116/ TS for body weights of 50kg. = 0.157 / TS for body weight of 70kg. TS = Duration of current exposure (fault clearance time). TS IB (50kg) IB (70kg). 0.2sec 259 mA 351 mA. 0.5sec 164 mA 222 mA. 1.0sec 116 mA 157 mA.

Figure 4 - Current flow under fault condition



13

1.8 feV~Vh dh frjks/kdrk fu/kkZfjr djus okys dkjdfeV~Vh dh frjks/kdrk fu/kkZfjr djus okys dkjdfeV~Vh dh frjks/kdrk fu/kkZfjr djus okys dkjdfeV~Vh dh frjks/kdrk fu/kkZfjr djus okys dkjd FACTORS WHICH DETERMINE RESISTIVITY OF SOIL

The resistivity of soil for earthing system depends upon the following factors:

Type of soil Moisture content Chemical composition of salt dissolved in the

contained water Concentration of salt Temperature of material Grain size and distribution of grain size Size and spacing of earth electrodes

1.8.1 feV~Vh dh frjks/kdrk de djus dh fof/kfeV~Vh dh frjks/kdrk de djus dh fof/kfeV~Vh dh frjks/kdrk de djus dh fof/kfeV~Vh dh frjks/kdrk de djus dh fof/k;k;k;k;k Methods of Reducing Resistivity of Soil

feV~Vh dh frjks/kdrk ds dkj feV~Vh dh frjks/kdrk ds dkj feV~Vh dh frjks/kdrk ds dkj feV~Vh dh frjks/kdrk ds dkj Types of soil resistivity

Sl. No.

Type of soil Resistivity in Ohm-cm

1 Loamy garden soil 500 - 5000 2 Clay 800 - 5000 3 Clay, Sans and Gravel mix 4000 - 25000 4 Sand and Gravel 6000 - 10000 5 Slates, Slab sand stone 1000 - 50000 6 Crystalline Rock 20000 - 100000



14

1.8.2 feV~Vh dk mipkjfeV~Vh dk mipkjfeV~Vh dk mipkjfeV~Vh dk mipkj Soil Treatment When the soil resistance is high, even the

multiple electrodes in large number may also fail to produce low resistance to earth. To reduce the resistivity of soil immediately surrounding the electrode some salt substances are made available as a solution with water. The substances are used salt sodium chloride (NaCl), Calcium chloride (CaCl2) Sodium carbonate (Na2CO3), copper sulphate (CuSO4) and soft cock and charcoal in suitable proportion.

Nearly 90% of resistance between electrode and soil is with in a radius of two meters from electrode/ rod. Treating this soil will result in required reduction in earth resistance by excavation of one meter diameter around top of the electrode/ rod to 30 cm deep and applying artificial soil treatment agency and watering sufficiently.

General practice to treat the soil surrounding the ground electrode with common salt, charcoal and soft cock in order to bring down the earth resistance. These conventional methods are effective in soils of moderately high resistivity up to 300 ohm-meter. When the soil resistivity exceeds this value, these conventional methods of chemical treatment will be inadequate to get desired value of earth resistance.



15

1.8.3 feV~Vh mipkj esa cSUVksukbV dk ;ksxfeV~Vh mipkj esa cSUVksukbV dk ;ksxfeV~Vh mipkj esa cSUVksukbV dk ;ksxfeV~Vh mipkj esa cSUVksukbV dk ;ksx Use of Bentonite in Soil Treatment Bentonite is clay with excellent electrical

properties. It swells to several times its original volume when suspended in water. It binds the water of crystallization and the water absorbed during the mixing process is retained over a long period. Bentonite suspension in water when used to surround the earth electrode virtually increases the electrode surface area.

Use of bentonite around the earth electrode results in reduction of ground resistance by about 25- 30 %.

Bentonite has a tremendous capacity to absorb water and retain it over along period.

Even during the summer months, bentonite suspension retains the moisture where as the natural soil dries up.

Bentonite may be used to advantage in rocky terrain.

1.8.4 feV~Vh mipkj esa feV~Vh mipkj esa feV~Vh mipkj esa feV~Vh mipkj esa eghu jk[k eghu jk[k eghu jk[k eghu jk[k dk ;kdk ;kdk ;kdk ;ksxsxsxsx Use of fly ash in soil treatment

As per CPRI studies reveals that fly ash from thermal stations has equivalent chemical composition and hence can be used for the electrical installations in areas of high ground resistivity. Fly ash can also be used as a chemical treatment material to reduce soil resistivity.



16

1.8.5 vFkZ frjks/kdrk ij vkvFkZ frjks/kdrk ij vkvFkZ frjks/kdrk ij vkvFkZ frjks/kdrk ij vknznznznzrk dk Hkkork dk Hkkork dk Hkkork dk Hkko Effect of Moisture Content on Earth Resistivity

Moisture content is expressed in percentage by weight of dry soil. Dry earth weights about 1440 kg/m3. Therefore about 144 kg (10%) of water is required per cubic meter of soil to have 10% of moisture content. About 20% moisture the resistivity is very little affected below 20% moisture the resistivity increases very abruptly with decrease in moisture. Moisture content of about 17% to 18% by weight of dry soil is the optimum requirement. Availability of moisture assists formation of electrolyte by dissolving salt content in soils and there by enhance the conductivity of soil. More water content can not improve soil resistivity.

Resistivity(Ohm-cm) Moisture content(% by weight) Top Soil Sandy Loam

0 1000x106 1000x106 2.5 250000 150000 5 165000 43000 10 53000 18500 20 12000 6300 30 6400 4200

1.8.6 rkieku dk Hkkorkieku dk Hkkorkieku dk Hkkorkieku dk Hkko Effect of Temperature

The temperature coefficient of resistivity for soil is negative, but is negligible for temperatures above freezing point. At about 200 C the water in the soil begins to freeze and introduce a tremendous increase in the temperature coefficient. The resistivity changes 9% per degree C. Below 0 degree C resistivity rises abnormally.



17

Effect of Temperature on Resistivity

0C 0F Resistivity(Ohm-cm) 20 68 7,200 10 50 9,900 0 32(Water) 13,000 0 32(Ice) 30,000 -5 23 79,000 -15 14 330,000

1.8.7 feV~Vh dh frjks/kdrk feV~Vh dh frjks/kdrk feV~Vh dh frjks/kdrk feV~Vh dh frjks/kdrk ddddk tax ij k tax ij k tax ij k tax ij HkkoHkkoHkkoHkko Effect of Soil Resistivity on Corrosion

Resistivity plays an important role in so far as the corrosion performance of earthing rods is concerned. It is observed that soils having resistivity of less than 25 ohm-meter are severely corrosive in nature while corrosion rate is of less importance in soils of resistivity over 200 ohm- meter. The methods adopted to safe guard earthing conductors against corrosion depends upon

a. Material of the conductor b. Corrosivity of the soil c. Size of the grounding system

Range of soil resistivity (Ohm-metre)

Class of soil

Less than 25 Severely corrosive 25 50 Moderately corrosive 50 -100 Mildly Above 100 Very mildly corrosive



18

1.8.8 lrg ij iRFkj pwjk dh irZ ds Qk;ns lrg ij iRFkj pwjk dh irZ ds Qk;ns lrg ij iRFkj pwjk dh irZ ds Qk;ns lrg ij iRFkj pwjk dh irZ ds Qk;ns Advantages of Crushed Rock Used as a Surface Layer It provides high resistivity surface layer It serves as impediment to the movement of

reptiles and there by help in minimizing the hazards which can be caused by them

It prevents the formation of pools of oil from oil insulated and oil cooled electrical equipment

It discourages the growth of weeds It helps retention of moisture on the underlying

soil and thus helps in maintaining the resistivity of the subsoil at lower value.

It discourages running of persons in the switchyard and saves them from the risk of being subjected to possible high step potentials.

1.8.9 iRFkj pwjk dh eghu irZ dk Hkko iRFkj pwjk dh eghu irZ dk Hkko iRFkj pwjk dh eghu irZ dk Hkko iRFkj pwjk dh eghu irZ dk Hkko Effect of Thin Layer of Crushed Rock

In outdoor switchyard, a thin layer of crushed rock is spread on the surface.

The resistivity of gravel () is 2000 ohm-meter while that of soil is 100 ohm-meter. Since of gravel is high, only a high voltage can force the current through the body to cause injuries. The gravel act like insulator & throws the electric field generated by GPR back to soil.



19

1.9 vFkZ bysDVksM yxkus dk LFkku vFkZ bysDVksM yxkus dk LFkku vFkZ bysDVksM yxkus dk LFkku vFkZ bysDVksM yxkus dk LFkku LOCATION OF EARTH ELECTRODE

The location of earth electrode should be chosen in one of the following types of soil in the order of preference given on next page

Wet marshy ground. Clay, loamy soil and arable land Clay and loam mixed with varying proportions of

sand, gravel and stones. Damp and wet sand, peat.

Dry sand, gravel chalk limestone, granite, very stone ground and all locations where virgin rock is very close to the surface should be avoided.



20

2.0 vFvFvFvFkZ bysDVksM ds fMtk;u] lkbt ,oa dkjkZ bysDVksM ds fMtk;u] lkbt ,oa dkjkZ bysDVksM ds fMtk;u] lkbt ,oa dkjkZ bysDVksM ds fMtk;u] lkbt ,oa dkj DESIGN, SIZE AND TYPES OF EARTH ELECTRODE

2.1 bysDVksM ,oa vFkZ ds e/;bysDVksM ,oa vFkZ ds e/;bysDVksM ,oa vFkZ ds e/;bysDVksM ,oa vFkZ ds e/; frjks/k frjks/k frjks/k frjks/k ELECTRODE RESISTANCE TO EARTH

Conventional practices to measure the earth resistance is by using ohms law.

For electrode resistance to earth, current is injected to earth by electrode and electric field travels through the earth. The voltage appears at certain distance from electrode and the resulting impedance is electrode resistance to earth.

This is similar to CT, where the flow of primary current results in voltage appearing across CT secondary. This drives the current through the connected relay (burden) as shown in figure- 5

IF

CT RV

x

v1

Resistance area of driven earth rod

Figure-5



21

2.2 bysDVksM frjks/k dks Hkkfor djus okys dkjdbysDVksM frjks/k dks Hkkfor djus okys dkjdbysDVksM frjks/k dks Hkkfor djus okys dkjdbysDVksM frjks/k dks Hkkfor djus okys dkjd INFLUENCING FACTORS FOR ELECTRODE RESISTANCE:

The major factor is the length, diameter or width (Cross section) has very minor influence.

The resistance of pipe electrode is given by

R = ( / 2 piL) [LN {8L / ( x 2.7183)}]. Where, L = Length in meter. (Pipe)

LN=Nominal length (buried conductor)

= Diameter in meter

Let, consider the case of a length = 6 meter.

For = 2.5 Cm, R = 16.4 .

For = 10 Cm, R = 15.3 .

So, it is observed that 300% increase in diameter, resistance decreases by app 7% only.

The electrode resistance is not much dependent on type of electrode materials like Cu, Al or GI. Resistance is the function of physical dimension, mainly length.

A horizontal earth strip of 75mm x 10mm Cu and 45mm x 10mm GI both of same length will offer almost same electrode resistance.



22

2.3 bysDVksM dk lkbtbysDVksM dk lkbtbysDVksM dk lkbtbysDVksM dk lkbt ELECTRODE SIZE

The choices for materials & size are only with respect to the amount of fault current to be discharged to earth.

The current density (A/mm2) as per IS-3043. Materials Cu Al GI 0.5 sec rating 290 178 113 1 sec rating 205 126 80

Earthing grid for EHV switchyards is designed for 0.5 sec duty & for others 1sec duty is selected.

2.4 vFkZ bysDVksM dh vFkZ bysDVksM dh vFkZ bysDVksM dh vFkZ bysDVksM dh fMtkbufMtkbufMtkbufMtkbu DESIGN OF EARTH ELECTRODES

2.4.1 bysDVksM frjks/kdbysDVksM frjks/kdbysDVksM frjks/kdbysDVksM frjks/kd ij vkdkj dk Hkko ij vkdkj dk Hkko ij vkdkj dk Hkko ij vkdkj dk Hkko Effect of Shape on Electrode Resistance

With electrodes, the greater part of the fall in potential occurs in the soil within about 2m of the electrode surface, since it is here that the current density is highest. To obtain a low overall resistance the current density should be as low as possible in the medium adjacent to the electrode and should decrease rapidly with distance from the electrode. This requirement is met by making the dimensions in one direction large compared with those in the other two. Thus we find that a pipe, rod or strip will have much lower resistance than a plate of equal surface area. The resistance is not, however, inversely proportion to the surface area of the electrode.

The theoretical principles relating to calculation of resistance of earth electrodes are dealt with in the



23

resistance of any electrode in the earth is in fact related to the capacitance of that electrode and its image in free space since it can bare shown that the lines of current flow are identical with the electrostatic lines of force which would result if the earth were a dielectric and the electrode with its image in the earths surface where a considered as a condenser in free space.

This relationship is given by

100 R= -------- 4piC

Where R= Resistance in an infinite medium = Resistance of the medium in ohm-meter C = Capacitance of the electrode and its

image in free space. In the practical case the medium is divided into

two by the plane of earths surface so that 100 R= -------- 2piC

Thus, if the capacitance in free space of any form of electrode is known together with the resistivity of the surrounding soil, the resistance of the electrode can be calculated. This capacitance is known for some simple forms of electrodes.

Applying this principle, resistance of pipe and rod electrodes, strip electrodes and plate electrodes can be calculated.



24

2.5 vFkZ bysDVksMvFkZ bysDVksMvFkZ bysDVksMvFkZ bysDVksM EARTH ELECTRODE

It is a metal pipe, rod or other conductor which makes an effective connection with the general mass of the earth.

When a fault is passing, the potential of the electrode is much above the general mass of the earth. The potential exists over an area in the vicinity of the electrode. The potential gradient i.e. the voltage drop between two points on the earth surface is high close around the electrode. It decreases as moved away from the electrode. Each electrode has a resistance area within which the voltage gradient exists.

The resistance areas of two earth electrode should not overlap each other; otherwise the effectiveness of the electrode is reduced as shown in figure-6. The recommended distance between the two electrodes is twice of its length minimum, if the rod length is L, separation distance shall be 2L as shown in figure-7 on next page.

To obtain low effective earth grid resistance, electrodes are connected in parallel. The total resistance will be half of individual resistance.



25

Fig-6

Fig-7

I

OOvveerr llaappppiinngg rreessiissttaannccee aarreeaass ooff ttwwoo eeaarrtthh

rrooddss

L

SSeeppaarraattiioonn ddiissttaannccee

2L

I



26

2.6 bysDVksM ds dkjbysDVksM ds dkjbysDVksM ds dkjbysDVksM ds dkj TYPES OF ELECTRODES

Types of earth electrodes are used as follows:

2.6.1 IysV bysDVksM IysV bysDVksM IysV bysDVksM IysV bysDVksM Plate Electrode

Plate electrode may be made of copper, galvanized iron or steel. If electrode made of copper the minimum size is 60 cm x 60 cm x 3.15 mm. If of galvanized iron or steel, the minimum size should be 60 cm x 60 cm x 6.3 mm.

Plate electrode shall be buried such that its top edge is at a depth not less than 1.5 m from the surface of the ground. Where the resistance of one plate electrode is higher than the required value, two or more plates shall be used in parallel. In such a case two plates shall be separated from each other by not less than 8.0 m. Plate shall preferably be set vertically. Use of plate electrode is recommended only where the current carrying capacity is the prime consideration i.e. generating stations and substations.

If necessary, plate electrodes shall have a galvanized iron water pipe buried vertically and adjacent to the electrode. One end of the pipe shall be at least 5 cm above the surface of the ground and need not be more than 10 cm .The internal diameter of the pipe shall be at least 5 cm and need not be such that it should be able to reach the center of the plate. In no case, however, shall it be more than the depth of the bottom edge of the plate.

Plates to be buried vertically in pits and surrounded by finely divided coke, crushed coal or



27

charcoal at least 150 mm all round the plates. Plates should not be less than 12.2 m apart and should be buried at sufficient depth to ensure that they are always surrounded by moist earth as shown in figure-8

Figure- 8 Plate Earth Electrode

12.7mm dia GI PIPE

COPPER OR GI WIRE

BOLT, NUT, CHECK NUT AND WASHER TO BE OF COPPER FOR COPPER PLATE AND GI FOR GI PLATE

GROUND LEVEL CI COVER

WIRE MESH

CEMENT CONCRETE

10mm DIA GI PIPE

CHARCOAL

1.5m (Min.)

15 cm

60 cm 90 cm

15 cm

60cm X 60cm X 6.30mm GI PLATE OR 60cm X 60 cm X 3.15mm COPPER PLATE

50cm

70 cm

CI FRAME

FUNNEL

View of section A

A



28

2.6.1.1 IysV bysDVksM dh fMtkIysV bysDVksM dh fMtkIysV bysDVksM dh fMtkIysV bysDVksM dh fMtk;u ;u ;u ;u Design of plate electrode

In designing plate electrodes, the resistance may be calculated from, the following formula

pi R= ------ ------- ohms

4 A Where

= Resistivity of soil in ohm-meter A = Area of both sides of plate in m2

In practice little gain is obtained by increasing the plate area of on side by more than 1.75 m2

2.6.2 IkkbZi bysDVksM IkkbZi bysDVksM IkkbZi bysDVksM IkkbZi bysDVksM Pipe Electrode

It should be made of B class G.I pipe. The internal diameter should not be smaller than 38 mm and it should be 100 mm for cast Iron pipe. The length of the pipe electrode should not less than 2.5 m. It should be embedded vertically. Where hard rock is encountered it can be inclined to vertical. The inclination shall not more than 300 from the vertical.

To reduce the depth of burial of an electrode without increasing the resistance, a number of pipes shall be connected together in parallel. The resistance in this case is practically proportional to the reciprocal of the number of electrodes used so long as each is situated outside the resistance area of the other. The distance between two electrodes in such a case shall preferably be not less than twice the length of the electrode as shown in figure 9



29

Figure 9 Pipe Electrode

2500

m

m (M

in.)

CHARCOAL OR COKE AND SALT IN ALTERNATE LAYER OF 300

300mm

CLAMP

40MM DIA G.I. PIPE

200mm

300 300

8 SWG

G L

G.I. WIRE

12MM DIA HOLES



30

2.6.2.1 IkkbZi bysDVksM dh fMtk;u IkkbZi bysDVksM dh fMtk;u IkkbZi bysDVksM dh fMtk;u IkkbZi bysDVksM dh fMtk;u Design of pipe electrode

In designing drive rod or pipe electrodes, the resistance may be calculated from the following formula:

100 4L R= -------- loge ------ ohms 2piL d

Where

= Resistivity of the soil in ohm-meter L = Length of rod or pipe in cm, and D = Diameter of rod or pipe in cm.

Consideration of the above formula will show that theoretical resistance to earth of a driven rod electrode depends to a large degree upon its buried length and to a lesser extent upon its diameter.

2.6.3 iV~Vh bysDVksM iV~Vh bysDVksM iV~Vh bysDVksM iV~Vh bysDVksM Strip Electrode

Where strip electrode is used for earthing, it should not be less than 25 mm x 1.60 mm, if made of copper and 25 mm x 4 mm if made of G.I. or steel. The length of the buried conductor should not be less than 15 m. laid in a trench not less than 0.5 m depth. If round conductors are used, their cross-sectional area shall not be smaller than 3.0 mm2 if of copper and 6 mm2 if of galvanized iron or steel.

The electrodes shall be widely distributed as possible, preferably in a single straight or circular trench or in a number of trenches radiating from a point. If the conditions necessitate use of more than one



31

strip, they shall be laid either in parallel trenches or in radial trenches as shown figure-10

Resistance for strip or horizontal wire electrode is measured by RYDERs formula:-

R = ( / 2 piL) [LN (8L/T) +LN (L/h) - 2 + (2h/L)-(h2/L2)].

Where,

L = Length in meter.(electrode) LN= Nominal length (buried conductor) h = Depth in meter. T = Width in meter (for strip).

2.6.4 dscy 'khFk dscy 'khFk dscy 'khFk dscy 'khFk Cable Sheaths

Where an extensive underground cable system is available, lead sheathed and steel armored 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 earth electrode system is generally less than one ohm.

L

h

Figure 10 Strip Electrode



32

2.7 vfFkZax vfFkZax vfFkZax vfFkZax dsdsdsds dkj dkj dkj dkj TYPES OF EARTHING

The various types of earthing are as follows:

2.7.1 ffffLVLVLVLVi vfFkZaxi vfFkZaxi vfFkZaxi vfFkZax Strip Earthing

In this system of earthing strip electrodes of cross section not less than 25 mm x 1.6 mm if of copper and 25 mm x 4 mm if galvanized iron or steel are buried in horizontal trenches of minimum of depth 0.5 meter. If round conductors are used, their cross-sectional area shall not be smaller than 3.0 mm2 if of copper and 6 mm2 if of galvanized iron or steel. The length of buried conductor shall be sufficient to give the required earth resistance. It shall not be less than 15 meters. The electrodes shall be as widely distributed as possible, preferably in a single or circular trench or in a number of trenches radiating from a point, if conditions require use of more than one strip, they shall be laid either in parallel trenches or in radial trenches.

This type of earthing is used at places which have rocky earth bed because at such placed excavations work for plate earthing is difficult.

2.7.2 jkWM jkWM jkWM jkWM vfFkZaxvfFkZaxvfFkZaxvfFkZax Rod Earthing

In this system of earthing 12.5 mm diameter solid rod of copper or 16 mm diameter solid rod of galvanized iron or steel; or hollow section 25 mm G I pipes of length not less than 2.5 meters are driven vertically into the earth either manually or by pneumatic hammer. In order to increase the embedded length of electrodes under the ground, which is sometimes necessary to reduce the earth resistance to



33

desired value, more than one rod sections are hammered on above the other.

This system of earthing is suitable for areas which are sandy in character. This system of earthing is very cheap as no excavation work is involved.

2.7.3 ikbi ikbi ikbi ikbi vfFkZaxvfFkZaxvfFkZaxvfFkZax Pipe Earthing

Pipe earthing is the best form of earthing and is very cheap in cost. In this method of earthing, a galvanized and perforated pipe of approved length and diameter is placed up right in a permanently wet soil.

The size of the pipe depends upon the current to be carried and the type of the soil. Usually the pipe used for this purpose is of diameter 38 mm and 2.5 meters in length for ordinary soil or of greater length in case of dry and rocky soil. The depth at which the pipe must be buried depends upon the moisture of the ground. The pipe is placed at a depth of 3.75 meters (minimum). The pipe is provided with a tapered casing at the lower end in order to facilitate the driving. The pipe at the bottom is surrounded by broken pieces of coke to increase the effective area of the earth and to the earth and to decrease the earth resistance respectively. Another pipe of 19 mm diameter and minimum length 1.25 meter is connected at the top to G I pipe through reducing socket.

In our country in summer the moisture in the soil decrease which cause increase in earth resistance. So a cement concrete work, is done in order to keep the water arrangement accessible, and in summer to have an effective earth, 3 or 4 buckets of water are put



34

through the funnel connected to 19 mm diameter pipe, which is further connected to G I pipe.

The earth wire (either G I wire or G I Strip of sufficient cross section to carry faulty current safely) is carried in a G I pipe of diameter 13 mm at a depth of about 60 mm from the ground).

2.7.4 IysV IysV IysV IysV vfFkZaxvfFkZaxvfFkZaxvfFkZax Plate Earthing In plate earthing an earthing plate either of

copper of dimensions 60 cm x 60 cm x 3 mm or of galvanized iron of dimensions 60 cm x 60 cm x 6 mm is buried into the ground with its face vertical at a depth of not less than 3 meters from ground level. The earth plate is embedded in alternate layers of coke and salt for a minimum thickness of 15 cm. The earth wire (G I wire for G I plate earthing and copper wire for copper plate earthing) is securely bolted to an earth plate with the help of a bolt, nut and washer made of material of that of earth plate (made of copper in case of copper plate earthing and of galvanized iron in case of G I plate earthing).

A small masonry brick wall enclosure with a cast iron cover on top or an R C C pipe round the earth plate is provided to facilitate its identification and for carrying out periodical inspection and tests.

For smaller installations G I pipe earthing is used and for larger stations and transmission lines, where the fault current, likely to be high, plate earthing is used.



35

2.8 vfFkZaxvfFkZaxvfFkZaxvfFkZax yhM yhM yhM yhM EARTHING LEAD

It is the conductor by which the final connection to the earth is made. Its size should be of sufficient cross sectional area so that it will not fuse under worst fault condition.

The earthing lead should be terminated on a soldered lug and secured perfectly to the body at the point of connection to the earth plate. There should be a clean metal to metal surface contact which will remain intact permanently without deterioration or corrosion.

3.0 vfFkZaxvfFkZaxvfFkZaxvfFkZax flLVe flLVe flLVe flLVe EARTHING SYSTEM

3.1 vfFkZaxvfFkZaxvfFkZaxvfFkZax dk oxhZdj.k dk oxhZdj.k dk oxhZdj.k dk oxhZdj.k CLASSIFICATION OF EARTHING

The earthing can be classified as (1) System earthing

(2) Equipment earthing

3.1.1 flLVe vfFkZaxflLVe vfFkZaxflLVe vfFkZaxflLVe vfFkZax System Earthing

System earthing is designed to maintain protection of the system by ensuring the potential on each conductor to be restricted to a value consistent with the level of insulation applied.

It is very important that earthing should be ensured, in such a manner to operate the protective device fast and efficiently in case of any earth fault.



36

The system resistance should be such that, when any fault occurs against which earthing is designed, should protect or operate the gear to achieve the faulty main or plant harmless.

In such cases, the faulty main or plant is generally isolated with the help of circuit breakers or fuses.

In case of overhead equipments it becomes very difficult to arrange the value of earth resistance of the system to achieve protection when the conductor falls due to breakage and makes a good contact with the ground.

3.1.1.1 U;wVy U;wVy U;wVy U;wVy vfFkZaxvfFkZaxvfFkZaxvfFkZax flLVe dh fof/k flLVe dh fof/k flLVe dh fof/k flLVe dh fof/k Methods of Earthing System Neutral

A. Solid Earthing B. Resistance Earthing C. Reactance Earthing D. Arc-suppression Coil or Peterson Coil

Earthing

A. Bksl vfFkZax Bksl vfFkZax Bksl vfFkZax Bksl vfFkZax Solid Earthing

When the fault current is expected to be low and not likely to cause damage to plant, cables and loss of stability of system, the earthing may be done directly through metallic conductor from system neutral to the main earthing ring without any impedance in the circuit. It should be ensured that the impedance between the N and E is so low so that if an earth fault occurs in one phase of the system



37

sufficient current will flow to operate to protective devices as shown in figure- 11

B. frjks/k vfFkZaxfrjks/k vfFkZaxfrjks/k vfFkZaxfrjks/k vfFkZax Resistance Earthing

Resistance earthing is generally used when the fault current is likely to be so high as to cause damage to transformers. If a resistance is inserted between the neutral and earth, quick action protective devices are also used as shown in figure-12. The resistors shall comprise of metallic resistance units supported in insulation in a metal frame or shall be a liquid resistor of a weak aqueous solution either of zinc chloride or sodium carbonate.

Figure - 11 Solid Grounded Neutral

Figure 12 Resistance Grounded

N

N



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Metallic resistors have a constant resistance which does not change with time liquid resistors have to be treated frequently specially after the clearance of a fault. Metallic resistors are slightly inductive and this is a disadvantage with overhead lines traveling waves and impulses are subject to positive reflection and this is likely to unduly stress the insulation of the equipment and cause breakdown. Use of liquid resistors is recommended only at voltages above 6.6 kV. All neutral earthing resistances should be designed to carry their rated current for a short period, usually 30 seconds.

The earth resistance shall be of such a value if a fault is outside the equipment, the fault current will be restricted to the rated full load current if the equipment. If the earth resistance is too low, for any occurrence of the earth fault, the equipment will be subjected to shock due to load resulting from the power loss in the resistor.

C. fj;DVsalfj;DVsalfj;DVsalfj;DVsal vfFkZax vfFkZax vfFkZax vfFkZax Reactance Earthing

When the zero sequence reactance of generators or transformers is as low as to cause excessive fault current, usually reactance earthing is used. A single phase reactor is inserted between the neutral and the earth to limit fault current to the maximum of three phase short circuit current. Here the current due to earth fault on one phase is limited to minimize damage the equipment. Care should



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be taken to see that dangerously high transient voltage during system fault or switching operations do not occur due to high value of reactance of earthing reactor as shown in figure-13.

D. vkdZ lsku Dokby vkdZ lsku Dokby vkdZ lsku Dokby vkdZ lsku Dokby vfFkZaxvfFkZaxvfFkZaxvfFkZax Arc-Suppression Coil Earthing

In high voltage systems with isolated neutrals over voltages caused by switching surges or by lighting may cause a line to each. Considerable current will be drawn through the arc to charge the system capacitance to earth. The arc is quenched at zero voltage but may restrict at a higher voltage. This successive restricting if the arc often causes very high voltages to be built upon the transmission lines, and is known as arcing grounds. To avoid isolation of system under earth fault conditions, arc-suppression coils are sometimes used. Arc-suppression coil, also known as Peterson coil, is a tuned earthing reactor as shown in figure- 14. It is to the system capacitance in such a way as to make the reactance of the zero sequence

Figure- 13 Reactance grounding

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networks practically infinite, so that no fault current flows to the earth and there is no tendency for arcing grounds to occur. With the use of Peterson coil, arc current is reduced to such a small value that it is usually self-extinguishing, which increases continuity in service

3.1.2 midj.kmidj.kmidj.kmidj.k vfFkZax vfFkZax vfFkZax vfFkZax Equipment Earthing It pertains to those electrical conductors, by

which all metallic structures through which the energized conductor passes will be inter connected.

The purpose of equipment earthing is; To maintain low potential difference between

nearby metallic structure in any area to achieve freedom from electrical shock to person or animal etc.

Figure- 14 Resonant grounding

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To provide an effective and easy path over which short circuit current involving ground can flow without heating or sparking or fire to combustible atmosphere.

All housings of electrical conductors, equipment enclosure, motor frame shall be interconnected by equipment earthing and two separate and distinct connection to be made to main earthing.

3.2 lc LVsku esa vfFkZax .kkyhlc LVsku esa vfFkZax .kkyhlc LVsku esa vfFkZax .kkyhlc LVsku esa vfFkZax .kkyh EARTHING SYSTEM IN SUB STATION

The earthing system comprises of earthing (or) grid, earthing electrodes, earthing conductors and earth connections.

3.2.1 vFkZ eSV ;k fxzMvFkZ eSV ;k fxzMvFkZ eSV ;k fxzMvFkZ eSV ;k fxzM Earth Mat or Grid

The primary requirement of earthing is to have a very low earth resistance. If the individual electrodes driven in the soil are measured it will have a fairly high resistance. But if these individual electrodes area inter linked inside the soil, it increases the area in constant with soil and creates a number or paralleled paths and hence the value of earth resistance in the interlink state, which is called combined earth resistance, will be much lower than the individual resistance.

However interlinking of earth pit electrodes is necessary. The sub-station involves many earthing through individual electrodes. In order to have uniform interconnection, a mat or grid or earthing conductor is formed inside the soil. Thus a mat is spread underneath the sub-station. Hence if a ground electrode is driven in the soil, the interlinking can be done by a small link



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between that electrode and earth mat running nearby. The spreading of such a mat in the soil also ensures the object of earthing that and surface under and around the sub-station is kept at as nearly absolute earth potential as possible.

3.2.2 vFkZ eSvFkZ eSvFkZ eSvFkZ eSV dk fuekZ.kV dk fuekZ.kV dk fuekZ.kV dk fuekZ.k Construction of Earth Mat

The sub-station site including the fence is segregated at intervals, of say four meters width along with length and breadth wise. Trenches of one meter, to 1.5 meter depth and one meter width is dug along these lines. The earthing conductors of sufficient sizes (as per fault current) are placed at the bottom of these trenches. All the crossing and joints are braced. The trenches are then filled up with soil of uniform fine mass of earth mixed with required chemicals depending upon the soil resistivity.

If location of equipment is fixed, the intervals are also arranged that the earth mat passes nearby the equipment location to facilitate for easy interlinking.

It is preferable to extend the mat beyond the fence for about one meter that fence can also be suitably earthed and made safe for touching.

Normally the earth mat is buried horizontally at a depth of about half a meter below the surface of the ground and ground rods at suitable points.



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3.2.3 lc LVsku esa foflc LVsku esa foflc LVsku esa foflc LVsku esa fofHkUu vFkZ eSV dk dusDkuHkUu vFkZ eSV dk dusDkuHkUu vFkZ eSV dk dusDkuHkUu vFkZ eSV dk dusDku Earth Mat connection in a Sub-Station The neutral point of such system through its own

independent earth. Equipment frame work and other non-current

carrying parts of the electrical equipments in the sub station.

All extraneous metallic frame work not associated with equipment.

Handle of the operating pipe. Fence if it is within 2 m from earth mat.

3.3 lc LVskulc LVskulc LVskulc LVsku esa fofHkUu midj.kksa dh vfFkZaxesa fofHkUu midj.kksa dh vfFkZaxesa fofHkUu midj.kksa dh vfFkZaxesa fofHkUu midj.kksa dh vfFkZax EARTHING OF VARIOUS EQUIPMENT IN THE SUB-STATIONS

3.3.1 vkblksysVj ,oa fLopstvkblksysVj ,oa fLopstvkblksysVj ,oa fLopstvkblksysVj ,oa fLopst Isolators and switches A flexible earth conductor is provided between

the handle and earthing conductor attached to the mounting bracket and the handle of switches is connected to earthing mat by means of two separate distinct connections made with MS flat. One connection is made with the nearest longitudinal conductor, while the other is made to the nearest transverse conductor of the mat.

3.3.2 ykbVfuax vjsLVjykbVfuax vjsLVjykbVfuax vjsLVjykbVfuax vjsLVj Lightning Arrestors

Conductors as short and straight as practicable to ensure minimum impedance shall directly connect the bases of the lightning arrestors to the earth grid. In addition, there shall be as direct a connection as practicable from the earth side of lightning arrestors to the frame of the equipment being protected.



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Individual ground electrodes should be provided for each lighting arrestor for the reason that large grounding system in itself may be relatively of little use for lightning protection. These ground electrodes should be connected to the main earth system. In the case of lighting arrestors mounted near transformers, earthing conductor shall be located clear off the tank and coolers in order to avoid possible oil leakage caused by arcing.

3.3.3 lfdZV czsdjlfdZV czsdjlfdZV czsdjlfdZV czsdj Circuit Breakers

For every breaker there will be five earth connections to the earth mat with MS flat (i) breaker body (ii) relay panel (iii) CTs of the breaker (iv) Two side of the breaker structure.

3.3.4 ikikikikoj oj oj oj VklQkeZjVklQkeZjVklQkeZjVklQkeZj Power Transformers

The tank of each transformer shall be directly connected to the main grid. In addition there shall be as direct a connection as practicable from the tank to the earth side of projecting lightning arrestors.

The transformer track rails shall be earthed either separately or by bonding at each end of the track and at intervals not exceeding 60.96 meter (200 feet). The earthing of neutral bushing shall be by two separate strips to the earth grid and shall likewise be run clear to rank cell and coolers.

3.3.5 djsaV VklQkeZj djsaV VklQkeZj djsaV VklQkeZj djsaV VklQkeZj ,oa iksVsfUk;y ,oa iksVsfUk;y ,oa iksVsfUk;y ,oa iksVsfUk;y VklQkeZjVklQkeZjVklQkeZjVklQkeZj Current Transformers and Potential Transformers

The supporting structures of Current Transformer and Potential Transformer unit of bases, all bolted cover plates to which the bushings are attached connected to the earthing mat by means of two



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separate distinct connections made with MS flat. One connection is made with the nearest longitudinal conductor, while the other is made to the nearest transverse conductor of the mat.

3.3.6 vU; midj.kvU; midj.kvU; midj.kvU; midj.k Other Equipments

All equipments, structures, and metallic frames of switches and isolators are to be earthed separately as shown in figure- 15.

3.3.6.1 ?ksjk?ksjk?ksjk?ksjk Fences

The Sub-station fence should be generally too far outside the substation equipment and grounded separately from the station ground. The station and the fence ground should not be linked. To avoid any risk to the person walking near the fence inside the station, no metal parts connecting connected to the station ground, should be near to the fence five feet and it is desirable to cover the strip about ten feet wide inside the fence by a layer of crushed stone which keeps its high resistively even under wet condition. If the distance between the fence and station structures, can not be increased at least five feet and if the fence is too near the substation equipment structure etc., the station fence should be connected to the fence ground, otherwise a person touching the fence and the station ground simultaneously would be subjected to a very high potential under fault conditions.

Figure - 15



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In a fence very near to the station area, high shock voltage can be avoided by ensuring good contact between the fence stations and by grounding the fence at intervals. The station fence should not be connected to the station ground but should be grounded separately. If however, the fence is close to the metal parts of substation, it should be connected to the station ground.

3.3.6.2 xzxzxzxzkUkUkUkUM rkjM rkjM rkjM rkj Ground Wire

All ground wires over a station shall be connected to the station earth grid. In order that the station earth potentials during fault conditions are not applied to transmission line ground wires and towers, all ground wires coming to the station shall be broken at and insulated on the station side of the first tower or pole external to the station by means of 10 disc insulator.

3.3.6.3 ddddsfcy ,oa liksZVsfcy ,oa liksZVsfcy ,oa liksZVsfcy ,oa liksZV Cables and Supports

Metal sheathed cables within the station earth grid area shall be connected to that grid. Multi-core cables shall be connected to the grid at least at one point. Single core cables normally shall be connected to the grid at one point only. Where cables which are connected to the station earth grid pass under a metallic station perimeter fence, they shall be laid at a depth of not less than 762 mm (2-6) below the fence, or shall be enclosed in an insulating pipe for a distance of not less than 1524 mm (5) on each side of the fence.



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3.3.6.4 iSuy ,oa D;wfcdyiSuy ,oa D;wfcdyiSuy ,oa D;wfcdyiSuy ,oa D;wfcdy Panels and Cubicles

Each panel or cubicle should be provided near the base with a frame earth bar of copper to which shall be connected the metal bases and covers of switches and contactor unit. The frame earth bar shall in turn be connected to the earth grid by an earthing conductor.

3.4 forj.k forj.k forj.k forj.k VklQkeZjVklQkeZjVklQkeZjVklQkeZj LVdpj dh vfFkZax LVdpj dh vfFkZax LVdpj dh vfFkZax LVdpj dh vfFkZax DISTRIBUTION TRANSFORMER STRUCTURE EATHING 1. For earthing three earth pits in triangular

formation at a distance of six meter from each other are to be provided.

2. Earth pit should be digged for 45 cm x 45 cm size and 5 ft. depth.

3. 3 Nos. of 40 mm dia and 2.9 mm thickness and 3 mts. (10 ft) length of earth pipe should be used for earthing. This earth pipe is erected in 5 ft. depth earth pit and for the balance length of earth pipe is driven by hammering into the ground.

4. When a pipe is driven into the earth, the earth surrounding the pipe can be considered to be consisting of concentric cylinders of earth which will be bigger in size and area, as they are away from the pipe. The current can travel into the earth with large area having little resistance.

5. 3 m. length of electrode will have contact with the earth area of 3 m in radius. Hence to have better effect 3 m pipe should be fixed at a



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distance of 6 m (i.e.) twice the distance of pipe length.

6. For better earth connection, one G I clamp should be welded to the earth pipe and the other clamp bolted with 2 nos. 11/2 x G I bolt nuts and 4 nos. G. I. washers to the earth pipe.

7. Two separate distinct connections through G I wire should be made from the transformer neutral bushing to the earth pit No. 2.

8. Two separate distinct connections through GI wire should be made from the transformer HT lightning Arrestor to the earth pit No. 1. As far as possible this earth wire should not have contact with other earth wire connections. If needed PVC sleeves can be used for insulation.

9. Two separate distinct connections through GI wire from the following parts of the structure should be made to the earth pit No. 3 as shown in figure- 16. Metal part of the disc and stay. Top channel. AB switch frame, metal part of the

insulator, side Arms. HG fuses frame and metal part of the

insulator. LT cross arm, metal part of the insulator,

open type fuse frame. AB switch guide and operating pipe ( At

the top and bottom )



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Transformer body. Belting angle. Seating channel LT lightning arrestor.

The above earth connections should be made as far as possible without joints. Wherever joints are necessary, GI sleeves should be used by proper crimping.

10. The earth pits No. 2 and 3 can be interlinked to serve as parallel path and lower the earth resistance.

11. If the earth resistance of the earth pit No. 1 is high, then another earth pit No. 4 can be formed as a counter poise earth and linked with the HT lightning arrestor pit.



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Figure- 16 Earthing of Distribution Transformer Structure

B Y R N

SEATING CHANNEL

HG FUSE

AB SWITCH

H.T.LA's

EARTH PIT NO.2

EARTH PIT NO.3

EARTH PIT NO.1

EARTHING

EARTHING

TRANSFORMER



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3.5 forj.k ykbu LVforj.k ykbu LVforj.k ykbu LVforj.k ykbu LVdpj dh vfFkZadpj dh vfFkZadpj dh vfFkZadpj dh vfFkZaxxxx EARTHING OF DISTRIBUTION LINE STRUCTURES

The following procedure is adopted for the earthing of HT and LT line supports.

3.5.1 ,p Vh ykbu LVdpj ,p Vh ykbu LVdpj ,p Vh ykbu LVdpj ,p Vh ykbu LVdpj HT Line Structures

Lines carried on metal poles: Every fifth pole and all supports provided with

mass or block concrete foundation shall be earthed.

Lines carried on R.C.C. & P.S.C. poles The metal cross arm and the insulator pin shall be

bound together and earthed at every pole.

3.5.2 ,y Vh ykbu LV,y Vh ykbu LV,y Vh ykbu LV,y Vh ykbu LVdpj dpj dpj dpj LT Line Structures (with Multiple Earthed Neutral)

Lines carried on metal poles: Every fifth pole and all supports provided with

mass or block concrete foundation shall be earthed.

Lines carried on R.C.C. & P.S.C. poles The metal cross arm and the insulator pins shall

be bound together and earthed at every fifth pole. All special structures carrying switches, transformers, fuses etc., should be earthed.

3.5.3 vU; LVdpj vU; LVdpj vU; LVdpj vU; LVdpj Other Structures The supports on either side of a road, railway or

river crossing span should be earthed. All supports (metal, wood or R.C.C. ) of both H T

and LT lines running through inhabited locations,



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road crossings and along such other places where earthing of all poles is considered desirable from safety considerations should be earthed.

In special locations, railway and telegraph crossings, special structures etc., pipe earth should be adopted (i.e.) and earthing should be done by means of a 25 mm GI pipe driven 2.5 to 3 meter into the ground.

At other locations coil earthing may be adopted which consists of either 10 meter length of 6 or 8 SWG GI wire compressed into a coil of one meter length and diameter 75 to 100 mm and buried 1.5 meter deep or as per REC standard or pole earthing with 8 SWG GI wire of 75 feet length wound as a coil to have 115 turns of 75/50 mm dia as to have good contact with soil is to be provided.

Whenever the distribution line structures pass close to well or a permanently moist place, an earth should be provided in the well or the marshy place and connected to distribution line support.

All tapping poles, terminal poles, stay poles, streetlight poles and service connection tapping poles should be earthed.

Only if the above requirements are met out we can say that LT is with multiple earth neutral system. The ohmic resistance of the earth should be as low as possible and should not exceed 10 ohms.



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3.6 miHkksDmiHkksDmiHkksDmiHkksDrk ds ifjlj esa vfFkZaxrk ds ifjlj esa vfFkZaxrk ds ifjlj esa vfFkZaxrk ds ifjlj esa vfFkZax EARTHING AT CONSUMERS PREMISES

As per rule 33 (i) of I.E. rules 1956, the supplier shall provide and maintain at the consumers premises for the consumers use a suitable earthed terminal in an accessible position at the point of commencement of supply.

a) ijh lfoZl dusDku ykbu ijh lfoZl dusDku ykbu ijh lfoZl dusDku ykbu ijh lfoZl dusDku ykbu Overhead Service Connection Lines: 1. The earthed terminal may be a 32 mm x 3

mm or near about, consisting of copper plate with three number (16 mm) studs.

2. One of the studs on the earthed terminal should be connected to the neutral wire of the twin core supply lead.

3. The bearer wire should be connected to the second stud of the earthed terminal.

4. The consumers installation should be connected to the third stud of the earthed terminal.

5. The bearer wire should not be used as the earth lead. The bearer wire should be earthed at both the pole ends and the consumers premises and by connecting it to the overhead neutral wire and to the earthed terminal respectively.

6. The size of the bearer wire should be stranded 7/20 G.I. or near about size.

7. The bearer wire and the W.P. cable should be bunched together by porcelain reel insulators or alkathene clips intervals of 6.1 meter.



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b) Hkwfexr dscy Hkwfexr dscy Hkwfexr dscy Hkwfexr dscy Underground Cables 1. The terminal block for earth connection

may be of size 31 mm x 3 mm or near about, consisting of copper plate with three number 12.5 mm copper or brass studs with lock nuts or spring washers.

2. The neutral core of the cable, the lead sheath, the steel armour and the cable box should be connected to one of the studs on the earthed terminal.

3. The metal part of the boards meter should be connected to the second stud of the earthed terminal.

4. The consumers installation should be connected to the third stud of the terminal.

3.6.1 fyV dh vfFkZax fyV dh vfFkZax fyV dh vfFkZax fyV dh vfFkZax Earthing in Lifts Frames of motors, winding machine, control

panel, cases and covers of tappet switch and similar electrical apparatus, which normally carry the main current, shall be all earthed.

The exposed metal parts of electrical apparatus installed on a lift car shall be sufficiently bonded and earthed.

3.6.2 ?kjsyw midj.kksa

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Handbook on Electrical Earthing

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