handbook on earthing system for power supply installations.pdf

7/23/2019 Handbook on Earthing system for power supply installations.pdf

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

Handbook on Earthing System for Power Supply Installation November 2010

1

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dSeVsd@bZ@10&11@vfFkZax&ih,lvkbZ@1-0CAMTECH/E/10-11/EARTHING-PSI/1.0

uoEcj 2010November 2010

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Earthing System For Power Supply Installations

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CAMTECH/E/10-11/PSI/1.0

November 2010 Handbook on Earthing System for Power Supply Installation

2

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Handbook on Earthing System

For

Power Supply Installations

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4

FOREWORD

Earthing for substation and switching station (power

supply installation) of TRD plays very important role in

smooth functioning of equipment, safety of personnel and

equipment. 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 earthing

system for power supply installation containing construction

of earthing, earthing arrangement at traction substation,

maintenance schedules, maintenance free earthing,

exothermic welding and dos & dont.

I hope this handbook will prove to be useful for themaintenance personnel working in TRD department.

CAMTECH, Gwalior S.C. Singhal

Date:24.12.2010 Executive Director


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6

PREFACE

Provision of adequate earthing in substations and

switching stations is essential for the safety of operating

personnel as well as of equipment. It is also necessary for

functioning of the system operation. By means of earthing,

electrical equipment are connected to the general mass of the

earth, which has a very low resistance.

This handbook on earthing system for power supply

installation has been prepared by CAMTECH with the

objective of making our maintenance personnel aware of

maintenance free earthing and exothermic welding.

It is clarified that this handbook does not supersede any

existing provisions laid down by RDSO, Railway Board or AC

traction manual. This handbook is for guidance only and it isnot 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: 30 thNovember 2010 Jt. Director Electrical

e - mail id : [email protected]


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CONTENTS

Sr.No. Description Page No.

Foreword ivPreface viContents viiiCorrection Slip xii

1.0 INTRODUCTION 01

1.1 PURPOSE OF SUBSTATION EARTHING 01

1.2 EARTHING SYSTEM 02

1.3 TERMINOLOGY 02

1.4 DISTINCTION BETWEEN GROUNDING AND

EARTHING 06

1.5 MAXIMUM PERMISSIBLE EARTH RESISTANCE 07

1.6 GENERAL REQUIREMENT FOR EARTHING 07

1.7 FACTORS WHICH DETERMINE RESISTIVITYOF SOIL 09

2.0 EARTHING ARRANGEMENTS AT SUBSTATION 15

2.1 EARTH ELECTRODES 15

2.2 PARAMETERS AFFECTING THE DESIGN OFEARTHING MAT 17

2.3 DESIGN PROCEDURE 172.4 EARTHINGMAT 18

2.5 BURRIED RAIL 20

2.6 SYSTEM EARTHING 21

2.7 EQUIPMENT EARTHING 22

2.8 SIZE OF EARTHING CONDUCTOR 29

2.9 EARTHING ARRANGEMENT AT SWITCHING 29STATION


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Sr. No. Description Page No.

3.0 MAINTENANCE SCHEDULE OF

CONVENTIONAL EARTH ELECTRODE 323.1 QUARETLY SCHEDULE 323.2 HALF YEARLY SCHEDULE 33

3.3 YEARLY SCHEDULE 34

3.4 MEASUREMENT OF EARTH ELECTRODERESISTANCE 34

3.5 DETAILS OF EARTH TESTER(HAND DRIVEN) 36

4.0 MAINTENANCE FREE EARTHING 384.1 EARTH RESISTANCE 38

4.2 APPLICATION 39

4.3 MAINTENANCE FREE EARTHING SYSTEM 39

5.0 EXOTHERMIC WELDING CONNECTION FORBONDING, GROUNDING/ EARTHING 49

5.1 GENERAL REQUIREMENTS 50

5.2 WELDING MATERIAL 51

5.3 SPECIFICATION OF WELDING TOOLS 52

5.4 WELDING TOOLS 54

6.0 DOS & DONTS 57

6.1 DO S 57

6.2 DONT 58

ANNEXURE-1

Photographs of Exothermic Welding Procedure 59ANNEXURE-2

Photographs of Process of Hole Drilling in Rail 62

ANNEXURE-3 63

Specification of Rail Drilling Machine

REFERENCES 68


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12

ISSUE OF CORRECTION SLIP

The correction slips to be issued in future for this

handbook will be numbered as follows:

CAMTECH/E/10-11/Earthing-PSI/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

Provision of adequate earthing in substations and

switching stations is essential for the safety of operating

personnel as well as of equipment. It is also necessary

for functioning of the system. By means of earthing

electrical equipment are connected to the general mass

of the earth, which has a very low resistance.

1.1 lcLVsku dh vfFklcLVsku dh vfFklcLVsku dh vfFklcLVsku dh vfFkZax ds mn~ns;Zax ds mn~ns;Zax ds mn~ns;Zax ds mn~ns;@@@@PURPOSE OFSUBSTATION EARTHING

The object of an earthing system in a substation

is to provide, under and around the substation a surface

that shall be at a uniform potential and as near to zero

or absolute earth potential as possible. The provision of

such a surface of uniform potential under and around

the substation ensures human safety from electric shock

in case of short circuit or development of any abnormalcondition in the equipment installed.

The primary requirements of a good earthing

system in a substation are:

a. It stabilizes circuit potential with respect to groundand limits the overall potential rise.

b. It protects life and property from over voltage.c. It provides low impedance path to fault currents

to ensure prompt and consistent operation of

protective devices during ground faults.

d. It keeps the maximum voltage gradient along the

surface inside and around the substation within

safe limits during ground fault.


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1.2 vfFkZax .kkyhvfFkZax .kkyhvfFkZax .kkyhvfFkZax .kkyh@@@@EARTHING SYSTEM

All the non current carrying parts of the

electrical equipment in the substation are connectedto the earthing mat. Under normal conditions the

ground rods contribute a little towards lowering the

ground resistance. It maintains a low value of

resistance under all weather conditions.

The mat is connected to the following

equipment in sub station:a. The neutral point of system through its own

independent earth.

b. Equipment frame work and other non current

carrying parts.

c. All extraneous metallic frameworks not

associated with equipment.

d. Lightning arrestors through their independent

earths.

e. Handles of operating pipes.

f. Fence if it is within 2m from earth mat.

1.3 ifjHkkf"kd 'kCnkoyh@ifjHkkf"kd 'kCnkoyh@ifjHkkf"kd 'kCnkoyh@ifjHkkf"kd 'kCnkoyh@TERMINOLOGY

The following terms are commonly used in

earthing system:

1.3.1 Hkw@Hkw@Hkw@Hkw@Earth

The conductive mass of the earth, whose

electrical potential at any point is conventionally taken

as zero.


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1.3.2 vFkZ bYvFkZ bYvFkZ bYvFkZ bYkSDkSDkSDkSDVksMVksMVksMVksM@@@@Earth electrode

A galvanized iron (GI) pipe or group of pipes in

intimate contact with and providing an electricalconnection to earth.

1.3.3 vFkvFkvFkvFkZ fxzM@Z fxzM@Z fxzM@Z fxzM@Earth grid

A system of grounding electrodes consisting of

inter-connected connectors buried in the earth to

provide a common ground for electrical devices and

metallic structures.

1.3.4 vFkvFkvFkvFkZZZZeSV@eSV@eSV@eSV@Earth Mat

A grounding system formed by a grid of

horizontally buried conductors and which serves to

dissipate the earth fault current to earth and also as an

equi-potential bonding conductor system.

1.3.5 vfFkZax midj.kvfFkZax midj.kvfFkZax midj.kvfFkZax midj.k@@@@Equipment EarthingIt 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.1.3.6 flLVeflLVeflLVeflLVe 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

this to ensure the security of the system.


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1.3.7 LVsi ,oaVp oksYVrkLVsi ,oaVp oksYVrkLVsi ,oaVp oksYVrkLVsi ,oaVp oksYVrk@@@@Step & Touch Potential

Step & touch potential refer to the potential

experienced by a person standing on a surface whenearth mat buried up to 750 mm below, surface has risen

to ground potential rise (GPR).

Touch potential is the difference between GPR &

the surface potential at the point where person is

standing, while his hand is in contact with grounded

structure.

Step potential is the difference in surface potentialexperienced by a person bridging a distance of one

meter with his feet without contacting any other

grounded object as shown in figure - 1

If resistance offered by each foot is R, then for

step potential the resistance is 2R while for touch

potential is R/2.

Figure 1


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1.4.1 xzkfUMaxxzkfUMaxxzkfUMaxxzkfUMax@@@@GroundingGrounding 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.

1.4.2 vfFkZaxvfFkZaxvfFkZaxvfFkZax@@@@Earthing

Earthing implies connection of non currentcarrying 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 - 2

Figure 2

EARTHING

GENERATOR TRANSFORMER

NEUTRAL GROUNDINGNEUTRAL GROUNDING


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At each power supply installation the combinedresistance of the earthing system shall not be more than

the following values:-

i) Traction Sub Station - 0.5 Ohms

ii) Switching Station - 2.0 Ohms

iii) BT and AT station - 10 Ohms

1.6 vfFkZaxvfFkZaxvfFkZaxvfFkZaxdh lkekU;dh lkekU;dh lkekU;dh lkekU; vko;drk;savko;drk;savko;drk;savko;drk;saGENERAL REQUIREMENTS 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:3043-1987 - Code of practice for

earthing (latest)

ii) National Electricity Code - 1985 of Bureau

of Indian Standards

iii) IEEE guide for safety in a.c. substation

grounding No. ANSI/IEEE standard 80-

1986.

iv) Indian Electricity Rule 1956 (Latestedition)

In cases where direct earthing may prove harmfulrather than providing safety, relaxation may be

obtained from the competent authority.

Earth electrodes shall be provided at substations,switching stations and consumer premises in

accordance with the requirements.


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As far as possible all earth connections shall bevisible for inspection.

All connections shall be carefully made. If theyare 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 thetesting of individual earth electrode is possible. It

is recommended that the value of any earth

system resistance shall not be more than specified

value.

The minimum size of earthing lead (discharge rodcable) used on installations shall have a nominal

cross-section area not less than 40 mm2 multi

stranded copper to be able to withstand short

circuit current.

It is recommended drawing showing the main

earth connection and earth electrode be preparedfor each installation.

No addition to the existing load whethertemporary 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 tosuch addition.

All materials, fittings etc. used in earthing shallconfirm 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.


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1.7 feVfeVfeVfeV~~~~VhVhVhVh dh frjks/kdrk fu/kkZfjr djus odh frjks/kdrk fu/kkZfjr djus odh frjks/kdrk fu/kkZfjr djus odh frjks/kdrk fu/kkZfjr djus okys dkjdkys dkjdkys dkjdkys dkjd@@@@FACTORS WHICH DETERMINE RESISTIVITY OFSOIL

The resistivity of soil for earthing system

depends upon the following factors:

Type of soil

Moisture content

Chemical composition of salt dissolved in thecontained water

Concentration of salt

Temperature of material

Grain size and distribution of grain size

Size and spacing of earth electrodes

1.7.1 feVfeVfeVfeV~~~~VhVhVhVh dh frjks/kdrkdh frjks/kdrkdh frjks/kdrkdh frjks/kdrk de djus dhde djus dhde djus dhde djus dh

fof/k;k@fof/k;k@fof/k;k@fof/k;k@Methods of Reducing Resistivity of SoilfeVfeVfeVfeV~~~~Vh dh frjks/kdrk ds dkj@Vh dh frjks/kdrk ds dkj@Vh dh frjks/kdrk ds dkj@Vh dh frjks/kdrk ds dkj@Types of soilresistivity

S.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


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1.7.2 feV~Vh mipkj@feV~Vh mipkj@feV~Vh mipkj@feV~Vh mipkj@Soil Treatment

a. When the soil resistance is high, even the

multiple electrodes in large number may also failto 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.

b. Nearly 90% of resistance between electrode andsoil 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.

c. General practice to treat the soil surrounding theground 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 upto 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.


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11

1.7.3 feV~Vh mipkjfeV~Vh mipkjfeV~Vh mipkjfeV~Vh mipkj esaesaesaesa ccccsUVksukbZV dk mi;ksxsUVksukbZV dk mi;ksxsUVksukbZV dk mi;ksxsUVksukbZV dk mi;ksx@@@@Use ofBentonite in Soil Treatment

Bentonite is clay with excellent electricalproperties. It swells to several times its originalvolume 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 resultsin reduction of ground resistance by about 25- 30

%.

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

Even during the summer months, bentonite

suspension retains the moisture where as the naturalsoil dries up.

Bentonite may be used to advantage in rockyterrain.

1.7.4 feV~Vh mipkjfeV~Vh mipkjfeV~Vh mipkjfeV~Vh mipkjesaeghu jk[k dk mi;kesaeghu jk[k dk mi;kesaeghu jk[k dk mi;kesaeghu jk[k dk mi;ksxsxsxsx@@@@Use of fly ashin 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.


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12

1.7.5 vFkZ frjks/kdrk ij vknzrkvFkZ frjks/kdrk ij vknzrkvFkZ frjks/kdrk ij vknzrkvFkZ frjks/kdrk ij vknzrk dk Hkkodk Hkkodk Hkkodk Hkko@@@@Effect ofMoisture Content on Earth Resistivity

Moisture content is expressed in percentage byweight of dry soil. Dry earth weights about 1440 kg/m

3.

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 moistureassists 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 1000x10

6

2.5 250000 150000

5 165000 43000

10 53000 18500

20 12000 6300

30 6400 4200

1.7.6 rki dk Hkko@rki dk Hkko@rki dk Hkko@rki dk Hkko@Effect of TemperatureThe 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 raises

abnormally.


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13

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

1.7.7 vFkZ frjks/kdrk dk taxvFkZ frjks/kdrk dk taxvFkZ frjks/kdrk dk taxvFkZ frjks/kdrk dk tax ij Hkko@ij Hkko@ij Hkko@ij Hkko@Effect of SoilResistivity 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 ofresistivity 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


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14

1.7.8 lrg ij iRFkj pwjk dh irZ dsQk;ns@lrg ij iRFkj pwjk dh irZ ds Qk;ns@lrg ij iRFkj pwjk dh irZ dsQk;ns@lrg ij iRFkj pwjk dh irZ ds Qk;ns@Advantagesof Crushed Rock Used as a Surface Layer

It provides high resistivity surface layer

It serves as impediment to the movement of reptilesand there by help in minimizing the hazards

which can be caused by them

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

It discourages the growth of weeds

It helps retention of moisture on the underlying soiland thus helps in maintaining the resistivity of thesubsoil at lower value.

It discourages running of persons in the switchyardand saves them from the risk of being subjected

to possible high step potentials.

1.7.9 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 RockIn 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 tosoil.

1.7.10 vFkZ bYkSDVksM yxkusdk LFkkvFkZ bYkSDVksM yxkusdk LFkkvFkZ bYkSDVksM yxkusdk LFkkvFkZ bYkSDVksM yxkusdk LFkku@u@u@u@Location of EarthElectrode

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:


27/82



15

a. Wet marshy ground.

b. Clay, loamy soil and arable land

c. Clay and loam mixed with varying proportions of

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

Dry sand,gravel chalk limestone, granite, verystone ground and all locations where virgin rock is very

close to the surface should be avoided.

2.0 lcLVskulcLVskulcLVskulcLVsku ijijijij vfFkZaxvfFkZaxvfFkZaxvfFkZax O;oLFkk@O;oLFkk@O;oLFkk@O;oLFkk@EARTHING

ARRANGEMENTS AT SUB STATION

2.1 vFkZ bYvFkZ bYvFkZ bYvFkZ bYkSDkSDkSDkSDVksMVksMVksMVksM@@@@Earth Electrodes

The earth electrodes are made of mild steel

galvanized perforated pipe of not less than 40 mm

nominal bore, of about 4 m length provided with a

spike at one end and welded lug suitable for taking

directly MS flat of required size at the other end. The

pipe is embedded as far as possible vertically into the

ground, except when hard rock is encountered, where it

may be buried inclined to the vertical, the inclination

being limited to 300

from the vertical. The connection

of MS flat to each electrode is made through MS links

by bolted joints to enable isolation of the electrode for

testing purpose.

Earth electrodes shall be embedded as apart aspossible from each other. Mutual separation between

two electrodes shall not be less than 6.0 m i.e. twice the

length of the electrode as shown in figure- 3

In high embankments, it may be difficult to

achieve earth resistance even after chemical treatment

of electrodes. In these locations, use electrodes longer

than 4 m so as to reach the parent soil is recommended.


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16

As far as possible, earth electrodes for traction

substation/ switching stations shall be installed within

and adjacent to perimeter fence.

75

150

3000

150 150

75 75

48.4

75

150mmLONG

75mmX8mm

M.S.FLATWELDED

12mmDIAHOLES

ATSIM

ILAR

SPACIN

GFOR

ENTIRE

LENGTH

200

200

500

50

50

GROU

NDLEVEL

LAYEROFSAND

LID

FOREARTHBOX200

80X(RCC)

50mmTHIC

K200

EARTHBOX

(RCC)

AREATOBEWELL

RAMMED

2X75mm

X8mm

100mmX

10mm

80mmX12mm

Alternateequal

Layersofsalt,

charcoalorcock

350

150

150

EARTH

ELECTRODE

12mmDIA

HOLES

Pipetomedriven

thesoilforbetter

holding

50

175 3

500

Figure -3


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17

2.2 vfFvfFvfFvfFkZax eSVkZax eSVkZax eSVkZax eSV fMt+kbufMt+kbufMt+kbufMt+kbu dks Hkkfor djusokys dkjd@dksHkkfor djusokysdkjd@dks Hkkfor djusokys dkjd@dksHkkfor djusokysdkjd@PARAMETERS AFFECTING THE DESIGN OFEARTHING MAT

Several variable factors are involved in the

design of earthing mat conductor. Earthing mat for each

substation has to be designed individually. The earthing

mat has to be designed as per the site conditions to have

low overall impedance and a current carrying capacity

consistent with the fault current magnitude. The

following parameters influence the design of earthing

mat:1. Magnitude of fault current

2. Duration of fault

3. Soil resistivity

4. Resistivity of surface material

5. Shock duration

6. Material of earthing mat conductor

7. Earthing mat geometry

2.3 fMtfMtfMtfMt++++k;u f;k@k;u f;k@k;u f;k@k;u f;k@DESIGN PROCEDUREThe following steps are involved in the design

of earthing mat:

i. The layout of the sub station, determine the area tobe covered by the earthing mat.

ii. Determine the soil resistivity at the sub station site.The resistivity of the earth varies within

extremely wide limits, between 1 and 10,000

ohm-meter. The resistivity of the soil at many

station sites has been found to be non- uniform.

Variation of the resistivity of the soil with

depth is more predominant as compared to the

variation with horizontal distances. To design


30/82



18

the most economical and technically sound

grounding system for large stations, it is

necessary to obtain accurate data on the soil

resistivity and on its variation at the station site.Resistivity measurements at the site will reveal

whether the soil is homogeneous or non-

uniform. In case the soil is found uniform,

conventional methods are applicable for the

computation of earth resistivity. When the soil

is found non- uniform, either a gradual

variation or a two-layer model may be adopted

for the computation of earth resistivity.

2.4 vvvvffffFkFkFkFkZaxZaxZaxZaxeSVeSVeSVeSV@@@@EARTHING MATAn earthing mat is formed by means of bare mild

steel rod of appropriate size buried at a depth of about

600 mm below the ground level and connected to earth

electrodes. The connection between the earth electrodes

and the mat shall be by means of two separate and

distinct connections made with 75 mm x 8 mm MS flat.

The connection between the MS flat and the MS rod

shall be made by welding, while that between the earth

electrode and the MS flats through MS links by bolted

joints. The earth electrodes are provided at the outer

periphery of the mat. As far as possible the earthing

mat conductors shall not pass through the foundationblock of the equipment.

All crossings between longitudinal conductors

and transverse conductors are jointed by welding. The

longitudinal and transverse conductors of the earthing

grid shall be suitably spaced so as to keep the step and


31/82



19

touch voltage within the acceptable limit. However the

overall length of the earthing grid conductors shall not

be less than the calculated length as shown in figure 4.

BURIEDGRIDCONDUCTORWITHIN1METREOFFENCE

MS ROD

M S FLAT

EARTH

TRACTIONSUB.-STN.FENCIG

ELECTRODE

CROSSING

WELDED AT

75 mm X 8mm MS FLATS

FROM POWERTRANSFORMER

SECONDARY WINDINGTERMINAL

BURIES RAIL

75 mm X 8mm MS FLATS TO

NON TRACK CIRCUITED RAILNEUTRAL POINT OF

IMPEDANCE BOND

Figure - 4


32/82



20

The size of the earthing grid conductors are decided

based on the incoming system voltage and fault level.

The fault level considered is taking into account the

anticipated increase in fault current during the life spanof the station. The system voltage, fault level and

conductor size are given below:

Sr.No

System

voltage

(KV)

Fault level (MVA) Diameter of

grid conductor

(GI rod) in mm

1 66

upto 4000

above 4000 upto 5000above 5000 upto 6000

32

3640

2 110

upto 6000

above 6000 upto 8000


32

36

40

3 132upto 7000


32

36

4 220

upto 12000



32

36

40

2.5 xMxMxMxM+h gqbZ+h gqbZ+h gqbZ+h gqbZ jsy@jsy@jsy@jsy@BURIED RAIL

A steel rail of section 52 kg/m (the one used for

the railways track) and length about 13 m is buried nearthe track at the traction substation at a depth of about

one meter to form part of the earthing system. Two

separate and distinct connections are made by means of

75 mm x 8 mm MS flat between the earthing mat and

the buried rail. The buried rail are connected by means

of two separate and distinct connections made with 75

mm x 8 mm MS flat to the traction rail(s) in a single-

rail track circulated section and to the neutral points(s)


33/82



21

of the impedance bond(s) in a double-rail track

circuited section.

In case where the feeding post is locatedseparately away from the traction substation, the buried

rail is provided at the feeding post (where one terminal

of the secondary winding of the traction transformer of

the substation is grounded).

2.6 flLVeflLVeflLVeflLVe vvvvffffFkFkFkFkZaxZaxZaxZax@@@@SYSTEM EARTHING

One terminal of thesecondary winding (25 kV

winding) of each traction power

transformer are earthed directly by

connecting it to the earthing grid

by means of one 75 mm x 8 mm

MS flat , and to the buried rail by

means of another 75 mm x 8 mm

GI flat as shown figure in - 5

One designated terminal of the secondary of

each potential, current and auxiliary transformer are

connected to the earthing grid by means of two separate

and distinct earth connection made with 50 mm x 6 mm

MS flat as shown in figure in 6,7& 8.

Figure - 6 Figure - 7 Figure - 8

Figure- 5


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22

2.7 midj.kmidj.kmidj.kmidj.k vvvvffffFkFkFkFkZax@Zax@Zax@Zax@EQUIPMENT EARTHING

2.7.1 ikWoj VkUlQkeZj dhikWoj VkUlQkeZj dhikWoj VkUlQkeZj dhikWoj VkUlQkeZj dh vvvvffffFkFkFkFkZax@Zax@Zax@Zax@ Earthing of Power

Transformer

The metallic frame work (tank) of each

power transformer is directly connected to

the main earthing mat by means of two

separate distinct connections made with 50

mm x 6 mm MS flat for the 25 kV side and

75 mm x 8 mm MS flat for the primary

side. One connection is made with the

nearest longitudinal conductor, while theother is made to the nearest transverse

conductor of the mat.

In addition there is a direct connection from the

tank to the earth side of lightning arrestors. The

transformer tank rails are earthed either separately or by

bonding at each end of the track as shown in figure- 9.

2.7.2 lfdZVlfdZVlfdZVlfdZV cszdj rFkk buVIVjcszdj rFkk buVIVjcszdj rFkk buVIVjcszdj rFkk buVIVj dh vfFkZaxdh vfFkZaxdh vfFkZaxdh vfFkZax@@@@ Earthing ofCircuit Breakers and Interrupter

The supporting structures of each circuit

breaker unit are connected to the

earthing mat by means of two separate

distinct connections made with 50 mm

x 6 mm MS flat for the 25 kV side and

75 mm x 8 mm MS flat for the primaryside. One connection is made with the

nearest longitudinal conductor, while

the other is made to the nearest

transverse conductor of the mat.

Cubicles, doors and cable glands

are also connected to the earthing mat

as shown in figure 10 & 11.

Figure - 9

Figure -10 & 11.


35/82



23

Fi ure -14

2.7.3 vkblksysVj dhvkblksysVj dhvkblksysVj dhvkblksysVj dh vvvvffffFkFkFkFkZax@Zax@Zax@Zax@Earthing ofIsolators

A flexible earth conductor is

provided between the handle andearthing 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 50

mm x 6 mm MS flat for the 25 kV

side and 75 mm x 8 mm MS flat for

the primary side .One connection ismade with the nearest longitudinal

conductor, while the other is made to the nearest

transverse conductor of the mat as shown in figure 12

& 13

2.7.4 ykbfVaxykbfVaxykbfVaxykbfVax vvvvjsLVj dhjsLVj dhjsLVj dhjsLVj dh vvvvffffFkFkFkFkZaxZaxZaxZax@@@@Earthing of LightningArrestor

In addition to the earth

electrodes provided for the main

earthing mat, an independent earth

electrode is provided for each

lightning arrester. This earth

electrode is connected to the ground

terminal of the lightning arrester, as

well as to the main earthing mat bymeans of two separate distinct

connections made with 50 mm x 6

mm MS flat for the 25 kV side lightning arresters, and

with 75 mm x 8 mm MS flat for the primary side

lighting arresters. The earth electrode is provided as

close as possible to the lightning arrester and the

Figure -13


36/82



24

connection shall be as short and straight as possible

avoiding unnecessary bends.

For lightning arresters provided for the traction

power transformers, there is also be a connection as

direct as possible from the ground terminal of the

lighting arrester to the frame of the transformer of the

being protected. This connection is also be made by

two separate and distinct connections made with 50

mm x 6 mm MS flat for the 25 kV side lightning

arresters, and with 75 mm x 8 mm MS flat for the

primary side lighting arresters as shown in figure- 14

2.7.5 djsaV ,oadjsaV ,oadjsaV ,oadjsaV ,oa iksVsfUk;y VkUlQiksVsfUk;y VkUlQiksVsfUk;y VkUlQiksVsfUk;y VkUlQWWWWkeZj dhkeZj dhkeZj dhkeZj dhvvvvffffFkFkFkFkZax@Zax@Zax@Zax@Earthing of Current Transformer andPotential Transformer

The supporting structures of

Current Transformer and PotentialTransformer unit of bases, all bolted

cover plates to which the bushings

are attached connected to the earthing

mat by means of two separate distinct

connections made with 50 mm x 6

mm MS flat for the 25 kV side and

75 mm x 8 mm MS flat for the

primary side. One connection is madewith the nearest longitudinal

conductor, while the other is made to

the nearest transverse conductor of

the mat as shown in figure 15 & 16

Figure -15& 16


37/82



25

2.7.6 daVksy :e dsvUnj dhdaVksy :e ds vUnj dhdaVksy :e dsvUnj dhdaVksy :e ds vUnj dh vvvvffffFkFkFkFkZax@Zax@Zax@Zax@ Earthing insideControl Room

An earthing ring shall beprovided inside the control

room by means of 50 mm x 6

mm MS flat which shall be run

along the wall on teak wood

blocks fixed to the wall at a

height of about 300 mm from

the floor level as shown in

figure- 17.

The earthing ring is

connected to the main earthing

grid by means of two separate

and distinct connections made

with 50 mm x 6 mm MS flat.

The earthing ring is also be

connected to independent earth

electrode by means of two

separate and distinct

connections made with 50 mm

x 6 mm MS flat as shown in

figure 18.

The metallic framework of control and relay

panels, LT AC and DC distribution boards, battery

chargers, remote control equipment cabinets and such

other equipment are connected to the earthing ring by

means of two separate and distinct connection made

with 8 SWG galvanized iron wire. The connections are

being taken along the wall and in the floor. All recesses

shall be covered with cement plaster after finishing the

work.

Figure- 18

Figure - 17


38/82



26

Connection between the MS flats is made by welding as

shown in figure 19, 20, 21& 22.

Figure- 19 Figure - 20

Figure- 21 Figure- 22


39/82



27

2.7.7 vU; midj.kksadhvU; midj.kksadhvU; midj.kksadhvU; midj.kksadh vvvvffffFkFkFkFkZax@Zax@Zax@Zax@Earthing of otherEquipment

a. vFkZ Lhu dh vfFkZaxvFkZ Lhu dh vfFkZaxvFkZ Lhu dh vfFkZaxvFkZ Lhu dh vfFkZax@@@@Earthing of Earth Screen

The area covered

by outdoor substation are

shielded against direct

strokes of lightning by an

overhead earth screen

comprising 19/2.5 mm

galvanized steel standardwire strung across the

pinnacles of the metallic

structures. The earth

screen wires are strung at a height as indicated

in the approved traction substation layouts (not

less than 2.5 m above the live conductor) and

shall be solidly connected to the traction

substation earthing grid at each termination by

means of 50 m x 6 mm GI flat as shown in

figure 23.

b. QsfUlax LraHkksa vkSj isuyksa dhQsfUlax LraHkksa vkSj isuyksa dhQsfUlax LraHkksa vkSj isuyksa dhQsfUlax LraHkksa vkSj isuyksa dh vvvvffffFkFkFkFkZaxZaxZ a xZax@@@@Earthing of fencing uprights and panels

Each metallic fencingupright is connected to the

traction substation main

earthing grid by means of

two separate and distinct

connections made with 50

mm x 6 mm MS flat. In

addition, all the metallic

fencing panels shall be connected to the uprights

Fi ure- 24

Figure- 23

Fi ure- 24

Figure- 24


40/82



28

by means of two separate and distinct

connections made with

6 SWG G I wire. The

entire metallic doorpanels are also be

connected to the

supporting uprights by

means of two distinct

connections made with

6 SWG I wire as shown

in figure- 24 & 25

All metallic part

of structures, masts and

metallic frames is

connected to the

traction substation

main earthing mat by

means of two separate

and distinct connections

made with 50 mm x 6 mm MS flats shown in

figure- 26.

2.7.8 vkW;y fQYVjsku IykaVvkW;y fQYVjsku IykaVvkW;y fQYVjsku IykaVvkW;y fQYVjsku IykaVds fy,ds fy,ds fy,ds fy, 240oksYV ,lh 50oksYV ,lh 50oksYV ,lh 50oksYV ,lh 50gVZt dsikWbaV dhgVZt dsikWbaV dhgVZt dsikWbaV dhgVZt dsikWbaV dh vvvvffffFkFkFkFkZax@Zax@Zax@Zax@Earthing at the pointof 240 V ac 50 Hz supply for oil filtration plant

The 240 V ac 50 Hz

distribution board for power

supply to oil filtration plant is

connected to the main earthing

grid by means of two distinct

connections made with 50 mm

x 6 mm MS flat as shown in

figure - 27

Figure- 26

Figure- 25

Figure- 27


41/82



29

2.8 vvvvffffFkFkFkFkZax pkyd dk lkbZax pkyd dk lkbZax pkyd dk lkbZax pkyd dk lkbZt@Zt@Zt@Zt@SIZE OF EARTHINGCONDUCTOR

Size of earthing conductor depends on systemvoltage and fault level as given below:

Sr.

No

Equipment

(KV)

System voltage and

fault level

Ground

conductor size

1 Equipment

on the

primary

side of

tractionpower

transformer

66 KV, upto 3000 MVA




66 KV, above 3000 upto

6000 MVA


10000 MVA


12000 MVA

220 KV, above 10000

pto 20000 MVA

50mm x 6mm

75mm x 8mm

2 Equipment on the secondary side of

traction power transformer

50mm x 6mm

3 Fencing uprights/steel structures 50mm x 6mm

4 Doors/fencing panels 6 SWG G I wire

2.9 ffffLofpax LVskuksa ijLofpax LVskuksa ijLofpax LVskuksa ijLofpax LVskuksa ij vvvvffffFkFkFkFkZax O;oLFkk@Zax O;oLFkk@Zax O;oLFkk@Zax O;oLFkk@EARTHINGARRANGEMENT AT SWITCHING STATION

A minimum number of three earth electrodes

(excluding the one to be provided separately for the

remote control cubicle earthing) are provided at each

switching station, and they are interconnected by means

of 50 mm x 6 mm MS flat forming a closed loop main

earthing ring. This ring is connected by two separate


42/82



30

and distinct connections made with 50 mm x 6 mm MS

flat, to the non-track circuited rail in a single rail track

circuited section and to the neutral point of the

impedance bond in a double rail track circuited sectionof the nearest track, so as to limit the potential gradient

developing in the vicinity of the switching station in the

event of a fault.

2.9.1 flLVeflLVeflLVeflLVe vvvvffffFkFkFkFkZax@Zax@Zax@Zax@System Earthing

One designated terminal of the secondary of

each potential transformer, current transformer andauxiliary transformer are connected to the main

earthing ring by means of two separate and distinct

connections made with 50 mm x 6mm MS flat as

shown in figure -28 &29.

2.9.2midj.kmidj.kmidj.kmidj.k vvvvffffFkFkFkFkZax@Zax@Zax@Zax@Equipment earthingAll masts,

structures, fencing

uprights and all

outdoor equipment

pedestals including

auxiliary transformer

tank is connected to

the earthing ring by

means of two

separate and distinct




43/82



31

connections made with 50mm x 6mm MS flat. All

fencing panels are connected to the supporting uprights

by means of two separate and distinct connections made

with 6 SWG G.I. wire. All the metallic door panels areconnected to the supporting uprights by means of two

separate and distinct connections made with 6 SWG

G.I. wire as shown in figure- 30&31

The metal casing of potential and current

transformers are connected to the mast/ structures by

means of two separate and distinct connections made

with 50mm x 6mm MS flat as shown in figure 32&33.

The ground terminal of

lightning arrester shall be

connected directly to the earth

electrodes by means of twoseparate and distinct connections

made with 50mm x 6mm MS flat.

The earth electrode shall be so

placed that the earthing leads from

the lightning arrester may be

brought to the earth electrodes by as short and straight a

path as possible as shown in figure 34.


Fi ure- 34


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32

3.0 ijEijkxrijEijkxrijEijkxrijEijkxr vFkvFkvFkvFkZ byZ byZ byZ byssssDVksM dk vuqj{k.kDVksM dk vuqj{k.kDVksM dk vuqj{k.kDVksM dk vuqj{k.kMAINTENANCE SCHEDULE FOR CONVENTIONALEARTH ELECTRODE

3.1 ====SSSSekfld vuqj{k.k@ekfld vuqj{k.k@ekfld vuqj{k.k@ekfld vuqj{k.k@QUARTERLY SCHEDULE

Sr. Items Inspection Action to be taken

1 Earth flat

connections to

structures,equipment, rail

and earth

electrodes with nut

and check nuts

Check for

proper

continuityand

tightness

If found broken or

loose, it should be

immediatelyreplaced /tightened

2 Bolts and nuts of

the connections

Check for

rust and

dirt

Rust and dirt should

be cleaned and apply

grease

3 MS links by bolted

joints between

earth electrode and

75mmx 8mm MS

flat

Check for

tightness

If found broken or

loose, it should be

immediately

replaced/ tightened

4 Projection of theearth electrode

Check forground

level and

proper soil

It should be 175 10mm above the

ground level


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3.2 v)Zv)Zv)Zv)Z okf"kZd vuqj{k.k@okf"kZd vuqj{k.k@okf"kZd vuqj{k.k@okf"kZd vuqj{k.k@HALF YEARLY SCHEDULE

Carry out following work in addition to quarterly

schedule:

Sr. Items Inspection Action to be taken

1. Measure

earth

resistance

Individual earth

electrode and

record the value

By earth resistance

meter. It should be

permissible limit

2. Measure

earthresistance

combined

Combined earth

resistance withearthing flats,

connected to the

equipment,

structures and

earth electrode

TSS 0.5

Switching station 2.0

AT Station 10.0

3. Sump Check up general

condition

including dryness.

If the surrounding area

is too dry, water

should be poured intothe sump to keep the

soil moist.


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3.3 okf"kZd vuqj{k.k@okf"kZd vuqj{k.k@okf"kZd vuqj{k.k@okf"kZd vuqj{k.k@YEARLY SCHEDULE

Carry out following work in addition to

quarterly and half yearly schedule:

Sr. Item Inspection Action to be taken

1. Earth

pits

Check up

the

electrode

for proper

earthcontinuity.

Remove the hardened top

layer of the earth pit for a

depth of 1 meter, mix with

coke and loamy soil (non-

sandy) and ram the earth.Repair the earth. Repair

sides and top cover of the

earth pits. Avoid use of salt

as far as possible to avoid

rusting of earth pipe.

3.4 vFkZvFkZvFkZvFkZ bybybybySSSSDVksM dsfrjks/k dk ekiu@DVksM dsfrjks/k dk ekiu@DVksM dsfrjks/k dk ekiu@DVksM dsfrjks/k dk ekiu@MEASUREMENT OF EARTH ELECTRODERESISTANCE

Fall of potential method

In this method two auxiliary earth electrodes,

besides the test electrode, are placed at suitable

distances from the test electrode as shown in figure35.

A measured current is passed between the electrode A

to be tested and an auxiliary current electrode C andthe potential difference between the electrode A and

the auxiliary potential electrode B is measured. The

resistance of the test electrode A is then given by:

R = V/I

Where, R = Resistance of the test electrode in

ohms,

V = Reading of the voltmeter in volts,


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35

I = Reading of the ammeter in amperes

In most cases, there will be stray currents flowing in

the soil and unless some steps are taken to eliminate their

effect, they may produce serious errors in the measuredvalue. If the testing current is of the same frequency as the

stray current, this elimination becomes very difficult. It is

better to use an earth tester incorporating a hand driven

generator. These earth testers usually generate direct

current, and have rotary current reverser and synchronous

rectifier mounted on the generator shaft so that alternating

current is applied to the test circuit and the resulting

potentials are rectified for measurement by a direct reading

moving coil ohm meter. The presence of stray currents in

the soil is indicated by a wandering of the instrument

pointer, but an increase or decrease of generator handle

speed will cause this to disappear.

At the time of test, where possible, the test electrode

shall be separated from the earthing system. The auxiliaryelectrode consists of 12.5 mm diameter mild steel rod

driven up to one meter into the ground.

AMMETER

VOLTMETER

A

V

CURRENT

SOURCE

A B C

TESTELECTRODE

POTENTIAL

ELECTRODECURRENT

ELECTRODE

X = 1m

Figure 35


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3.5 gLr pkfyr vFkZ VsLVj dk fooj.k@gLr pkfyr vFkZ VsLVj dk fooj.k@gLr pkfyr vFkZ VsLVj dk fooj.k@gLr pkfyr vFkZ VsLVj dk fooj.k@DETAILS OFEARTH TESTER (Hand Driven)

Earth resistance meter are employed formeasurements of earth resistance in Traction sub

station, switching stations and other electrical

installations. An Earth resistance meter comprises a

hand driven magneto type D.C. Generator, a current

reverser, rotary rectifier and ohm meter.

The current reverser and rotary rectifier are

driven along with D.C. Generator by driving systems

which incorporate a clutch mechanism for

unidirectional rotation and a governor for speed control.

The function of current reverser is to change the

direction of flow of current in the soil and that of rotary

rectifier is to maintain unidirectional current in the

potential coils of the ohm meter.

The ohm meter consists of a current coil and a

potential coil mounted on a common spindle and placed

in the magnetic field of a permanent magnet. The

current coil is connected in series with the earth

electrodes and current electrodes. The potential coil is

connected across the earth electrode and the potential

electrode through the rotary rectifier. While measuring

the earth resistance the terminals C1, P1 are connected

to the main earth electrode P2 to the potential electrodeand C2 to the current electrode. The potential and

current electrodes are temporary electrodes placed in

the ground 50 to 75 feet apart and 50 to 75 feet & from

the earth electrode as shown in below figure- 35 A

When the megger is operated an ac current is

produced in the coil. The voltage drop produced in the

earth electrode is applied across the potential coil. The


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37

current coil produces a torque in the clock wise

direction, and the potential coil produces a torque in

anti-clock wise direction. The current applied to the

current coil is inversely proportional to the earthresistance and the voltage drop applied across the

potential coil is directly proportional to the earth

resistance the torque opposes each other and brings the

moving system to rest when they are equal. The pointer

indicates the earth resistance values on a calibrated

scale.

Figure 35 A

Cu rrent Term inal: or C2Poten tial Terminal: P1 or P2

Test EarthElectrode

PotentialElectrode

CurrentElectrode

C1

P1

C2

P2

50 - 70 feet 50 - 70 feet


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38

4.0 vuqj{k.k eqDr vfFkZax@vuqj{k.k eqDr vfFkZax@vuqj{k.k eqDr vfFkZax@vuqj{k.k eqDr vfFkZax@MAINTENANCE FREEEARTHING

In conventional earthing system GI pipe is usedas earth electrode. It is provided with charcoal and salt

as conducting media, which provides a reasonable

earth. Corrosion of metallic parts is comparatively fast.

It also requires maintenance by way of watering of

earth pits and chiseling of corrosion prone parts and

their replacement. Required monitoring may not always

be feasible in certain crowded and inaccessible areas.

With technological developments in this field,

modern maintenance free and durable earthing system

employs steel conductors as electrode which are copper

claded and utilize graphitic compounds and non

corrosive salts as Ground Enhancing Material which

do not lead to corrosion. Such earth pits also do not

require the usual watering schedules to maintain the

earth resistance with in limits. Maintenance free earthsare to be constructed as per RDSOs specification

no.RDSO/ PE/ SPEC/ 0109-2008 (REV0).

Where the earth pits are not easily accessible for

schedule maintenance, maintenance free earth pits shall

be provided. In areas where clusters of earth pits are

required to keep the earth resistance low, provision of

maintenance free earth pits should be made duringinitial installation.

4.1 vFkZ frjks/kvFkZ frjks/kvFkZ frjks/kvFkZ frjks/k@@@@EARTH RESISTANCE

The earth resistance value at earth bus bar

should be less than 0.5 ohms for major electrical

equipment & installation.


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39

4.2 mi;ksx@mi;ksx@mi;ksx@mi;ksx@APPLICATIONS

This earthing system may be used in following

locations. Sub stations & switching stations Remote Terminal Units Transformer & Generator neutral earths Lightning arrester earths Equipment earths including panels

4.3 vuqj{k.k eqDr vfFkZax .kkyh@vuqj{k.k eqDr vfFkZax .kkyh@vuqj{k.k eqDr vfFkZax .kkyh@vuqj{k.k eqDr vfFkZax .kkyh@MAINTENANCE

FREE EARTHING SYSTEM

This earthing system includes earth electrode

installation in suitable pit, construction of earth pit with

cover for the installation, connection of earth electrode

with equi-potential earth bus and connection of

equipment to equi- potential earth bus.

4.3.1 vFkZvFkZvFkZvFkZ bySDVksM@bySDVksM@bySDVksM@bySDVksM@Earth ElectrodeThe material for earth electrode used in this type

of earthing, has a good electrical conductivity and it

does not corrode in a wide range of soil conditions.

There are basic two types of earth electrodes

used in this earthing system.

i. jkWM Vkbi vjkWM Vkbi vjkWM Vkbi vjkWM Vkbi vFkZFkZFkZFkZbySDVksM@bySDVksM@bySDVksM@bySDVksM@Rod type earthelectrode

The copper bonded stainless steel rod

(low carbon high tensile steel alloy) earth

electrode shall conform to the standard BS 4360

Grade 43A or EN10025:2-004 S275IR

molecularly bonded by 99.99% pure high


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40

conductivity copper on outer surface with

copper coating thickness 250 micron or more.

The earth rod shall have following

characteristics/ specifications.

a.The minimum length of earth electrode shallbe 3.0 meters long. The length of electrode

may be increased in multiple of 1.0 meter to

reduce earth resistance.

b.To increase the length, pieces of similar rodshall be either exothermally welded to basic

3.0 meter electrode or connected using

socket of suitable size. These sockets shall

be molecularly bonded by 99.99% pure

high conductivity copper on inner & outer

surface with copper coating thickness 250

micron or more.

c.The diameter of earth electrode shall not beless than 17 mm.

d.The copper bonding thickness on stainlesssteel rod shall be 250 microns or more, hot

dipped or electroplated.

e.Copper bus bar of size 250 mm x 50 mm x 6mm having electrical conductivity of 101%

IACS minimum 99.9% copper content shallbe exothermically welded to rod with 4

holes of 12 mm dia. (2 on each side) for

connecting earthing conductor.

f.Current carrying capacity of earth electrodeshould be such as to have more than 15kA

for one second.


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41

ii. llllaaaadsfUnzr ikbi vFkZ bySDVksM@dsfUnzr ikbi vFkZ bySDVksM@dsfUnzr ikbi vFkZ bySDVksM@dsfUnzr ikbi vFkZ bySDVksM@Concentricpipe earth electrode

MS pipe with 25 - 50 mm diameter, class B,ISI mark as per IS: 1239, length 2000 mmor 3000 mm is used as primary conductor

as shown in table -1

MS pipe with 40 - 100 mm diameter, class B,ISI mark as per IS: 1239, length 2000 mm

or 3000 mm is used as secondary conductor

(electrode) as shown in table -1

Table - 1Sr. Current

capacity

Primary

conductor

diameter

Secondary conductor

(Electrode) dimensions

(dia x length)

1 3kA 25 mm 40 mm x 2000 mm

2 5kA 25 mm 40 mm x 3000 mm

3 15kA 25 mm 50 mm x 3000 mm

4 40kA 40 mm 60 mm x 3000 mm5 50kA 50 mm 100 mm x 3000 mm

NOTE For more than 50kA applications, multiple

electrodes of 50kA capacity are installed and

connected.

For hermetically filling inside the cavity betweensecondary conductor and primary conductor,

crystalline compound is to be injected in theelectrode assembly. It is a combination of high

conductivity metal alloys, copper & aluminium

powder, conductive carbon/cement and bonding

material etc. mixed in different proportion. The

mixture is forced (pressurized) filled inside the

earth electrode in the paste form and after

solidification of the same, the end caps are welded.


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42

The metal alloys shall help in conducting the

current and conductive carbon gives anti corrosive

property. Bonding material provides strength to the

mixture. Resistivity of the mixture shall be lessthan 0.2 ohm - meter. Resistivity shall be tested by

making a 20 cm cube of the material and checking

resistance across the opposite face of the cube.

Complete electrodes shall be molecularly bondedby 99.99% pure, high conductivity copper on outer

surface with copper coating thickness 300 micron

or more.

Its surface shall be cleaned and free from anyvisible oxide layer or foreign material.

Copper bus bar of size 250 mm x 50 mm x 6 mmhaving electrical conductivity 101% , minimum

99.9% copper content shall be preferable to

exothermically welded to earth electrode or

connected with the help of two number stainless

steel nut bolts of appropriate size having 4 holes of12 mm dia ( 2 on each side) for connecting earthing

conductor as shown in figure - 36

EarthEhancement

Earth Electrode

(Dimension as perdesign)

ExothermicWelding

25 x 3mm Copper Strip (To be duplicated)

300 x 25 x 6mm Copper bus

ExothermicWelding

ExothermicWelding

Ground Level

5 ft x 5ft x 10ft pit or 300mm bore

(Earth Electrode Installation)

To Equipment

Figure - 36


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43

4.3.2 vFkZ bugsUlesaV lkexzhvFkZ bugsUlesaV lkexzhvFkZ bugsUlesaV lkexzhvFkZ bugsUlesaV lkexzh@@@@Earth Enhancement Material

Earth enhancement material is a superior

conductive material that improves earthing

effectiveness especially in area of poor conductivity asshown in figure 36A. It improves conductivity of the

earth electrode and ground contact area. It has

following characteristics:

a. It should low resistivity bellow 0.2 ohm- meters.

b. It shall not depend on the continuous presence ofwater to maintain its conductivity.

c. It should be a little alkaline in nature with pHvalue >7 but


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44

e. It has a capacity to retain >10% moisture at105

0C.

f. It should have water solubility


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45

4.3.3 cSdfQy lkexzh@cSdfQy lkexzh@cSdfQy lkexzh@cSdfQy lkexzh@Backfill material

The excavated soil can be used if it is free from

sand, gravel and stones. Small proportion of sand inthe soil may be permissible. Material like sand, salt,

coke breeze, cinders and ash are not used because of

its acidic and corrosive nature.

While backfilling the soil shall be thoroughly

compacted with at least 5 kg compactor, in case the

soil is dry small quantity of water may be sprinkled

only to make it moist enough suitable for

compacting. Large quantity of water may make thesoil muddy which is not suitable for compacting and

after drying the soil may contain voids which may

permanently increase earth resistance.

4.3.4 le foHko cl ,oavFkZ pkyd@le foHko cl ,oavFkZ pkyd@le foHko cl ,oavFkZ pkyd@le foHko cl ,oavFkZ pkyd@Equi- Potential Bus& Earth Conductor

A copper bus bar of size 300 mm x 25 mm x 6 mmto be installed in this equipment room as equi

potential bus connected with copper strip of size

25 mm x 3 mm (suitable length) from instrument

to the bus bar. The connecting terminal of the

earth electrodes to the bus bar must be connected

by copper strip of 25 mm x 3 mm (suitable

length) buried inside a trench of 300 mm width x

600 mm (depth from the nearest wall). It is

duplicate earth conductor.

The maximum specific resistance of the copperstrip earthing conductor shall be 17.241 x 10

-7

ohm cm at 200C and having electrical

conductivity of 101% IACS i.e. minimum 99.9%

copper content.


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46

At a temperature of 200C, its density shall be 8.89gm/cm

3.

A single length of copper strip shall be used foreach duplicate earthing conductor and no joint

shall be permitted. The joint shall be made by

exothermic welding of at least 10 mm

overlapping portion of the strips.

It shall be connected to earth electrode and earthbus bar with the help of exothermic welding or at

least two number stainless steel nut bolts of

appropriate size.

4.3.5 vFkZvFkZvFkZvFkZ bdkbZbdkbZbdkbZbdkbZ dk fuekZ.k@dk fuekZ.k@dk fuekZ.k@dk fuekZ.k@Construction of unit Earth

Make 5ft x 5ft x 10ft earth pit. If it not possible to

make such a pit due to non availability of clearspace a minimum 300 mm bore up to 10ft deep

can be made using earth auger or any other

method. Each pit larger than specified size can be

made, if required.

Sleeve the soil digged and remove the gravels andstones. If soil quality is good then add some

quantity of earth enhancement material in the soilfor using as backfill.

If the soil seems unusable (containing largequantity of gravel, stones, murum, sand etc) then

replace the soil with black cotton soil.

Insert the electrode at the center of the earth pit andarrange to keep it vertical in the pit


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47

Arrange for adequate quantity of water supply forthe earth pit (600 liters).

Fill the pit with the backfill and keep on adding theearth enhancement material surrounding the

electrode and simultaneously watering the pit

with a steel bar or pipe, keep on poking the soil

gel and stirring intermittently for removing the air

pockets and proper settlement of the pit. The

procedure to be repeated till completion of the

filling of the earth pit along with the packing

material and sufficient watering adequateramming.

The pit should be very compactly rammed andwatering for 2-3 days and addition of soil if

required be done.

Construct inspection chamber with cover for theinstallation.

Measure the earth resistance as per IS 3043:1987code of practice.

4.3.6 cgq vFkZ fiVksa}kjk vFkZ fjax dk fuekZ.k@cgq vFkZ fiVksa}kjk vFkZ fjax dk fuekZ.k@cgq vFkZ fiVksa}kjk vFkZ fjax dk fuekZ.k@cgq vFkZ fiVksa}kjk vFkZ fjax dk fuekZ.k@Construction of Ring Earth by providing multiple

earth pits

Wherever it is not possible to achieve requiredearth resistance with one earth electrode/ pit due todifficult/ rocky soil conditions, provision of ring

earth consisting of more than one earth pit is

required. The number of pits required can be

decided based on the resistance achieved for the

earth pits already installed.


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48

The distance between two successive earthelectrodes shall be min. 3.0 mtrs/length of electrode

which ever is higher, and max. up to twice the

length of the earth electrode.

These earth pits shall inter linked using 25 x 3 mmcopper strip to from a loop preferably using

exothermic welding or with the help of at least two

numbers of stainless steel nut bolts of appropriate

size.

The interconnecting strips shall be buried not less

than 800 mm (0.8 m) below the ground level. Thisinterconnecting strip shall also be covered with

earth enhancing compound.

4.3.7 fujh{k.k d{k@fujh{k.k d{k@fujh{k.k d{k@fujh{k.k d{k@Inspection chamber

A 300 mm x 300 mm x 300 mm (inside dimension)concrete box (wall thickness min. 50 mm) with

smooth cement plaster finish shall be provided on

the top of pit. A concrete lid of 25 to 50 mm thick,

with pulling hooks, painted black shall be provided

to cover the earth pit. PVC sleeve of appropriate

size shall be provided in concrete wall to take out

earthing connections.

The masonry work shall be white washed inside

and outside.

Care shall be taken regarding level of the floorsurrounding the earth so that the connector is not

too deep in the masonry or projecting out of it.

On backside of the cover, date of test and averageresistance value shall also be written with yellow

paint on black background with date.


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49

5.0 ckWfUMax] xzkmfUMax@ vfFkckWfUMax] xzkmfUMax@ vfFkckWfUMax] xzkmfUMax@ vfFkckWfUMax] xzkmfUMax@ vfFkZax dsfy, ,DlksFkfeZdZax dsfy, ,DlksFkfeZdZax dsfy, ,DlksFkfeZdZax dsfy, ,DlksFkfeZdoSoSoSoSfYMax la;kstufYMax la;kstufYMax la;kstufYMax la;kstu@@@@EXOTHERMIC WELDINGCONNECTION FOR BONDING, GROUNDING/EARTHING

The Exothermic welding connection process for

grounding/ earthing covers the requirements and

acceptance principle for weld material, welding tool

and tool kit, bonds/ track bond wires for making

permanent electrical bond connection between two

conductors as per RDSO specification no. TI/ SPC/OHE/ EXOTHRM BOND/ 0100 (04/10).

These weld materials including tool kit shall be useful

in following railway application on 25kV electric

traction system.

(a) Traction bond connection to rail & OHE structures

enroute.(b) Traction rail continuity connection.

(c) Earthing connections like at TSS, SP/ SSPs,SCADA/ RTU and other important electrical

installation.

(d) Bus bar connections etc.

(e) Copper to copper, copper to steel connection.(f) Important bonds which are not disturbed during

track relaying.

(g) Sections where 90UTS rails have been provided.

(h) Theft prone areas.


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50

5.1 lkekU; vko;drk;salkekU; vko;drk;salkekU; vko;drk;salkekU; vko;drk;sa@@@@GENERAL REQUIREMENTS

Welding material along with tool kit shall be

capable of providing a permanent bondingconnection involving copper or aluminium or steel.

The final bond shall be achieved in such a way that

homogeneous molecular bonding of two involved

metals takes place perfectly.

Exothermic welding process should be a steadyburn, no pops and no drastic color changes (white

hot color during burn, turns orange as the reaction

stops and the molten metal grows orange).

Welding connection shall be without voids orpinhole, and no porosity should be detected. The

process shall be consistent and continuous.

It shall be permanent, solid metal, maintenance freeconnection and shall not be affected by high current

surge. It shall not loosen or corrode at the point of weld.

There shall be no additional contact surface or

mechanical pressures involved.

It shall be vibration proof and corrosion freeelectrical connection.

It shall not brittle with age and shall be able tosustain physical impacts, which may be

experienced under the field conditions.

Electrical resistance and other properties shall notdeteriorate with ageing and under severe Indian

climatic conditions.

It shall have a current carrying capacity equal to or

greater than that of the conductors


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51

Life of the joints shall not be less than 10 yearsirrespective of life of conductors, equi-potential

earth bus bars or ground rods used.

5.2 oooossssfYfYfYfYaaaaMx lkexzh@Mx lkexzh@Mx lkexzh@Mx lkexzh@WELDING MATERIAL

The weld material shall be designed for twospecific types of jointing applications

Rail welding application

Earthing & bonding application.

It shall not contain phosphorous or any caustic,toxic or explosive substance. For rail welding

application, the welding material shall not contain

tin besides the above cited elements. Its

components shall not volatile or sensitive to shock,

nor capable of spontaneous ignition by friction.

The reaction of the welding material shall be

completed within 10 seconds and on completion ofjointing & bonding process. The resulting weld

nugget shall free of dissolved gases or low boiling

point elements, e.g., small voids or blisters on the

surface.

Weld materials shall provide a final bond withminimum tensile strengths of 40,000 psi tested

using ASTM ES-94a (Test Methods for Tension

Testing of Metallic Material).

The weld material for traction bond and studwelding for rail application shall be free from tin.

As presence of metallic tin allows for wetting of

rail surface and provides for unnecessary

penetration that can lead to metallurgical damage to

the rail.


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52

The weld material for earthing & bondingapplication shall be tested as provision of IEEE-837

by NABL/ILAC Member labs.

5.3 osfYaMxosfYaMxosfYaMxosfYaMx VwYl dh fofk"VVwYl dh fofk"VVwYl dh fofk"VVwYl dh fofk"Vrk;sa@rk;sa@rk;sa@rk;sa@SPECIFICATIONOF WELDING TOOLS

It shall be suitable for bonding as per the sizesspecified for bonding equi-potential earth bus bars

(EEBB) to g

handbook on earthing system for power supply installations.pdf

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