Handbook on Electrical Earthing

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

1 x gs rq ds oy dk;Z ky;hu mi;ks (For Official Use Only)

Hkkjr ljdkj GOVERNMENT OF INDIA jsy ea =ky; MINISTRY OF RAILWAYS

fo|qr vfFkZax ij gLriqfLrdk Handbook on Electrical Earthing y{; lewg % fo|qr lkekU; ls s ds vuq ok vuqj{k.k deZpkjh kjh l TARGET GROUP: General Services Services Maintenance Staff

dSeVsd@bZ d@bZ@10&11 10&11 &11@ @bZ @bZ,y&vfFkZ ,y&vfFkZ ax x @1 @1 -0 -0 CAMTECH/E/10-11/El-EARTHING/1.0

fnlEcj 2010 December 2010

egkjktiqj , Xokfy;j Xokfy;j & 474 005 Maharajpur, GWALIOR GWALIOR - 474 005

Handbook on Electrical Electrical Earthing Earthing

December, December, 2010

2 Earthing/1.0

CAMTECH/E/10-11/El-

fo|qr vfFkZax ij gLriqfLrdk Handbook on Electrical Earthing

xq.koRrk uhfr jsyks yks es a es ;k=h a ;k=h vkSj eky ;krk;kr dh c<+ rh ek¡x dks iw iw jk djus ds fy, xq.koRrk Áca/k/k Á.kkyh es a vuql/kku] a/kku] fMtkbuks a vkSj ekudks a esa mRd`"Vrk rFkk lr~r lq/kkjks a ds ds ek/;e ls lkafof/kd fof/kd vkSj fu;ked vis{kkvks {kkvks a dks dks iw iw jk djrs gq, lqjf{kr] vk/kqfud vkSj fdQk;rh jsy ÁkS|ksfxdh fxdh dk fodkl djuk A

QUALITY POLICY “To develop safe, modern and cost effective Railway Technology complying with Statutory and Regulatory requirements, through excellence in Research, Designs and Standards and Continual improvements in Quality Management System to cater to growing demand of passenger and freight traffic on the railways”. December, 2010 Earthing

Handbook on on Electrical


3

ÁkDdFku ÁkDdFku fo|qr laLFkkiukvks a ,oa midj.kks a dh vfFkZ ax] x] fo|qr ç.kkyh ds lqjf{kr dk;Z djus ds lkFk&lkFk deZpkfj;ks a dh lqj{kk es a egRoiw .kZ .kZ Hkwfedk vnk djrh gSA ;g QkYV dja sV dks fuEu bEihMsUl iFk Ánku djrk gS vkSj xzkmaM QkYV gksus u ij s ij lqj{kk ;a =ks a dk Rofjr ifjpkyu lqfuf’pr djrk gSA dSeVsd }kjk **fo|qr vfFkZ ax Á.kkyh** ij ;g gLriqfLrdk fo|qr lkekU; ls okvks a ds deZpkfj;ks a dks tkudkjh nsus us ds ds mn~ mn~ns’; ls cukbZ cukbZ xbZ gSA bl gLriqfLrdk es a vfFka vfFka Zx dh la jpuk] lcLVs’ku dh vfFkZ ax O;oLFkk] vuqj{k.k 'kSM~;w y] y] vuqj{k.k eqDr vfFkZ ax] x] bR;kfn dk fooj.k fn;k x;k gSA eq>s vk’kk gS fd ;g gLriqfLrdk fo|qr lkekU; ls okvks a ds ds vuq vuqj{k.k deZpkfj;ks a ds ds fy, fy, mi;ksxh fl) gksxh xh A dSeVsd d]] Xokfy;j fnukad 27 tuojh]] 201 20111 277]] tuojh


,l lh-lh ?ky ?ky ky ,l lh -lh -fla -fla?ky dk;Zdkjh kjh funs’kd ’kd


4 Earthing/1.0

CAMTECH/E/10-11/El-

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 Date: 27.01. 2011

December, 2010 Earthing

S.C. Singhal Executive Director

Handbook on Electrical


5

Hkwf edk edk vfFkZax Á.kkyh dk mfpr j[kj[kko ,oa vuq j{k.k fo|qr midj.kks a dh vPNh ,oa fo’oluh; lq j{kk Á.kkyh dks lqfuf’pr djus ,oa ifjpkyd dks fo|qr >Vds ls cpkus ds fy, vko’;d gSA dSeVsd }kjk fo|q r vfFkZ ax ij ;g gLriqfLrdk gekjs vuq j{k.k deZpkjh;ks a dks dk;Z {ks = es a vfFkZax Á.kkyh ls voxr djkus ds mn~n’s ; ls cukbZ xbZ gSA ;g Li"V fd;k tkrk gS fd ;g gLriqfLrdk vfFkZax dk vkbZ- ,l- dksM ¼ IS: 3043½] vkbZ bZ fu;ekoyh] vkjMh,lvks ;k jsyos cksMZ }kjk fofuZfn"V fdlh Hkh fo/kku dks foLFkkfir ugha djrhA ;g gLriqfLrdk ds oy ekxZn’kZu gs rq gS ,oa ;g ,d oS/kkfud nLrkost+ ugha gSA eS]a dk;Z{ = ks ds mu lHkh deZpkfj;ks a dk vkHkkjh g¡w ftUgks aus bl gLriqfLrdk dks cukus es a gekjh lgk;rk dh A rduhdh mUu;urk vkSj lh[kuk ,d lrr~ izfdz;k gSA vr% bl gLriqfLrdk es a tksM+u s @ lq/kkjus ds fy;s ges a fy[kus es a Lora = eglw l djs a A bl fn’kk es a ge vkids ;ksxnku dh ljkguk djs axAs

dSeVsd] Xokfy;j fnukad 31 31] fnlEcj] fnl Ecj] 2010

Handbook on Electrical Earthing

ih;w" k xqIrk la- funs’kd ¼fo|qr½

December, 2010

6 Earthing/1.0

CAMTECH/E/10-11/El-

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 Date:31.12.2010


Peeyoosh Gupta Jt.DirectorElectrical e - mailid:[email protected]



7

fo"k; lwp h h ØØ - la laØ

1.0

2.0

fooj.k fooj.k i`"B "B laçkDdFku iii v Hkwf edk vii fo"k; lwp h la’kks/ku ifpZ;ksa dk izdk’ku xi ÁLrkouk 01 1-1 vfFkZ ax ds ykHk 02 1-2 ikfjHkkf"kd 'kCnkoyh 03 1-3 Hkw pkyd dh rjg 05 1-4 vfFkZ ax ls lEcfU/kr egRoiw. kZ Hkkjrh; fo|qr fu;e 05 1-5 vfFkZ ax ds fy, lkekU; vko’;drk;sa 07 1-6 xzkÅf.Max ,oa vfFkZax es a vUrj 10 1-7 fo|qr dk >Vdk ,oa ekuo vo;o 11 1-8 feV~Vh dh Áfrjks/kdrk fu/kkZfjr djus okys dkjd 13 1-9 vFkZ bysDVªksM yxkus dk LFkku 19 vFkZ bysDVªksM ds fMtk;u] lkbt ,oa Ádkj 20 2-1 bysDVªkM s ,oa vFkZ ds e/; Áfrjks/k 20 2-2 bysDVªkM s Áfrjks/k dks ÁHkkfor djus okys 21 dkjd 2-3 bysDVªkM s dk lkbt 22 2-4 vFkZ bysDVªksM dh fMtkbu 22 2-5 vFkZ bysDVªksM 24 Handbook on Electrical Earthing

December, 2010

8 Earthing/1.0

CAMTECH/E/10-11/El-

CONTENTS Sr.No.

1.0

Description

Foreword Preface Contents Correction Slip INTRODUCTION

iv vi viii xii 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

GENERAL REQUIREMENT FOR EARTHING

07

DIFFERENCE BETWEEN GROUNDING AND EARTHING

10

HUMAN ELEMENT & ELECTRIC SHOCK

11

FACTORS WHICH DETERMINE RESISTIVITY OF SOIL

13

LOCATION OF EARTH ELECTRODE

19

1.5 1.6 1.7 1.8 1.9

2.0

Page No.

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




ØØ - la laØ

fooj.k fooj.k

2-6 2-7 2-8 3-0 vfFkZax 3-1 3-2 3-3

4-0 5-0

6-0

bysDVªkM s ds Ádkj vfFkZ ax ds çdkj vfFkZ ax yhM flLVe flLVe vfFkZ ax dk oxhZdj.k lc LVs’ku esa vfFkZax Á.kkyh lc LVs’ku esa fofHkUu midj.kks a dh vfFkZ ax 3-4 forj.k Vªk¡lQkeZj LVªdpj d h vfFkZax 3-5 forj.k ykbu LVªdpj dh vfFkZ ax 3-6 miHkksDrk ds ifjlj es a vfFkZ ax 3-7 vkS|ksfxd ifjlj esa vfFkZ ax 3-8 viw. kZ vfFkZ ax ds [krjs 3-9 lko/kkfu;k¡ ijh{k.k ,oa vuqj{k.k {k.k 4-1 vfFkZ ax Á.kkyh dk ijh{k.k 4-2 vuqj{k.k 'ksM~;wy vuqj{k.k jfgr vfFkZ ax 5-1 vFkZ çfrjks/k 5-2 mi;ksx 5-3 vuqj{k.k jfgr vfFkZax flLVe D;k djs a vkSj D;k u djs a 6-1 D;k djsa 6-2 D;k u djsa lanHkZ Handbook on Electrical Earthing

9

i`"B la26 32 35 35 35 41 43 47 51 53 56 57 58 59 59 62 65 66 66 66 77 77 78 80 December, 2010

10 Earthing/1.0

CAMTECH/E/10-11/El-

Sr.No.

3.0

Description 2.6

TYPES OF ELECTRODES

26

2.7

TYPES OF EARTHING

32

2.8

EARTHING LEAD

35

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

DISTRIBUTION TRANSFORMER STRUCTURE EATHING

47

EARTHING OF DISTRIBUTION LIENE STRUCTURES

51

3.6

EARTHING AT CONSUMER’S PREMISES

53

3.7

EARTHING IN INDUSTRIAL PREMISES

56

3.8

DANGERS OF IMPERFECT EARTHING

57

3.9

PRECAUTIONS

58

3.4 3.5

4.0

5.0

6.0

Page No.

TESTING & MAINTENANCE

59

4.1

TESTING OF EARTHING SYSTEM

59

4.2

MAINTENANCE SCHEDULES

62

MAINTENANCE FREE EARTHING

65

5.1

EARTH RESISTANCE

66

5.2

APPLICATIONS

66

5.3

MAINTENANCE FREE EARTHING SYSTEM

66

DO’S & DON’TS

77

6.1 DO’S

77

6.2 DON’T

78

REFRENCES

81




11

la’kks/ku ifpZ;ksa dk izdk’ku k’ku bl gLriqfLrdk ds fy;s Hkfo"; es a izdkf’kr gksu s okyh la’kks/ku ifpZ; aks dks fuEukuq lkj la[;kafdr fd;k tk;sxkA dSeVsd@bZ@10&11 10&11@ @bZ,y y&vfFkZ &vfFkZ ax@1 fnukad ------------ @1--0@ lh,l # XX fnuka

tgkWa “XX” lEcfU/kr la’kks/ku iphZ dh dze la[;k gS ¼ 01 ls izkjEHk gksdj vkxs dh vks j½

izdkf’kr la’kks/ku ifpZ;k¡ dz la -la-

izdk’ku la’kksf/kr Ik`"B la[;k dh rkjh[k ;k rkjh[k rFkk en la[;k


fVIi.kh fVIi.kh

December, 2010

12 Earthing/1.0

CAMTECH/E/10-11/El-

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 issue

of Page no. and Item no. modified

Remarks



1.0

1

ÁLrkouk 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.


December, 2010


2

1.1

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.

vfFkZax 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.

December, 2010



1.2

3

ikfjHkkf"kd 'kCnkoyh TERMINOLOGY The following terms are commonly used in earthing systems:

1.2.1

vFkZ

Earth

The conductive mass of the earth, whose electrical potential at any point is conventionally taken as zero. 1.2.2

vFkZ bySDVªksM Earth electrode A Galvanized Iron (GI) pipe in intimate contact with and providing an electrical connections to earth.

1.2.3

vfFkZax 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.k midj.k vfFkZx a

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

Á.kkyh vfFkZx a 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.


December, 2010


4

1.2.6

Vp oksYVst Touch Voltage (E Touch) The potential difference between a ground metallic structure and a point on the earth’s surface separated by a distance equal to the normal maximum horizontal reach of a person, approximately one meter as shown in figure-1

Figure-1

1.2.7

LVsi 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-2

December, 2010



1.2.8

5

eS’k oksYVst Mesh Voltage (E mesh) The maximum touch voltage to be found within a mesh of an earthing grid.

1.3

vFkZ pkyd

EARTH AS CONDUCTOR

•

Resistivity (ρ) of earth is 100 Ω-M.

•

Resistivity (ρ) of copper is 1.7x 10

•

Resistivity (ρ) of G. I. is 1.7x 10

-7

-8

Ω-M.

Ω-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

vf fFkZ ax kjrh; fo|qr fu;e Hk k jrh; k fFkZ a ls lEcfU/kr egkRoiw. kZ Hkk IMPORTANT INDIAN ELECTRICITY RULE RELATED TO EARTHING

fu;e 33

RULE No: 33 Earth terminals on consumer’s premises.

fu;e 61 (A)

RULE No: 61.

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.


December, 2010


6

(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

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

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.

December, 2010



•

7

th

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

fu;e 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

RULE No: 92. Protection against lightening

1.5

•

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.

vfFkZax ds fy, lkekU; vko’;drk;s a 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)


December, 2010


8

ii)

National Electricity Code - 1985 of BIS

iii)



IEEE guide for safety in AC substation grounding no. ANSI/IEEE standard, 801986. 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 2 areas of not less than 3.0 mm if of copper and 2 6.0 mm 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.

December, 2010



9



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


December, 2010


10

the earth electrode mechanical damage.

•

to

protect

it

against

All the earth wires run along the various subcircuits 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 2 mm (14 SWG) or half of the size of the sub circuit conductor.

1.6

xzkÅf.Max ,oa vfFkZax es a vUrj

1.6.1

xzkÅfUMa ÅfU Max

DIFFERENCE BETWEEN GROUNDING AND EARTHING 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.

December, 2010



1.6.2

11

vfFkZax 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 GENERATOR

TRANSFORMER

NEUTRAL GROUNDING

NEUTRAL GROUNDING

EARTHING

Figure-3

1.7

fo|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).


December, 2010


12

Allowable body current IB (Amperes) for two body weights, as per IEEE STD:-80 are given: = 0.116/ √TS for body weights of 50kg.

IB

= 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 December, 2010



1.8

13

feV~Vh dh Áfrjks/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 Áfrjks/kdrk de djus dh fof/k;k ¡¡ fof/k ;k¡ ;k Methods of Reducing Resistivity of Soil

feV~Vh dh Áfrjks/kdrk ds Ádkj Types of soil resistivity Sl. Type of soil No.

Resistivity 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


in

December, 2010


14

1.8.2

feV~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.

December, 2010

Handbook on on Electrical Earthing Earthing


1.8.3

15

feV~Vh mipkj es a cS cSUVksukbV ukbV dk Á;ksxx Use of Bentonite in Soil Treatment

1.8.4



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.

feV~Vh mipkj es a eghu eghu jk[k dk Á;k Á;ks sx x s 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 Áfrjks/kdrk /kdrk ij vk vknz nz rk rk dk ÁHkko Effect of Moisture Content on Earth Resistivity

Moisture content is expressed in percentage by 3 weight of dry soil. Dry earth weights about 1440 kg/m . 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. resistivit y. Moisture content(% by weight) 0 2.5 5 10 20 30 1.8.6

Resistivity(Ohm-cm) Top Soil Sandy Loam 6

1000x10 250000 165000 53000 12000 6400

1000x10 150000 43000 18500 6300 4200

6

rkieku dk ÁHkko Effect of Temperature The temperature coefficient of resistivity for soil is negative, but is negligible for temperatures above 0 freezing point. At about 20 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.

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17

Effect of Temperature on Resistivity 0

C 20 10 0 0 -5 -15 1.8.7

0

F 68 50 32(Water) 32(Ice) 23 14

Resistivity(Ohm-cm) 7,200 9,900 13,000 30,000 79,000 330,000

feV~Vh dh Áfrjks/kdrk dk d k tax ij ÁHkko d 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. b. c.

Material of the conductor Corrosivity of the soil Size of the grounding system

Range of soil resistivity (Ohm-metre) Less than 25 25 – 50 50 -100 Above 100


Class of soil Severely corrosive Moderately corrosive Mildly Very mildly corrosive

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18

1.8.8

1.8.9

lrg ij iRFkj pw jk 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.

iRFkj pw jk dh eghu irZ dk ÁHkko 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 ( ρ) is 2000 ohm-meter while that of soil is 100 ohm-meter. Since ρ 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.

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1.9

19

vFkZ bysDVªksM 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.


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20

2.0

vFkZ kZ bysDVªksM ds fMtk;u] lkbt ,oa Ádkj vF DESIGN, SIZE ELECTRODE

2.1

AND

TYPES

OF

EARTH

bysDVªkM s ,oa vFkZ ds e/; Áfrjks/k ELECTRODE RESISTANCE TO EARTH Conventional practices to measure the earth resistance is by using ohm’s 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

V

R

v 1 x

Resistance area of driven earth rod

Figure-5 December, 2010



2.2

21

bysDVªkM s Áfrjks/k dks ÁHkkfor 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 = (ρ / 2 πL) [LN {8L / (ϕ x 2.7183)}]. Where, L = Length in meter. (Pipe) LN=Nominal length (buried conductor)

ϕ = Diameter in meter Let, consider the case of a length = 6 meter. For ϕ = 2.5 Cm, R = 16.4 Ω . For ϕ = 10 Cm, R = 15.3 Ω. 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.


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22

2.3

bysDVªkM s dk lkbt ELECTRODE SIZE The choices for materials & size are only with respect to the amount of fault current to be discharged to earth. 2

The current density (A/mm ) 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 bysDVªkM s dh fMtkbu DESIGN

OF EARTH

ELECTRODES 2.4.1

bysDVªkM s Áfrjks/kd kd ij vkdkj dk ÁHkko 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 December, 2010



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 earth’s surface where a considered as a condenser in free space. This relationship is given by 100ρ R= -------4πC Where R= ρ= C=

Resistance in an infinite medium Resistance of the medium in ohm-meter Capacitance of the electrode and its image in free space. In the practical case the medium is divided into two by the plane of earth’s surface so that 100ρ R= -------2πC 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.


December, 2010


24

2.5

vFkZ bysDVªksM 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.

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25

I

Over llapping r resistance a areas o of two e earth rods

Fig-6

I

L

2L

Separation d distance Fig-7


December, 2010


26

2.6

bysDVªkM s ds Ádkj TYPES OF ELECTRODES Types of earth electrodes are used as follows:

2.6.1

IysV bysDVªksM

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 December, 2010



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 CI COVER

GROUND LEVEL

CI FRAME

WIRE MESH

50cm FUNNEL CEMENT CONCRETE 70 cm

1.5m (Min.)

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

10mm DIA GI PIPE

CHARCOAL

12.7mm dia GI PIPE

A 60 cm

COPPER OR GI WIRE

90 cm

15 cm

15 cm

Figure- 8 Plate Earth Electrode BOLT, NUT, CHECK NUT AND WASHER TO BE OF COPPER FOR COPPER PLATE AND GI FOR GI PLATE

View of section ‘A’ Handbook on Electrical Earthing

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28

2.6.1.1 IysV

bysDVªksM dh fMtk fMtk;u ;u

Design of plate electrode

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

ρ

π

R= ------ ------- ohms 4 √A Where ρ = Resistivity of soil in ohm-meter 2 A= Area of both sides of plate in m In practice little gain is obtained by increasing 2 the plate area of on side by more than 1.75 m 2.6.2

IkkbZi bysDVªksM

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 0 inclination shall not more than 30 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 December, 2010



G

29

L

300mm

200mm

CLAMP

8 SWG G.I. WIRE

40MM DIA G.I. PIPE ) . n i M ( m m 0 0 5 2

CHARCOAL OR COKE AND SALT IN ALTERNATE LAYER OF 300 12MM DIA HOLES

300

300

Figure – 9 Pipe Electrode Handbook on Electrical Earthing

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30

2.6.2.1 IkkbZi

bysDVªksM dh fMtk;u

Design of pipe electrode

In designing drive rod or pipe electrodes, the resistance may be calculated from the following formula: 100ρ 4L R= -------- loge ------ ohms 2πL d Where

ρ= L= D=

Resistivity of the soil in ohm-meter Length of rod or pipe in cm, and 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 bysDVªksM

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 2 2 be smaller than 3.0 mm if of copper and 6 mm 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 December, 2010



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 RYDER’s formula:R = (ρ / 2 πL) [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).

h

L

Figure – 10 Strip Electrode

2.6.4

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.


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32

2.7

vfFkZax ds çdkj TYPES OF EARTHING The various types of earthing are as follows:

2.7.1

LVª fLV ªiª vfFkZax Strip Earthing LV ff 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 cross2 sectional area shall not be smaller than 3.0 mm if of 2 copper and 6 mm 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 vfFkZax 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

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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 vfFkZx ax Pipe 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 vfFkZax x Plate 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.

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2.8

35

EARTHING LEAD vfFkZax x yhM EARTHING 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

EARTHING SYSTEM vfFkZax x flLVe EARTHING

3.1

vfFkZax j.k x dk oxhZdj.k CLASSIFICATION OF EARTHING The earthing can be classified as (1) (2)

3.1.1

System earthing Equipment earthing

flLVe vfFkZaxx 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;wV ªy vfFkZ ax 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.

Bks l 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

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37

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

Figure - 11 Solid Grounded Neutral B.

Áfrjks/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.

N

Figure – 12


Resistance Grounded

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38

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;DVs a l l 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

December, 2010



39

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 figure13.

N

Figure- 13 Reactance grounding D.

vkdZ lÁs’ku Dokby vfFkZx a 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. Arcsuppression 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 Handbook on Electrical Earthing

December, 2010


40

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 selfextinguishing, which increases continuity in service

N

Figure- 14 Resonant grounding 3.1.2

midj.k a Equipment Earthing midj.k vfFkZx •

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;

December, 2010

To maintain low potential difference between nearby metallic structure in any area to achieve freedom from electrical shock to person or animal etc. Handbook on Electrical Earthing


3.2

41

•

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.

lc LVs’ku es a vfFkZax Á.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 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


December, 2010


42

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 eSV 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

43

lc LVs’ku es a fof fofHkUu HkUu vFkZ eSV dk dusD’ku Earth Mat connection in a Sub-Station

3.3

•

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 associated with equipment.

• •

Handle of the operating pipe.

frame

work

not

Fence if it is within 2 m from earth mat.

lc LVs’ku es a fofHkUu midj.kks a dh vfFkZax EARTHING OF VARIOUS EQUIPMENT IN THE SUB-STATIONS

3.3.1

vkblksysVj ,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 vjs LVj 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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44

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 cz sdj 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

ikoj oj Vªk¡lQkeZj Power Transformers ik 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

djs aV Vªk¡lQkeZj ,oa iksVsfU’k;y Vªk¡lQkeZj 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 December, 2010



45

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.k Other Equipments

All equipment’s, structures, and metallic frames of switches and isolators are to be earthed separately as shown in figure- 15. 3.3.6.1

Figure - 15

?ks jk 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.


December, 2010


46

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

xz kÅUM kÅU M rkj Ground Wire kÅU 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

ds d sf cy ,oa liks ZV Cables and Supports d 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.

December, 2010



3.3.6.4

47

iSuy ,oa D;wf cdy 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

Vªk¡lQkeZj LVªdpj

dh

vfFkZx a

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


December, 2010


48

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 ½ 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.

• • •

December, 2010

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 ) Handbook on Electrical Earthing


• • • •

49

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.


December, 2010


50

H.T.LA's

AB SWITCH

EARTHING HG FUSE

TRANSFORMER

B Y R N

SEATING CHANNEL EARTHING

EARTH PIT NO.2

EARTH PIT NO.1

EARTH PIT NO.3

Figure- 16 Earthing of Distribution Transformer Structure

December, 2010



3.5

51

forj.k ykbu LVª ªd pj dh vfFkZ ax LV ªdpj x 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 LVªdpj

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

• 3.5.2

The metal cross arm and the insulator pin shall be bound together and earthed at every pole.

,y Vh ykbu LV LVª ªd pj ªdpj

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; LVªdpj 



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,


December, 2010


52











December, 2010

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.



3.6

53

miHkksD rk rk ds ifjlj es a vfFkZax EARTHING AT CONSUMER’S PREMISES As per rule 33 (i) of I.E. rules 1956, the supplier shall provide and maintain at the consumer’s premises for the consumer’s use a suitable earthed terminal in an accessible position at the point of commencement of supply.

a)

Åijh lfoZl dusD’ku 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 consumer’s 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 consumer’s 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. Handbook on Electrical Earthing

December, 2010


54

b)

3.6.1

Hkwf exr 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 board’s meter should be connected to the second stud of the earthed terminal.

4.

The consumer’s installation should be connected to the third stud of the terminal.

fy¶V 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.kks a ,oa f fQfVa QfVax aks dh vfFkZax Earthing of Domestic Fittings and Appliances

Earthing of domestic appliances arises in case they have only functional insulation. Appliances having reinforced or double insulation need not be earthed.

December, 2010



1.

Iyx ,oa lkWdsV

55

Plug and Sockets

All plugs and sockets shall be of threepin type, one of the pins being connected to earth. 2.

ykbV fQfVax Light Fittings If the bracket type lamp holders are of metallic construction, it is recommended that they should be earthed. All pedestal lamp fittings of metallic construction shall be earthed.

3.

ia[ks ,oa jsxqysVj Fans And Regulators Bodies of all table fans, pedestal fans, exhaust fans, etc., shall be earthed by the use of three pin plugs. The covers of the regulators, if of metallic construction shall be earthed by means of a separate earth wire.

4.

dqfdax ajst Cooking Ranges Bodies of hot plates, kettles, toasters, heaters, ovens and water boilers shall all be earthed by the use of three pin plugs. However, if fixed wiring has been used, then a separate earth wire shall be used for earthing these appliances.

5.

Luku ?kj Bath Room The body of automatic electric water heaters shall be earthed by the use of a three-pin plug or by a separate earth wire, if fixed wiring has been done. All non electrical metal work including the bathtub, metal pipes, sinks and tanks shall be bonded together and earthed.


December, 2010


56

6.

okf’k ’ka ax e’khu e’khu ,oa fM’k ok’kj okf ’k Washing Machine & Dish Washer

From the point of view of good reception it is recommended that washing machine & dish washer should be earthed through an electrode different from that of the main earth system for other electrical appliances. However, if it is not possible to have separate earth electrode, washing machine & dish washer may be earthed through the main earth system. 3.7

vkS|ksfxd fxd ifjlj ifjlj es a vfFkZax EARTHING IN INDUSTRIAL PREMISES

In factories and workshops all metal conduits, truncking, cable sheaths, switchgear, distribution fuse boards, starters, motors and all other parts made of metal shall be bonded together and connected to an efficient earth system. The electricity regulations made under the factories act require that adequate precautions shall be taken to prevent non-current-carrying metalwork of the installation from becoming electrically charged. In larger installations, having one or more substations, it is recommended to parallel all earth continuity system. 3.7.1

fo|qr vkdZ osfYMax midj.kks a dh vfFkZx a Earthing of Electric Arc Welding Equipment All components shall be effectively bonded and connected to earth, the transformers and separate regulators forming multi-operator sets and capacitors for power factor correction, if used, shall be included in the bonding.

December, 2010



57

All terminals on the output side of a motor generator set shall be insulated from the car case and control panel, as the generator is not connected electrically to a motor and therefore the welding circuit is electrically separate from the supply circuit including the earth. In case of transformers sets, which for welding purpose are double wound, an ‘earth and work’ terminal shall be provided. In single-phase sets this terminal shall be connected to one end of the secondary winding and in case of three-phase sets this shall be connected to neutral point of the secondary winding. 3.7.2

ih,ylhlh midj.k dh vyx ls vfFkZax Separate Earthing For Power Line Communication (PLCC) Equipment

Carrier

Providing separate earthing is not sufficient if earthing is not effective. Earth resistance of such earthing should not be more than 0.5 Ohm. If earthing is not efficient, it will have effect in communication signaling (poor signaling). Separate earthing is provided to avoid flow of fault current through PLCC components and for human safety. Hence ineffective earthing may cause failure of components and also danger to human beings. 3.8

viw. kZ vfFkZax ds [krjs DANGERS OF IMPERFECT EARTHING

•

If the transformer neutral is not earthed properly, in the event of an earth fault in the system a condition will occur resulting in high voltages resulting in irreparable damage to the transformer.


December, 2010


58

3.9

•

If lightning arrester earth is not proper, in the event of lightning discharge, the lightning arrester will become in-effective and this may cause injury elsewhere in the equipment.

•

In case air brake (AB) switch handle is not earthed properly, during the operation of the AB switch, the touch voltage limits may not exceed and cause injury to the operator.

•

In-effective earthing in a distribution system may, not only result in life hazards but also may affect metering.

lko/kkfu;k¡ lko/kkfu;k ¡¡ PRECAUTIONS •

Always see that earthing requirements are observed without any compromise.

•

Never use an earth return to serve as neutral when tapping single phase supply.

•

Consumer neutral must always be maintained pucca and proper linkage with the system neutral ensured.

•

Remember that earth wire will carry current during fault conditions and hence adequate size of the wire should be used for earthing.

•

Remember that removal of the earth or improper maintenance of the earth system will cause single voltages at some points which may cause over fluxing conditions on the feeding power transformer and cause irreparable damages inside the power transformers.

December, 2010



59

4.0

ijh{k.k ,oa vuqj{k.k TESTING & MAINTENANCE

4.1

vfFkZax Á.kkyh dk ijh{k.k TESTING OF EARTHING SYSTEM

4.1.1

vFkZ b bys ysDVªksM ds Áfrjks/k dk dk ekiu ekiu Measurement of Earth Electrode Resistance

4.1.1.1

iksV asf’k;y fxjkoV fof/k

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 figure–17. A measured current is passed between the electrode ‘A’ to be tested and an auxiliary current electrode ‘C’ and the potential difference between the electrode ‘A’ and the auxiliary potential electrode ‘B’ 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, 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 measured value. 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 Handbook on Electrical Earthing

December, 2010


60

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 auxiliary electrode consists of 12.5 mm diameter mild steel rod driven up to one meter into the ground. A

AMMETER

CURRENT SOURCE

V

VOLTMETER X = 1m

B

A

TEST ELECTRODE

C

POTENTIAL ELECTRODE

CURRENT ELECTRODE

Figure- 17 4.1.1.2

gLrpkfyr vFkZ Vs LVj dk fooj.k Details

of Earth

Tester (Hand Driven)

Earth resistance meter are employed for measurements 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 December, 2010



61

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 electrode and 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- 18. 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 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 earth resistance 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.


December, 2010


62

Current Terminal: or C2 Potential Terminal: P1 or P2

C1

C2

P1

P2

50 - 70 feet

Test Ea rth Electrode

50 - 70 feet

Potential Electrode

Current Electrode

Figure- 18 4.2 4.2.1

vuqj{k.k {k.k 'ksM~;w y MAINTENANCE SCHEDULE ekfld Monthly Schedule ekfld 'ksM~;wy Sr

Items

Inspection

1

Earth connections & Earth electrodes

2

Bolts and nuts of the connections

Visual check for connections, overheating, tapped hole, rigidness and any sign of deterioration Check for rust and dirt

December, 2010

Action to be taken If found abnormal or loose, it should be attended immediately

Rust and dirt should be cleaned and apply grease, if required.



4.2.2

=Sekfld w kfld 'ksM~;y

63

Quarterly Schedule

Carry out following work in addition to monthly schedule: Sr

Items

Inspection

1

Earth wire

Check joints

2

Earth conductor

Check for cross section area

3

MS links by bolted joints between earth electrode and MS flat

Check for tightness

4

Projection of earth electrode

5

Power equipment

Check for ground level and proper soil Check for two separate distinct connection to the earth


the

for

Action to be taken Never be twisted together for making joints. All joints should be soldered together solidly. It should not be less then minimum permissible limit. If found broken or loose, it should be immediately replaced/ tightened. It should be 175± 10 mm above the ground level.

If it is not found, it should be made.

December, 2010


64

4.2.3

v)Z okf"kZ okf"kZd 'ksM~;wy

Half Yearly Schedule

Carry out following work in addition to monthly and quarterly schedule: Sr. 1.

Items Measure earth resistance

2.

Measure earth resistance combined

3.

Sump

December, 2010

Inspection Measure individual earth electrode resistance. It should be checked preferably during May & December. Combined earth resistance with earthing flats, connected to the equipment, structures and earth electrode.

Check general condition including dryness.

Action to be taken By earth resistance meter. It should be within permissible limit. Record the value and updated on the pit using paint.

Large Substation – 0.5 Ω Small Substation 2.0Ω Other Installation -8.0Ω

up If the surrounding area is too dry, water should be poured into the sump to keep the soil moist.



4.2.4

okf"kZd 'ksM~;wy

65

Yearly Schedule

Carry out following work in addition to monthly, quarterly and half yearly schedule: Sr. 1.

5.0

Item Earth pits

Inspection Check up the electrode for proper earth continuity.

Action to be taken Remove the hardened top layer of the earth pit for a depth of one 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.

vuqj{k.k jfgr vfFkZ ax MAINTENANCE FREE EARTHING In conventional earthing system GI pipe is used as 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 replacement. It also requires monitoring which 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


December, 2010


66

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 earths are to be constructed as per RDSO’s specification no.RDSO/ PE/ SPEC/ 0109-2008 (REV‘0’). 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 during initial installation.

5.1

vFkZ çfrjks/k

EARTH RESISTANCE

The earth resistance value at earth bus bar should be less than 0.5 ohms for major electrical equipment & installation.

5.2

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

5.3

vuqj{k.k jfgr vfFkZ ax flLVe 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.

December, 2010



5.3.1

vFkZ b byS ySDVªkM s

67

Earth Electrode

The 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 Ádkj dk vFkZ bySDVªkM s

Rod type earth

electrode

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 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 shall be 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 rod shall 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 be less than 17 mm. Handbook on Electrical Earthing

December, 2010


68

d. The copper bonding thickness on stainless steel rod shall be 250 microns or more, hot dipped or electroplated. e. Copper bus bar of size 250 mm x 50 mm x 6 mm having electrical conductivity of 101% IACS minimum 99.9% copper content shall be 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 electrode should be such as to have more than 15kA for one second.

ladsfUnzr ikbi vFkZ bySDVªksM

ii.

Concentric pipe earth electrode  MS pipe with 25 - 50 mm diameter, class B, ISI mark as per IS: 1239, length 2000 mm or 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 Sr.

Current capacity

Table - 1 Primary Secondary conductor conductor (Electrode) dimensions diameter (dia x length)

1 2 3 4 5

3kA 5kA 15kA 40kA 50kA

25 mm 25 mm 25 mm 40 mm 50 mm

40 mm x 2000 mm 40 mm x 3000 mm 50 mm x 3000 mm 60 mm x 3000 mm 100 mm x 3000 mm

NOTE – For more than 50kA applications, multiple electrodes

of 50kA capacity are installed and connected. December, 2010





69

For hermetically filling inside the cavity between secondary conductor and primary conductor, crystalline compound is to be injected in the electrode assembly. It is a combination of high conductivity metal alloys, copper & aluminum 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. 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 less than 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 bonded by 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 any visible oxide layer or foreign material.



Copper bus bar of size 250 mm x 50 mm x 6 mm having 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 of 12 mm dia (2 on each side) for connecting earthing conductor as shown in figure – 19.


December, 2010


70

25 x 3mm Copper Strip (To be duplicated) 300 x 25 x 6mm Copper bus

Ground Le

To Equipment Exothermic Welding

Exothermic Welding Exothermic Welding

Earth Electrode (Dimension as per design)

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

(Earth Electrode Installation)

Figure - 19

5.3.2

vFkZ bugsUles aV lkexzh Earth Enhancement Material

Figure- 20 December, 2010



71

Earth enhancement material is a superior conductive material that improves earthing effectiveness especially in area of poor conductivity as shown in figure-20. 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 of water to maintain its conductivity.

c.

It should be a little alkaline in nature with pH value >7 but <9.

d.

It should have better hygroscopic properties to absorb moisture. It should absorb and release the moisture in dry weather condition and help in maintaining the moisture around the earth electrode.

e.

It has a capacity to retain >10% moisture at 0 105 C.

f.

It should have water solubility <5%.

g.

It should not decompose or leach out with time.

h.

It shall be thermally stable between -10 C to + 0 60 C ambient temperature.

i.

It should be non toxic, non reactive, non explosive and non corrosive.

j.

It shall not pollute the soil or local water table and shall meet environmental friendly requirement for landfill.

k.

It should expand & swell considerably and shall remove entrapped air to create strong connection between earth electrode and soil.

0


December, 2010


72

5.3.3

l.

It should be diffused into soil pores and shall create conductive roots enlarging conductive zone of earth pit.

m.

It shall be permanent & maintenance free and in its “set form”, shall maintain constant earth resistance with time.

n.

It shall not replacement.

o.

It shall not cause burns, irritation to eye, skin etc.

p.

Minimum quantity of earth enhancement material to be required:  For 5ft x 5ft x 10ft earth pit – min 75 kgs per pit. For 300 mm bore type earth pit – min 50  kgs per pit

cSfDQy inkFkZ

require

periodic

charging

or

Backfill material

The excavated soil can be used if it is free from sand, gravel and stones. Small proportion of sand in the 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 the soil muddy which is not suitable for compacting and after drying the soil may contain voids which may permanently increase earth resistance. December, 2010



5.3.4

73

le foHko cl ,oa vFkZ pkyd pkyd Equi- Potential Bus & Earth Conductor 

A copper bus bar of size 300 mm x 25 mm x 6 mm to 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 copper -7 strip earthing conductor shall be 17.241 x 10 0 ohm cm at 20 C and having electrical conductivity of 101% IACS i.e. minimum 99.9% copper content.



At a temperature of 20 C, its density shall be 8.89 3 gm/cm .



A single length of copper strip shall be used for each 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 earth bus bar with the help of exothermic welding or at least two number stainless steel nut bolts of appropriate size.

0


December, 2010


74

5.3.5

bdkbZ vFkZ dh fuekZ.kk 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 clear space 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 and stones. If soil quality is good then add some quantity of earth enhancement material in the soil for using as backfill.



If the soil seems unusable (containing large quantity of gravel, stones, murum, sand etc) then replace the soil with black cotton soil.



Insert the electrode at the center of the earth pit and arrange to keep it vertical in the pit’



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



Fill the pit with the backfill and keep on adding the earth 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 adequate ramming.

December, 2010



5.3.6

75



The pit should be very compactly rammed and watering for 2-3 days and addition of soil if required be done.



Construct inspection chamber with cover for the installation.



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

cgq vFkZ fiVks a }kjk vFkZ fjax dk fuekZ.k Construction of Ring Earth by Providing Multiple Earth Pits 

Wherever it is not possible to achieve required earth resistance with one earth electrode/ pit due to difficult/ 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.



The distance between two successive earth electrodes 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 mm copper 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. This interconnecting strip shall also be covered with earth enhancing compound.




December, 2010


76

5.3.7

fujh{k.k ujh{k.k pSEcj Inspection chamber ff 

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 floor surrounding 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 average resistance value shall also be written with yellow paint on black background with date.

December, 2010



77

6.0

D;k djs a vkSj D;k u djs a

6.1

D;k djs a

DO’S & DON’TS

DO’S:

1.

Ensure that the electrodes are embedded below permanent moisture level.

2.

Inspect earth electrode regularly.

3.

Ensure that every earth wire shall be of copper, galvanized iron or steel.

4.

Ensure good and reliable electrical connection between earthing leads and earth electrodes.

5.

Ensure that path of earth wire should be as far as possible, out of reach of any person.

6.

Consider all parameters while designing earth system.

7.

Ensure the size of earth wire should be proper and according to IS 3043 (Code of practice for earthing and IE rules)

8.

Ensure that all materials, fittings etc. used in earthing system shall confirm to IS specification wherever they exist.

9.

Ensure safety installations.

10.

Ensure that as far as possible all earth terminals should be visible.

11.

Ensure that voltage between neutral to earth should be below 2.0 volts.


while

working

on

electrical

December, 2010


78

6.2

D;k u djs a

DON’T

1.

Don’t connect single pole switch or fuse in a neutral circuit. Always connect it in the live or phase wire circuit.

2.

Don’t renew a blown fuse until you are satisfied as to the cause of its blowing and also as to the removal of the cause.

3.

Don’t use copper or aluminum wire as substitute for fuse wire.

4.

Don’t touch or tamper with any electrical gear or conductor unless you have made sure that it is dead and earthed. High voltage apparatus may give shock or flashover without touching.

5.

Don’t disconnect earthing connections or render ineffective the safety gadgets installed on mains and apparatus till you are at work.

6.

Don’t expose your eyes to an electric arc. Painful injury may result even with short exposure.

7.

Don’t take unnecessary risk with electricity. Low voltage under certain circumstances can be dangerous.

8.

Don’t use paint, enamel and grease on the electrodes.

9.

Don’t use neutral conductor as earth wire.

10.

Don’t use water pipe line for earthing.

11.

Don’t make series connections for earth path.

December, 2010






NOTES

79



December, 2010


80

lUnHkZ 1- Hkkjrh; ekud “vfFkZax ds fy, dksM vkWQ çsfDVl ” vkbZ,l 3043&1937 A 2- ,lh lcLVs’ku xzkmfUMax es a lqj{kk ds fy,

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December, 2010



81

REFERENCES 1. Indian standard code of practice for earthing IS: 30431937 2. IEEE guide for safety in AC sub station grounding IEEE 80 3. Standard for qualifying permanent connections used in substation grounding IEEE 837. 4. Indian Electricity Rules 1956 with latest amendments. 5. Maintenance free earthing RDSO specification no. RDSO/PE/SPEC/PS/0109-2008( REV ‘0’) 6. “Earthing Practice in a Nutshell” report written by Chief Engineer (Personnel) Chennai. 7. Suggestions given by participants from various Railways th during seminar conducted on 8 December 2010 at IRCAMTECH/ Gwalior.


December, 2010


82

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Handbook on Electrical Earthing

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