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Handbook on Electrical Earthing December, 2010
1
Hkkjr ljdkj GOVERNMENT OF INDIA jsy ea=ky;jsy ea=ky;jsy
ea=ky;jsy ea=ky; MINISTRY OF RAILWAYS
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& Xokfy;j & Xokfy;j & 474 005474 005474 005474 005
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CAMTECH/E/10-11/El-EARTHING/1.0
fnlEcj 2010fnlEcj 2010fnlEcj 2010fnlEcj 2010 December 2010
dsoy dk;Zky;hu mi;ksx gsrq (For Official Use Only)
fo|qr vfFkZax ij gLriqfLrdk Handbook on Electrical Earthing
y{; lewg % fo|qr lkekU; ly{; lewg % fo|qr lkekU; ly{; lewg %
fo|qr lkekU; ly{; lewg % fo|qr lkekU; lsok ds vuqj{k.k deZpkjhsok
ds vuqj{k.k deZpkjhsok ds vuqj{k.k deZpkjhsok ds vuqj{k.k deZpkjh
TARGET GROUP: General Services Maintenance Staff
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December, 2010 Handbook on Electrical Earthing
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fo|qr vfFkZax ij gLriqfLrdkfo|qr vfFkZax ij gLriqfLrdkfo|qr
vfFkZax ij gLriqfLrdkfo|qr vfFkZax ij gLriqfLrdk Handbook on
Electrical Earthing
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;k=h vkSj eky ;krk;kr dh c
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CAMTECH/E/10-11/El-Earthing/1.0
Handbook on Electrical Earthing December, 2010
3
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nku djrk gS vkSj xzkmaM QkYV gksus ij lqj{kk ;a=ksa dk Rofjr
ifjpkyu lqfufpr djrk gSA
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ds mn~ns; ls cukbZ xbZ gSA
bl gLriqfLrdk esa vfFkaZx dh lajpuk] lcLVsku dh
vfFkZax O;oLFkk] vuqj{k.k 'kSM~;wy] vuqj{k.k eqDr vfFkZax]
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lsokvksa ds vuqj{k.k deZpkfj;ksa ds fy, mi;ksxh fl) gksxh A
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2fnukad 2fnukad 27777] ] ] ] tuojhtuojhtuojhtuojh] 201] 201] 201]
2011111 dk;Zdkjh funskd dk;Zdkjh funskd dk;Zdkjh funskd dk;Zdkjh
funskd
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FOREWORD
Earthing of electrical installations/ equipments plays very
important role in safe functioning of system as well as safety of
personnel. It provides low impedance path to fault currents and
ensures prompt and consistent operation of protective devices
during ground faults.
CAMTECH has prepared this handbook on electrical earthing system
for general services to disseminate knowledge to working
personnel.
The handbook contains construction of earthing, earthing
arrangement at substation, maintenance schedules, maintenance free
earthing etc.
I hope this handbook will prove to be useful to maintenance
personnel working in general services department.
CAMTECH, Gwalior S.C. Singhal Date: 27.01. 2011 Executive
Director
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HkwfedkHkwfedkHkwfedkHkwfedk
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vPNh ,oa fooluh; lqj{kk .kkyh dks lqfufpr djus ,oa ifjpkyd dks
fo|qr >Vds ls cpkus ds fy, vko;d gSA dSeVsd }kjk fo|qr vfFkZax
ij ;g gLriqfLrdk gekjs vuqj{k.k deZpkjh;ksa dks dk;Z {ks= esa
vfFkZax .kkyh ls voxr djkus ds mn~ns; ls cukbZ xbZ gSA
;g Li"V fd;k tkrk gS fd ;g gLriqfLrdk vfFkZax dk
vkbZ- ,l- dksM IS: 3043] vkbZ bZ fu;ekoyh] vkjMh,lvks ;k jsyos
cksMZ }kjk fofuZfn"V fdlh Hkh fo/kku dks foLFkkfir ugha djrhA ;g
gLriqfLrdk dsoy ekxZnkZu gsrq gS ,oa ;g ,d oS/kkfud nLrkost+ ugha
gSA
eSa] dk;Z{ks= ds mu lHkh deZpkfj;ksa dk vkHkkjh gw ftUgksaus
bl gLriqfLrdk dks cukus esa gekjh lgk;rk dh A rduhdh mUu;urk
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Lora= eglwl djsa A bl fnkk esa ge vkids ;ksxnku dh ljkguk
djsaxsA
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ih;w"k xqIrk ih;w"k xqIrk ih;w"k xqIrk ih;w"k xqIrk fnukad 3fnukad
3fnukad 3fnukad 31111] ] ] ] fnlfnlfnlfnlEcj] 2010 Ecj] 2010 Ecj]
2010 Ecj] 2010 la la la la---- funskd fo|qr funskd fo|qr funskd
fo|qr funskd fo|qr
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December, 2010 Handbook on Electrical Earthing
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PREFACE
The proper upkeep and maintenance of earthing system is
necessary to ensure good and reliable protection system for
electrical equipment and to avoid shock to the operator. This
handbook on Electrical Earthing has been prepared by CAMTECH with
the objective of making our maintenance personnel aware of earthing
systems to be adopted in field.
It is clarified that this handbook does not supersede any
existing provisions of IS code of earthing (IS: 3043), IE Rules and
other existing provisions laid down by RDSO or Railway Board. This
handbook is for guidance only and it is not a statutory
document.
I am sincerely thankful to all field personnel who helped us in
preparing this handbook.
Technological up-gradation & learning is a continuous
process. Please feel free to write to us for any addition/
modification in this handbook. We shall highly appreciate your
contribution in this direction.
CAMTECH, Gwalior Peeyoosh Gupta Date:31.12.2010
Jt.DirectorElectrical e - mailid:[email protected]
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fo"k; lwphfo"k; lwphfo"k; lwphfo"k; lwph
---- la la la la---- fooj.kfooj.kfooj.kfooj.k i`ii``i`"B la"B
la"B la"B la----
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xi
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lEcfU/kr egRoiw.kZ Hkkjrh;
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xzkf.Max ,oa vfFkZax esa vUrj 10 1-7 fo|qr dk >Vdk ,oa ekuo vo;o
11 1-8 feV~Vh dh frjks/kdrk fu/kkZfjr djus
okys dkjd 13 1-9 vFkZ bysDVksM yxkus dk LFkku 19
2.0 vFkZ bysDVksM ds fMtk;u] lkbt ,oa dkjvFkZ bysDVksM ds
fMtk;u] lkbt ,oa dkjvFkZ bysDVksM ds fMtk;u] lkbt ,oa dkjvFkZ
bysDVksM ds fMtk;u] lkbt ,oa dkj 20 20 20 20 2-1 bysDVksM ,oa vFkZ
ds e/; frjks/k 20 2-2 bysDVksM frjks/k dks Hkkfor djus okys 21
dkjd 2-3 bysDVksM dk lkbt 22 2-4 vFkZ bysDVksM dh fMtkbu 22 2-5
vFkZ bysDVksM 24
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CONTENTS
Sr.No. Description Page No.
Foreword iv Preface vi Contents viii Correction Slip xii
1.0 INTRODUCTION 01 1.1 ADVANTAGES OF EARTHING 02 1.2
TERMINOLOGY 03 1.3 EARTH AS CONDUCTOR 05 1.4 IMPORTANT INDIAN
ELECTRICITY
RULE RELATED TO EARTHING 05 1.5 GENERAL REQUIREMENT
FOR EARTHING 07 1.6 DIFFERENCE BETWEEN
GROUNDING AND EARTHING 10 1.7 HUMAN ELEMENT & ELECTRIC
SHOCK 11 1.8 FACTORS WHICH DETERMINE
RESISTIVITY OF SOIL 13 1.9 LOCATION OF EARTH ELECTRODE 19
2.0 DESIGN, SIZE AND TYPES OF EARTH ELECTRODE 20 2.1 ELECTRODE
RESISTANCE TO EARTH 20 2.2 INFLUENCING FACTORS FOR
ELECTRODE RESISTANCE 21 2.3 ELECTRODE SIZE 22 2.4 DESIGN OF
EARTH ELECTRODES 22 2.5 EARTH ELECTRODE 24
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---- la la la la---- fooj.kfooj.kfooj.kfooj.k i`"B lai`"B lai`"B
lai`"B la----
2-6 bysDVksM ds dkj 26 2-7 vfFkZax ds dkj 32 2-8 vfFkZax yhM
35
3333----0000 vfFkZax flLVevfFkZax flLVevfFkZax flLVevfFkZax
flLVe 35353535 3-1 vfFkZax dk oxhZdj.k 35 3-2 lc LVsku esa vfFkZax
.kkyh 41 3-3 lc LVsku esa fofHkUu midj.kksa dh
vfFkZax 43 3-4 forj.k VklQkeZj LVdpj dh vfFkZax 47 3-5 forj.k
ykbu LVdpj dh vfFkZax 51 3-6 miHkksDrk ds ifjlj esa vfFkZax 53 3-7
vkS|ksfxd ifjlj esa vfFkZax 56 3-8 viw.kZ vfFkZax ds [krjs 57 3-9
lko/kkfu;k 58
4444----0000 ijh{k.k ,oa vuqj{k.kijh{k.k ,oa vuqj{k.kijh{k.k ,oa
vuqj{k.kijh{k.k ,oa vuqj{k.k 59595959 4-1 vfFkZax .kkyh dk ijh{k.k
59 4-2 vuqj{k.k 'ksM~;wy 62
5555----0 0 0 0 vuqj{k.k jfgr vfFkZax vuqj{k.k jfgr vfFkZax
vuqj{k.k jfgr vfFkZax vuqj{k.k jfgr vfFkZax 65656565 5-1 vFkZ
frjks/k 66 5-2 mi;ksx 66 5-3 vuqj{k.k jfgr vfFkZax flLVe 66
6666----0000 D;k djsa vkSj D;k u djsa D;k djsa vkSj D;k u djsa
D;k djsa vkSj D;k u djsa D;k djsa vkSj D;k u djsa 77777777 6-1 D;k
djsa 77 6-2 D;k u djsa 78
lanHkZlanHkZlanHkZlanHkZ 80808080
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Sr.No. Description Page No.
2.6 TYPES OF ELECTRODES 26 2.7 TYPES OF EARTHING 32 2.8 EARTHING
LEAD 35
3.0 EARTHING SYSTEM 35 3.1 CLASSIFICATION OF EARTHING 35 3.2
EARTHING SYSTEM IN SUB STATION 41 3.3 EARTHING OF VARIOUS EQUIPMENT
IN
THE SUB-STATIONS 43 3.4 DISTRIBUTION TRANSFORMER
STRUCTURE EATHING 47 3.5 EARTHING OF DISTRIBUTION LIENE
STRUCTURES 51 3.6 EARTHING AT CONSUMERS PREMISES 53 3.7 EARTHING
IN INDUSTRIAL PREMISES 56 3.8 DANGERS OF IMPERFECT EARTHING 57 3.9
PRECAUTIONS 58
4.0 TESTING & MAINTENANCE 59 4.1 TESTING OF EARTHING SYSTEM
59 4.2 MAINTENANCE SCHEDULES 62
5.0 MAINTENANCE FREE EARTHING 65 5.1 EARTH RESISTANCE 66 5.2
APPLICATIONS 66 5.3 MAINTENANCE FREE EARTHING SYSTEM 66
6.0 DOS & DONTS 77 6.1 DOS 77 6.2 DONT 78 REFRENCES 81
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lakks/ku ifpZ;ksa dk izdkkulakks/ku ifpZ;ksa dk izdkkulakks/ku
ifpZ;ksa dk izdkkulakks/ku ifpZ;ksa dk izdkku
bl gLriqfLrdk ds fy;s Hkfo"; esa izdkfkr gksus okyh lakks/ku
ifpZ;ksa dks fuEukuqlkj la[;kafdr fd;k tk;sxkA
dSeVsd@bZ@dSeVsd@bZ@dSeVsd@bZ@dSeVsd@bZ@10&1110&1110&1110&11@@@@bZ,ybZ,ybZ,ybZ,y&vfFkZax&vfFkZax&vfFkZax&vfFkZax@1@1@1@1----0000@@@@
lh,l lh,l lh,l lh,l # XX fnukadfnukadfnukadfnukad-------------
tgkWa XX lEcfU/kr lakks/ku iphZ dh dze la[;k gS 01
ls izkjEHk gksdj vkxs dh vksj
izdkfkr lakks/ku ifpZ;k izdkfkr lakks/ku ifpZ;k izdkfkr lakks/ku
ifpZ;k izdkfkr lakks/ku ifpZ;k
dzdzdzdz----lalalala---- izdkku izdkku izdkku izdkku dh rkjh[kdh
rkjh[kdh rkjh[kdh rkjh[k
lakksf/kr Ik`"B la[;k lakksf/kr Ik`"B la[;k lakksf/kr Ik`"B
la[;k lakksf/kr Ik`"B la[;k rFkk en la[;krFkk en la[;krFkk en
la[;krFkk en la[;k
fVIi.khfVIi.khfVIi.khfVIi.kh
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ISSUE OF CORRECTION SLIP
The correction slips to be issued in future for this handbook
will be numbered as follows:
CAMTECH/E/10-11/El-Earthing/1.0/ C.S. # XX date---
Where XX is the serial number of the concerned correction slip
(starting from 01 onwards).
CORRECTION SLIPS ISSUED
Sr. No. Date of issue
Page no. and Item no. modified
Remarks
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1.0 LrkoukLrkoukLrkoukLrkouk INTRODUCTION
Earthing is a connection done through a metal link between the
body of any electrical appliance, or neutral point, as the case may
be, to the deeper ground through these metal links, normally of MS
flat, CI flat, GI wire penetrated to the earth grid. Object of
earthing is that all parts of apparatus other than live parts shall
be at earth potential.
Earthing eliminates the possibility of any dangerous potential
rise on the body of electrical equipment. It drains away the charge
on the equipment through an earth connection. When an earth fault
is occurres such as winding insulation failure etc. causes a heavy
current flow into the general mass of the earth. This causes
blowing out of fuse or operation/ tripping of protective devices.
The potential under and around of the object shall be uniform
nearly to zero w.r.t. earth.
Apart from this it is to ensure that operators or working
personnel shall be at earth potential at all times, so that there
will be no potential difference to cause shock or injury to a
person, whenever any short circuit takes place.
The primary requirements of a good earthing system are: a. It
stabilizes circuit potential with respect to
ground potential and limits the potential rise. b. It protects
men & materials from injury or
damage due to over voltage or touching.
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c. It provides low impedance path to fault currents to ensure
prompt & consistent operation of protective devices during
earth fault.
d. It keeps the maximum voltage gradient along the surface
inside & around the substation within safe limits during earth
fault.
e. It protects underground cables from overall ground potential
rise & voltage gradient during ground fault in the system.
1.1 vfFkZax ds ykHkvfFkZax ds ykHkvfFkZax ds ykHkvfFkZax ds ykHk
ADVANTAGES OF EARTHING
For efficient/effective operation of any power system, it is
essential to connect the neutral to suitable earth connection. The
following are the few advantages: Reduced operation &
maintenance cost Reduction in magnitude of transient over voltages.
Improved lightning protection. Simplification of ground fault
location. Improved system and equipment fault protection. Improved
service reliability Greater safety for personnel & equipment
Prompt and consistent operation of protective
devices during earth fault.
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1.2 ikfjHkkf"kd 'kCnkoyhikfjHkkf"kd 'kCnkoyhikfjHkkf"kd
'kCnkoyhikfjHkkf"kd 'kCnkoyh TERMINOLOGY The following terms are
commonly used in
earthing systems:
1.2.1 vFkZ vFkZ vFkZ vFkZ Earth The conductive mass of the
earth, whose
electrical potential at any point is conventionally taken as
zero.
1.2.2 vFkZ bySDVksMvFkZ bySDVksMvFkZ bySDVksMvFkZ bySDVksM Earth
electrode A Galvanized Iron (GI) pipe in intimate contact
with and providing an electrical connections to earth.
1.2.3 vfFkZavfFkZavfFkZavfFkZax fxzMx fxzMx fxzMx fxzM Earthing
grid A system of a number of interconnected,
horizontal bare conductors buried in the earth, providing a
common ground for electrical devices and metallic structures,
usually in one specific location.
1.2.4 midj.kmidj.kmidj.kmidj.k vfFkZax vfFkZax vfFkZax vfFkZax
Equipment Earthing It comprises earthing of all metal work of
electrical equipment other than parts which are normally live or
current carrying. This is done to ensure effective operation of the
protective gear in the event of leakage through such metal work,
the potential of which with respect to neighboring objects may
attain a value which would cause danger to life or risk of
fire.
1.2.5 .kkyh .kkyh .kkyh .kkyh vfFkZaxvfFkZaxvfFkZaxvfFkZax
System Earthing Earthing done to limit the potential of live
conductors with respect to earth to values which the insulation
of the system is designed to withstand and to ensure the security
of the system.
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1.2.6 Vp oksYVstVp oksYVstVp oksYVstVp oksYVst Touch Voltage (E
Touch)
The potential difference between a ground metallic structure and
a point on the earths surface separated by a distance equal to the
normal maximum horizontal reach of a person, approximately one
meter as shown in figure-1
1.2.7 LVsi oksYVstLVsi oksYVstLVsi oksYVstLVsi oksYVst Step
Voltage (E Step)
The potential difference between two points on the earth's
surface separated by distance of one pace that will be assumed to
be one meter in the direction of maximum potential gradient as
shown in figure -2
Figure-1
Figure-2
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1.2.8 eSk oksYVsteSk oksYVsteSk oksYVsteSk oksYVst Mesh Voltage
(E mesh)
The maximum touch voltage to be found within a mesh of an
earthing grid.
1.3 vFkZ pkyd vFkZ pkyd vFkZ pkyd vFkZ pkyd EARTH AS CONDUCTOR
Resistivity () of earth is 100-M. Resistivity () of copper is 1.7x
10 -8 -M. Resistivity () of G. I. is 1.7x 10 -7 -M.
Take as reference, 25x 4 mm copper strip. To obtain the same
resistance, the size of G.I. will be 65mm x10mm. The corresponding
figure for earth is 800mtrs x 800mtrs (158 acres.)Hence, it shows
metallic conductor is a preferred alternative conductor to earth to
bring the fault current back to source.
1.4 vvvvffffFkZFkZFkZFkZ axaxaxax ls lEcfU/kr egkRoiw.kZ Hkls
lEcfU/kr egkRoiw.kZ Hkls lEcfU/kr egkRoiw.kZ Hkls lEcfU/kr
egkRoiw.kZ Hkkkkkjrh; fo|qr fu;ejrh; fo|qr fu;ejrh; fo|qr fu;ejrh;
fo|qr fu;e IMPORTANT INDIAN ELECTRICITY RULE RELATED TO
EARTHING
fu;e 33 fu;e 33 fu;e 33 fu;e 33 RULE No: 33 Earth terminals on
consumers premises.
fu;e 61 fu;e 61 fu;e 61 fu;e 61 RULE No: 61. (A) Max:
permissible resistance of earthing system.
Large power station : 0.5 ohms. Major sub-station : 1.0 ohms.
Small sub-station : 2.0 ohms. In all other cases : 8.0 ohms. The
earth continuity
inside an installation : 1.0 ohms.
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December, 2010 Handbook on Electrical Earthing
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 fu;e 67 fu;e 67 fu;e 67 RULE No: 67. Connection to
earth
All equipments associated with HV/EHV installation shall be
earthed by not less than two distinct and separate connections with
the earth having its own electrode, except an earth mat.
Testing of such earth resistance not less than once in every
year during a dry day of a dry season shall be conducted &
recorded
fu;e 90 fu;e 90 fu;e 90 fu;e 90 RULE No: 90. Earthing In
distribution system, all metal supports and all
reinforced/ pre-stressed cement concrete supports of overhead
line and metallic fittings attached shall be permanently and
effectively earthed.
Each stay wire shall be similarly earthed, unless insulators
have been provided in it at a height not less than three mtrs from
the ground.
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Every 5th pole as a minimum shall be grounded, if the
foundations are not cements concrete blocks.
fu;e fu;e fu;e fu;e 91 91 91 91 RULE No: 91. Safety and
protective device Every overhead line erected over any part of
street
or public place shall be protected with a device, approved by
the EIG, for rendering the line electrically harmless in case it
brakes.
The owner of every high and extra high overhead line shall be
protected to the satisfaction of the EIG, to prevent unauthorized
persons from ascending any of the supports of such overhead
lines.
fu;e 92 fu;e 92 fu;e 92 fu;e 92 RULE No: 92. Protection against
lightening The owner of every overhead line which is so
exposed, as may be liable to injury from lightening, shall
adopted efficient means for diverting to earth, any electrical
surge during lightening.
The earthing lead for any lightening arrester shall not pass
through any iron or steel pipe but shall be taken as directed as
possible from the lightening arrester to a separate earthing
electrode/ mat.
1.5 vfFkZax vfFkZax vfFkZax vfFkZax ds fy, lkekU; vko;drk;sa ds
fy, lkekU; vko;drk;sa ds fy, lkekU; vko;drk;sa ds fy, lkekU;
vko;drk;sa GENERAL REQUIREMENT FOR EARTHING Earthing shall
generally be carried out in
accordance with the requirement of I.E. rules, 1956, as amended
from time to time and the relevant regulation of the electricity
supply. Codes /Standard given below may also be referred : i)
IS:3043 - Code of practice for earthing
(latest)
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ii) National Electricity Code - 1985 of BIS iii) IEEE guide for
safety in AC substation
grounding no. ANSI/IEEE standard, 80-1986.
In cases where direct earthing may prove harmful rather than
provide safety, relaxation may be obtained from the competent
authority.
Earth electrodes shall be provided at generating stations,
substations and consumer premises in accordance with the
requirements.
As far as possible all earth connections shall be visible for
inspection.
All connections shall be carefully made. If they are not
properly made or are inadequate for the purpose for which they are
intended, loss of life or serious personnel injury may result.
Each earth system shall be so devised that the testing of
individual earth electrode is possible. It is recommended that the
value of any earth system resistance shall not be more than 5 ohms
unless otherwise specified.
The minimum size of earthing lead used on any installations
shall have a nominal cross-section at areas of not less than 3.0
mm2 if of copper and 6.0 mm2 if of galvanized iron or steel. The
actual size will depend on the maximum fault current which the
earthing lead will be required to carry safely.
It is recommended that a drawing showing the main earth
connection and earth electrode be prepared for each
installation.
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No addition to the existing load whether temporary or permanent
shall be made, which may exceed the assessed earth fault or its
duration until it is ascertained that the existing arrangement of
earthing is capable of carrying the new value of earth fault
current resulting due to such addition.
All materials, fittings etc. used in earthing shall confirm to
Indian Standard specification wherever these exist. In the case of
material for which Indian standard specifications does not exists,
the material shall be approved by the competent authority.
An earthing electrode shall not be situated with in a distance
of 1.5 meter from the building whose installation system is being
earthed.
The earthing electrode shall always be placed in vertical
position inside the earth or pit so that it may not be in contact
with all the different earth layers.
The sensitivity of the protective equipment, system voltage and
the maximum fault current directly relate to permissible value of
earth resistance. In case the earth exceeds the permissible value,
then in the event of earth fault, the fault current may not reach a
sufficient value to operate the protective equipment (such as fuses
or relays) and dangerous condition may arise.
The earth wire and earth electrode will be of same material. The
earth wire shall be taken through G.I. pipe of 13 mm diameter for
at least 30 cm length above and below ground surface to
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the earth electrode to protect it against mechanical damage.
All the earth wires run along the various sub-circuits shall be
terminated and looped firmly at the main board and from main board
the main earth shall be taken to earth electrode. The loop earth
wires used shall not be either less than 2.9 mm2 (14 SWG) or half
of the size of the sub circuit conductor.
1.6 xzkf.Max ,oa vfFkZax esa vUrjxzkf.Max ,oa vfFkZax esa
vUrjxzkf.Max ,oa vfFkZax esa vUrjxzkf.Max ,oa vfFkZax esa vUrj
DIFFERENCE BETWEEN GROUNDING AND EARTHING
1.6.1 xzkfUxzkfUxzkfUxzkfUMax Max Max Max Grounding
Grounding implies connection of current carrying parts to
ground. It is mostly either generator or transformer neutral. Hence
it is generally called neutral grounding. Grounding is for
equipment safety.
There are three requirements for grounding: a. Shall provide a
low impedance path for the
return of fault current, so that an over current protection
device can act quickly to clear the circuit.
b. Shall maintain a low potential difference between exposed
metal parts to avoid personnel hazards.
c. Shall control over voltage.
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1.6.2 vfFkZaxvfFkZaxvfFkZaxvfFkZax Earthing
Earthing implies connection of non current carrying parts to
ground like metallic enclosures. Earthing is for human safety.
Under balanced operating conditions of power systems, earthing
system does not play any role. But under any ground fault
condition, it enables the ground fault current to return back to
the source without endangering human safety as shown in figure
-.3
1.7 fo|qr dk >Vdk ,oa ekuo vo;ofo|qr dk >Vdk ,oa ekuo
vo;ofo|qr dk >Vdk ,oa ekuo vo;ofo|qr dk >Vdk ,oa ekuo vo;o
HUMAN ELEMENT & ELECTRIC SHOCK
Electric shock is possible only when the human body bridges two
points of unequal potential as shown in fgure-4.
Maximum tolerable current for human body is 160 mA for one
second. If this limit exceeds, it will result in death due to
ventricular fibrillation (heart attack).
EARTHING
GENERATOR TRANSFORMER
NEUTRAL GROUNDINGNEUTRAL GROUNDING
Figure-3
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Allowable body current IB (Amperes) for two body weights, as per
IEEE STD:-80 are given:
IB = 0.116/ TS for body weights of 50kg. = 0.157 / TS for body
weight of 70kg. TS = Duration of current exposure (fault clearance
time). TS IB (50kg) IB (70kg). 0.2sec 259 mA 351 mA. 0.5sec 164 mA
222 mA. 1.0sec 116 mA 157 mA.
Figure 4 - Current flow under fault condition
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1.8 feV~Vh dh frjks/kdrk fu/kkZfjr djus okys dkjdfeV~Vh dh
frjks/kdrk fu/kkZfjr djus okys dkjdfeV~Vh dh frjks/kdrk fu/kkZfjr
djus okys dkjdfeV~Vh dh frjks/kdrk fu/kkZfjr djus okys dkjd FACTORS
WHICH DETERMINE RESISTIVITY OF SOIL
The resistivity of soil for earthing system depends upon the
following factors:
Type of soil Moisture content Chemical composition of salt
dissolved in the
contained water Concentration of salt Temperature of material
Grain size and distribution of grain size Size and spacing of earth
electrodes
1.8.1 feV~Vh dh frjks/kdrk de djus dh fof/kfeV~Vh dh frjks/kdrk
de djus dh fof/kfeV~Vh dh frjks/kdrk de djus dh fof/kfeV~Vh dh
frjks/kdrk de djus dh fof/k;k;k;k;k Methods of Reducing Resistivity
of Soil
feV~Vh dh frjks/kdrk ds dkj feV~Vh dh frjks/kdrk ds dkj feV~Vh
dh frjks/kdrk ds dkj feV~Vh dh frjks/kdrk ds dkj Types of soil
resistivity
Sl. No.
Type of soil Resistivity in Ohm-cm
1 Loamy garden soil 500 - 5000 2 Clay 800 - 5000 3 Clay, Sans
and Gravel mix 4000 - 25000 4 Sand and Gravel 6000 - 10000 5
Slates, Slab sand stone 1000 - 50000 6 Crystalline Rock 20000 -
100000
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1.8.2 feV~Vh dk mipkjfeV~Vh dk mipkjfeV~Vh dk mipkjfeV~Vh dk
mipkj Soil Treatment When the soil resistance is high, even the
multiple electrodes in large number may also fail to produce low
resistance to earth. To reduce the resistivity of soil immediately
surrounding the electrode some salt substances are made available
as a solution with water. The substances are used salt sodium
chloride (NaCl), Calcium chloride (CaCl2) Sodium carbonate
(Na2CO3), copper sulphate (CuSO4) and soft cock and charcoal in
suitable proportion.
Nearly 90% of resistance between electrode and soil is with in a
radius of two meters from electrode/ rod. Treating this soil will
result in required reduction in earth resistance by excavation of
one meter diameter around top of the electrode/ rod to 30 cm deep
and applying artificial soil treatment agency and watering
sufficiently.
General practice to treat the soil surrounding the ground
electrode with common salt, charcoal and soft cock in order to
bring down the earth resistance. These conventional methods are
effective in soils of moderately high resistivity up to 300
ohm-meter. When the soil resistivity exceeds this value, these
conventional methods of chemical treatment will be inadequate to
get desired value of earth resistance.
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1.8.3 feV~Vh mipkj esa cSUVksukbV dk ;ksxfeV~Vh mipkj esa
cSUVksukbV dk ;ksxfeV~Vh mipkj esa cSUVksukbV dk ;ksxfeV~Vh mipkj
esa cSUVksukbV dk ;ksx Use of Bentonite in Soil Treatment Bentonite
is clay with excellent electrical
properties. It swells to several times its original volume when
suspended in water. It binds the water of crystallization and the
water absorbed during the mixing process is retained over a long
period. Bentonite suspension in water when used to surround the
earth electrode virtually increases the electrode surface area.
Use of bentonite around the earth electrode results in reduction
of ground resistance by about 25- 30 %.
Bentonite has a tremendous capacity to absorb water and retain
it over along period.
Even during the summer months, bentonite suspension retains the
moisture where as the natural soil dries up.
Bentonite may be used to advantage in rocky terrain.
1.8.4 feV~Vh mipkj esa feV~Vh mipkj esa feV~Vh mipkj esa feV~Vh
mipkj esa eghu jk[k eghu jk[k eghu jk[k eghu jk[k dk ;kdk ;kdk ;kdk
;ksxsxsxsx Use of fly ash in soil treatment
As per CPRI studies reveals that fly ash from thermal stations
has equivalent chemical composition and hence can be used for the
electrical installations in areas of high ground resistivity. Fly
ash can also be used as a chemical treatment material to reduce
soil resistivity.
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1.8.5 vFkZ frjks/kdrk ij vkvFkZ frjks/kdrk ij vkvFkZ frjks/kdrk
ij vkvFkZ frjks/kdrk ij vknznznznzrk dk Hkkork dk Hkkork dk Hkkork
dk Hkko Effect of Moisture Content on Earth Resistivity
Moisture content is expressed in percentage by weight of dry
soil. Dry earth weights about 1440 kg/m3. Therefore about 144 kg
(10%) of water is required per cubic meter of soil to have 10% of
moisture content. About 20% moisture the resistivity is very little
affected below 20% moisture the resistivity increases very abruptly
with decrease in moisture. Moisture content of about 17% to 18% by
weight of dry soil is the optimum requirement. Availability of
moisture assists formation of electrolyte by dissolving salt
content in soils and there by enhance the conductivity of soil.
More water content can not improve soil resistivity.
Resistivity(Ohm-cm) Moisture content(% by weight) Top Soil Sandy
Loam
0 1000x106 1000x106 2.5 250000 150000 5 165000 43000 10 53000
18500 20 12000 6300 30 6400 4200
1.8.6 rkieku dk Hkkorkieku dk Hkkorkieku dk Hkkorkieku dk Hkko
Effect of Temperature
The temperature coefficient of resistivity for soil is negative,
but is negligible for temperatures above freezing point. At about
200 C the water in the soil begins to freeze and introduce a
tremendous increase in the temperature coefficient. The resistivity
changes 9% per degree C. Below 0 degree C resistivity rises
abnormally.
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Effect of Temperature on Resistivity
0C 0F Resistivity(Ohm-cm) 20 68 7,200 10 50 9,900 0 32(Water)
13,000 0 32(Ice) 30,000 -5 23 79,000 -15 14 330,000
1.8.7 feV~Vh dh frjks/kdrk feV~Vh dh frjks/kdrk feV~Vh dh
frjks/kdrk feV~Vh dh frjks/kdrk ddddk tax ij k tax ij k tax ij k
tax ij HkkoHkkoHkkoHkko Effect of Soil Resistivity on Corrosion
Resistivity plays an important role in so far as the corrosion
performance of earthing rods is concerned. It is observed that
soils having resistivity of less than 25 ohm-meter are severely
corrosive in nature while corrosion rate is of less importance in
soils of resistivity over 200 ohm- meter. The methods adopted to
safe guard earthing conductors against corrosion depends upon
a. Material of the conductor b. Corrosivity of the soil c. Size
of the grounding system
Range of soil resistivity (Ohm-metre)
Class of soil
Less than 25 Severely corrosive 25 50 Moderately corrosive 50
-100 Mildly Above 100 Very mildly corrosive
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1.8.8 lrg ij iRFkj pwjk dh irZ ds Qk;ns lrg ij iRFkj pwjk dh irZ
ds Qk;ns lrg ij iRFkj pwjk dh irZ ds Qk;ns lrg ij iRFkj pwjk dh irZ
ds Qk;ns Advantages of Crushed Rock Used as a Surface Layer It
provides high resistivity surface layer It serves as impediment to
the movement of
reptiles and there by help in minimizing the hazards which can
be caused by them
It prevents the formation of pools of oil from oil insulated and
oil cooled electrical equipment
It discourages the growth of weeds It helps retention of
moisture on the underlying
soil and thus helps in maintaining the resistivity of the
subsoil at lower value.
It discourages running of persons in the switchyard and saves
them from the risk of being subjected to possible high step
potentials.
1.8.9 iRFkj pwjk dh eghu irZ dk Hkko iRFkj pwjk dh eghu irZ dk
Hkko iRFkj pwjk dh eghu irZ dk Hkko iRFkj pwjk dh eghu irZ dk Hkko
Effect of Thin Layer of Crushed Rock
In outdoor switchyard, a thin layer of crushed rock is spread on
the surface.
The resistivity of gravel () is 2000 ohm-meter while that of
soil is 100 ohm-meter. Since of gravel is high, only a high voltage
can force the current through the body to cause injuries. The
gravel act like insulator & throws the electric field generated
by GPR back to soil.
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1.9 vFkZ bysDVksM yxkus dk LFkku vFkZ bysDVksM yxkus dk LFkku
vFkZ bysDVksM yxkus dk LFkku vFkZ bysDVksM yxkus dk LFkku LOCATION
OF EARTH ELECTRODE
The location of earth electrode should be chosen in one of the
following types of soil in the order of preference given on next
page
Wet marshy ground. Clay, loamy soil and arable land Clay and
loam mixed with varying proportions of
sand, gravel and stones. Damp and wet sand, peat.
Dry sand, gravel chalk limestone, granite, very stone ground and
all locations where virgin rock is very close to the surface should
be avoided.
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2.0 vFvFvFvFkZ bysDVksM ds fMtk;u] lkbt ,oa dkjkZ bysDVksM ds
fMtk;u] lkbt ,oa dkjkZ bysDVksM ds fMtk;u] lkbt ,oa dkjkZ bysDVksM
ds fMtk;u] lkbt ,oa dkj DESIGN, SIZE AND TYPES OF EARTH
ELECTRODE
2.1 bysDVksM ,oa vFkZ ds e/;bysDVksM ,oa vFkZ ds e/;bysDVksM ,oa
vFkZ ds e/;bysDVksM ,oa vFkZ ds e/; frjks/k frjks/k frjks/k frjks/k
ELECTRODE RESISTANCE TO EARTH
Conventional practices to measure the earth resistance is by
using ohms law.
For electrode resistance to earth, current is injected to earth
by electrode and electric field travels through the earth. The
voltage appears at certain distance from electrode and the
resulting impedance is electrode resistance to earth.
This is similar to CT, where the flow of primary current results
in voltage appearing across CT secondary. This drives the current
through the connected relay (burden) as shown in figure- 5
IF
CT RV
x
v1
Resistance area of driven earth rod
Figure-5
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2.2 bysDVksM frjks/k dks Hkkfor djus okys dkjdbysDVksM frjks/k
dks Hkkfor djus okys dkjdbysDVksM frjks/k dks Hkkfor djus okys
dkjdbysDVksM frjks/k dks Hkkfor djus okys dkjd INFLUENCING FACTORS
FOR ELECTRODE RESISTANCE:
The major factor is the length, diameter or width (Cross
section) has very minor influence.
The resistance of pipe electrode is given by
R = ( / 2 piL) [LN {8L / ( x 2.7183)}]. Where, L = Length in
meter. (Pipe)
LN=Nominal length (buried conductor)
= Diameter in meter
Let, consider the case of a length = 6 meter.
For = 2.5 Cm, R = 16.4 .
For = 10 Cm, R = 15.3 .
So, it is observed that 300% increase in diameter, resistance
decreases by app 7% only.
The electrode resistance is not much dependent on type of
electrode materials like Cu, Al or GI. Resistance is the function
of physical dimension, mainly length.
A horizontal earth strip of 75mm x 10mm Cu and 45mm x 10mm GI
both of same length will offer almost same electrode
resistance.
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2.3 bysDVksM dk lkbtbysDVksM dk lkbtbysDVksM dk lkbtbysDVksM dk
lkbt ELECTRODE SIZE
The choices for materials & size are only with respect to
the amount of fault current to be discharged to earth.
The current density (A/mm2) as per IS-3043. Materials Cu Al GI
0.5 sec rating 290 178 113 1 sec rating 205 126 80
Earthing grid for EHV switchyards is designed for 0.5 sec duty
& for others 1sec duty is selected.
2.4 vFkZ bysDVksM dh vFkZ bysDVksM dh vFkZ bysDVksM dh vFkZ
bysDVksM dh fMtkbufMtkbufMtkbufMtkbu DESIGN OF EARTH ELECTRODES
2.4.1 bysDVksM frjks/kdbysDVksM frjks/kdbysDVksM
frjks/kdbysDVksM frjks/kd ij vkdkj dk Hkko ij vkdkj dk Hkko ij
vkdkj dk Hkko ij vkdkj dk Hkko Effect of Shape on Electrode
Resistance
With electrodes, the greater part of the fall in potential
occurs in the soil within about 2m of the electrode surface, since
it is here that the current density is highest. To obtain a low
overall resistance the current density should be as low as possible
in the medium adjacent to the electrode and should decrease rapidly
with distance from the electrode. This requirement is met by making
the dimensions in one direction large compared with those in the
other two. Thus we find that a pipe, rod or strip will have much
lower resistance than a plate of equal surface area. The resistance
is not, however, inversely proportion to the surface area of the
electrode.
The theoretical principles relating to calculation of resistance
of earth electrodes are dealt with in the
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resistance of any electrode in the earth is in fact related to
the capacitance of that electrode and its image in free space since
it can bare shown that the lines of current flow are identical with
the electrostatic lines of force which would result if the earth
were a dielectric and the electrode with its image in the earths
surface where a considered as a condenser in free space.
This relationship is given by
100 R= -------- 4piC
Where R= Resistance in an infinite medium = Resistance of the
medium in ohm-meter C = Capacitance of the electrode and its
image in free space. In the practical case the medium is divided
into
two by the plane of earths surface so that 100 R= --------
2piC
Thus, if the capacitance in free space of any form of electrode
is known together with the resistivity of the surrounding soil, the
resistance of the electrode can be calculated. This capacitance is
known for some simple forms of electrodes.
Applying this principle, resistance of pipe and rod electrodes,
strip electrodes and plate electrodes can be calculated.
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2.5 vFkZ bysDVksMvFkZ bysDVksMvFkZ bysDVksMvFkZ bysDVksM EARTH
ELECTRODE
It is a metal pipe, rod or other conductor which makes an
effective connection with the general mass of the earth.
When a fault is passing, the potential of the electrode is much
above the general mass of the earth. The potential exists over an
area in the vicinity of the electrode. The potential gradient i.e.
the voltage drop between two points on the earth surface is high
close around the electrode. It decreases as moved away from the
electrode. Each electrode has a resistance area within which the
voltage gradient exists.
The resistance areas of two earth electrode should not overlap
each other; otherwise the effectiveness of the electrode is reduced
as shown in figure-6. The recommended distance between the two
electrodes is twice of its length minimum, if the rod length is L,
separation distance shall be 2L as shown in figure-7 on next
page.
To obtain low effective earth grid resistance, electrodes are
connected in parallel. The total resistance will be half of
individual resistance.
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Fig-6
Fig-7
I
OOvveerr llaappppiinngg rreessiissttaannccee aarreeaass ooff
ttwwoo eeaarrtthh
rrooddss
L
SSeeppaarraattiioonn ddiissttaannccee
2L
I
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2.6 bysDVksM ds dkjbysDVksM ds dkjbysDVksM ds dkjbysDVksM ds dkj
TYPES OF ELECTRODES
Types of earth electrodes are used as follows:
2.6.1 IysV bysDVksM IysV bysDVksM IysV bysDVksM IysV bysDVksM
Plate Electrode
Plate electrode may be made of copper, galvanized iron or steel.
If electrode made of copper the minimum size is 60 cm x 60 cm x
3.15 mm. If of galvanized iron or steel, the minimum size should be
60 cm x 60 cm x 6.3 mm.
Plate electrode shall be buried such that its top edge is at a
depth not less than 1.5 m from the surface of the ground. Where the
resistance of one plate electrode is higher than the required
value, two or more plates shall be used in parallel. In such a case
two plates shall be separated from each other by not less than 8.0
m. Plate shall preferably be set vertically. Use of plate electrode
is recommended only where the current carrying capacity is the
prime consideration i.e. generating stations and substations.
If necessary, plate electrodes shall have a galvanized iron
water pipe buried vertically and adjacent to the electrode. One end
of the pipe shall be at least 5 cm above the surface of the ground
and need not be more than 10 cm .The internal diameter of the pipe
shall be at least 5 cm and need not be such that it should be able
to reach the center of the plate. In no case, however, shall it be
more than the depth of the bottom edge of the plate.
Plates to be buried vertically in pits and surrounded by finely
divided coke, crushed coal or
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charcoal at least 150 mm all round the plates. Plates should not
be less than 12.2 m apart and should be buried at sufficient depth
to ensure that they are always surrounded by moist earth as shown
in figure-8
Figure- 8 Plate Earth Electrode
12.7mm dia GI PIPE
COPPER OR GI WIRE
BOLT, NUT, CHECK NUT AND WASHER TO BE OF COPPER FOR COPPER PLATE
AND GI FOR GI PLATE
GROUND LEVEL CI COVER
WIRE MESH
CEMENT CONCRETE
10mm DIA GI PIPE
CHARCOAL
1.5m (Min.)
15 cm
60 cm 90 cm
15 cm
60cm X 60cm X 6.30mm GI PLATE OR 60cm X 60 cm X 3.15mm COPPER
PLATE
50cm
70 cm
CI FRAME
FUNNEL
View of section A
A
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2.6.1.1 IysV bysDVksM dh fMtkIysV bysDVksM dh fMtkIysV bysDVksM
dh fMtkIysV bysDVksM dh fMtk;u ;u ;u ;u Design of plate
electrode
In designing plate electrodes, the resistance may be calculated
from, the following formula
pi R= ------ ------- ohms
4 A Where
= Resistivity of soil in ohm-meter A = Area of both sides of
plate in m2
In practice little gain is obtained by increasing the plate area
of on side by more than 1.75 m2
2.6.2 IkkbZi bysDVksM IkkbZi bysDVksM IkkbZi bysDVksM IkkbZi
bysDVksM Pipe Electrode
It should be made of B class G.I pipe. The internal diameter
should not be smaller than 38 mm and it should be 100 mm for cast
Iron pipe. The length of the pipe electrode should not less than
2.5 m. It should be embedded vertically. Where hard rock is
encountered it can be inclined to vertical. The inclination shall
not more than 300 from the vertical.
To reduce the depth of burial of an electrode without increasing
the resistance, a number of pipes shall be connected together in
parallel. The resistance in this case is practically proportional
to the reciprocal of the number of electrodes used so long as each
is situated outside the resistance area of the other. The distance
between two electrodes in such a case shall preferably be not less
than twice the length of the electrode as shown in figure 9
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Figure 9 Pipe Electrode
2500
m
m (M
in.)
CHARCOAL OR COKE AND SALT IN ALTERNATE LAYER OF 300
300mm
CLAMP
40MM DIA G.I. PIPE
200mm
300 300
8 SWG
G L
G.I. WIRE
12MM DIA HOLES
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2.6.2.1 IkkbZi bysDVksM dh fMtk;u IkkbZi bysDVksM dh fMtk;u
IkkbZi bysDVksM dh fMtk;u IkkbZi bysDVksM dh fMtk;u Design of pipe
electrode
In designing drive rod or pipe electrodes, the resistance may be
calculated from the following formula:
100 4L R= -------- loge ------ ohms 2piL d
Where
= Resistivity of the soil in ohm-meter L = Length of rod or pipe
in cm, and D = Diameter of rod or pipe in cm.
Consideration of the above formula will show that theoretical
resistance to earth of a driven rod electrode depends to a large
degree upon its buried length and to a lesser extent upon its
diameter.
2.6.3 iV~Vh bysDVksM iV~Vh bysDVksM iV~Vh bysDVksM iV~Vh
bysDVksM Strip Electrode
Where strip electrode is used for earthing, it should not be
less than 25 mm x 1.60 mm, if made of copper and 25 mm x 4 mm if
made of G.I. or steel. The length of the buried conductor should
not be less than 15 m. laid in a trench not less than 0.5 m depth.
If round conductors are used, their cross-sectional area shall not
be smaller than 3.0 mm2 if of copper and 6 mm2 if of galvanized
iron or steel.
The electrodes shall be widely distributed as possible,
preferably in a single straight or circular trench or in a number
of trenches radiating from a point. If the conditions necessitate
use of more than one
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31
strip, they shall be laid either in parallel trenches or in
radial trenches as shown figure-10
Resistance for strip or horizontal wire electrode is measured by
RYDERs formula:-
R = ( / 2 piL) [LN (8L/T) +LN (L/h) - 2 + (2h/L)-(h2/L2)].
Where,
L = Length in meter.(electrode) LN= Nominal length (buried
conductor) h = Depth in meter. T = Width in meter (for strip).
2.6.4 dscy 'khFk dscy 'khFk dscy 'khFk dscy 'khFk Cable
Sheaths
Where an extensive underground cable system is available, lead
sheathed and steel armored cables may be used as earth electrodes
provided the bond across the joints is at least of the same
conductivity as of the sheath. The resistance of such an earth
electrode system is generally less than one ohm.
L
h
Figure 10 Strip Electrode
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2.7 vfFkZax vfFkZax vfFkZax vfFkZax dsdsdsds dkj dkj dkj dkj
TYPES OF EARTHING
The various types of earthing are as follows:
2.7.1 ffffLVLVLVLVi vfFkZaxi vfFkZaxi vfFkZaxi vfFkZax Strip
Earthing
In this system of earthing strip electrodes of cross section not
less than 25 mm x 1.6 mm if of copper and 25 mm x 4 mm if
galvanized iron or steel are buried in horizontal trenches of
minimum of depth 0.5 meter. If round conductors are used, their
cross-sectional area shall not be smaller than 3.0 mm2 if of copper
and 6 mm2 if of galvanized iron or steel. The length of buried
conductor shall be sufficient to give the required earth
resistance. It shall not be less than 15 meters. The electrodes
shall be as widely distributed as possible, preferably in a single
or circular trench or in a number of trenches radiating from a
point, if conditions require use of more than one strip, they shall
be laid either in parallel trenches or in radial trenches.
This type of earthing is used at places which have rocky earth
bed because at such placed excavations work for plate earthing is
difficult.
2.7.2 jkWM jkWM jkWM jkWM vfFkZaxvfFkZaxvfFkZaxvfFkZax Rod
Earthing
In this system of earthing 12.5 mm diameter solid rod of copper
or 16 mm diameter solid rod of galvanized iron or steel; or hollow
section 25 mm G I pipes of length not less than 2.5 meters are
driven vertically into the earth either manually or by pneumatic
hammer. In order to increase the embedded length of electrodes
under the ground, which is sometimes necessary to reduce the earth
resistance to
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desired value, more than one rod sections are hammered on above
the other.
This system of earthing is suitable for areas which are sandy in
character. This system of earthing is very cheap as no excavation
work is involved.
2.7.3 ikbi ikbi ikbi ikbi vfFkZaxvfFkZaxvfFkZaxvfFkZax Pipe
Earthing
Pipe earthing is the best form of earthing and is very cheap in
cost. In this method of earthing, a galvanized and perforated pipe
of approved length and diameter is placed up right in a permanently
wet soil.
The size of the pipe depends upon the current to be carried and
the type of the soil. Usually the pipe used for this purpose is of
diameter 38 mm and 2.5 meters in length for ordinary soil or of
greater length in case of dry and rocky soil. The depth at which
the pipe must be buried depends upon the moisture of the ground.
The pipe is placed at a depth of 3.75 meters (minimum). The pipe is
provided with a tapered casing at the lower end in order to
facilitate the driving. The pipe at the bottom is surrounded by
broken pieces of coke to increase the effective area of the earth
and to the earth and to decrease the earth resistance respectively.
Another pipe of 19 mm diameter and minimum length 1.25 meter is
connected at the top to G I pipe through reducing socket.
In our country in summer the moisture in the soil decrease which
cause increase in earth resistance. So a cement concrete work, is
done in order to keep the water arrangement accessible, and in
summer to have an effective earth, 3 or 4 buckets of water are
put
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through the funnel connected to 19 mm diameter pipe, which is
further connected to G I pipe.
The earth wire (either G I wire or G I Strip of sufficient cross
section to carry faulty current safely) is carried in a G I pipe of
diameter 13 mm at a depth of about 60 mm from the ground).
2.7.4 IysV IysV IysV IysV vfFkZaxvfFkZaxvfFkZaxvfFkZax Plate
Earthing In plate earthing an earthing plate either of
copper of dimensions 60 cm x 60 cm x 3 mm or of galvanized iron
of dimensions 60 cm x 60 cm x 6 mm is buried into the ground with
its face vertical at a depth of not less than 3 meters from ground
level. The earth plate is embedded in alternate layers of coke and
salt for a minimum thickness of 15 cm. The earth wire (G I wire for
G I plate earthing and copper wire for copper plate earthing) is
securely bolted to an earth plate with the help of a bolt, nut and
washer made of material of that of earth plate (made of copper in
case of copper plate earthing and of galvanized iron in case of G I
plate earthing).
A small masonry brick wall enclosure with a cast iron cover on
top or an R C C pipe round the earth plate is provided to
facilitate its identification and for carrying out periodical
inspection and tests.
For smaller installations G I pipe earthing is used and for
larger stations and transmission lines, where the fault current,
likely to be high, plate earthing is used.
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2.8 vfFkZaxvfFkZaxvfFkZaxvfFkZax yhM yhM yhM yhM EARTHING
LEAD
It is the conductor by which the final connection to the earth
is made. Its size should be of sufficient cross sectional area so
that it will not fuse under worst fault condition.
The earthing lead should be terminated on a soldered lug and
secured perfectly to the body at the point of connection to the
earth plate. There should be a clean metal to metal surface contact
which will remain intact permanently without deterioration or
corrosion.
3.0 vfFkZaxvfFkZaxvfFkZaxvfFkZax flLVe flLVe flLVe flLVe
EARTHING SYSTEM
3.1 vfFkZaxvfFkZaxvfFkZaxvfFkZax dk oxhZdj.k dk oxhZdj.k dk
oxhZdj.k dk oxhZdj.k CLASSIFICATION OF EARTHING
The earthing can be classified as (1) System earthing
(2) Equipment earthing
3.1.1 flLVe vfFkZaxflLVe vfFkZaxflLVe vfFkZaxflLVe vfFkZax
System Earthing
System earthing is designed to maintain protection of the system
by ensuring the potential on each conductor to be restricted to a
value consistent with the level of insulation applied.
It is very important that earthing should be ensured, in such a
manner to operate the protective device fast and efficiently in
case of any earth fault.
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The system resistance should be such that, when any fault occurs
against which earthing is designed, should protect or operate the
gear to achieve the faulty main or plant harmless.
In such cases, the faulty main or plant is generally isolated
with the help of circuit breakers or fuses.
In case of overhead equipments it becomes very difficult to
arrange the value of earth resistance of the system to achieve
protection when the conductor falls due to breakage and makes a
good contact with the ground.
3.1.1.1 U;wVy U;wVy U;wVy U;wVy vfFkZaxvfFkZaxvfFkZaxvfFkZax
flLVe dh fof/k flLVe dh fof/k flLVe dh fof/k flLVe dh fof/k Methods
of Earthing System Neutral
A. Solid Earthing B. Resistance Earthing C. Reactance Earthing
D. Arc-suppression Coil or Peterson Coil
Earthing
A. Bksl vfFkZax Bksl vfFkZax Bksl vfFkZax Bksl vfFkZax Solid
Earthing
When the fault current is expected to be low and not likely to
cause damage to plant, cables and loss of stability of system, the
earthing may be done directly through metallic conductor from
system neutral to the main earthing ring without any impedance in
the circuit. It should be ensured that the impedance between the N
and E is so low so that if an earth fault occurs in one phase of
the system
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sufficient current will flow to operate to protective devices as
shown in figure- 11
B. frjks/k vfFkZaxfrjks/k vfFkZaxfrjks/k vfFkZaxfrjks/k vfFkZax
Resistance Earthing
Resistance earthing is generally used when the fault current is
likely to be so high as to cause damage to transformers. If a
resistance is inserted between the neutral and earth, quick action
protective devices are also used as shown in figure-12. The
resistors shall comprise of metallic resistance units supported in
insulation in a metal frame or shall be a liquid resistor of a weak
aqueous solution either of zinc chloride or sodium carbonate.
Figure - 11 Solid Grounded Neutral
Figure 12 Resistance Grounded
N
N
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Metallic resistors have a constant resistance which does not
change with time liquid resistors have to be treated frequently
specially after the clearance of a fault. Metallic resistors are
slightly inductive and this is a disadvantage with overhead lines
traveling waves and impulses are subject to positive reflection and
this is likely to unduly stress the insulation of the equipment and
cause breakdown. Use of liquid resistors is recommended only at
voltages above 6.6 kV. All neutral earthing resistances should be
designed to carry their rated current for a short period, usually
30 seconds.
The earth resistance shall be of such a value if a fault is
outside the equipment, the fault current will be restricted to the
rated full load current if the equipment. If the earth resistance
is too low, for any occurrence of the earth fault, the equipment
will be subjected to shock due to load resulting from the power
loss in the resistor.
C. fj;DVsalfj;DVsalfj;DVsalfj;DVsal vfFkZax vfFkZax vfFkZax
vfFkZax Reactance Earthing
When the zero sequence reactance of generators or transformers
is as low as to cause excessive fault current, usually reactance
earthing is used. A single phase reactor is inserted between the
neutral and the earth to limit fault current to the maximum of
three phase short circuit current. Here the current due to earth
fault on one phase is limited to minimize damage the equipment.
Care should
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be taken to see that dangerously high transient voltage during
system fault or switching operations do not occur due to high value
of reactance of earthing reactor as shown in figure-13.
D. vkdZ lsku Dokby vkdZ lsku Dokby vkdZ lsku Dokby vkdZ lsku
Dokby vfFkZaxvfFkZaxvfFkZaxvfFkZax Arc-Suppression Coil
Earthing
In high voltage systems with isolated neutrals over voltages
caused by switching surges or by lighting may cause a line to each.
Considerable current will be drawn through the arc to charge the
system capacitance to earth. The arc is quenched at zero voltage
but may restrict at a higher voltage. This successive restricting
if the arc often causes very high voltages to be built upon the
transmission lines, and is known as arcing grounds. To avoid
isolation of system under earth fault conditions, arc-suppression
coils are sometimes used. Arc-suppression coil, also known as
Peterson coil, is a tuned earthing reactor as shown in figure- 14.
It is to the system capacitance in such a way as to make the
reactance of the zero sequence
Figure- 13 Reactance grounding
N
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networks practically infinite, so that no fault current flows to
the earth and there is no tendency for arcing grounds to occur.
With the use of Peterson coil, arc current is reduced to such a
small value that it is usually self-extinguishing, which increases
continuity in service
3.1.2 midj.kmidj.kmidj.kmidj.k vfFkZax vfFkZax vfFkZax vfFkZax
Equipment Earthing It pertains to those electrical conductors,
by
which all metallic structures through which the energized
conductor passes will be inter connected.
The purpose of equipment earthing is; To maintain low potential
difference between
nearby metallic structure in any area to achieve freedom from
electrical shock to person or animal etc.
Figure- 14 Resonant grounding
N
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To provide an effective and easy path over which short circuit
current involving ground can flow without heating or sparking or
fire to combustible atmosphere.
All housings of electrical conductors, equipment enclosure,
motor frame shall be interconnected by equipment earthing and two
separate and distinct connection to be made to main earthing.
3.2 lc LVsku esa vfFkZax .kkyhlc LVsku esa vfFkZax .kkyhlc LVsku
esa vfFkZax .kkyhlc LVsku esa vfFkZax .kkyh EARTHING SYSTEM IN SUB
STATION
The earthing system comprises of earthing (or) grid, earthing
electrodes, earthing conductors and earth connections.
3.2.1 vFkZ eSV ;k fxzMvFkZ eSV ;k fxzMvFkZ eSV ;k fxzMvFkZ eSV
;k fxzM Earth Mat or Grid
The primary requirement of earthing is to have a very low earth
resistance. If the individual electrodes driven in the soil are
measured it will have a fairly high resistance. But if these
individual electrodes area inter linked inside the soil, it
increases the area in constant with soil and creates a number or
paralleled paths and hence the value of earth resistance in the
interlink state, which is called combined earth resistance, will be
much lower than the individual resistance.
However interlinking of earth pit electrodes is necessary. The
sub-station involves many earthing through individual electrodes.
In order to have uniform interconnection, a mat or grid or earthing
conductor is formed inside the soil. Thus a mat is spread
underneath the sub-station. Hence if a ground electrode is driven
in the soil, the interlinking can be done by a small link
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between that electrode and earth mat running nearby. The
spreading of such a mat in the soil also ensures the object of
earthing that and surface under and around the sub-station is kept
at as nearly absolute earth potential as possible.
3.2.2 vFkZ eSvFkZ eSvFkZ eSvFkZ eSV dk fuekZ.kV dk fuekZ.kV dk
fuekZ.kV dk fuekZ.k Construction of Earth Mat
The sub-station site including the fence is segregated at
intervals, of say four meters width along with length and breadth
wise. Trenches of one meter, to 1.5 meter depth and one meter width
is dug along these lines. The earthing conductors of sufficient
sizes (as per fault current) are placed at the bottom of these
trenches. All the crossing and joints are braced. The trenches are
then filled up with soil of uniform fine mass of earth mixed with
required chemicals depending upon the soil resistivity.
If location of equipment is fixed, the intervals are also
arranged that the earth mat passes nearby the equipment location to
facilitate for easy interlinking.
It is preferable to extend the mat beyond the fence for about
one meter that fence can also be suitably earthed and made safe for
touching.
Normally the earth mat is buried horizontally at a depth of
about half a meter below the surface of the ground and ground rods
at suitable points.
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3.2.3 lc LVsku esa foflc LVsku esa foflc LVsku esa foflc LVsku
esa fofHkUu vFkZ eSV dk dusDkuHkUu vFkZ eSV dk dusDkuHkUu vFkZ eSV
dk dusDkuHkUu vFkZ eSV dk dusDku Earth Mat connection in a
Sub-Station The neutral point of such system through its own
independent earth. Equipment frame work and other
non-current
carrying parts of the electrical equipments in the sub
station.
All extraneous metallic frame work not associated with
equipment.
Handle of the operating pipe. Fence if it is within 2 m from
earth mat.
3.3 lc LVskulc LVskulc LVskulc LVsku esa fofHkUu midj.kksa dh
vfFkZaxesa fofHkUu midj.kksa dh vfFkZaxesa fofHkUu midj.kksa dh
vfFkZaxesa fofHkUu midj.kksa dh vfFkZax EARTHING OF VARIOUS
EQUIPMENT IN THE SUB-STATIONS
3.3.1 vkblksysVj ,oa fLopstvkblksysVj ,oa fLopstvkblksysVj ,oa
fLopstvkblksysVj ,oa fLopst Isolators and switches A flexible earth
conductor is provided between
the handle and earthing conductor attached to the mounting
bracket and the handle of switches is connected to earthing mat by
means of two separate distinct connections made with MS flat. One
connection is made with the nearest longitudinal conductor, while
the other is made to the nearest transverse conductor of the
mat.
3.3.2 ykbVfuax vjsLVjykbVfuax vjsLVjykbVfuax vjsLVjykbVfuax
vjsLVj Lightning Arrestors
Conductors as short and straight as practicable to ensure
minimum impedance shall directly connect the bases of the lightning
arrestors to the earth grid. In addition, there shall be as direct
a connection as practicable from the earth side of lightning
arrestors to the frame of the equipment being protected.
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Individual ground electrodes should be provided for each
lighting arrestor for the reason that large grounding system in
itself may be relatively of little use for lightning protection.
These ground electrodes should be connected to the main earth
system. In the case of lighting arrestors mounted near
transformers, earthing conductor shall be located clear off the
tank and coolers in order to avoid possible oil leakage caused by
arcing.
3.3.3 lfdZV czsdjlfdZV czsdjlfdZV czsdjlfdZV czsdj Circuit
Breakers
For every breaker there will be five earth connections to the
earth mat with MS flat (i) breaker body (ii) relay panel (iii) CTs
of the breaker (iv) Two side of the breaker structure.
3.3.4 ikikikikoj oj oj oj VklQkeZjVklQkeZjVklQkeZjVklQkeZj Power
Transformers
The tank of each transformer shall be directly connected to the
main grid. In addition there shall be as direct a connection as
practicable from the tank to the earth side of projecting lightning
arrestors.
The transformer track rails shall be earthed either separately
or by bonding at each end of the track and at intervals not
exceeding 60.96 meter (200 feet). The earthing of neutral bushing
shall be by two separate strips to the earth grid and shall
likewise be run clear to rank cell and coolers.
3.3.5 djsaV VklQkeZj djsaV VklQkeZj djsaV VklQkeZj djsaV
VklQkeZj ,oa iksVsfUk;y ,oa iksVsfUk;y ,oa iksVsfUk;y ,oa
iksVsfUk;y VklQkeZjVklQkeZjVklQkeZjVklQkeZj Current Transformers
and Potential Transformers
The supporting structures of Current Transformer and Potential
Transformer unit of bases, all bolted cover plates to which the
bushings are attached connected to the earthing mat by means of
two
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separate distinct connections made with MS flat. One connection
is made with the nearest longitudinal conductor, while the other is
made to the nearest transverse conductor of the mat.
3.3.6 vU; midj.kvU; midj.kvU; midj.kvU; midj.k Other
Equipments
All equipments, structures, and metallic frames of switches and
isolators are to be earthed separately as shown in figure- 15.
3.3.6.1 ?ksjk?ksjk?ksjk?ksjk Fences
The Sub-station fence should be generally too far outside the
substation equipment and grounded separately from the station
ground. The station and the fence ground should not be linked. To
avoid any risk to the person walking near the fence inside the
station, no metal parts connecting connected to the station ground,
should be near to the fence five feet and it is desirable to cover
the strip about ten feet wide inside the fence by a layer of
crushed stone which keeps its high resistively even under wet
condition. If the distance between the fence and station
structures, can not be increased at least five feet and if the
fence is too near the substation equipment structure etc., the
station fence should be connected to the fence ground, otherwise a
person touching the fence and the station ground simultaneously
would be subjected to a very high potential under fault
conditions.
Figure - 15
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In a fence very near to the station area, high shock voltage can
be avoided by ensuring good contact between the fence stations and
by grounding the fence at intervals. The station fence should not
be connected to the station ground but should be grounded
separately. If however, the fence is close to the metal parts of
substation, it should be connected to the station ground.
3.3.6.2 xzxzxzxzkUkUkUkUM rkjM rkjM rkjM rkj Ground Wire
All ground wires over a station shall be connected to the
station earth grid. In order that the station earth potentials
during fault conditions are not applied to transmission line ground
wires and towers, all ground wires coming to the station shall be
broken at and insulated on the station side of the first tower or
pole external to the station by means of 10 disc insulator.
3.3.6.3 ddddsfcy ,oa liksZVsfcy ,oa liksZVsfcy ,oa liksZVsfcy
,oa liksZV Cables and Supports
Metal sheathed cables within the station earth grid area shall
be connected to that grid. Multi-core cables shall be connected to
the grid at least at one point. Single core cables normally shall
be connected to the grid at one point only. Where cables which are
connected to the station earth grid pass under a metallic station
perimeter fence, they shall be laid at a depth of not less than 762
mm (2-6) below the fence, or shall be enclosed in an insulating
pipe for a distance of not less than 1524 mm (5) on each side of
the fence.
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3.3.6.4 iSuy ,oa D;wfcdyiSuy ,oa D;wfcdyiSuy ,oa D;wfcdyiSuy ,oa
D;wfcdy Panels and Cubicles
Each panel or cubicle should be provided near the base with a
frame earth bar of copper to which shall be connected the metal
bases and covers of switches and contactor unit. The frame earth
bar shall in turn be connected to the earth grid by an earthing
conductor.
3.4 forj.k forj.k forj.k forj.k VklQkeZjVklQkeZjVklQkeZjVklQkeZj
LVdpj dh vfFkZax LVdpj dh vfFkZax LVdpj dh vfFkZax LVdpj dh vfFkZax
DISTRIBUTION TRANSFORMER STRUCTURE EATHING 1. For earthing three
earth pits in triangular
formation at a distance of six meter from each other are to be
provided.
2. Earth pit should be digged for 45 cm x 45 cm size and 5 ft.
depth.
3. 3 Nos. of 40 mm dia and 2.9 mm thickness and 3 mts. (10 ft)
length of earth pipe should be used for earthing. This earth pipe
is erected in 5 ft. depth earth pit and for the balance length of
earth pipe is driven by hammering into the ground.
4. When a pipe is driven into the earth, the earth surrounding
the pipe can be considered to be consisting of concentric cylinders
of earth which will be bigger in size and area, as they are away
from the pipe. The current can travel into the earth with large
area having little resistance.
5. 3 m. length of electrode will have contact with the earth
area of 3 m in radius. Hence to have better effect 3 m pipe should
be fixed at a
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distance of 6 m (i.e.) twice the distance of pipe length.
6. For better earth connection, one G I clamp should be welded
to the earth pipe and the other clamp bolted with 2 nos. 11/2 x G I
bolt nuts and 4 nos. G. I. washers to the earth pipe.
7. Two separate distinct connections through G I wire should be
made from the transformer neutral bushing to the earth pit No.
2.
8. Two separate distinct connections through GI wire should be
made from the transformer HT lightning Arrestor to the earth pit
No. 1. As far as possible this earth wire should not have contact
with other earth wire connections. If needed PVC sleeves can be
used for insulation.
9. Two separate distinct connections through GI wire from the
following parts of the structure should be made to the earth pit
No. 3 as shown in figure- 16. Metal part of the disc and stay. Top
channel. AB switch frame, metal part of the
insulator, side Arms. HG fuses frame and metal part of the
insulator. LT cross arm, metal part of the insulator,
open type fuse frame. AB switch guide and operating pipe (
At
the top and bottom )
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Transformer body. Belting angle. Seating channel LT lightning
arrestor.
The above earth connections should be made as far as possible
without joints. Wherever joints are necessary, GI sleeves should be
used by proper crimping.
10. The earth pits No. 2 and 3 can be interlinked to serve as
parallel path and lower the earth resistance.
11. If the earth resistance of the earth pit No. 1 is high, then
another earth pit No. 4 can be formed as a counter poise earth and
linked with the HT lightning arrestor pit.
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Figure- 16 Earthing of Distribution Transformer Structure
B Y R N
SEATING CHANNEL
HG FUSE
AB SWITCH
H.T.LA's
EARTH PIT NO.2
EARTH PIT NO.3
EARTH PIT NO.1
EARTHING
EARTHING
TRANSFORMER
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3.5 forj.k ykbu LVforj.k ykbu LVforj.k ykbu LVforj.k ykbu LVdpj
dh vfFkZadpj dh vfFkZadpj dh vfFkZadpj dh vfFkZaxxxx EARTHING OF
DISTRIBUTION LINE STRUCTURES
The following procedure is adopted for the earthing of HT and LT
line supports.
3.5.1 ,p Vh ykbu LVdpj ,p Vh ykbu LVdpj ,p Vh ykbu LVdpj ,p Vh
ykbu LVdpj HT Line Structures
Lines carried on metal poles: Every fifth pole and all supports
provided with
mass or block concrete foundation shall be earthed.
Lines carried on R.C.C. & P.S.C. poles The metal cross arm
and the insulator pin shall be
bound together and earthed at every pole.
3.5.2 ,y Vh ykbu LV,y Vh ykbu LV,y Vh ykbu LV,y Vh ykbu LVdpj
dpj dpj dpj LT Line Structures (with Multiple Earthed Neutral)
Lines carried on metal poles: Every fifth pole and all supports
provided with
mass or block concrete foundation shall be earthed.
Lines carried on R.C.C. & P.S.C. poles The metal cross arm
and the insulator pins shall
be bound together and earthed at every fifth pole. All special
structures carrying switches, transformers, fuses etc., should be
earthed.
3.5.3 vU; LVdpj vU; LVdpj vU; LVdpj vU; LVdpj Other Structures
The supports on either side of a road, railway or
river crossing span should be earthed. All supports (metal, wood
or R.C.C. ) of both H T
and LT lines running through inhabited locations,
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52
road crossings and along such other places where earthing of all
poles is considered desirable from safety considerations should be
earthed.
In special locations, railway and telegraph crossings, special
structures etc., pipe earth should be adopted (i.e.) and earthing
should be done by means of a 25 mm GI pipe driven 2.5 to 3 meter
into the ground.
At other locations coil earthing may be adopted which consists
of either 10 meter length of 6 or 8 SWG GI wire compressed into a
coil of one meter length and diameter 75 to 100 mm and buried 1.5
meter deep or as per REC standard or pole earthing with 8 SWG GI
wire of 75 feet length wound as a coil to have 115 turns of 75/50
mm dia as to have good contact with soil is to be provided.
Whenever the distribution line structures pass close to well or
a permanently moist place, an earth should be provided in the well
or the marshy place and connected to distribution line support.
All tapping poles, terminal poles, stay poles, streetlight poles
and service connection tapping poles should be earthed.
Only if the above requirements are met out we can say that LT is
with multiple earth neutral system. The ohmic resistance of the
earth should be as low as possible and should not exceed 10
ohms.
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3.6 miHkksDmiHkksDmiHkksDmiHkksDrk ds ifjlj esa vfFkZaxrk ds
ifjlj esa vfFkZaxrk ds ifjlj esa vfFkZaxrk ds ifjlj esa vfFkZax
EARTHING AT CONSUMERS PREMISES
As per rule 33 (i) of I.E. rules 1956, the supplier shall
provide and maintain at the consumers premises for the consumers
use a suitable earthed terminal in an accessible position at the
point of commencement of supply.
a) ijh lfoZl dusDku ykbu ijh lfoZl dusDku ykbu ijh lfoZl dusDku
ykbu ijh lfoZl dusDku ykbu Overhead Service Connection Lines: 1.
The earthed terminal may be a 32 mm x 3
mm or near about, consisting of copper plate with three number
(16 mm) studs.
2. One of the studs on the earthed terminal should be connected
to the neutral wire of the twin core supply lead.
3. The bearer wire should be connected to the second stud of the
earthed terminal.
4. The consumers installation should be connected to the third
stud of the earthed terminal.
5. The bearer wire should not be used as the earth lead. The
bearer wire should be earthed at both the pole ends and the
consumers premises and by connecting it to the overhead neutral
wire and to the earthed terminal respectively.
6. The size of the bearer wire should be stranded 7/20 G.I. or
near about size.
7. The bearer wire and the W.P. cable should be bunched together
by porcelain reel insulators or alkathene clips intervals of 6.1
meter.
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b) Hkwfexr dscy Hkwfexr dscy Hkwfexr dscy Hkwfexr dscy
Underground Cables 1. The terminal block for earth connection
may be of size 31 mm x 3 mm or near about, consisting of copper
plate with three number 12.5 mm copper or brass studs with lock
nuts or spring washers.
2. The neutral core of the cable, the lead sheath, the steel
armour and the cable box should be connected to one of the studs on
the earthed terminal.
3. The metal part of the boards meter should be connected to the
second stud of the earthed terminal.
4. The consumers installation should be connected to the third
stud of the terminal.
3.6.1 fyV dh vfFkZax fyV dh vfFkZax fyV dh vfFkZax fyV dh
vfFkZax Earthing in Lifts Frames of motors, winding machine,
control
panel, cases and covers of tappet switch and similar electrical
apparatus, which normally carry the main current, shall be all
earthed.
The exposed metal parts of electrical apparatus installed on a
lift car shall be sufficiently bonded and earthed.
3.6.2 ?kjsyw midj.kksa