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7/23/2019 Handbook on Earthing system for power supply installations.pdf
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CAMTECH/E/10-11/Earthing-PSI/1.0
Handbook on Earthing System for Power Supply Installation November 2010
1
Hkkjr ljdkj GOVERNMENT OF INDIAjsy ea=ky;s as ajsy ea=ky; MINISTRY OF RAILWAYS
egkjktiqjegkjktiqjegkjktiqjegkjktiqj,Xokfy;j &Xokfy;j &Xokfy;j &Xokfy;j & 474 005474 005474 005474 005
Maharajpur, GWALIOR - 474 005
dSeVsd@bZ@10&11@vfFkZax&ih,lvkbZ@1-0CAMTECH/E/10-11/EARTHING-PSI/1.0
uoEcj 2010November 2010
dsoy dk;Zky;hu mi;ksx gsrq(For Official Use Only)
IkkWoj lIykbZ laLFkkiukvksadh vfFkZax .kkyhIkkWoj lIykbZ laLFkkiukvksadh vfFkZax .kkyhIkkWoj lIykbZ laLFkkiukvksadh vfFkZax .kkyhIkkWoj lIykbZ laLFkkiukvksadh vfFkZax .kkyhij gLriqfLrdkij gLriqfLrdkij gLriqfLrdkij gLriqfLrdkHandbook on
Earthing System For Power Supply Installations
Yk{; lewg% VhvkjMh vuqj{k.k deZpkjhYk{; lewg% VhvkjMh vuqj{k.k deZpkjhYk{; lewg% VhvkjMh vuqj{k.k deZpkjhYk{; lewg% VhvkjMh vuqj{k.k deZpkjhTARGET GROUP: TRD Maintenance Staff
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ij gLriqfLrdkij gLriqfLrdkij gLriqfLrdkij gLriqfLrdk
Handbook on Earthing System
For
Power Supply Installations
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FOREWORD
Earthing for substation and switching station (power
supply installation) of TRD plays very important role in
smooth functioning of equipment, safety of personnel and
equipment. It provides low impedance path to fault currents
and ensures prompt and consistent operation of protective
devices during ground faults.
CAMTECH has prepared this handbook on earthing
system for power supply installation containing construction
of earthing, earthing arrangement at traction substation,
maintenance schedules, maintenance free earthing,
exothermic welding and dos & dont.
I hope this handbook will prove to be useful for themaintenance personnel working in TRD department.
CAMTECH, Gwalior S.C. Singhal
Date:24.12.2010 Executive Director
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dSeVsd] Xokfy;jdSeVsd] Xokfy;jdSeVsd] Xokfy;jdSeVsd] Xokfy;j ih;w"k xqIrkih;w"k xqIrkih;w"k xqIrkih;w"k xqIrkfnukad 30] uoEcj] 2010fnukad 30] uoEcj] 2010fnukad 30] uoEcj] 2010fnukad 30] uoEcj] 2010 la la la la---- funskd fo|qrfunskd fo|qrfunskd fo|qrfunskd fo|qr
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PREFACE
Provision of adequate earthing in substations and
switching stations is essential for the safety of operating
personnel as well as of equipment. It is also necessary for
functioning of the system operation. By means of earthing,
electrical equipment are connected to the general mass of the
earth, which has a very low resistance.
This handbook on earthing system for power supply
installation has been prepared by CAMTECH with the
objective of making our maintenance personnel aware of
maintenance free earthing and exothermic welding.
It is clarified that this handbook does not supersede any
existing provisions laid down by RDSO, Railway Board or AC
traction manual. This handbook is for guidance only and it isnot a statutory document.
I am sincerely thankful to all field personnel who helped
us in preparing this handbook.
Technological up-gradation & learning is a continuous
process. Please feel free to write to us for any addition/
modification in this handbook. We shall highly appreciate your
contribution in this direction.
CAMTECH, Gwalior ( Peeyoosh Gupta)
Date: 30 thNovember 2010 Jt. Director Electrical
e - mail id : direlcamtech@gmail.com
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CONTENTS
Sr.No. Description Page No.
Foreword ivPreface viContents viiiCorrection Slip xii
1.0 INTRODUCTION 01
1.1 PURPOSE OF SUBSTATION EARTHING 01
1.2 EARTHING SYSTEM 02
1.3 TERMINOLOGY 02
1.4 DISTINCTION BETWEEN GROUNDING AND
EARTHING 06
1.5 MAXIMUM PERMISSIBLE EARTH RESISTANCE 07
1.6 GENERAL REQUIREMENT FOR EARTHING 07
1.7 FACTORS WHICH DETERMINE RESISTIVITYOF SOIL 09
2.0 EARTHING ARRANGEMENTS AT SUBSTATION 15
2.1 EARTH ELECTRODES 15
2.2 PARAMETERS AFFECTING THE DESIGN OFEARTHING MAT 17
2.3 DESIGN PROCEDURE 172.4 EARTHINGMAT 18
2.5 BURRIED RAIL 20
2.6 SYSTEM EARTHING 21
2.7 EQUIPMENT EARTHING 22
2.8 SIZE OF EARTHING CONDUCTOR 29
2.9 EARTHING ARRANGEMENT AT SWITCHING 29STATION
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3-0 ijEijkxr vFkZ bysDVksM dk vuqj{k.k 32
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4-0 vuqj{k.k eqDr vfFkZax 38
4-1 vFkZ frjks/k 384-2 mi;ksx 394-3 vuqj{k.k eqDr vfFkZax .kkyh 39
5-0 ckWfUMax] xzkmfUMax@ vfFkZax dsfy, 49,DlksFkfeZd oSfYMax la;kstu5-1 lkekU; vko;drk;sa 505-2 osfYaMx lkexzh 515-3 osfYaMx VwYl dh fofk"Vrk;sa 525-4 osfYaMx vkSt+kj 54
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Sr. No. Description Page No.
3.0 MAINTENANCE SCHEDULE OF
CONVENTIONAL EARTH ELECTRODE 323.1 QUARETLY SCHEDULE 323.2 HALF YEARLY SCHEDULE 33
3.3 YEARLY SCHEDULE 34
3.4 MEASUREMENT OF EARTH ELECTRODERESISTANCE 34
3.5 DETAILS OF EARTH TESTER(HAND DRIVEN) 36
4.0 MAINTENANCE FREE EARTHING 384.1 EARTH RESISTANCE 38
4.2 APPLICATION 39
4.3 MAINTENANCE FREE EARTHING SYSTEM 39
5.0 EXOTHERMIC WELDING CONNECTION FORBONDING, GROUNDING/ EARTHING 49
5.1 GENERAL REQUIREMENTS 50
5.2 WELDING MATERIAL 51
5.3 SPECIFICATION OF WELDING TOOLS 52
5.4 WELDING TOOLS 54
6.0 DOS & DONTS 57
6.1 DO S 57
6.2 DONT 58
ANNEXURE-1
Photographs of Exothermic Welding Procedure 59ANNEXURE-2
Photographs of Process of Hole Drilling in Rail 62
ANNEXURE-3 63
Specification of Rail Drilling Machine
REFERENCES 68
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lakks/ku ifpZ;ksadk izdkkulakks/ku ifpZ;ksadk izdkkulakks/ku ifpZ;ksadk izdkkulakks/ku ifpZ;ksadk izdkku
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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/Earthing-PSI/1.0/ C.S. # XX date---
Where XX is the serial number of the concerned
correction slip (starting from 01 onwards).
CORRECTION SLIPS ISSUED
Sr. No. Date of
issue
Page no. and Item
no. modified
Remarks
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1.0 LrkoukLrkoukLrkoukLrkouk@@@@INTRODUCTION
Provision of adequate earthing in substations and
switching stations is essential for the safety of operating
personnel as well as of equipment. It is also necessary
for functioning of the system. By means of earthing
electrical equipment are connected to the general mass
of the earth, which has a very low resistance.
1.1 lcLVsku dh vfFklcLVsku dh vfFklcLVsku dh vfFklcLVsku dh vfFkZax ds mn~ns;Zax ds mn~ns;Zax ds mn~ns;Zax ds mn~ns;@@@@PURPOSE OFSUBSTATION EARTHING
The object of an earthing system in a substation
is to provide, under and around the substation a surface
that shall be at a uniform potential and as near to zero
or absolute earth potential as possible. The provision of
such a surface of uniform potential under and around
the substation ensures human safety from electric shock
in case of short circuit or development of any abnormalcondition in the equipment installed.
The primary requirements of a good earthing
system in a substation are:
a. It stabilizes circuit potential with respect to groundand limits the overall potential rise.
b. It protects life and property from over voltage.c. It provides low impedance path to fault currents
to ensure prompt and consistent operation of
protective devices during ground faults.
d. It keeps the maximum voltage gradient along the
surface inside and around the substation within
safe limits during ground fault.
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1.2 vfFkZax .kkyhvfFkZax .kkyhvfFkZax .kkyhvfFkZax .kkyh@@@@EARTHING SYSTEM
All the non current carrying parts of the
electrical equipment in the substation are connectedto the earthing mat. Under normal conditions the
ground rods contribute a little towards lowering the
ground resistance. It maintains a low value of
resistance under all weather conditions.
The mat is connected to the following
equipment in sub station:a. The neutral point of system through its own
independent earth.
b. Equipment frame work and other non current
carrying parts.
c. All extraneous metallic frameworks not
associated with equipment.
d. Lightning arrestors through their independent
earths.
e. Handles of operating pipes.
f. Fence if it is within 2m from earth mat.
1.3 ifjHkkf"kd 'kCnkoyh@ifjHkkf"kd 'kCnkoyh@ifjHkkf"kd 'kCnkoyh@ifjHkkf"kd 'kCnkoyh@TERMINOLOGY
The following terms are commonly used in
earthing system:
1.3.1 Hkw@Hkw@Hkw@Hkw@Earth
The conductive mass of the earth, whose
electrical potential at any point is conventionally taken
as zero.
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1.3.2 vFkZ bYvFkZ bYvFkZ bYvFkZ bYkSDkSDkSDkSDVksMVksMVksMVksM@@@@Earth electrode
A galvanized iron (GI) pipe or group of pipes in
intimate contact with and providing an electricalconnection to earth.
1.3.3 vFkvFkvFkvFkZ fxzM@Z fxzM@Z fxzM@Z fxzM@Earth grid
A system of grounding electrodes consisting of
inter-connected connectors buried in the earth to
provide a common ground for electrical devices and
metallic structures.
1.3.4 vFkvFkvFkvFkZZZZeSV@eSV@eSV@eSV@Earth Mat
A grounding system formed by a grid of
horizontally buried conductors and which serves to
dissipate the earth fault current to earth and also as an
equi-potential bonding conductor system.
1.3.5 vfFkZax midj.kvfFkZax midj.kvfFkZax midj.kvfFkZax midj.k@@@@Equipment EarthingIt comprises earthing of all metal work of electrical
equipment other than parts which are normally live or
current carrying. This is done to ensure effective
operation of the protective gear in the event of leakage
through such metal work, the potential of which with
respect to neighboring objects may attain a value which
would cause danger to life or risk of fire.1.3.6 flLVeflLVeflLVeflLVe vfFkZaxvfFkZaxvfFkZaxvfFkZax@@@@ System Earthing
Earthing done to limit the potential of live
conductors with respect to earth to values which the
insulation of the system is designed to withstand and
this to ensure the security of the system.
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1.3.7 LVsi ,oaVp oksYVrkLVsi ,oaVp oksYVrkLVsi ,oaVp oksYVrkLVsi ,oaVp oksYVrk@@@@Step & Touch Potential
Step & touch potential refer to the potential
experienced by a person standing on a surface whenearth mat buried up to 750 mm below, surface has risen
to ground potential rise (GPR).
Touch potential is the difference between GPR &
the surface potential at the point where person is
standing, while his hand is in contact with grounded
structure.
Step potential is the difference in surface potentialexperienced by a person bridging a distance of one
meter with his feet without contacting any other
grounded object as shown in figure - 1
If resistance offered by each foot is R, then for
step potential the resistance is 2R while for touch
potential is R/2.
Figure 1
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1.4 xzxzxzxzkkkkfUMax rFkk vfFkZaxfUMax rFkk vfFkZaxfUMax rFkk vfFkZaxfUMax rFkk vfFkZax esaesaesaesa vUrj@vUrj@vUrj@vUrj@DISTINCTIONBETWEEN GROUNDING AND EARTHING
1.4.1 xzkfUMaxxzkfUMaxxzkfUMaxxzkfUMax@@@@GroundingGrounding implies connection of current
carrying parts to ground. It is mostly either generator or
transformer neutral. Hence it is generally called neutral
grounding. Grounding is for equipment safety.
There are three requirements for grounding:
a. Shall provide a low impedance path for the return
of fault current, so that an over current protection
device can act quickly to clear the circuit.
b. Shall maintain a low potential difference between
exposed metal parts to avoid personnel hazards.
c. Shall control over voltage.
1.4.2 vfFkZaxvfFkZaxvfFkZaxvfFkZax@@@@Earthing
Earthing implies connection of non currentcarrying parts to ground like metallic enclosures.
Earthing is for human safety.
Under balanced operating conditions of power
systems, earthing system does not play any role. But
under any ground fault condition, it enables the ground
fault current to return back to the source without
endangering human safety as shown in figure - 2
Figure 2
EARTHING
GENERATOR TRANSFORMER
NEUTRAL GROUNDINGNEUTRAL GROUNDING
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1.5 mPpremPpremPpremPpre vuqKs; vFkZ frjks/k@vuqKs; vFkZ frjks/k@vuqKs; vFkZ frjks/k@vuqKs; vFkZ frjks/k@MAXIMUMPERMISSIBLE EARTH RESISTANCE
At each power supply installation the combinedresistance of the earthing system shall not be more than
the following values:-
i) Traction Sub Station - 0.5 Ohms
ii) Switching Station - 2.0 Ohms
iii) BT and AT station - 10 Ohms
1.6 vfFkZaxvfFkZaxvfFkZaxvfFkZaxdh lkekU;dh lkekU;dh lkekU;dh lkekU; vko;drk;savko;drk;savko;drk;savko;drk;saGENERAL REQUIREMENTS FOR EARTHING Earthing shall generally be carried out in
accordance with the requirement of I.E. rules,
1956, as amended from time to time and the
relevant regulation of the electricity supply.
Codes /Standard given below may also be
referred :
i) IS:3043-1987 - Code of practice for
earthing (latest)
ii) National Electricity Code - 1985 of Bureau
of Indian Standards
iii) IEEE guide for safety in a.c. substation
grounding No. ANSI/IEEE standard 80-
1986.
iv) Indian Electricity Rule 1956 (Latestedition)
In cases where direct earthing may prove harmfulrather than providing safety, relaxation may be
obtained from the competent authority.
Earth electrodes shall be provided at substations,switching stations and consumer premises in
accordance with the requirements.
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8
As far as possible all earth connections shall bevisible for inspection.
All connections shall be carefully made. If theyare not properly made or are inadequate for the
purpose for which they are intended, loss of life
or serious personnel injury may result.
Each earth system shall be so devised that thetesting of individual earth electrode is possible. It
is recommended that the value of any earth
system resistance shall not be more than specified
value.
The minimum size of earthing lead (discharge rodcable) used on installations shall have a nominal
cross-section area not less than 40 mm2 multi
stranded copper to be able to withstand short
circuit current.
It is recommended drawing showing the main
earth connection and earth electrode be preparedfor each installation.
No addition to the existing load whethertemporary or permanent shall be made, which
may exceed the assessed earth fault or its duration
until it is ascertained that the existing
arrangement of earthing is capable of carrying the
new value of earth fault current resulting due tosuch addition.
All materials, fittings etc. used in earthing shallconfirm to Indian Standard specification
wherever these exist. In the case of material for
which Indian Standard specifications does not
exists, the material shall be approved by the
competent authority.
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1.7 feVfeVfeVfeV~~~~VhVhVhVh dh frjks/kdrk fu/kkZfjr djus odh frjks/kdrk fu/kkZfjr djus odh frjks/kdrk fu/kkZfjr djus odh frjks/kdrk fu/kkZfjr djus okys dkjdkys dkjdkys dkjdkys dkjd@@@@FACTORS WHICH DETERMINE RESISTIVITY OFSOIL
The resistivity of soil for earthing system
depends upon the following factors:
Type of soil
Moisture content
Chemical composition of salt dissolved in thecontained water
Concentration of salt
Temperature of material
Grain size and distribution of grain size
Size and spacing of earth electrodes
1.7.1 feVfeVfeVfeV~~~~VhVhVhVh dh frjks/kdrkdh frjks/kdrkdh frjks/kdrkdh frjks/kdrk de djus dhde djus dhde djus dhde djus dh
fof/k;k@fof/k;k@fof/k;k@fof/k;k@Methods of Reducing Resistivity of SoilfeVfeVfeVfeV~~~~Vh dh frjks/kdrk ds dkj@Vh dh frjks/kdrk ds dkj@Vh dh frjks/kdrk ds dkj@Vh dh frjks/kdrk ds dkj@Types of soilresistivity
S.No Type of soil Resistivity in
Ohm-cm
1 Loamy garden soil 500 - 5000
2 Clay 800 - 5000
3 Clay, Sans and Gravel mix 4000 - 25000
4 Sand and Gravel 6000 - 10000
5 Slates, Slab sand stone 1000 - 50000
6 Crystalline Rock 20000 - 100000
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1.7.2 feV~Vh mipkj@feV~Vh mipkj@feV~Vh mipkj@feV~Vh mipkj@Soil Treatment
a. When the soil resistance is high, even the
multiple electrodes in large number may also failto produce low resistance to earth. To reduce the
resistivity of soil immediately surrounding the
electrode some salt substances are made available
as a solution with water. The substances are used
salt sodium chloride (NaCl), Calcium chloride
(CaCl2) Sodium carbonate (Na2CO3), copper
sulphate (CuSO4) and soft cock and charcoal in
suitable proportion.
b. Nearly 90% of resistance between electrode andsoil is with in a radius of two meters from
electrode/ rod. Treating this soil will result in
required reduction in earth resistance by
excavation of one meter diameter around top of
the electrode/ rod to 30 cm deep and applying
artificial soil treatment agency and watering
sufficiently.
c. General practice to treat the soil surrounding theground electrode with common salt, charcoal and
soft cock in order to bring down the earth
resistance. These conventional methods are
effective in soils of moderately high resistivity upto 300 ohm-meter. When the soil resistivity
exceeds this value, these conventional methods of
chemical treatment will be inadequate to get
desired value of earth resistance.
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1.7.3 feV~Vh mipkjfeV~Vh mipkjfeV~Vh mipkjfeV~Vh mipkj esaesaesaesa ccccsUVksukbZV dk mi;ksxsUVksukbZV dk mi;ksxsUVksukbZV dk mi;ksxsUVksukbZV dk mi;ksx@@@@Use ofBentonite in Soil Treatment
Bentonite is clay with excellent electricalproperties. It swells to several times its originalvolume when suspended in water. It binds the water
of crystallization and the water absorbed during the
mixing process is retained over a long period.
Bentonite suspension in water when used to
surround the earth electrode virtually increases the
electrode surface area.
Use of bentonite around the earth electrode resultsin reduction of ground resistance by about 25- 30
%.
Bentonite has a tremendous capacity to absorbwater and retain it over along period.
Even during the summer months, bentonite
suspension retains the moisture where as the naturalsoil dries up.
Bentonite may be used to advantage in rockyterrain.
1.7.4 feV~Vh mipkjfeV~Vh mipkjfeV~Vh mipkjfeV~Vh mipkjesaeghu jk[k dk mi;kesaeghu jk[k dk mi;kesaeghu jk[k dk mi;kesaeghu jk[k dk mi;ksxsxsxsx@@@@Use of fly ashin soil treatment
As per CPRI studies reveals that fly ash from
thermal stations has equivalent chemical composition
and hence can be used for the electrical installations in
areas of high ground resistivity. Fly ash can also be
used as a chemical treatment material to reduce soil
resistivity.
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1.7.5 vFkZ frjks/kdrk ij vknzrkvFkZ frjks/kdrk ij vknzrkvFkZ frjks/kdrk ij vknzrkvFkZ frjks/kdrk ij vknzrk dk Hkkodk Hkkodk Hkkodk Hkko@@@@Effect ofMoisture Content on Earth Resistivity
Moisture content is expressed in percentage byweight of dry soil. Dry earth weights about 1440 kg/m
3.
Therefore about 144 kg (10%) of water is required per
cubic meter of soil to have 10% of moisture content.
About 20% moisture the resistivity is very little
affected below 20% moisture the resistivity increases
very abruptly with decrease in moisture. Moisture
content of about 17% to 18% by weight of dry soil is
the optimum requirement. Availability of moistureassists formation of electrolyte by dissolving salt
content in soils and there by enhance the conductivity
of soil. More water content can not improve soil
resistivity.
Resistivity(Ohm-cm)Moisture content(% by
weight) Top Soil Sandy Loam
0 1000x106 1000x10
6
2.5 250000 150000
5 165000 43000
10 53000 18500
20 12000 6300
30 6400 4200
1.7.6 rki dk Hkko@rki dk Hkko@rki dk Hkko@rki dk Hkko@Effect of TemperatureThe temperature coefficient of resistivity for soil
is negative, but is negligible for temperatures above
freezing point. At about 200
C the water in the soil
begins to freeze and introduce a tremendous increase in
the temperature coefficient. The resistivity changes 9%
per degree C. Below 0 degree C resistivity raises
abnormally.
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Effect of Temperature on Resistivity
0C 0F Resistivity(Ohm-cm)20 68 7,200
10 50 9,900
0 32(Water) 13,000
0 32(Ice) 30,000
-5 23 79,000
-15 14 330,000
1.7.7 vFkZ frjks/kdrk dk taxvFkZ frjks/kdrk dk taxvFkZ frjks/kdrk dk taxvFkZ frjks/kdrk dk tax ij Hkko@ij Hkko@ij Hkko@ij Hkko@Effect of SoilResistivity on Corrosion
Resistivity plays an important role in so far as
the corrosion performance of earthing rods is
concerned. It is observed that soils having resistivity of
less than 25 ohm-meter are severely corrosive in nature
while corrosion rate is of less importance in soils ofresistivity over 200 ohm- meter. The methods adopted
to safe guard earthing conductors against corrosion
depends upon
a. Material of the conductor
b. Corrosivity of the soil
c. Size of the grounding system
Range of soil resistivity
(Ohm-metre)
Class of soil
Less than 25 Severely corrosive
25 50 Moderately corrosive
50 -100 Mildly
Above 100 Very mildly corrosive
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1.7.8 lrg ij iRFkj pwjk dh irZ dsQk;ns@lrg ij iRFkj pwjk dh irZ ds Qk;ns@lrg ij iRFkj pwjk dh irZ dsQk;ns@lrg ij iRFkj pwjk dh irZ ds Qk;ns@Advantagesof Crushed Rock Used as a Surface Layer
It provides high resistivity surface layer
It serves as impediment to the movement of reptilesand there by help in minimizing the hazards
which can be caused by them
It prevents the formation of pools of oil from oilinsulated and oil cooled electrical equipment
It discourages the growth of weeds
It helps retention of moisture on the underlying soiland thus helps in maintaining the resistivity of thesubsoil at lower value.
It discourages running of persons in the switchyardand saves them from the risk of being subjected
to possible high step potentials.
1.7.9 iRFkj pwjk dh eghu irZ dk Hkko@iRFkj pwjk dh eghu irZ dk Hkko@iRFkj pwjk dh eghu irZ dk Hkko@iRFkj pwjk dh eghu irZ dk Hkko@Effect of Thin
Layer of Crushed RockIn outdoor switchyard, a thin layer of crushed
rock is spread on the surface.
The resistivity of gravel () is 2000 ohm-meter
while that of soil is 100 ohm-meter. Since of gravel is
high, only a high voltage can force the current through
the body to cause injuries. The gravel act like insulator
& throws the electric field generated by GPR back tosoil.
1.7.10 vFkZ bYkSDVksM yxkusdk LFkkvFkZ bYkSDVksM yxkusdk LFkkvFkZ bYkSDVksM yxkusdk LFkkvFkZ bYkSDVksM yxkusdk LFkku@u@u@u@Location of EarthElectrode
The location of earth electrode should be chosen
in one of the following types of soil in the order of
preference given on next page:
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a. Wet marshy ground.
b. Clay, loamy soil and arable land
c. Clay and loam mixed with varying proportions of
sand, gravel and stones.d. Damp and wet sand, peat.
Dry sand,gravel chalk limestone, granite, verystone ground and all locations where virgin rock is very
close to the surface should be avoided.
2.0 lcLVskulcLVskulcLVskulcLVsku ijijijij vfFkZaxvfFkZaxvfFkZaxvfFkZax O;oLFkk@O;oLFkk@O;oLFkk@O;oLFkk@EARTHING
ARRANGEMENTS AT SUB STATION
2.1 vFkZ bYvFkZ bYvFkZ bYvFkZ bYkSDkSDkSDkSDVksMVksMVksMVksM@@@@Earth Electrodes
The earth electrodes are made of mild steel
galvanized perforated pipe of not less than 40 mm
nominal bore, of about 4 m length provided with a
spike at one end and welded lug suitable for taking
directly MS flat of required size at the other end. The
pipe is embedded as far as possible vertically into the
ground, except when hard rock is encountered, where it
may be buried inclined to the vertical, the inclination
being limited to 300
from the vertical. The connection
of MS flat to each electrode is made through MS links
by bolted joints to enable isolation of the electrode for
testing purpose.
Earth electrodes shall be embedded as apart aspossible from each other. Mutual separation between
two electrodes shall not be less than 6.0 m i.e. twice the
length of the electrode as shown in figure- 3
In high embankments, it may be difficult to
achieve earth resistance even after chemical treatment
of electrodes. In these locations, use electrodes longer
than 4 m so as to reach the parent soil is recommended.
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As far as possible, earth electrodes for traction
substation/ switching stations shall be installed within
and adjacent to perimeter fence.
75
150
3000
150 150
75 75
48.4
75
150mmLONG
75mmX8mm
M.S.FLATWELDED
12mmDIAHOLES
ATSIM
ILAR
SPACIN
GFOR
ENTIRE
LENGTH
200
200
500
50
50
GROU
NDLEVEL
LAYEROFSAND
LID
FOREARTHBOX200
80X(RCC)
50mmTHIC
K200
EARTHBOX
(RCC)
AREATOBEWELL
RAMMED
2X75mm
X8mm
100mmX
10mm
80mmX12mm
Alternateequal
Layersofsalt,
charcoalorcock
350
150
150
EARTH
ELECTRODE
12mmDIA
HOLES
Pipetomedriven
thesoilforbetter
holding
50
175 3
500
Figure -3
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2.2 vfFvfFvfFvfFkZax eSVkZax eSVkZax eSVkZax eSV fMt+kbufMt+kbufMt+kbufMt+kbu dks Hkkfor djusokys dkjd@dksHkkfor djusokysdkjd@dks Hkkfor djusokys dkjd@dksHkkfor djusokysdkjd@PARAMETERS AFFECTING THE DESIGN OFEARTHING MAT
Several variable factors are involved in the
design of earthing mat conductor. Earthing mat for each
substation has to be designed individually. The earthing
mat has to be designed as per the site conditions to have
low overall impedance and a current carrying capacity
consistent with the fault current magnitude. The
following parameters influence the design of earthing
mat:1. Magnitude of fault current
2. Duration of fault
3. Soil resistivity
4. Resistivity of surface material
5. Shock duration
6. Material of earthing mat conductor
7. Earthing mat geometry
2.3 fMtfMtfMtfMt++++k;u f;k@k;u f;k@k;u f;k@k;u f;k@DESIGN PROCEDUREThe following steps are involved in the design
of earthing mat:
i. The layout of the sub station, determine the area tobe covered by the earthing mat.
ii. Determine the soil resistivity at the sub station site.The resistivity of the earth varies within
extremely wide limits, between 1 and 10,000
ohm-meter. The resistivity of the soil at many
station sites has been found to be non- uniform.
Variation of the resistivity of the soil with
depth is more predominant as compared to the
variation with horizontal distances. To design
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the most economical and technically sound
grounding system for large stations, it is
necessary to obtain accurate data on the soil
resistivity and on its variation at the station site.Resistivity measurements at the site will reveal
whether the soil is homogeneous or non-
uniform. In case the soil is found uniform,
conventional methods are applicable for the
computation of earth resistivity. When the soil
is found non- uniform, either a gradual
variation or a two-layer model may be adopted
for the computation of earth resistivity.
2.4 vvvvffffFkFkFkFkZaxZaxZaxZaxeSVeSVeSVeSV@@@@EARTHING MATAn earthing mat is formed by means of bare mild
steel rod of appropriate size buried at a depth of about
600 mm below the ground level and connected to earth
electrodes. The connection between the earth electrodes
and the mat shall be by means of two separate and
distinct connections made with 75 mm x 8 mm MS flat.
The connection between the MS flat and the MS rod
shall be made by welding, while that between the earth
electrode and the MS flats through MS links by bolted
joints. The earth electrodes are provided at the outer
periphery of the mat. As far as possible the earthing
mat conductors shall not pass through the foundationblock of the equipment.
All crossings between longitudinal conductors
and transverse conductors are jointed by welding. The
longitudinal and transverse conductors of the earthing
grid shall be suitably spaced so as to keep the step and
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touch voltage within the acceptable limit. However the
overall length of the earthing grid conductors shall not
be less than the calculated length as shown in figure 4.
BURIEDGRIDCONDUCTORWITHIN1METREOFFENCE
MS ROD
M S FLAT
EARTH
TRACTIONSUB.-STN.FENCIG
ELECTRODE
CROSSING
WELDED AT
75 mm X 8mm MS FLATS
FROM POWERTRANSFORMER
SECONDARY WINDINGTERMINAL
BURIES RAIL
75 mm X 8mm MS FLATS TO
NON TRACK CIRCUITED RAILNEUTRAL POINT OF
IMPEDANCE BOND
Figure - 4
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The size of the earthing grid conductors are decided
based on the incoming system voltage and fault level.
The fault level considered is taking into account the
anticipated increase in fault current during the life spanof the station. The system voltage, fault level and
conductor size are given below:
Sr.No
System
voltage
(KV)
Fault level (MVA) Diameter of
grid conductor
(GI rod) in mm
1 66
upto 4000
above 4000 upto 5000above 5000 upto 6000
32
3640
2 110
upto 6000
above 6000 upto 8000
above 8000 upto 10000
32
36
40
3 132upto 7000
above 7000 upto 10000
32
36
4 220
upto 12000
above 12000 upto 16000
above 16000 upto 20000
32
36
40
2.5 xMxMxMxM+h gqbZ+h gqbZ+h gqbZ+h gqbZ jsy@jsy@jsy@jsy@BURIED RAIL
A steel rail of section 52 kg/m (the one used for
the railways track) and length about 13 m is buried nearthe track at the traction substation at a depth of about
one meter to form part of the earthing system. Two
separate and distinct connections are made by means of
75 mm x 8 mm MS flat between the earthing mat and
the buried rail. The buried rail are connected by means
of two separate and distinct connections made with 75
mm x 8 mm MS flat to the traction rail(s) in a single-
rail track circulated section and to the neutral points(s)
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of the impedance bond(s) in a double-rail track
circuited section.
In case where the feeding post is locatedseparately away from the traction substation, the buried
rail is provided at the feeding post (where one terminal
of the secondary winding of the traction transformer of
the substation is grounded).
2.6 flLVeflLVeflLVeflLVe vvvvffffFkFkFkFkZaxZaxZaxZax@@@@SYSTEM EARTHING
One terminal of thesecondary winding (25 kV
winding) of each traction power
transformer are earthed directly by
connecting it to the earthing grid
by means of one 75 mm x 8 mm
MS flat , and to the buried rail by
means of another 75 mm x 8 mm
GI flat as shown figure in - 5
One designated terminal of the secondary of
each potential, current and auxiliary transformer are
connected to the earthing grid by means of two separate
and distinct earth connection made with 50 mm x 6 mm
MS flat as shown in figure in 6,7& 8.
Figure - 6 Figure - 7 Figure - 8
Figure- 5
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2.7 midj.kmidj.kmidj.kmidj.k vvvvffffFkFkFkFkZax@Zax@Zax@Zax@EQUIPMENT EARTHING
2.7.1 ikWoj VkUlQkeZj dhikWoj VkUlQkeZj dhikWoj VkUlQkeZj dhikWoj VkUlQkeZj dh vvvvffffFkFkFkFkZax@Zax@Zax@Zax@ Earthing of Power
Transformer
The metallic frame work (tank) of each
power transformer is directly connected to
the main earthing mat by means of two
separate distinct connections made with 50
mm x 6 mm MS flat for the 25 kV side and
75 mm x 8 mm MS flat for the primary
side. One connection is made with the
nearest longitudinal conductor, while theother is made to the nearest transverse
conductor of the mat.
In addition there is a direct connection from the
tank to the earth side of lightning arrestors. The
transformer tank rails are earthed either separately or by
bonding at each end of the track as shown in figure- 9.
2.7.2 lfdZVlfdZVlfdZVlfdZV cszdj rFkk buVIVjcszdj rFkk buVIVjcszdj rFkk buVIVjcszdj rFkk buVIVj dh vfFkZaxdh vfFkZaxdh vfFkZaxdh vfFkZax@@@@ Earthing ofCircuit Breakers and Interrupter
The supporting structures of each circuit
breaker unit are connected to the
earthing mat by means of two separate
distinct connections made with 50 mm
x 6 mm MS flat for the 25 kV side and
75 mm x 8 mm MS flat for the primaryside. One connection is made with the
nearest longitudinal conductor, while
the other is made to the nearest
transverse conductor of the mat.
Cubicles, doors and cable glands
are also connected to the earthing mat
as shown in figure 10 & 11.
Figure - 9
Figure -10 & 11.
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Fi ure -14
2.7.3 vkblksysVj dhvkblksysVj dhvkblksysVj dhvkblksysVj dh vvvvffffFkFkFkFkZax@Zax@Zax@Zax@Earthing ofIsolators
A flexible earth conductor is
provided between the handle andearthing conductor attached to the
mounting bracket and the handle of
switches is connected to earthing
mat by means of two separate
distinct connections made with 50
mm x 6 mm MS flat for the 25 kV
side and 75 mm x 8 mm MS flat for
the primary side .One connection ismade with the nearest longitudinal
conductor, while the other is made to the nearest
transverse conductor of the mat as shown in figure 12
& 13
2.7.4 ykbfVaxykbfVaxykbfVaxykbfVax vvvvjsLVj dhjsLVj dhjsLVj dhjsLVj dh vvvvffffFkFkFkFkZaxZaxZaxZax@@@@Earthing of LightningArrestor
In addition to the earth
electrodes provided for the main
earthing mat, an independent earth
electrode is provided for each
lightning arrester. This earth
electrode is connected to the ground
terminal of the lightning arrester, as
well as to the main earthing mat bymeans of two separate distinct
connections made with 50 mm x 6
mm MS flat for the 25 kV side lightning arresters, and
with 75 mm x 8 mm MS flat for the primary side
lighting arresters. The earth electrode is provided as
close as possible to the lightning arrester and the
Figure -13
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connection shall be as short and straight as possible
avoiding unnecessary bends.
For lightning arresters provided for the traction
power transformers, there is also be a connection as
direct as possible from the ground terminal of the
lighting arrester to the frame of the transformer of the
being protected. This connection is also be made by
two separate and distinct connections made with 50
mm x 6 mm MS flat for the 25 kV side lightning
arresters, and with 75 mm x 8 mm MS flat for the
primary side lighting arresters as shown in figure- 14
2.7.5 djsaV ,oadjsaV ,oadjsaV ,oadjsaV ,oa iksVsfUk;y VkUlQiksVsfUk;y VkUlQiksVsfUk;y VkUlQiksVsfUk;y VkUlQWWWWkeZj dhkeZj dhkeZj dhkeZj dhvvvvffffFkFkFkFkZax@Zax@Zax@Zax@Earthing of Current Transformer andPotential Transformer
The supporting structures of
Current Transformer and PotentialTransformer unit of bases, all bolted
cover plates to which the bushings
are attached connected to the earthing
mat by means of two separate distinct
connections made with 50 mm x 6
mm MS flat for the 25 kV side and
75 mm x 8 mm MS flat for the
primary side. One connection is madewith the nearest longitudinal
conductor, while the other is made to
the nearest transverse conductor of
the mat as shown in figure 15 & 16
Figure -15& 16
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2.7.6 daVksy :e dsvUnj dhdaVksy :e ds vUnj dhdaVksy :e dsvUnj dhdaVksy :e ds vUnj dh vvvvffffFkFkFkFkZax@Zax@Zax@Zax@ Earthing insideControl Room
An earthing ring shall beprovided inside the control
room by means of 50 mm x 6
mm MS flat which shall be run
along the wall on teak wood
blocks fixed to the wall at a
height of about 300 mm from
the floor level as shown in
figure- 17.
The earthing ring is
connected to the main earthing
grid by means of two separate
and distinct connections made
with 50 mm x 6 mm MS flat.
The earthing ring is also be
connected to independent earth
electrode by means of two
separate and distinct
connections made with 50 mm
x 6 mm MS flat as shown in
figure 18.
The metallic framework of control and relay
panels, LT AC and DC distribution boards, battery
chargers, remote control equipment cabinets and such
other equipment are connected to the earthing ring by
means of two separate and distinct connection made
with 8 SWG galvanized iron wire. The connections are
being taken along the wall and in the floor. All recesses
shall be covered with cement plaster after finishing the
work.
Figure- 18
Figure - 17
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Connection between the MS flats is made by welding as
shown in figure 19, 20, 21& 22.
Figure- 19 Figure - 20
Figure- 21 Figure- 22
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2.7.7 vU; midj.kksadhvU; midj.kksadhvU; midj.kksadhvU; midj.kksadh vvvvffffFkFkFkFkZax@Zax@Zax@Zax@Earthing of otherEquipment
a. vFkZ Lhu dh vfFkZaxvFkZ Lhu dh vfFkZaxvFkZ Lhu dh vfFkZaxvFkZ Lhu dh vfFkZax@@@@Earthing of Earth Screen
The area covered
by outdoor substation are
shielded against direct
strokes of lightning by an
overhead earth screen
comprising 19/2.5 mm
galvanized steel standardwire strung across the
pinnacles of the metallic
structures. The earth
screen wires are strung at a height as indicated
in the approved traction substation layouts (not
less than 2.5 m above the live conductor) and
shall be solidly connected to the traction
substation earthing grid at each termination by
means of 50 m x 6 mm GI flat as shown in
figure 23.
b. QsfUlax LraHkksa vkSj isuyksa dhQsfUlax LraHkksa vkSj isuyksa dhQsfUlax LraHkksa vkSj isuyksa dhQsfUlax LraHkksa vkSj isuyksa dh vvvvffffFkFkFkFkZaxZaxZ a xZax@@@@Earthing of fencing uprights and panels
Each metallic fencingupright is connected to the
traction substation main
earthing grid by means of
two separate and distinct
connections made with 50
mm x 6 mm MS flat. In
addition, all the metallic
fencing panels shall be connected to the uprights
Fi ure- 24
Figure- 23
Fi ure- 24
Figure- 24
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by means of two separate and distinct
connections made with
6 SWG G I wire. The
entire metallic doorpanels are also be
connected to the
supporting uprights by
means of two distinct
connections made with
6 SWG I wire as shown
in figure- 24 & 25
All metallic part
of structures, masts and
metallic frames is
connected to the
traction substation
main earthing mat by
means of two separate
and distinct connections
made with 50 mm x 6 mm MS flats shown in
figure- 26.
2.7.8 vkW;y fQYVjsku IykaVvkW;y fQYVjsku IykaVvkW;y fQYVjsku IykaVvkW;y fQYVjsku IykaVds fy,ds fy,ds fy,ds fy, 240oksYV ,lh 50oksYV ,lh 50oksYV ,lh 50oksYV ,lh 50gVZt dsikWbaV dhgVZt dsikWbaV dhgVZt dsikWbaV dhgVZt dsikWbaV dh vvvvffffFkFkFkFkZax@Zax@Zax@Zax@Earthing at the pointof 240 V ac 50 Hz supply for oil filtration plant
The 240 V ac 50 Hz
distribution board for power
supply to oil filtration plant is
connected to the main earthing
grid by means of two distinct
connections made with 50 mm
x 6 mm MS flat as shown in
figure - 27
Figure- 26
Figure- 25
Figure- 27
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2.8 vvvvffffFkFkFkFkZax pkyd dk lkbZax pkyd dk lkbZax pkyd dk lkbZax pkyd dk lkbZt@Zt@Zt@Zt@SIZE OF EARTHINGCONDUCTOR
Size of earthing conductor depends on systemvoltage and fault level as given below:
Sr.
No
Equipment
(KV)
System voltage and
fault level
Ground
conductor size
1 Equipment
on the
primary
side of
tractionpower
transformer
66 KV, upto 3000 MVA
110 KV, upto 5000 MVA
132 KV, upto 6000 MVA
220 KV, upto 10000 MVA
66 KV, above 3000 upto
6000 MVA
110 KV, above 5000 upto
10000 MVA
132 KV, above 6000 upto
12000 MVA
220 KV, above 10000
pto 20000 MVA
50mm x 6mm
75mm x 8mm
2 Equipment on the secondary side of
traction power transformer
50mm x 6mm
3 Fencing uprights/steel structures 50mm x 6mm
4 Doors/fencing panels 6 SWG G I wire
2.9 ffffLofpax LVskuksa ijLofpax LVskuksa ijLofpax LVskuksa ijLofpax LVskuksa ij vvvvffffFkFkFkFkZax O;oLFkk@Zax O;oLFkk@Zax O;oLFkk@Zax O;oLFkk@EARTHINGARRANGEMENT AT SWITCHING STATION
A minimum number of three earth electrodes
(excluding the one to be provided separately for the
remote control cubicle earthing) are provided at each
switching station, and they are interconnected by means
of 50 mm x 6 mm MS flat forming a closed loop main
earthing ring. This ring is connected by two separate
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and distinct connections made with 50 mm x 6 mm MS
flat, to the non-track circuited rail in a single rail track
circuited section and to the neutral point of the
impedance bond in a double rail track circuited sectionof the nearest track, so as to limit the potential gradient
developing in the vicinity of the switching station in the
event of a fault.
2.9.1 flLVeflLVeflLVeflLVe vvvvffffFkFkFkFkZax@Zax@Zax@Zax@System Earthing
One designated terminal of the secondary of
each potential transformer, current transformer andauxiliary transformer are connected to the main
earthing ring by means of two separate and distinct
connections made with 50 mm x 6mm MS flat as
shown in figure -28 &29.
2.9.2midj.kmidj.kmidj.kmidj.k vvvvffffFkFkFkFkZax@Zax@Zax@Zax@Equipment earthingAll masts,
structures, fencing
uprights and all
outdoor equipment
pedestals including
auxiliary transformer
tank is connected to
the earthing ring by
means of two
separate and distinct
Figure- 28 Figure- 29
Figure- 30 Figure- 31
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connections made with 50mm x 6mm MS flat. All
fencing panels are connected to the supporting uprights
by means of two separate and distinct connections made
with 6 SWG G.I. wire. All the metallic door panels areconnected to the supporting uprights by means of two
separate and distinct connections made with 6 SWG
G.I. wire as shown in figure- 30&31
The metal casing of potential and current
transformers are connected to the mast/ structures by
means of two separate and distinct connections made
with 50mm x 6mm MS flat as shown in figure 32&33.
The ground terminal of
lightning arrester shall be
connected directly to the earth
electrodes by means of twoseparate and distinct connections
made with 50mm x 6mm MS flat.
The earth electrode shall be so
placed that the earthing leads from
the lightning arrester may be
brought to the earth electrodes by as short and straight a
path as possible as shown in figure 34.
Figure- 32 Figure- 33
Fi ure- 34
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3.0 ijEijkxrijEijkxrijEijkxrijEijkxr vFkvFkvFkvFkZ byZ byZ byZ byssssDVksM dk vuqj{k.kDVksM dk vuqj{k.kDVksM dk vuqj{k.kDVksM dk vuqj{k.kMAINTENANCE SCHEDULE FOR CONVENTIONALEARTH ELECTRODE
3.1 ====SSSSekfld vuqj{k.k@ekfld vuqj{k.k@ekfld vuqj{k.k@ekfld vuqj{k.k@QUARTERLY SCHEDULE
Sr. Items Inspection Action to be taken
1 Earth flat
connections to
structures,equipment, rail
and earth
electrodes with nut
and check nuts
Check for
proper
continuityand
tightness
If found broken or
loose, it should be
immediatelyreplaced /tightened
2 Bolts and nuts of
the connections
Check for
rust and
dirt
Rust and dirt should
be cleaned and apply
grease
3 MS links by bolted
joints between
earth electrode and
75mmx 8mm MS
flat
Check for
tightness
If found broken or
loose, it should be
immediately
replaced/ tightened
4 Projection of theearth electrode
Check forground
level and
proper soil
It should be 175 10mm above the
ground level
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3.2 v)Zv)Zv)Zv)Z okf"kZd vuqj{k.k@okf"kZd vuqj{k.k@okf"kZd vuqj{k.k@okf"kZd vuqj{k.k@HALF YEARLY SCHEDULE
Carry out following work in addition to quarterly
schedule:
Sr. Items Inspection Action to be taken
1. Measure
earth
resistance
Individual earth
electrode and
record the value
By earth resistance
meter. It should be
permissible limit
2. Measure
earthresistance
combined
Combined earth
resistance withearthing flats,
connected to the
equipment,
structures and
earth electrode
TSS 0.5
Switching station 2.0
AT Station 10.0
3. Sump Check up general
condition
including dryness.
If the surrounding area
is too dry, water
should be poured intothe sump to keep the
soil moist.
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3.3 okf"kZd vuqj{k.k@okf"kZd vuqj{k.k@okf"kZd vuqj{k.k@okf"kZd vuqj{k.k@YEARLY SCHEDULE
Carry out following work in addition to
quarterly and half yearly schedule:
Sr. Item Inspection Action to be taken
1. Earth
pits
Check up
the
electrode
for proper
earthcontinuity.
Remove the hardened top
layer of the earth pit for a
depth of 1 meter, mix with
coke and loamy soil (non-
sandy) and ram the earth.Repair the earth. Repair
sides and top cover of the
earth pits. Avoid use of salt
as far as possible to avoid
rusting of earth pipe.
3.4 vFkZvFkZvFkZvFkZ bybybybySSSSDVksM dsfrjks/k dk ekiu@DVksM dsfrjks/k dk ekiu@DVksM dsfrjks/k dk ekiu@DVksM dsfrjks/k dk ekiu@MEASUREMENT OF EARTH ELECTRODERESISTANCE
Fall of potential method
In this method two auxiliary earth electrodes,
besides the test electrode, are placed at suitable
distances from the test electrode as shown in figure35.
A measured current is passed between the electrode A
to be tested and an auxiliary current electrode C andthe potential difference between the electrode A and
the auxiliary potential electrode B is measured. The
resistance of the test electrode A is then given by:
R = V/I
Where, R = Resistance of the test electrode in
ohms,
V = Reading of the voltmeter in volts,
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I = Reading of the ammeter in amperes
In most cases, there will be stray currents flowing in
the soil and unless some steps are taken to eliminate their
effect, they may produce serious errors in the measuredvalue. If the testing current is of the same frequency as the
stray current, this elimination becomes very difficult. It is
better to use an earth tester incorporating a hand driven
generator. These earth testers usually generate direct
current, and have rotary current reverser and synchronous
rectifier mounted on the generator shaft so that alternating
current is applied to the test circuit and the resulting
potentials are rectified for measurement by a direct reading
moving coil ohm meter. The presence of stray currents in
the soil is indicated by a wandering of the instrument
pointer, but an increase or decrease of generator handle
speed will cause this to disappear.
At the time of test, where possible, the test electrode
shall be separated from the earthing system. The auxiliaryelectrode consists of 12.5 mm diameter mild steel rod
driven up to one meter into the ground.
AMMETER
VOLTMETER
A
V
CURRENT
SOURCE
A B C
TESTELECTRODE
POTENTIAL
ELECTRODECURRENT
ELECTRODE
X = 1m
Figure 35
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3.5 gLr pkfyr vFkZ VsLVj dk fooj.k@gLr pkfyr vFkZ VsLVj dk fooj.k@gLr pkfyr vFkZ VsLVj dk fooj.k@gLr pkfyr vFkZ VsLVj dk fooj.k@DETAILS OFEARTH TESTER (Hand Driven)
Earth resistance meter are employed formeasurements of earth resistance in Traction sub
station, switching stations and other electrical
installations. An Earth resistance meter comprises a
hand driven magneto type D.C. Generator, a current
reverser, rotary rectifier and ohm meter.
The current reverser and rotary rectifier are
driven along with D.C. Generator by driving systems
which incorporate a clutch mechanism for
unidirectional rotation and a governor for speed control.
The function of current reverser is to change the
direction of flow of current in the soil and that of rotary
rectifier is to maintain unidirectional current in the
potential coils of the ohm meter.
The ohm meter consists of a current coil and a
potential coil mounted on a common spindle and placed
in the magnetic field of a permanent magnet. The
current coil is connected in series with the earth
electrodes and current electrodes. The potential coil is
connected across the earth electrode and the potential
electrode through the rotary rectifier. While measuring
the earth resistance the terminals C1, P1 are connected
to the main earth electrode P2 to the potential electrodeand C2 to the current electrode. The potential and
current electrodes are temporary electrodes placed in
the ground 50 to 75 feet apart and 50 to 75 feet & from
the earth electrode as shown in below figure- 35 A
When the megger is operated an ac current is
produced in the coil. The voltage drop produced in the
earth electrode is applied across the potential coil. The
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current coil produces a torque in the clock wise
direction, and the potential coil produces a torque in
anti-clock wise direction. The current applied to the
current coil is inversely proportional to the earthresistance and the voltage drop applied across the
potential coil is directly proportional to the earth
resistance the torque opposes each other and brings the
moving system to rest when they are equal. The pointer
indicates the earth resistance values on a calibrated
scale.
Figure 35 A
Cu rrent Term inal: or C2Poten tial Terminal: P1 or P2
Test EarthElectrode
PotentialElectrode
CurrentElectrode
C1
P1
C2
P2
50 - 70 feet 50 - 70 feet
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4.0 vuqj{k.k eqDr vfFkZax@vuqj{k.k eqDr vfFkZax@vuqj{k.k eqDr vfFkZax@vuqj{k.k eqDr vfFkZax@MAINTENANCE FREEEARTHING
In conventional earthing system GI pipe is usedas earth electrode. It is provided with charcoal and salt
as conducting media, which provides a reasonable
earth. Corrosion of metallic parts is comparatively fast.
It also requires maintenance by way of watering of
earth pits and chiseling of corrosion prone parts and
their replacement. Required monitoring may not always
be feasible in certain crowded and inaccessible areas.
With technological developments in this field,
modern maintenance free and durable earthing system
employs steel conductors as electrode which are copper
claded and utilize graphitic compounds and non
corrosive salts as Ground Enhancing Material which
do not lead to corrosion. Such earth pits also do not
require the usual watering schedules to maintain the
earth resistance with in limits. Maintenance free earthsare to be constructed as per RDSOs specification
no.RDSO/ PE/ SPEC/ 0109-2008 (REV0).
Where the earth pits are not easily accessible for
schedule maintenance, maintenance free earth pits shall
be provided. In areas where clusters of earth pits are
required to keep the earth resistance low, provision of
maintenance free earth pits should be made duringinitial installation.
4.1 vFkZ frjks/kvFkZ frjks/kvFkZ frjks/kvFkZ frjks/k@@@@EARTH RESISTANCE
The earth resistance value at earth bus bar
should be less than 0.5 ohms for major electrical
equipment & installation.
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4.2 mi;ksx@mi;ksx@mi;ksx@mi;ksx@APPLICATIONS
This earthing system may be used in following
locations. Sub stations & switching stations Remote Terminal Units Transformer & Generator neutral earths Lightning arrester earths Equipment earths including panels
4.3 vuqj{k.k eqDr vfFkZax .kkyh@vuqj{k.k eqDr vfFkZax .kkyh@vuqj{k.k eqDr vfFkZax .kkyh@vuqj{k.k eqDr vfFkZax .kkyh@MAINTENANCE
FREE EARTHING SYSTEM
This earthing system includes earth electrode
installation in suitable pit, construction of earth pit with
cover for the installation, connection of earth electrode
with equi-potential earth bus and connection of
equipment to equi- potential earth bus.
4.3.1 vFkZvFkZvFkZvFkZ bySDVksM@bySDVksM@bySDVksM@bySDVksM@Earth ElectrodeThe material for earth electrode used in this type
of earthing, has a good electrical conductivity and it
does not corrode in a wide range of soil conditions.
There are basic two types of earth electrodes
used in this earthing system.
i. jkWM Vkbi vjkWM Vkbi vjkWM Vkbi vjkWM Vkbi vFkZFkZFkZFkZbySDVksM@bySDVksM@bySDVksM@bySDVksM@Rod type earthelectrode
The copper bonded stainless steel rod
(low carbon high tensile steel alloy) earth
electrode shall conform to the standard BS 4360
Grade 43A or EN10025:2-004 S275IR
molecularly bonded by 99.99% pure high
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conductivity copper on outer surface with
copper coating thickness 250 micron or more.
The earth rod shall have following
characteristics/ specifications.
a.The minimum length of earth electrode shallbe 3.0 meters long. The length of electrode
may be increased in multiple of 1.0 meter to
reduce earth resistance.
b.To increase the length, pieces of similar rodshall be either exothermally welded to basic
3.0 meter electrode or connected using
socket of suitable size. These sockets shall
be molecularly bonded by 99.99% pure
high conductivity copper on inner & outer
surface with copper coating thickness 250
micron or more.
c.The diameter of earth electrode shall not beless than 17 mm.
d.The copper bonding thickness on stainlesssteel rod shall be 250 microns or more, hot
dipped or electroplated.
e.Copper bus bar of size 250 mm x 50 mm x 6mm having electrical conductivity of 101%
IACS minimum 99.9% copper content shallbe exothermically welded to rod with 4
holes of 12 mm dia. (2 on each side) for
connecting earthing conductor.
f.Current carrying capacity of earth electrodeshould be such as to have more than 15kA
for one second.
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ii. llllaaaadsfUnzr ikbi vFkZ bySDVksM@dsfUnzr ikbi vFkZ bySDVksM@dsfUnzr ikbi vFkZ bySDVksM@dsfUnzr ikbi vFkZ bySDVksM@Concentricpipe earth electrode
MS pipe with 25 - 50 mm diameter, class B,ISI mark as per IS: 1239, length 2000 mmor 3000 mm is used as primary conductor
as shown in table -1
MS pipe with 40 - 100 mm diameter, class B,ISI mark as per IS: 1239, length 2000 mm
or 3000 mm is used as secondary conductor
(electrode) as shown in table -1
Table - 1Sr. Current
capacity
Primary
conductor
diameter
Secondary conductor
(Electrode) dimensions
(dia x length)
1 3kA 25 mm 40 mm x 2000 mm
2 5kA 25 mm 40 mm x 3000 mm
3 15kA 25 mm 50 mm x 3000 mm
4 40kA 40 mm 60 mm x 3000 mm5 50kA 50 mm 100 mm x 3000 mm
NOTE For more than 50kA applications, multiple
electrodes of 50kA capacity are installed and
connected.
For hermetically filling inside the cavity betweensecondary conductor and primary conductor,
crystalline compound is to be injected in theelectrode assembly. It is a combination of high
conductivity metal alloys, copper & aluminium
powder, conductive carbon/cement and bonding
material etc. mixed in different proportion. The
mixture is forced (pressurized) filled inside the
earth electrode in the paste form and after
solidification of the same, the end caps are welded.
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The metal alloys shall help in conducting the
current and conductive carbon gives anti corrosive
property. Bonding material provides strength to the
mixture. Resistivity of the mixture shall be lessthan 0.2 ohm - meter. Resistivity shall be tested by
making a 20 cm cube of the material and checking
resistance across the opposite face of the cube.
Complete electrodes shall be molecularly bondedby 99.99% pure, high conductivity copper on outer
surface with copper coating thickness 300 micron
or more.
Its surface shall be cleaned and free from anyvisible oxide layer or foreign material.
Copper bus bar of size 250 mm x 50 mm x 6 mmhaving electrical conductivity 101% , minimum
99.9% copper content shall be preferable to
exothermically welded to earth electrode or
connected with the help of two number stainless
steel nut bolts of appropriate size having 4 holes of12 mm dia ( 2 on each side) for connecting earthing
conductor as shown in figure - 36
EarthEhancement
Earth Electrode
(Dimension as perdesign)
ExothermicWelding
25 x 3mm Copper Strip (To be duplicated)
300 x 25 x 6mm Copper bus
ExothermicWelding
ExothermicWelding
Ground Level
5 ft x 5ft x 10ft pit or 300mm bore
(Earth Electrode Installation)
To Equipment
Figure - 36
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4.3.2 vFkZ bugsUlesaV lkexzhvFkZ bugsUlesaV lkexzhvFkZ bugsUlesaV lkexzhvFkZ bugsUlesaV lkexzh@@@@Earth Enhancement Material
Earth enhancement material is a superior
conductive material that improves earthing
effectiveness especially in area of poor conductivity asshown in figure 36A. It improves conductivity of the
earth electrode and ground contact area. It has
following characteristics:
a. It should low resistivity bellow 0.2 ohm- meters.
b. It shall not depend on the continuous presence ofwater to maintain its conductivity.
c. It should be a little alkaline in nature with pHvalue >7 but
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e. It has a capacity to retain >10% moisture at105
0C.
f. It should have water solubility
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4.3.3 cSdfQy lkexzh@cSdfQy lkexzh@cSdfQy lkexzh@cSdfQy lkexzh@Backfill material
The excavated soil can be used if it is free from
sand, gravel and stones. Small proportion of sand inthe soil may be permissible. Material like sand, salt,
coke breeze, cinders and ash are not used because of
its acidic and corrosive nature.
While backfilling the soil shall be thoroughly
compacted with at least 5 kg compactor, in case the
soil is dry small quantity of water may be sprinkled
only to make it moist enough suitable for
compacting. Large quantity of water may make thesoil muddy which is not suitable for compacting and
after drying the soil may contain voids which may
permanently increase earth resistance.
4.3.4 le foHko cl ,oavFkZ pkyd@le foHko cl ,oavFkZ pkyd@le foHko cl ,oavFkZ pkyd@le foHko cl ,oavFkZ pkyd@Equi- Potential Bus& Earth Conductor
A copper bus bar of size 300 mm x 25 mm x 6 mmto be installed in this equipment room as equi
potential bus connected with copper strip of size
25 mm x 3 mm (suitable length) from instrument
to the bus bar. The connecting terminal of the
earth electrodes to the bus bar must be connected
by copper strip of 25 mm x 3 mm (suitable
length) buried inside a trench of 300 mm width x
600 mm (depth from the nearest wall). It is
duplicate earth conductor.
The maximum specific resistance of the copperstrip earthing conductor shall be 17.241 x 10
-7
ohm cm at 200C and having electrical
conductivity of 101% IACS i.e. minimum 99.9%
copper content.
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At a temperature of 200C, its density shall be 8.89gm/cm
3.
A single length of copper strip shall be used foreach duplicate earthing conductor and no joint
shall be permitted. The joint shall be made by
exothermic welding of at least 10 mm
overlapping portion of the strips.
It shall be connected to earth electrode and earthbus bar with the help of exothermic welding or at
least two number stainless steel nut bolts of
appropriate size.
4.3.5 vFkZvFkZvFkZvFkZ bdkbZbdkbZbdkbZbdkbZ dk fuekZ.k@dk fuekZ.k@dk fuekZ.k@dk fuekZ.k@Construction of unit Earth
Make 5ft x 5ft x 10ft earth pit. If it not possible to
make such a pit due to non availability of clearspace a minimum 300 mm bore up to 10ft deep
can be made using earth auger or any other
method. Each pit larger than specified size can be
made, if required.
Sleeve the soil digged and remove the gravels andstones. If soil quality is good then add some
quantity of earth enhancement material in the soilfor using as backfill.
If the soil seems unusable (containing largequantity of gravel, stones, murum, sand etc) then
replace the soil with black cotton soil.
Insert the electrode at the center of the earth pit andarrange to keep it vertical in the pit
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Arrange for adequate quantity of water supply forthe earth pit (600 liters).
Fill the pit with the backfill and keep on adding theearth enhancement material surrounding the
electrode and simultaneously watering the pit
with a steel bar or pipe, keep on poking the soil
gel and stirring intermittently for removing the air
pockets and proper settlement of the pit. The
procedure to be repeated till completion of the
filling of the earth pit along with the packing
material and sufficient watering adequateramming.
The pit should be very compactly rammed andwatering for 2-3 days and addition of soil if
required be done.
Construct inspection chamber with cover for theinstallation.
Measure the earth resistance as per IS 3043:1987code of practice.
4.3.6 cgq vFkZ fiVksa}kjk vFkZ fjax dk fuekZ.k@cgq vFkZ fiVksa}kjk vFkZ fjax dk fuekZ.k@cgq vFkZ fiVksa}kjk vFkZ fjax dk fuekZ.k@cgq vFkZ fiVksa}kjk vFkZ fjax dk fuekZ.k@Construction of Ring Earth by providing multiple
earth pits
Wherever it is not possible to achieve requiredearth resistance with one earth electrode/ pit due todifficult/ rocky soil conditions, provision of ring
earth consisting of more than one earth pit is
required. The number of pits required can be
decided based on the resistance achieved for the
earth pits already installed.
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The distance between two successive earthelectrodes shall be min. 3.0 mtrs/length of electrode
which ever is higher, and max. up to twice the
length of the earth electrode.
These earth pits shall inter linked using 25 x 3 mmcopper strip to from a loop preferably using
exothermic welding or with the help of at least two
numbers of stainless steel nut bolts of appropriate
size.
The interconnecting strips shall be buried not less
than 800 mm (0.8 m) below the ground level. Thisinterconnecting strip shall also be covered with
earth enhancing compound.
4.3.7 fujh{k.k d{k@fujh{k.k d{k@fujh{k.k d{k@fujh{k.k d{k@Inspection chamber
A 300 mm x 300 mm x 300 mm (inside dimension)concrete box (wall thickness min. 50 mm) with
smooth cement plaster finish shall be provided on
the top of pit. A concrete lid of 25 to 50 mm thick,
with pulling hooks, painted black shall be provided
to cover the earth pit. PVC sleeve of appropriate
size shall be provided in concrete wall to take out
earthing connections.
The masonry work shall be white washed inside
and outside.
Care shall be taken regarding level of the floorsurrounding the earth so that the connector is not
too deep in the masonry or projecting out of it.
On backside of the cover, date of test and averageresistance value shall also be written with yellow
paint on black background with date.
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5.0 ckWfUMax] xzkmfUMax@ vfFkckWfUMax] xzkmfUMax@ vfFkckWfUMax] xzkmfUMax@ vfFkckWfUMax] xzkmfUMax@ vfFkZax dsfy, ,DlksFkfeZdZax dsfy, ,DlksFkfeZdZax dsfy, ,DlksFkfeZdZax dsfy, ,DlksFkfeZdoSoSoSoSfYMax la;kstufYMax la;kstufYMax la;kstufYMax la;kstu@@@@EXOTHERMIC WELDINGCONNECTION FOR BONDING, GROUNDING/EARTHING
The Exothermic welding connection process for
grounding/ earthing covers the requirements and
acceptance principle for weld material, welding tool
and tool kit, bonds/ track bond wires for making
permanent electrical bond connection between two
conductors as per RDSO specification no. TI/ SPC/OHE/ EXOTHRM BOND/ 0100 (04/10).
These weld materials including tool kit shall be useful
in following railway application on 25kV electric
traction system.
(a) Traction bond connection to rail & OHE structures
enroute.(b) Traction rail continuity connection.
(c) Earthing connections like at TSS, SP/ SSPs,SCADA/ RTU and other important electrical
installation.
(d) Bus bar connections etc.
(e) Copper to copper, copper to steel connection.(f) Important bonds which are not disturbed during
track relaying.
(g) Sections where 90UTS rails have been provided.
(h) Theft prone areas.
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5.1 lkekU; vko;drk;salkekU; vko;drk;salkekU; vko;drk;salkekU; vko;drk;sa@@@@GENERAL REQUIREMENTS
Welding material along with tool kit shall be
capable of providing a permanent bondingconnection involving copper or aluminium or steel.
The final bond shall be achieved in such a way that
homogeneous molecular bonding of two involved
metals takes place perfectly.
Exothermic welding process should be a steadyburn, no pops and no drastic color changes (white
hot color during burn, turns orange as the reaction
stops and the molten metal grows orange).
Welding connection shall be without voids orpinhole, and no porosity should be detected. The
process shall be consistent and continuous.
It shall be permanent, solid metal, maintenance freeconnection and shall not be affected by high current
surge. It shall not loosen or corrode at the point of weld.
There shall be no additional contact surface or
mechanical pressures involved.
It shall be vibration proof and corrosion freeelectrical connection.
It shall not brittle with age and shall be able tosustain physical impacts, which may be
experienced under the field conditions.
Electrical resistance and other properties shall notdeteriorate with ageing and under severe Indian
climatic conditions.
It shall have a current carrying capacity equal to or
greater than that of the conductors
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Life of the joints shall not be less than 10 yearsirrespective of life of conductors, equi-potential
earth bus bars or ground rods used.
5.2 oooossssfYfYfYfYaaaaMx lkexzh@Mx lkexzh@Mx lkexzh@Mx lkexzh@WELDING MATERIAL
The weld material shall be designed for twospecific types of jointing applications
Rail welding application
Earthing & bonding application.
It shall not contain phosphorous or any caustic,toxic or explosive substance. For rail welding
application, the welding material shall not contain
tin besides the above cited elements. Its
components shall not volatile or sensitive to shock,
nor capable of spontaneous ignition by friction.
The reaction of the welding material shall be
completed within 10 seconds and on completion ofjointing & bonding process. The resulting weld
nugget shall free of dissolved gases or low boiling
point elements, e.g., small voids or blisters on the
surface.
Weld materials shall provide a final bond withminimum tensile strengths of 40,000 psi tested
using ASTM ES-94a (Test Methods for Tension
Testing of Metallic Material).
The weld material for traction bond and studwelding for rail application shall be free from tin.
As presence of metallic tin allows for wetting of
rail surface and provides for unnecessary
penetration that can lead to metallurgical damage to
the rail.
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The weld material for earthing & bondingapplication shall be tested as provision of IEEE-837
by NABL/ILAC Member labs.
5.3 osfYaMxosfYaMxosfYaMxosfYaMx VwYl dh fofk"VVwYl dh fofk"VVwYl dh fofk"VVwYl dh fofk"Vrk;sa@rk;sa@rk;sa@rk;sa@SPECIFICATIONOF WELDING TOOLS
It shall be suitable for bonding as per the sizesspecified for bonding equi-potential earth bus bars
(EEBB) to g
Recommended