92113333 Earthing Concepts

7/31/2019 92113333 Earthing Concepts

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EARTHING CONCEPTSEARTHING CONCEPTS


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Earthing in a EHV SubstationEarthing in a EHV Substation

Providing adequate Earthing in aProviding adequate Earthing in asubstation is an important safety measure.substation is an important safety measure.

Earthing means connecting the electricalEarthing means connecting the electrical

equipment to the general mass of earth ofequipment to the general mass of earth oflow resistance.low resistance.

Objective is to provide under and aroundObjective is to provide under and around

the substation a surface of uniformthe substation a surface of uniformpotentialpotential-- At near zero or absolute earth potential-- At near zero or absolute earth potential


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Earthing in a EHV SubstationEarthing in a EHV Substation

1.1. Objective:Objective: The touch and step potential shall be withinThe touch and step potential shall be within

limits under all conditions including faultlimits under all conditions including fault

conditioncondition Grounding resistance shall be lower.Grounding resistance shall be lower. Effective earthing system shall aim at providingEffective earthing system shall aim at providing

protection to life and property against dangerousprotection to life and property against dangerouspotentials under fault conditionspotentials under fault conditions


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Earthing in a EHV SubstationEarthing in a EHV SubstationI.E.Rules 1956I.E.Rules 1956

Rule 92Rule 92 Every substation /generating station exposed to lightningEvery substation /generating station exposed to lightning

shall adopt efficient means for diverting the electricalshall adopt efficient means for diverting the electricalsurges due to lightning to earthsurges due to lightning to earth

Earth lead of any lightning arrestor shall not pass throughEarth lead of any lightning arrestor shall not pass throughany iron or steel pipe.any iron or steel pipe. It shall be taken directly, as far as possible, to a separateIt shall be taken directly, as far as possible, to a separate

earth electrode and/or junction of the earth mat.earth electrode and/or junction of the earth mat. Bends Shall be avoided where ever practicableBends Shall be avoided where ever practicable Earth screen if provided for lightning protection shall beEarth screen if provided for lightning protection shall be

connected to main earth grid.connected to main earth grid.


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Earthing in a EHV SubstationEarthing in a EHV SubstationI.E.Rules 1956I.E.Rules 1956

Functioning of earthing in a substationFunctioning of earthing in a substation It shall be capable of passing maximum earth fault currentIt shall be capable of passing maximum earth fault current The passage of fault current does not result in any thermal orThe passage of fault current does not result in any thermal or

mechanical damage to the insulation of connected plant /mechanical damage to the insulation of connected plant /equipmentequipment

Every exposed conductor part and extraneous conductive partEvery exposed conductor part and extraneous conductive partmay be connected to the earth.may be connected to the earth.

There is no danger to the personnelThere is no danger to the personnel Ensure equi-potential bonding within the power systemEnsure equi-potential bonding within the power system No dangerous potential gradients (step or touch or transferNo dangerous potential gradients (step or touch or transfer

potentials) shall occur under normal or abnormal operatingpotentials) shall occur under normal or abnormal operatingconditionsconditions To minimize electromagnetic interference between power andTo minimize electromagnetic interference between power and

control/ communication systemcontrol/ communication system


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Earthing SystemEarthing System

Points to be earthed in a substationPoints to be earthed in a substation The neutral point of each separate systemThe neutral point of each separate system

should have an independent earth, in turnshould have an independent earth, in turninterconnected with the station grounding mat.interconnected with the station grounding mat.

Equipment frame work and other non-currentEquipment frame work and other non-currentparts (two connections)parts (two connections) All extraneous metallic frame works notAll extraneous metallic frame works not

associated with equipment ( two connections)associated with equipment ( two connections)

Lightning arrestors should have independentLightning arrestors should have independentearths, in turn connected to the stationearths, in turn connected to the stationgrounding grid.grounding grid.


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Earthing SystemEarthing SystemPoints to be earthed-contdPoints to be earthed-contd

Over head lightning screen shall also be connectedOver head lightning screen shall also be connectedto main ground mat.to main ground mat.

Operating handles of Isolators with a auxiliary earthOperating handles of Isolators with a auxiliary earth

mat underneath, if necessary.mat underneath, if necessary. Peripheral fencingPeripheral fencing Buildings inside the switch yard.Buildings inside the switch yard. Transformer Neutrals shall be connectedTransformer Neutrals shall be connected

directly to the earth electrode by twodirectly to the earth electrode by twoindependent MS stripsindependent MS strips


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Earthing and grounding -distinctionEarthing and grounding -distinction

Grounding:- connection of current carrying partsGrounding:- connection of current carrying partsto ground. Ex :Generator or transformer neutral.to ground. Ex :Generator or transformer neutral.

This is for equipment safety.This is for equipment safety. In a resistance grounded system it limits theIn a resistance grounded system it limits the

core damage in stator of rotating machines.core damage in stator of rotating machines. In solidly grounded system substantial groundIn solidly grounded system substantial ground

fault current flows enabling fault detection andfault current flows enabling fault detection andfaster clearance.faster clearance.


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Earthing and grounding -distinctionEarthing and grounding -distinction

Earthing:- connection of non currentEarthing:- connection of non currentcarrying parts to ground. Ex : Metalliccarrying parts to ground. Ex : Metallicenclosure.enclosure.

This is for human safety.This is for human safety. Earthing system plays no role underEarthing system plays no role under

balanced power system conditions.balanced power system conditions.

Under ground fault conditions, enablesUnder ground fault conditions, enablesground fault current to return back toground fault current to return back tosource without endangering human safety.source without endangering human safety.


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Basics of EarthingBasics of EarthingResistivity of earthResistivity of earth

Resistivity of earth:-Resistivity of earth:- Mother earth is a bad conductor.Mother earth is a bad conductor. Resistivity is normally around 100 ohm mt.Resistivity is normally around 100 ohm mt. GI of 65x10mm section will have same resistanceGI of 65x10mm section will have same resistance

as copper of 25x4mm section.as copper of 25x4mm section. Corresponding figure for earth is 800x800mtCorresponding figure for earth is 800x800mt

(158acres)(158acres) Metallic conductor is a preferred alternative toMetallic conductor is a preferred alternative to

earth to bring the fault current back to source.earth to bring the fault current back to source.


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Electric field Earth resistanceElectric field Earth resistance Current flows through a series of hemi-spherical shellsCurrent flows through a series of hemi-spherical shells

of earth of continuously increasing cross sections.of earth of continuously increasing cross sections.

Almost 95% of final resistance is contributed by soilAlmost 95% of final resistance is contributed by soilwithin 5mts of the electrode.within 5mts of the electrode.

If current is discharged from a grid towards anotherIf current is discharged from a grid towards anothergrid at B100 km away, only soil with in 5 to10 mts of thegrid at B100 km away, only soil with in 5 to10 mts of theelectrode contributes maximum resistance.electrode contributes maximum resistance.

Earth beyond, offers very minimum resistance.Earth beyond, offers very minimum resistance.

This is the concept of treating the soil around electrodeThis is the concept of treating the soil around electrodeof an earth pit.of an earth pit.


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Electric field Earth resistanceElectric field Earth resistance

Earth with its huge mass offers equi-Earth with its huge mass offers equi-potential everywherepotential everywhere

A very large charge is required to changeA very large charge is required to changeearth potential everywhereearth potential everywhere

Disturbance due to current injection at aDisturbance due to current injection at apoint is felt, only locally.point is felt, only locally.


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Substation earthingSubstation earthingDesign of Earth matDesign of Earth mat

Design depends upon the following parametersDesign depends upon the following parameters Durational and magnitude of the fault currentDurational and magnitude of the fault current Resistivity of the surface layer of the soilResistivity of the surface layer of the soil

Resistivity of the soilResistivity of the soil Magnitude of current that the human body canMagnitude of current that the human body cansafely carrysafely carry

Permissible earth potential raise that may takePermissible earth potential raise that may takeplace due to the fault conditionsplace due to the fault conditions

Shock durationShock duration Material of Earth- mat conductor.Material of Earth- mat conductor. Earth- mat geometryEarth- mat geometry


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Substation earthingSubstation earthingDesign of Earth matDesign of Earth mat

Parameters for the calculation of MaximumParameters for the calculation of Maximumpermissible step and touch potentialpermissible step and touch potential Fault duration :- Fault clearing time of back upFault duration :- Fault clearing time of back up

protection is adoptedprotection is adopted Modern protection systems provides for fastModern protection systems provides for fast

acting back up protectionacting back up protection Considerable saving can be made by optimizingConsiderable saving can be made by optimizing

the size of the conductor of earthing grid bythe size of the conductor of earthing grid by

considering lesser fault duration.considering lesser fault duration. These will change the earth potential raise due toThese will change the earth potential raise due to

which Step and Touch potentials arise.which Step and Touch potentials arise.


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Earth mat parametersEarth mat parametersLet go currentLet go current

Maximum safe current a person can tolerateMaximum safe current a person can tolerateand still release grip of an energised object,and still release grip of an energised object,using muscles affected by the currentusing muscles affected by the current

The magnitude of let go current adopted inThe magnitude of let go current adopted incalculating maximum permissible step andcalculating maximum permissible step andtouch potentials (As per IEEE 80 1976)touch potentials (As per IEEE 80 1976)for manfor man 9 milli amps 9 milli amps

for woman 6milli ampsfor woman 6milli amps


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Substation EarthingSubstation EarthingNon-fibrillation currentNon-fibrillation current

Developed by Dalziel and approved by AIEE80-1963Developed by Dalziel and approved by AIEE80-1963Magnitude of power frequency alternating current (mA) that aMagnitude of power frequency alternating current (mA) that a

human body of average weight( 50kgs to 70 kgs) can withhuman body of average weight( 50kgs to 70 kgs) can withstand without ventricular fibrillation,stand without ventricular fibrillation,

I =I =

0.116 for a body of0.116 for a body of

50kgs wt.50kgs wt.

tt

I =I =0.157 for a body of 70.157 for a body of 70kgs wt.0kgs wt.

tt

Av. Value of human body resistance (dry) 8 to 9 K-ohmsAv. Value of human body resistance (dry) 8 to 9 K-ohms

Adopted value for designing Earthing system 1KohmsAdopted value for designing Earthing system 1Kohms


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Substation EarthingSubstation EarthingNon fibrillation current contdNon fibrillation current contd

Non fibrillating current adopted for earth grid design in India.Non fibrillating current adopted for earth grid design in India.Magnitude of power frequency alternating current that aMagnitude of power frequency alternating current that ahuman body of average weight( 50kgs to 70 kgs) can withhuman body of average weight( 50kgs to 70 kgs) can withstand without ventricular fibrillation,stand without ventricular fibrillation,

I =I =0.1650.165

tt I = rms current throughI = rms current through human body in ampshuman body in amps t =durtation of shock in secondst =durtation of shock in seconds Assumption /considerations in deriving the above equationAssumption /considerations in deriving the above equation

--The duration of shock is from 8 milli-seconds to 3 seconds--The duration of shock is from 8 milli-seconds to 3 seconds


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Substation EarthingSubstation EarthingFault duration and magnitudeFault duration and magnitude

During a line to earth or double line earth faultDuring a line to earth or double line earth faultcurrent through earthing system causescurrent through earthing system causes

a)a) Heating of earthing conductorHeating of earthing conductor

b)b) Potential gradients in the soilPotential gradients in the soil For earthing design single line to ground fault isFor earthing design single line to ground fault is

considered asconsidered as Most of the faults are of this typeMost of the faults are of this type Current through earth in case of single line toCurrent through earth in case of single line to

earth fault is higher that in the later case.earth fault is higher that in the later case.


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Substation EarthingSubstation EarthingFault duration and magnitude-contd.Fault duration and magnitude-contd.

For determining maximum permissible step and touchFor determining maximum permissible step and touchpotentialspotentials

Fault duration corresponding to maximum faultFault duration corresponding to maximum fault

clearing time of back up protection relays areclearing time of back up protection relays areconsideredconsidered Normally in modern sub station clearance time ofNormally in modern sub station clearance time of

primary protection is 0.2 sec, ie., 200 milli sec andprimary protection is 0.2 sec, ie., 200 milli sec and

clearance time for back up protection is 0.5 sec, ie.,clearance time for back up protection is 0.5 sec, ie.,500 milli sec500 milli sec A fault duration time of 0.5 sec (500 mill sec) isA fault duration time of 0.5 sec (500 mill sec) is

adopted for designadopted for design


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Earthing conductor once placed underEarthing conductor once placed underearth may not be inspected normally.earth may not be inspected normally.

Prudent to make it capable of carryingPrudent to make it capable of carryingmaximum possible current for maximummaximum possible current for maximumtime.time.

If felt necessary and if it is economical,If felt necessary and if it is economical,fault duration of 1 sec can be adopted forfault duration of 1 sec can be adopted fordesign.design.

Substation EarthingFault duration and magnitude-contd.


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Substation EarthingSubstation EarthingSoil resistivitySoil resistivity

To design most economically and technicallyTo design most economically and technicallysound earthing system accurate data of soilsound earthing system accurate data of soilresistivity and its variation with in substation soilresistivity and its variation with in substation soilis essential.is essential.

Resistivity of soil in many substations has beenResistivity of soil in many substations has beenfound varying -at times between 1 and 10,000found varying -at times between 1 and 10,000ohm meters.ohm meters.

Variation in soil Resistivity with depth is moreVariation in soil Resistivity with depth is morepredominant as compared to variation inpredominant as compared to variation inhorizontal distances.horizontal distances.


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Large variations in stratification of earth layers willLarge variations in stratification of earth layers willresult in large variations in earth resistivity.result in large variations in earth resistivity.

Highly refined techniques for the determination ofHighly refined techniques for the determination of

resistivity of homogeneous soil( non uniform soil)resistivity of homogeneous soil( non uniform soil)is available.is available.

As resistivity of soil varies widely based on moistureAs resistivity of soil varies widely based on moisturecontent earth resistivity readings to be obtained incontent earth resistivity readings to be obtained in

summer or dry season.summer or dry season. Weiner's 4 electrode method is generally adopted forWeiner's 4 electrode method is generally adopted for

testing.testing.


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Substation Earthing-Substation Earthing- Soil resistivitySoil resistivityWeiner's 4 electrode methodWeiner's 4 electrode method

Earth resistivity tests shall be carried out at least inEarth resistivity tests shall be carried out at least in8 directions8 directions

If results obtained indicate wide variation, test shallIf results obtained indicate wide variation, test shallbe conducted in more number directions.be conducted in more number directions.

Four electrodes are driven into earth along aFour electrodes are driven into earth along astraight line at equal intervals.straight line at equal intervals.

Current is passed through two outer electrodes andCurrent is passed through two outer electrodes andearth.earth.

Voltage difference is measured between two innerVoltage difference is measured between two innerelectrodes.electrodes.


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Current flowing through the earth produces areCurrent flowing through the earth produces areelectric field proportional to current density andelectric field proportional to current density andresistivity of soil.resistivity of soil.

Voltage measured is proportional to the ratio ofVoltage measured is proportional to the ratio ofvoltage to the current i.e Rvoltage to the current i.e R

== 44ssRR - __- __ss____

11++ 22ss______ s+es+e

ss++4e4e


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WhereWhere= Resistivity of soil in ohm-meter= Resistivity of soil in ohm-meters= Distance between two successive electrodess= Distance between two successive electrodesin meterin meterR= Ratio of voltage to current or electrodeR= Ratio of voltage to current or electroderesistances in ohmresistances in ohme= depth of burial of electrodes in meterse= depth of burial of electrodes in meters

In case depth of burial of the electrodes in theIn case depth of burial of the electrodes in the

ground (e) is negligible compared to electrodesground (e) is negligible compared to electrodesspacing. This formula is the adjustedspacing. This formula is the adjusted ==22ssRR(This formula is normally adopted in AP Transco Ltd.)(This formula is normally adopted in AP Transco Ltd.)


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Substation EarthingSubstation EarthingMeasurement ofMeasurement of Soil resistivitySoil resistivity

There point methodThere point method Two temporary electrodes spikes are driven in to theTwo temporary electrodes spikes are driven in to the

earth at 150ft and 75ft respectively from earthearth at 150ft and 75ft respectively from earthelectrode under test.electrode under test.

Former is for current and the later is for voltage.Former is for current and the later is for voltage. Ohmic values of earth electrode resistances areOhmic values of earth electrode resistances areobtained using earth meagerobtained using earth meager

R =R = log 10 (4L/P)log 10 (4L/P) wherewhere22

RR == Electrode resistance in ohmElectrode resistance in ohmL = Length in cms of the rod driven under groundL = Length in cms of the rod driven under groundD = Dia in cms of the rodD = Dia in cms of the rod

= Earth resistivity in ohm-meter= Earth resistivity in ohm-meter


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Resistance of the earthing systemResistance of the earthing system

R =R = ++

4r4r LL

== Soil resistivity in ohm meterSoil resistivity in ohm meter

L =L = Length of conductor buried in metersLength of conductor buried in metersr =r = radius in meters of circle having the sameradius in meters of circle having the same

area as that occupied by the earth mat.area as that occupied by the earth mat.

The value of the R should be less than theThe value of the R should be less than theimpendence to ground values stated belowimpendence to ground values stated below


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Earthing SystemEarthing SystemPermissible resistance of earthing systemPermissible resistance of earthing system

Primary requirements : Impendence to ground (resistance of earthingPrimary requirements : Impendence to ground (resistance of earthingsystem)system)

Small substations 2 OhmsSmall substations 2 Ohms

EHV substations up to 220 kV 1 OhmEHV substations up to 220 kV 1 Ohm

Power stations and 400 kV substations 0.5 OhmsPower stations and 400 kV substations 0.5 Ohms Distribution transformer - 5 Ohms.Distribution transformer - 5 Ohms.

In order to avoid abnormal shift of the neutral potential, earth resistanceIn order to avoid abnormal shift of the neutral potential, earth resistanceof the station earthing system shall be normally less than or equal toof the station earthing system shall be normally less than or equal to

1ohm.1ohm.


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Substation EarthingSubstation EarthingStep and touch potentialStep and touch potential

Step potential - Difference in surface potentialsStep potential - Difference in surface potentialsexperienced by a man bridging a distance of 1experienced by a man bridging a distance of 1mt with his feet, with out contracting any othermt with his feet, with out contracting any othergrounded object.grounded object.

Touch potentialpotential difference betweenTouch potentialpotential difference betweenthe earth potential raise and the surfacethe earth potential raise and the surfacepotential at the point where a person ispotential at the point where a person isstanding touching an earthed structure.standing touching an earthed structure.

Tolerable touch potential of human body isTolerable touch potential of human body isless than tolerable step potential.less than tolerable step potential.


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Substation EarthingSubstation EarthingStep and touch potential-contdStep and touch potential-contd

In any switch yard, chances of exposure to TouchIn any switch yard, chances of exposure to Touchpotential is higher than that to step potential.potential is higher than that to step potential.

Resistance offered by the feet of a person againstResistance offered by the feet of a person againstTouch potential is much less compared to thatTouch potential is much less compared to thatagainst Step potential.against Step potential.

Hence Touch potential is more critical for designHence Touch potential is more critical for designwhile Step potential is usually academic.while Step potential is usually academic.


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Substation EarthingSubstation EarthingStep and touch potential- contd.Step and touch potential- contd.

Step potential is independent of the diameterStep potential is independent of the diameter( cross- section) of the earthing conductor.( cross- section) of the earthing conductor.

For 400% increase in diameter, reduction in TouchFor 400% increase in diameter, reduction in Touchpotential is only 35%.potential is only 35%.

Thus cross- section has minor influence on TouchThus cross- section has minor influence on Touchand Step potentials.and Step potentials.

Length of earthing conductor has significant effectLength of earthing conductor has significant effecton Touch and Step potentials.on Touch and Step potentials.


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Substation EarthingSubstation EarthingStep and touch potentialStep and touch potential

Tolerable Step and touch potentials (CBIP Publication no. 223)Tolerable Step and touch potentials (CBIP Publication no. 223) E step (LMT) = 0.E step (LMT) = 0.116 (1000+1.5Cs(hs.K.)116 (1000+1.5Cs(hs.K.)s)s)(volts)(volts)

ttE touch (LMT) = 0.E touch (LMT) = 0.116 (1000+ 6Cs.(hs.K.)116 (1000+ 6Cs.(hs.K.)s)s)(volts)(volts)tt

Where Cs= Reduction factor for de-rating normalWhere Cs= Reduction factor for de-rating normalvalue of surface layer resisvity, a function of K.value of surface layer resisvity, a function of K.

K=K= ---- ss++ ss

,, ssare resistivities of soil and surface layer respectively.are resistivities of soil and surface layer respectively.cs =1 when crushed rock has resistivity equal to that of soilcs =1 when crushed rock has resistivity equal to that of soil..

Otherwise it is derived from reference graphs ( Cs. vs hs.)Otherwise it is derived from reference graphs ( Cs. vs hs.)

hs =hs =thickness of surface layer in meter.thickness of surface layer in meter.t = Duration of shock current flow in secs.t = Duration of shock current flow in secs.


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Substation EarthingSubstation EarthingStep and touch potential-contd.Step and touch potential-contd.

Tolerable Step and touch potentials as adopted by certainTolerable Step and touch potentials as adopted by certainutilities.utilities.

E step (LMT) = IBE step (LMT) = IB((RGRG +1.5Cs.+1.5Cs.s)s)(volts)------(volts)------(1)(1)

E touch (LMT) = IBE touch (LMT) = IB ((RGRG + 6Cs.+ 6Cs.s)s)(volts) ------(volts) ------(2)(2)RG= body resistance in Ohms= 1000RG= body resistance in Ohms= 1000

IB= Permissible body current of human beings.IB= Permissible body current of human beings. Cs=Reduction factor(0 to 1)=1-(k / (2h+0.09) ------(3)Cs=Reduction factor(0 to 1)=1-(k / (2h+0.09) ------(3) k=0.09x(1-k=0.09x(1- //s)s) s= surface layer resistivity ( taken as 2000 ohm- mt.)s= surface layer resistivity ( taken as 2000 ohm- mt.) h= Thickness of gravel in cm.h= Thickness of gravel in cm. = Soil resistivity ( taken as 100 ohm- mt.)= Soil resistivity ( taken as 100 ohm- mt.)


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Substation EarthingSubstation EarthingStep and touch potential-contd.Step and touch potential-contd.

Sample calculation forSample calculation forE step (LMT) and E touch (LMT)E step (LMT) and E touch (LMT) DataData Weight of the man =70kgsWeight of the man =70kgs

Fault duration =0.5 secFault duration =0.5 sec

Resistivity Soil =Resistivity Soil = =100 ohm-mt, Surface layer ==100 ohm-mt, Surface layer =s=2000s=2000ohm-mt,ohm-mt,h= Thickness of gravel in cm.=10cmh= Thickness of gravel in cm.=10cmFrom (3), Cs=0.705From (3), Cs=0.705From table in slideFrom table in slide 2424 for a 70 kgs man and for a shock duration offor a 70 kgs man and for a shock duration of

0.5 sec0.5 sec IIB= 222mAB= 222mA From (1)From (1) E step (LMT)= 691VE step (LMT)= 691V From (2)From (2) E touch (LMT) =2100VE touch (LMT) =2100V

E thi S tE thi S t


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Methodology of design as adopted in APTranscoMethodology of design as adopted in APTranscoSize of earth mat conductor (steel strip ) Shall be :Size of earth mat conductor (steel strip ) Shall be :

A (Steel) =A (Steel) = 0.0013 x I t sq. mm for bolted joints0.0013 x I t sq. mm for bolted joints

= 0.011 x I t sq. mm for welded joints= 0.011 x I t sq. mm for welded joints

Where A = Area of Cross sectionWhere A = Area of Cross sectionI = Fault current in Amps. at the stationI = Fault current in Amps. at the station

== Fault MVA x 1000Fault MVA x 1000

3 x system kV3 x system kV

and t = Time in seconds during which current isand t = Time in seconds during which current isappliedapplied

Earthing SystemEarthing SystemSize of earth mat conductorSize of earth mat conductor


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Earthing materialsEarthing materials Determination of size of conductor for earth mat.Determination of size of conductor for earth mat.- Based on thermal stability determined by an approximateBased on thermal stability determined by an approximate

formula of IEEE - 80-1986formula of IEEE - 80-1986A = I/A = I/ ( TCAP x10( TCAP x10 44)) I nI n (K(K

oo+ T+ T

mm))

ttcc x ix irr rr (K(Koo + T+ Taa))WhereWhere

In case of steelIn case of steelA = I x 12.3A = I x 12.3 tc mm for welded jointstc mm for welded joints

= I x 15.13 tc mm for bolted joints= I x 15.13 tc mm for bolted jointsIn case tc = Duration of current =1secIn case tc = Duration of current =1sec

A = 12.3 x I mm for welded jointsA = 12.3 x I mm for welded joints= 15.3 x I mm for bolted joints= 15.3 x I mm for bolted joints


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Earthing materialsEarthing materials Based on Mechanical ruggedness of conductor and for easy installation.Based on Mechanical ruggedness of conductor and for easy installation.

Ratio of max width to thickness =7.5Ratio of max width to thickness =7.5Thickness for flat shall not be less than = 3mm (AsThickness for flat shall not be less than = 3mm (Asadopted 5to 6mm)adopted 5to 6mm)Minimum dia for steel rod = 5mmMinimum dia for steel rod = 5mm

Standard sizes of conductor as, As per IS 1730 1989Standard sizes of conductor as, As per IS 1730 1989(I)10 x 6mm(I)10 x 6mm (II)20x6mm (II)20x6mm(II)30 x 6mm (IV)40 x 6mm(II)30 x 6mm (IV)40 x 6mm(IV)50 x 6mm (VI)60 x 6mm(IV)50 x 6mm (VI)60 x 6mm

(VI)50 x 8mm (VIII)65 x 8mm(VI)50 x 8mm (VIII)65 x 8mm(IX)75 x 12mm (X)100 x 16mm(IX)75 x 12mm (X)100 x 16mm- For 33kV Substations 75x8mm and 50x6mm- For 33kV Substations 75x8mm and 50x6mm


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Earthing materialsEarthing materialsUp to 220 kV substationUp to 220 kV substation Earth matEarth mata)a) Peripheral or main earth mat : 100x 16m MS flatPeripheral or main earth mat : 100x 16m MS flatb)b) Internal earth matInternal earth mat : 50x8m MS flat placed at 5 m apart: 50x8m MS flat placed at 5 m apartc)c) Branch connectionsBranch connections : cross section not less than 64.5 sq.m: cross section not less than 64.5 sq.md)d) Raisers : 50x8m MS flatRaisers : 50x8m MS flat

For 400 kV substationFor 400 kV substation Earth matEarth mata)a) Peripheral or main earth mat :40mm dia MS rod of 3mt. lengthPeripheral or main earth mat :40mm dia MS rod of 3mt. lengthb)b) Internal earth matInternal earth mat 50x8mm MS flat placed at 5m apart50x8mm MS flat placed at 5m apartc)c) Raisers : 50x8m MS flatRaisers : 50x8m MS flat

Where necessary, 40mm rods will be driven in to earth vertically along theWhere necessary, 40mm rods will be driven in to earth vertically along theperiphery of the earth mat.periphery of the earth mat.


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Pipe earthingPipe earthinga)a) EHT Substations :EHT Substations :(i) Cast iron pipes 125 mm(i) Cast iron pipes 125 mm

in diameterin diameter 2.75 m long and not less2.75 m long and not lessthan 9.5 mmthan 9.5 mm thick.thick.(ii) Pipes 50.8 mm in dia and 3.05 m long(ii) Pipes 50.8 mm in dia and 3.05 m long

1. Joints are to be kept down to the minimum1. Joints are to be kept down to the minimumnumbernumber2.2. All joints and connections in earth grid are to beAll joints and connections in earth grid are to be

brazed, riveted, sweated, bolted or welded.brazed, riveted, sweated, bolted or welded.

3.3. For rust protection welds shall be treated withFor rust protection welds shall be treated withBarium chromate.Barium chromate.


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EarthingEarthing

2.2. Welded surfaces to be painted with red lead andWelded surfaces to be painted with red lead andaluminium paint and then with bitumen.aluminium paint and then with bitumen.

3.3. Joints to be broken periodically shall be boltedJoints to be broken periodically shall be bolted

and joint faces tinned.and joint faces tinned.4.4. All exposed steel earthing conductors should beAll exposed steel earthing conductors should be

protected with bituminous paintprotected with bituminous paint

5.5. All joints in steel earthing system shall be weldedAll joints in steel earthing system shall be welded

except joints to be removed for testing shall beexcept joints to be removed for testing shall bebolted.bolted.


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Earthing systemEarthing systemLowering of earth impedanceLowering of earth impedance

2)2) Lowering of earth impedanceLowering of earth impedanceIn places where soil resistivity is high steps to be taken to reduceIn places where soil resistivity is high steps to be taken to reduceearth impedance by one or combination of following:-earth impedance by one or combination of following:-

a.a. Connection of substation grid with a remote ground grid andConnection of substation grid with a remote ground grid andadjacent grounding facilities.adjacent grounding facilities.

b.b. Use of deep driven ground rods or longer ground rods or maximumUse of deep driven ground rods or longer ground rods or maximumnumber of ground rods along the perimeter of the earth grid.number of ground rods along the perimeter of the earth grid.c.c. Use of foundation rods as auxiliary grids where feasibleUse of foundation rods as auxiliary grids where feasibled.d. Formation of auxiliary grids if soil of low earth resistivity is availableFormation of auxiliary grids if soil of low earth resistivity is available

close byclose bye.e. Max. touch potential occurs in the corner of mesh of the grid. NoMax. touch potential occurs in the corner of mesh of the grid. No

equipment are to be kept in such areas. higher values of touchequipment are to be kept in such areas. higher values of touchpotential than the tolerable limit can be accepted if step potentialpotential than the tolerable limit can be accepted if step potentialare within permissible limitsare within permissible limits

f.f. If equipment is to be kept at corners of the mesh. Auxiliary grids areIf equipment is to be kept at corners of the mesh. Auxiliary grids areto be created at those corner to limit touch potential.to be created at those corner to limit touch potential.


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Earthing SystemEarthing SystemEarthing of switch yard fencingEarthing of switch yard fencing

Two methods of fence earthingTwo methods of fence earthinga) Extension of substation earth grid up to 0.5 to 1.5 m beyond thea) Extension of substation earth grid up to 0.5 to 1.5 m beyond the

fence, bonding the fence to the grid at regular intervals.fence, bonding the fence to the grid at regular intervals.b) Keeping the fence beyond the perimeter of the switch yardb) Keeping the fence beyond the perimeter of the switch yard

earthing grid, providing its own earthing system not connectingearthing grid, providing its own earthing system not connectingto the main earthing gridto the main earthing grid..

In the former case substantial reduction in the effectiveIn the former case substantial reduction in the effectivesubstation earthing resistance is possible but at additional cost.substation earthing resistance is possible but at additional cost.

In the later case any inadvertent connection could give rise toIn the later case any inadvertent connection could give rise todangerous potential under fault condition unless special care isdangerous potential under fault condition unless special care istaken.taken.

Electrical isolation of fence into short section with individualElectrical isolation of fence into short section with individual

earthing is required where fence is closer to a single phaseearthing is required where fence is closer to a single phasereactor or an electrical plant generating large electromagneticreactor or an electrical plant generating large electromagneticfields.fields.


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Earthing SystemEarthing SystemEarthing of switch yard fencing- conEarthing of switch yard fencing- con

Methods of earthing of fencing As per CBIP reportMethods of earthing of fencing As per CBIP report A.A. Design permits extension of earth mat within 1.5mt inside perimeterDesign permits extension of earth mat within 1.5mt inside perimeter

fencingfencing Electrical isolation of fencing can be ensuredElectrical isolation of fencing can be ensured Isolate fencing for earth matIsolate fencing for earth mat

Running of independent earth conductor underneath boundary andRunning of independent earth conductor underneath boundary andconnecting it to fencing at frequent intervals.connecting it to fencing at frequent intervals.

B.B. Design permits extension of earth mat up to fencingDesign permits extension of earth mat up to fencing Calculated touch potential within safe limitCalculated touch potential within safe limit

Extending the earth mat up to perimeter fencing and connecting theExtending the earth mat up to perimeter fencing and connecting thefencing at frequent intervals to earth matfencing at frequent intervals to earth mat Spreading crushed metal 1.5mt beyond fencingSpreading crushed metal 1.5mt beyond fencing


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Earthing SystemEarthing SystemEarthing of switch yard fencing- conEarthing of switch yard fencing- con

C.C.

Design permits extension of earth mat up toDesign permits extension of earth mat up tofencingfencing

Calculated touch potential beyond the fenceCalculated touch potential beyond the fenceabove the permissible limit for touch potentialabove the permissible limit for touch potential

Termination of earth mat within 1.5 mt ofTermination of earth mat within 1.5 mt offencingfencing

Fence electrically isolated and independentlyFence electrically isolated and independentlyearthed by running an earthed conductorearthed by running an earthed conductorunderneath the fence connecting it to the fenceunderneath the fence connecting it to the fenceat frequent intervalsat frequent intervals


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Earthing of gas insulated substationEarthing of gas insulated substation

In GIS multi-components like buses, switch gearIn GIS multi-components like buses, switch gearassociated equipment are present in an earthedassociated equipment are present in an earthedmetallic housingmetallic housing

They are subjected to same magnitude of faultThey are subjected to same magnitude of faultcurrent and require low impendence earthingcurrent and require low impendence earthing

Compared to a conventional substation, as GISCompared to a conventional substation, as GISrequires only 25% of land area design of earthrequires only 25% of land area design of earth

mat is comparatively difficult.mat is comparatively difficult.

Metallic enclosures of GIS have inducedMetallic enclosures of GIS have inducedcurrents, specially during internal earth faults.currents, specially during internal earth faults.


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Earthing of gas insulated substationEarthing of gas insulated substation

Inductive voltage drop occurring with GIS assembly shall beInductive voltage drop occurring with GIS assembly shall betaken into account for the design of earth mattaken into account for the design of earth mat

Touch voltage criteria =Touch voltage criteria = (F(FAA))22+(E+(EGG))22 < E< ETT (max)(max)Where FWhere FAA = Actually calculated touch voltage= Actually calculated touch voltage

EEGG= Max value of metal to metal voltage difference= Max value of metal to metal voltage difference

on and between GIS enclosures oron and between GIS enclosures orbetweenbetween GIS enclosures and supportingGIS enclosures and supportingstructuresstructures

EETT (max) = maximum permissible touch(max) = maximum permissible touchvoltagevoltage

Metallic enclosures of GIS may be continuous or notMetallic enclosures of GIS may be continuous or not

In either case provision of earth bond frequently is essential toIn either case provision of earth bond frequently is essential tominimize hazards of touch potentialminimize hazards of touch potential In addition, earthing of GIS structures and service platforms atIn addition, earthing of GIS structures and service platforms at

frequent intervals are to be done.frequent intervals are to be done.

S b i E hi


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Substation EarthingSubstation EarthingCase studiesCase studies

Karimnagar132kV ssKarimnagar132kV ss Kamalapuram 132kV ss fencing givingKamalapuram 132kV ss fencing giving

shockshockAuxiliary Earth grid at RTPPAuxiliary Earth grid at RTPP


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Documents

92113333 Earthing Concepts