EARTHING CONCEPTSEARTHING CONCEPTS

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Earthing in a EHV SubstationEarthing in a EHV Substation

Providing adequate ‘Earthing’ in a Providing adequate ‘Earthing’ in a substation is an important safety measure.substation is an important safety measure.

Earthing means connecting the electrical Earthing means connecting the electrical equipment to the general mass of earth of equipment to the general mass of earth of low resistance.low resistance.

Objective is to provide under and around Objective is to provide under and around the substation a surface of uniform the substation a surface of uniform potential potential

-- 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 within The touch and step potential shall be within

limits under all conditions including fault limits under all conditions including fault conditioncondition

Grounding resistance shall be lower.Grounding resistance shall be lower. Effective earthing system shall aim at Effective earthing system shall aim at

providing protection to life and property providing protection to life and property against dangerous potentials under fault against dangerous potentials under fault conditionsconditions

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Earthing in a EHV Substation Earthing in a EHV Substation I.E.Rules 1956I.E.Rules 1956

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

lightning shall adopt efficient means for diverting lightning shall adopt efficient means for diverting the electrical surges due to lightning to earththe electrical surges due to lightning to earth

Earth lead of any lightning arrestor shall not pass Earth lead of any lightning arrestor shall not pass through any iron or steel pipe.through any iron or steel pipe.

It shall be taken directly, as far as possible, to a It shall be taken directly, as far as possible, to a separate earth electrode and/or junction of the separate earth electrode and/or junction of the earth mat.earth mat.

Bends Shall be avoided where ever practicableBends Shall be avoided where ever practicable Earth screen if provided for lightning protection Earth screen if provided for lightning protection

shall be connected to main earth grid. shall be connected to main earth grid.

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Earthing in a EHV Substation Earthing in a EHV Substation I.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 The passage of fault current does not result in any thermal

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

Every exposed conductor part and extraneous conductive Every exposed conductor part and extraneous conductive part may be connected to the earth.part 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 transfer No dangerous potential gradients (step or touch or transfer

potentials) shall occur under normal or abnormal operating potentials) shall occur under normal or abnormal operating conditionsconditions

To minimize electromagnetic interference between power To minimize electromagnetic interference between power and control/ communication systemand control/ 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 system The neutral point of each separate system

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

Equipment frame work and other non-current Equipment frame work and other non-current parts (two connections)parts (two connections)

All extraneous metallic frame works not All extraneous metallic frame works not associated with equipment ( two connections)associated with equipment ( two connections)

Lightning arrestors should have independent Lightning arrestors should have independent earths, in turn connected to the station earths, in turn connected to the station grounding grid.grounding grid.

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Earthing SystemEarthing SystemPoints to be earthed-cont’dPoints to be earthed-cont’d

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

Operating handles of Isolators with a auxiliary Operating handles of Isolators with a auxiliary earth mat underneath, if necessary.earth mat underneath, if necessary.

Peripheral fencingPeripheral fencing Buildings inside the switch yard.Buildings inside the switch yard. Transformer Neutrals shall be connected Transformer Neutrals shall be connected

directly to the earth electrode by two directly to the earth electrode by two independent MS stripsindependent MS strips

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Earthing and grounding -distinction Earthing and grounding -distinction

Grounding:- connection of current carrying parts Grounding:- connection of current carrying parts to 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 the In 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 ground In solidly grounded system substantial ground

fault current flows enabling fault detection and fault current flows enabling fault detection and faster clearance. faster clearance.

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Earthing and grounding -distinctionEarthing and grounding -distinction

Earthing:- connection of non current Earthing:- connection of non current carrying parts to ground. Ex : Metallic carrying parts to ground. Ex : Metallic enclosure.enclosure.

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

balanced power system conditions.balanced power system conditions. Under ground fault conditions, enables Under ground fault conditions, enables

ground fault current to return back to ground fault current to return back to source 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 resistance GI of 65x10mm section will have same resistance

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

(158acres)(158acres) Metallic conductor is a preferred alternative to Metallic 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 Current flows through a series of hemi-spherical

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

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

If current is discharged from a grid towards another If current is discharged from a grid towards another grid at B100 km away, only soil with in 5 to10 mts of grid at B100 km away, only soil with in 5 to10 mts of the electrode contributes maximum resistance.the 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 This is the concept of treating the soil around electrode of an earth pit. electrode 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 change A very large charge is required to change earth potential everywhereearth potential everywhere

Disturbance due to current injection at a Disturbance due to current injection at a point 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 can Magnitude of current that the human body can

safely carrysafely carry• Permissible earth potential raise that may take Permissible earth potential raise that may take

place 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 Maximum Parameters for the calculation of Maximum permissible step and touch potentialpermissible step and touch potential Fault duration :- Fault clearing time of back up Fault duration :- Fault clearing time of back up

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

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

the size of the conductor of earthing grid by the size of the conductor of earthing grid by considering lesser fault duration.considering lesser fault duration.

These will change the earth potential raise due to These 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 Maximum safe current a person can tolerate and still release grip of an tolerate and still release grip of an energised object, using muscles affected energised object, using muscles affected by the currentby the current

The magnitude of let go current adopted in The magnitude of let go current adopted in calculating maximum permissible step and calculating maximum permissible step and touch potentials (As per IEEE – 80 – 1976)touch potentials (As per IEEE – 80 – 1976)

for man for man – 9 milli amps – 9 milli ampsfor woman – 6milli ampsfor woman – 6milli amps

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Substation EarthingSubstation Earthing Non-fibrillation current Non-fibrillation current

Developed by Dalziel and approved by AIEE80-1963Developed by Dalziel and approved by AIEE80-1963 Magnitude of power frequency alternating current Magnitude of power frequency alternating current (mA) that a human body of average weight( 50kgs to 70 (mA) that a human body of average weight( 50kgs to 70 kgs) can with stand without ventricular fibrillation, kgs) can with stand without ventricular fibrillation,

I =I =0.116 for a body of 0.116 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-Av. Value of human body resistance (dry) – 8 to 9 K-ohmsohms

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

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Substation EarthingSubstation Earthing Non fibrillation current– contd Non fibrillation current– contd

Non fibrillating current adopted for earth grid design Non fibrillating current adopted for earth grid design in India.in India.

Magnitude of power frequency alternating current Magnitude of power frequency alternating current that a human body of average weight( 50kgs to 70 that a human body of average weight( 50kgs to 70 kgs) can with stand without ventricular fibrillation, kgs) can with stand without ventricular fibrillation,

I =I =0.165 0.165 √ √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 Assumption /considerations in deriving the above

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

secondsseconds

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Substation EarthingSubstation Earthing Fault duration and magnitude Fault duration and magnitude

During a line to earth or double line earth fault During a line to earth or double line earth fault current 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 For earthing design single line to ground fault

is considered asis considered as Most of the faults are of this typeMost of the faults are of this type Current through earth in case of single line Current through earth in case of single line

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

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

For determining maximum permissible step and For determining maximum permissible step and touch potentialstouch potentials

Fault duration corresponding to maximum fault Fault duration corresponding to maximum fault clearing time of back up protection relays are clearing time of back up protection relays are consideredconsidered

Normally in modern sub station clearance time Normally in modern sub station clearance time of primary protection is 0.2 sec, ie., 200 milli sec of primary protection is 0.2 sec, ie., 200 milli sec and clearance time for back up protection is 0.5 and clearance time for back up protection is 0.5 sec, ie., 500 milli secsec, ie., 500 milli sec

A fault duration time of 0.5 sec (500 mill sec) is A fault duration time of 0.5 sec (500 mill sec) is adopted for designadopted for design

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Earthing conductor once placed under Earthing conductor once placed under earth may not be inspected normally.earth may not be inspected normally.

Prudent to make it capable of carrying Prudent to make it capable of carrying maximum possible current for maximum maximum possible current for maximum time.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 for fault duration of 1 sec can be adopted for design.design.

Substation Earthing Fault duration and magnitude-contd.

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Substation EarthingSubstation Earthing Soil resistivity Soil resistivity

To design most economically and technically To design most economically and technically sound earthing system accurate data of soil sound earthing system accurate data of soil resistivity and its variation with in substation soil resistivity and its variation with in substation soil is essential.is essential.

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

Variation in soil Resistivity with depth is more Variation in soil Resistivity with depth is more predominant as compared to variation in predominant as compared to variation in horizontal distances.horizontal distances.

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Substation EarthingSubstation Earthing Soil resistivitySoil resistivity

Large variations in stratification of earth layers will Large variations in stratification of earth layers will result in large variations in earth resistivity.result in large variations in earth resistivity.

Highly refined techniques for the determination of Highly 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 moisture As resistivity of soil varies widely based on moisture content earth resistivity readings to be obtained in content earth resistivity readings to be obtained in summer or dry season.summer or dry season.

Weiner's 4 electrode method is generally adopted for Weiner's 4 electrode method is generally adopted for testing.testing.

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Substation Earthing- Substation Earthing- Soil resistivitySoil resistivity Weiner's 4 electrode methodWeiner's 4 electrode method

Earth resistivity tests shall be carried out at least Earth resistivity tests shall be carried out at least in 8 directionsin 8 directions

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

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

Current is passed through two outer electrodes Current is passed through two outer electrodes and earth.and earth.

Voltage difference is measured between two Voltage difference is measured between two inner electrodes.inner electrodes.

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Substation EarthingSubstation Earthing Soil resistivitySoil resistivity

Current flowing through the earth Current flowing through the earth produces are electric field proportional to produces are electric field proportional to current density and resistivity of soil.current density and resistivity of soil.

Voltage measured is proportional to the Voltage measured is proportional to the ratio of voltage to the current i.e Rratio of voltage to the current i.e R

ρρ== 44ssΠΠRR - __ - __ss____

11+ + 22ss___ ___ √s²+e²√s²+e²

√ √s²s²++4e4e²²

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Substation EarthingSubstation Earthing Soil resistivitySoil resistivity

Where Where ρρ= Resistivity of soil in ohm-meter= Resistivity of soil in ohm-meters= Distance between two successive electrodes s= Distance between two successive electrodes in meterin meterR= Ratio of voltage to current or electrode R= Ratio of voltage to current or electrode resistances 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 the In case depth of burial of the electrodes in the ground (e) is negligible compared to electrodes ground (e) is negligible compared to electrodes spacing. This formula is the adjusted spacing. This formula is the adjusted ρρ==22ΠΠssR R

(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 of Measurement of Soil resistivitySoil resistivity

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

earth at 150ft and 75ft respectively from earth earth at 150ft and 75ft respectively from earth electrode 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 are Ohmic values of earth electrode resistances are

obtained using earth meager obtained using earth meager R = R = ρρ log 10 (4L/P) log 10 (4L/P) where where

2 2 ΠΠ RR = = Electrode resistance in ohmElectrode resistance in ohm L = Length in cms of the rod driven under groundL = Length in cms of the rod driven under ground D = Dia in cms of the rodD = Dia in cms of the rod

ρρ = 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 = ρρ + + ρρ

4r4r L L

ρρ = = Soil resistivity in ohm meterSoil resistivity in ohm meter

L = L = Length of conductor buried in metersLength of conductor buried in meters

r =r = radius in meters of circle having the same radius 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 the The value of the R should be less than the impendence 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 Primary requirements : Impendence to ground (resistance of earthing system)(resistance of earthing 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 In order to avoid abnormal shift of the neutral

potential, earth resistance of the station earthing potential, earth resistance of the station earthing system shall be normally less than or equal to 1ohm.system shall be normally less than or equal to 1ohm.

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Substation EarthingSubstation Earthing Step and touch potential Step and touch potential

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

Touch potential- potential difference between Touch potential- potential difference between the earth potential raise and the surface the earth potential raise and the surface potential at the point where a person is potential at the point where a person is standing touching an earthed structure.standing touching an earthed structure.

Tolerable touch potential of human body is Tolerable touch potential of human body is less than tolerable step potential.less than tolerable step potential.

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Substation EarthingSubstation Earthing Step and touch potential-contd Step and touch potential-contd

In any switch yard, chances of exposure to In any switch yard, chances of exposure to ‘Touch potential’ is higher than that to ‘step ‘Touch potential’ is higher than that to ‘step potential’.potential’.

Resistance offered by the feet of a person Resistance offered by the feet of a person against ‘Touch potential’ is much less against ‘Touch potential’ is much less compared to that against ‘Step potential’.compared to that against ‘Step potential’.

Hence ‘Touch potential ’ is more critical for Hence ‘Touch potential ’ is more critical for design while Step potential is usually design while Step potential is usually academic.academic.

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

Step potential is independent of the diameter Step 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 Touch For 400% increase in diameter, reduction in Touch potential is only 35%.potential is only 35%.

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

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

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Substation EarthingSubstation Earthing Step and touch potential Step 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.)ρρs) s) (volts)(volts)

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

√ √tt Where Cs= Reduction factor for de-rating normal Where Cs= Reduction factor for de-rating normal

value of surface layer resisvity, a value of surface layer resisvity, a function of K.function of K.

K=K= ρρ-- -- ρρss ρρ+ + ρρss ρρ, , ρρss are 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 Earthing Step and touch potential-contd. Step and touch potential-contd.

Tolerable Step and touch potentials as adopted by certain Tolerable Step and touch potentials as adopted by certain utilities.utilities.

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

E touch (LMT) = IBE touch (LMT) = IB ( (RGRG + 6Cs. + 6Cs.ρρs) s) (volts) ------(volts) ------(2)(2)RG= body resistance in Ohms= 1000 RG= 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- ρρ//ρρs)s) ρρs= 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. ρρ= Soil resistivity ( taken as 100 ohm- mt.)= Soil resistivity ( taken as 100 ohm- mt.)

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

Sample calculation for Sample calculation for E step (LMT) and E touch (LMT)E step (LMT) and E touch (LMT) DataData Weight of the man =70kgs Weight of the man =70kgs Fault duration =0.5 secFault duration =0.5 sec

Resistivity Soil = Resistivity Soil = ρρ=100 ohm-mt, Surface layer ==100 ohm-mt, Surface layer =ρρs=2000 s=2000 ohm-mt,ohm-mt,

h= Thickness of gravel in cm.=10cmh= Thickness of gravel in cm.=10cm From (3), Cs=0.705From (3), Cs=0.705 From table in slideFrom table in slide 24 24 for a 70 kgs man and for a shock duration of for a 70 kgs man and for a shock duration of

0.5 sec 0.5 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

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Methodology of design as adopted in APTranscoMethodology of design as adopted in APTransco

Size 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 joints 0.0013 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 section

I = 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 is

appliedapplied

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 approximate Based 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 n I n (K(Ko o + T+ Tmm))

ttc c x ix iØØrr ρρr r (K(Ko o + T+ Taa)) WhereWhere In case of steelIn case of steel

A = I x 12.3 A = I x 12.3 √tc mm² for welded joints√tc mm² for welded joints = I x 15.13 √tc mm² for bolted joints= I x 15.13 √tc mm² for bolted joints

In case tc = Duration of current =1secIn case tc = Duration of current =1secA = 12.3 x I mm² for welded jointsA = 12.3 x I mm² for welded joints = 15.3 x I mm² for bolted joints= 15.3 x I mm² 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 (As Thickness for flat shall not be less than = 3mm (As adopted 5to 6mm)adopted 5to 6mm)

Minimum dia for steel rod = 5mm Minimum 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 6mm² (II)20x6mm²² (II)20x6mm²(II)30 x 6mm² (IV)40 x 6mm²(II)30 x 6mm² (IV)40 x 6mm²

(IV)50 x 6mm² (VI)60 x 6mm²(IV)50 x 6mm² (VI)60 x 6mm² (VI)50 x 8mm² (VIII)65 x 8mm²(VI)50 x 8mm² (VIII)65 x 8mm²

(IX)75 x 12mm² (X)100 x 16mm²(IX)75 x 12mm² (X)100 x 16mm²- For 33kV Substations 75x8mm and 50x6mm- For 33kV Substations 75x8mm and 50x6mm

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Earthing materials Earthing materials Up 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 mat Internal earth mat : 50x8m MS flat placed at 5 m apart : 50x8m MS flat placed at 5 m apartc)c) Branch connections Branch connections : cross section not less than 64.5 sq.m : cross section not less than 64.5 sq.md)d) Raisers : 50x8m MS flat Raisers : 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 mat Internal earth mat 50x8mm MS flat placed at 5m apart 50x8mm MS flat placed at 5m apart c)c) Raisers : 50x8m MS flatRaisers : 50x8m MS flat Where necessary, 40mm rods will be driven in to earth vertically along Where necessary, 40mm rods will be driven in to earth vertically along

the periphery of the earth mat.the periphery of the earth mat.

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

mm in diameter mm in diameter 2.75 2.75 m long and not less than 9.5 mm m long and not less than 9.5 mm

thick. thick. (ii) Pipes 50.8 mm in dia (ii) Pipes 50.8 mm in dia

and 3.05 m longand 3.05 m long

1. Joints are to be kept down to the minimum 1. Joints are to be kept down to the minimum numbernumber

2.2. All joints and connections in earth grid are to be All 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 with For rust protection welds shall be treated with Barium chromate.Barium chromate.

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EarthingEarthing

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

3.3. Joints to be broken periodically shall be bolted Joints to be broken periodically shall be bolted and joint faces tinned.and joint faces tinned.

4.4. All exposed steel earthing conductors should All exposed steel earthing conductors should be protected with bituminous paintbe protected with bituminous paint

5.5. All joints in steel earthing system shall be All joints in steel earthing system shall be welded except joints to be removed for testing welded except joints to be removed for testing shall be bolted. shall be bolted.

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Earthing system Earthing system Lowering of earth impedance Lowering of earth impedance

2)2) Lowering of earth impedanceLowering of earth impedance In places where soil resistivity is high steps to be taken to reduce In places where soil resistivity is high steps to be taken to reduce

earth 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 and Connection of substation grid with a remote ground grid and adjacent grounding facilities. adjacent grounding facilities.

b.b. Use of deep driven ground rods or longer ground rods or maximum Use of deep driven ground rods or longer ground rods or maximum number 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 available Formation 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. No Max. touch potential occurs in the corner of mesh of the grid. No

equipment are to be kept in such areas. higher values of touch equipment are to be kept in such areas. higher values of touch potential than the tolerable limit can be accepted if step potential potential than the tolerable limit can be accepted if step potential are within permissible limits are within permissible limits

f.f. If equipment is to be kept at corners of the mesh. Auxiliary grids are If equipment is to be kept at corners of the mesh. Auxiliary grids are to 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 fencing Earthing of switch yard fencing

Two methods of fence earthing Two methods of fence earthing a) Extension of substation earth grid up to 0.5 to 1.5 m beyond the a) 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 yard b) Keeping the fence beyond the perimeter of the switch yard

earthing grid, providing its own earthing system not earthing grid, providing its own earthing system not connecting to the main earthing gridconnecting to the main earthing grid..

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

In the later case any inadvertent connection could give rise to In the later case any inadvertent connection could give rise to dangerous potential under fault condition unless special care dangerous potential under fault condition unless special care is taken.is taken.

Electrical isolation of fence into short section with individual Electrical isolation of fence into short section with individual earthing is required where fence is closer to a single phase earthing is required where fence is closer to a single phase reactor or an electrical plant generating large electromagnetic reactor or an electrical plant generating large electromagnetic fields.fields.

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Earthing SystemEarthing SystemEarthing of switch yard fencing- con…Earthing 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 Design permits extension of earth mat within 1.5mt inside perimeter fencingperimeter fencing

• 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 Running of independent earth conductor underneath boundary

and connecting it to fencing at frequent intervals.and 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 Extending the earth mat up to perimeter fencing and connecting

the fencing at frequent intervals to earth matthe fencing 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- con…Earthing of switch yard fencing- con…

C.C.

• Design permits extension of earth mat up to Design permits extension of earth mat up to fencingfencing

• Calculated touch potential beyond the fence Calculated touch potential beyond the fence above the permissible limit for touch potential above the permissible limit for touch potential

• Termination of earth mat within 1.5 mt of Termination of earth mat within 1.5 mt of fencingfencing

• Fence electrically isolated and independently Fence electrically isolated and independently earthed by running an earthed conductor earthed by running an earthed conductor underneath the fence connecting it to the fence underneath the fence connecting it to the fence at frequent intervalsat frequent intervals

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Earthing of gas insulated substationEarthing of gas insulated substation

In GIS multi-components like buses, switch gear In GIS multi-components like buses, switch gear associated equipment are present in an earthed associated equipment are present in an earthed metallic housingmetallic housing

They are subjected to same magnitude of fault They are subjected to same magnitude of fault current and require low impendence earthingcurrent and require low impendence earthing

Compared to a conventional substation, as GIS Compared to a conventional substation, as GIS requires only 25% of land area design of earth requires only 25% of land area design of earth mat is comparatively difficult.mat is comparatively difficult.

Metallic enclosures of GIS have induced Metallic enclosures of GIS have induced currents, 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 be Inductive voltage drop occurring with GIS assembly shall be taken 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 FA A = 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 or on and between GIS enclosures or

between between GIS enclosures and GIS enclosures and supporting structuressupporting structures

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

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 to In either case provision of earth bond frequently is essential to

minimize hazards of touch potentialminimize hazards of touch potential In addition, earthing of GIS structures and service platforms at In addition, earthing of GIS structures and service platforms at

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

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Substation EarthingSubstation EarthingCase studiesCase studies

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

shock shock Auxiliary Earth grid at RTPPAuxiliary Earth grid at RTPP

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Thank youThank you