7-Grounding Systems.pdf

8/14/2019 7-Grounding Systems.pdf

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GROUNDING SYSTEMSGROUNDING SYSTEMS

Professor Ahdab ElmorshedyProfessor Ahdab Elmorshedy

Cairo University, Faculty of EngineeringCairo University, Faculty of Engineering

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What is Grounding?

Definition: (IEEE Standard 100) IEEEDictionary

A conducting connection, whetherintentional or accidental, by which anelectric circuit or equipment is connected to

the Earth, or to some conducting body ofrelatively large extent that serves in place ofthe Earth.


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The objective of a grounding system are:

1. To provide safety to personnel during normaland fault conditions by limiting step and touch

potential.

2. To assure correct operation of electrical/electronic devices.

3. To prevent damage to electrical/ electronicapparatus.

4. To dissipate lightning strokes.

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5. To stabilize voltage during transientconditions and to minimize the probability

of flashover during transients.

6. To divert stray RF energy from sensitiveaudio, video, control, and computer

equipment.


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A safe grounding design has two objectives:

1. To provide means to carry electric currentsinto the earth under normal and fault

conditions without exceeding anyoperating and equipment limits.

2. To assure that a person in the vicinity ofgrounded facilities is not exposed to the

danger of critical electric shock.

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They are:

equipmentequipment groundsgrounds (Safety)(Safety)

systemssystems groundsgrounds

lightninglightning groundsgrounds

maintenancemaintenance groundsgroundsstaticstatic groundsgrounds

electronicelectronic groundsgrounds

There are basically six groundingThere are basically six groundingsystems in usesystems in use


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Equipment GroundingProvide Personnel Safety

Interconnecting all non-current carrying

metal components to eliminate potentialdifferences between them.

Connecting the equipment grounding

conductor to Earth will eliminate potentialdifferences between metal components andEarth.


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Equipment Grounding

(Personnel Safety)

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System GroundingSystem Grounding

SystemSystem groundinggrounding meansmeans thethe connectionconnection

ofof groundground toto thethe neutralneutral pointspoints ofofcurrentcurrent carryingcarrying conductorsconductors suchsuch asas

thethe neutralneutral pointpoint ofof aa circuit,circuit, aatransformer,transformer, rotatingrotating machinery,machinery, oror aa

system,system, eithereither solidlysolidly oror withwith aa currentcurrent

limitinglimiting devicedevice..


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System Grounding

Provide equipment protection by operatingover-current devices to clear fault current

and providing a potential reference.

An intentional connection to equipment

ground from one of the current carryingconductors of an electrical distribution

system.

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System Grounding

(Equipment Protection)


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Lightning Protection System

Multiple rooftop air (lightning) terminals

Down conductors

Equalizing conductors

Air terminals that surround a building

for the exclusive purpose of intercepting,diverting, and dissipating direct

lightning strikes

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Lightning!

Factoids: ~ 200 M voltsV

~ 40,000 ampI

~ 30mst12~ 10 WP

____

++++

________

++++ ++++

Collisions producecharged particles.

The heavierparticles (-) sit

near the bottom ofthe cloud; the

lighter particles (+)near the top.

SteppedLeader

Negativelycharged

electronsbegin

zigzaggingdownward.

AttractionAs the steppedleader nears theground, it draws

a streamer ofpositive charge

upward.

FlowingCharge

As the leaderand the

streamer cometogether,powerful

electric currentbegins flowing

Contact!Intensewave ofpositivecharge, areturnstroke,travels

upward at108 m/s

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Stepped LeaderStepped Leader


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StreamersStreamers

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A TreeEXPLODING !!

Farm House

Lightning Power


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Lightning killed 504 sheep when a stroke hit the ground in this rocky

Pasture in Utah. A stroke to high-resistance soil can produce exceptionally

high voltage in the ground and spread over an unusually large area.

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Ground Electrode System

Soil

Ground Electrode Conductors

Connectors

Electrodes


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Soil CharacteristicsSoil Characteristics

TheThe soilsoil resistivityresistivityisis thethe singlesingle mostmost importantimportantfactorfactor affectingaffecting thethe resistanceresistance ofof thethe groundground

systemsystem..

SoilSoil typetype.. SoilSoil resistivityresistivity variesvaries widelywidely

dependingdepending onon soilsoil type,type, fromfrom asas lowlow asas11.mforfor moistmoistloamyloamy topsoiltopsoil toto almostalmost1010,,000000.m

forforsurfacesurface limestonelimestone..

MoistureMoisture contentcontent isis oneone ofof thethe controllingcontrollingfactorsfactors inin earthearth resistanceresistance becausebecause electricalelectrical

conductionconduction inin soilsoil isis essentiallyessentially electrolyticelectrolytic..

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Resistivities of Different Soils


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Typical variation in soil resistivity as a function of

moisture, temperature and salt content

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TheThe temperaturetemperature effecteffect onon soilsoil resistivityresistivity isis

almostalmost negligiblenegligible forfor temperaturestemperatures aboveabove thethefreezingfreezing pointspoints..

WhenWhen temperaturetemperature dropsdrops belowbelow waterwaterfreezingfreezing point,point, thethe resistivityresistivity increasesincreases

rapidlyrapidly..

CompactnessCompactness andand granularitygranularity affectsaffects soilsoil

resistivityresistivity inin thatthat denserdenser soilssoils generallygenerally havehavelowerlower resistivityresistivity..


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TheThe twotwofactorsfactors -- moisturemoisture andand saltsalt contentcontent --areare thethe mostmost influentialinfluential onesones onon soilsoil

resistivityresistivity forfor aa givengiven typetype ofof soilsoil..

ThereforeTherefore thethe chemicalchemical treatmenttreatment ofof soilsoil

surroundingsurrounding groundground rodsrods isis preferablepreferable andandinin somesome casescases thethe onlyonly economicallyeconomically soundsound

solutionsolution inin obtainingobtaining lowlow impedanceimpedance ofof thethegroundground systemsystem..

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TYPES OF GROUND ELECTRODESTYPES OF GROUND ELECTRODES

GroundGround electrodeselectrodes mustmust penetratepenetrate intointo thethe

moisturemoisture levellevel belowbelow thethe groundground levellevel..

TheyThey consistconsist ofof aa metalmetal whichwhich dodo notnot corrodecorrode..

BecauseBecause ofof itsits highhigh conductivityconductivity andand resistanceresistance

toto corrosioncorrosion,, coppercopper isis thethe mostmost commonlycommonlyusedused materialmaterial forfor groundground electrodeselectrodes..


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OtherOther popularpopular materialsmaterials areare hothot--galvanizedgalvanized

steel,steel, stainlessstainless steelsteel andand leadlead..

GroundGround electrodeselectrodes maymay bebe rods,rods, plates,plates, strips,strips,solidsolid sectionsection wirewire oror matsmats..

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Grounding Electrode

System


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Ufer Grounds-Concrete encased electrode.

Ufer grounds should never be used as thesole ground electrode.

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Copper Ground Mesh-Used to augment thegrounding system.

The mesh can be strategically placed to protectpersonnel against step and touch potentials.


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Connecting several electrodes improves the reliability of your

grounding electrode system.

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A) Single earth rodB) Multiple rods

C) Radial tapes

D) Crows foot earth

E) Earth enhancing compounds placedaround the earth rod

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Ground Conductor Considerations

Sizing-withstand maximum fault current for

the maximum clearing time.

Inductance -Flat strap conductors have lessinductance than their similarly sized roundconductor counterparts.


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Strength/Durability-Round conductors are

much stronger than thin flat strapconductor.

This should be a consideration when

backfilling trenches.

Exothermic Connections-Preferred type of

connection.

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The three components affecting groundingThe three components affecting groundingelectrode resistance:electrode resistance:

((11)) TheThe resistanceresistance ofof thethe electrodeelectrode whichwhich isisnegligiblenegligible..

((22)) TheThe resistanceresistance ofof thethe electrodeelectrode--toto--soilsoil

interfaceinterface areaarea whichwhich isis negligiblenegligible

((33)) TheThe resistanceresistance ofof thethe bodybody ofof earthearthimmediatelyimmediately surroundingsurrounding thethe electrodeelectrode..

TheThe mainmain partpart ofof anyany electrodeelectrode resistanceresistance isis thatthat

ofof thethe bodybody ofof earthearth surroundingsurrounding thethe electrodeelectrode..


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GROUND RESISTANCE OFGROUND RESISTANCE OFAN ELECTRODEAN ELECTRODE

Grounding point electrodeGrounding point electrode

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WhenWhen aa currentcurrentIIentersenters thethe groundground throughthrough suchsuchanan electrode,electrode, duedue toto itsits hemisphericalhemispherical base,base, thethe

currentcurrent flowsflows radialradial outwardoutward asas shownshown inin thethe

sketchsketch belowbelow..

A Hemispherical

electrode

22 x

dxdR

=


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IfIf isis thethe resistivityresistivity ofof thethe soil,soil, thethe

resistanceresistance offeredoffered byby aa hemisphericalhemispherical shellshell ofof

thicknessthickness dxdx atat aa radialradial distancedistance xx fromfrom thethe

electrodeelectrode isis givengiven byby

Hence,Hence, thethe resistanceresistance encounteredencountered byby thethe

groundground electrodeelectrode upup toto thethe depthdepth ofof rr11 isis

22

1

x

dxR

r

a

=

=

1

112 ra

R

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IfIf rr11 isis mademade ,, thethe totaltotal resistanceresistance ofof thethe

groundground electrodeelectrode willwill bebe

ThisThis isis thethe maximummaximum resistanceresistance ofof thethe groundground

electrodeelectrode..

IfIf aa currentcurrent II entersenters thethe groundground electrode,electrode, thethe

potentialpotential dropdrop upup toto thethe shellshell radiusradius ofof rr11willwillbebe givengiven byby

1ra

R

2=


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AtAt veryveryhighhigh valuesvalues ofof rr11,, thethe resistanceresistancevaluevalue willwill approachapproach thethe valuevalue RR ,, beyondbeyond

whichwhich truetrue groundground cancan bebe assumedassumed toto bebepresentpresent..

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Step and Touch voltages near a groundedStep and Touch voltages near a groundedstructure:structure:

E electrostatic stress (voltage gradient)E electrostatic stress (voltage gradient)

22

x

IiEx

==

==

1

211

22

1

1 raIdx

xIV

r

a

r


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Step potentialStep potential

StepStep potentialpotential isis thethe voltagevoltagebetweenbetween thethe feetfeet ofof aa personperson standingstanding nearnear

anan energizedenergized groundedgrounded objectobject..

TouchTouch potentialpotential

TouchTouch potentialpotential isis thethe voltagevoltage betweenbetween

thethe energizedenergized objectobject andand thethe feetfeet ofof aa personperson

inin contactcontact withwith thethe objectobject..

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Step and touch voltages


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Step and touch voltagesStep and touch voltages

Lightning Step VoltageLightning Step Voltage

200,000 Volts

0 Volts far away

Currentflow

thru earthgenerates

voltage

8,000 voltsacross feet(Typical)

Step Voltage


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DISTANCE

VOLT

S

500 kV

250

125

375

Electrocutedanimal

Lightning Step VoltageLightning Step Voltage

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Driven rodsDriven rods


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Resistance of driven rods:Resistance of driven rods: TheThe GroundGround ResistanceResistance (R)(R) of of aa singlesingle rod,rod, ofof

diameterdiameter (d)(d) andand drivendriven lengthlength ((ii)) drivendriven verticallyvertically intointothethe soilsoil ofof resistivityresistivity (),(), cancan bebe calculatedcalculated asas followsfollows::

where:where: Soil Resistivity in mSoil Resistivity in m

ll Buried Length of the electrode in mBuried Length of the electrode in mdd Diameter of the electrode in mDiameter of the electrode in m

TheThe rodrod isis assumedassumed asas carryingcarrying currentcurrent uniformlyuniformly alongalongitsits rodrod..

= 1

8ln

2 d

l

lR

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ExamplesExamples

(a) 20mm rod of 3m length and Soil resistivity(a) 20mm rod of 3m length and Soil resistivity

50 50 --m ....R=16.1 m ....R=16.1

(b) 25mm rod of 2m length and Soil resistivity(b) 25mm rod of 2m length and Soil resistivity

30 30 --m ....R=13.0 m ....R=13.0


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TheThe resistanceresistance ofof aa singlesingle rodrod isis notnotsufficientlysufficiently lowlow..

AA numbernumber ofof rodsrods areare connectedconnected inin parallelparallel..

TheyThey shouldshould bebe drivendriven farfar apartapart asas possiblepossible totominimizeminimize thethe overlapoverlap amongamong theirtheir areasareas ofof

influenceinfluence..

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ItIt isis necessarynecessary toto determinedetermine thethe netnet

reductionreduction inin thethe totaltotal resistanceresistance bybyconnectingconnecting rodsrods inin parallelparallel..

TheThe rodrod isis replacedreplaced byby aa hemisphericalhemispherical

electrodeelectrode havinghaving thethe samesame resistanceresistance..


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The interfacing hemisphere

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Ground Rod DiameterDoubling diameter reduces resistance only

10%.

Ground Rod Length

Doubling length reduces resistance 40%,actual reduction depends on soil resistivities

encountered in multi-layered soils.

Ground Rod Spacing

Approximately twice the length. (in good

soil).


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Equivalent hemisphereEquivalent hemisphere

R for large power station = 0.5 ohmR for large power station = 0.5 ohm

R for small power station = 5 ohmR for small power station = 5 ohm

R for towers = 10R for towers = 10--30 ohm30 ohm R for labs = 2 ohmR for labs = 2 ohm

= 18ln

22 d

l

lreq

=

18

lnd

l

lreq

56

Rods too close


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Two groundTwo ground electrodeselectrodes

EquivalentEquivalenthemispherehemisphere

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Two driven rods:Two driven rods:

Actual valueActual value

Ideal valueIdeal value

OverlappingOverlappingcoefficient (screening)coefficient (screening)

+=+=dr

I

d

I

r

IV

11

42

)2/(

2

)2/(1

+==drI

Vqactual

11

4Re

r

qideal

4

Re =

1Re

Re==

qactual

qideal

actual

ideal


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Ground Rod Spacing

Rod: 3/4inch x 10feet

= 100 .m

One Ground Rod Two Ground Rods

R = 32 Spacing 20 feet: R = 17.4

60


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GroundGround--grid Mesh Electrodesgrid Mesh Electrodes

AA commoncommon methodmethod forfor obtainingobtaining aa low lowgroundground resistanceresistance atat highhigh voltagevoltage

substationssubstations isis toto useuse interconnectedinterconnected groundgroundgridsgrids..

AA typicaltypical gridgrid systemsystem forfor aa substationsubstationwouldwould comprisecomprise 44//00 barebare solidsolid coppercopper

conductorsconductors buriedburied atat aa depthdepth ofof 3030--6060 cm,cm,spacedspaced inin aa gridgrid patternpattern ofof aboutabout 33--1010 mm..

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AtAt eacheach junction,junction, thethe conductorsconductors areare securelysecurelybondedbonded togethertogether..

GridGrid meshesmeshes areare usedused toto complementcomplement rodsrods oror cancanbebe usedused separatelyseparately whenwhen deepdeep drivendriven rodsrods areareimpracticalimpractical duedue toto soilsoil considerationsconsiderations..

GridGrid meshesmeshes areare usedused forfor thethe groundinggrounding ininsubstationssubstations toto createcreate anan equipotentialequipotential platformplatform andand

toto handlehandle thethe highhigh faultfault currentscurrents returningreturning toto thethetransformertransformer neutralsneutrals..


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Perimeter Electrode Grounding Grid

Vertical ElectrodeLateral ElectrodeLateral Electrode

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TheyThey areare particularlyparticularly usefuluseful whenwhen multiplemultipleinjectioninjection pointspoints areare required,required, atat aa substationsubstation..

GroundingGrounding resistanceresistance ofof buriedburied gridgrid meshesmeshescancan bebe considerablyconsiderably lowerlower thanthan thosethoseimplementedimplemented usingusing verticalvertical groundground rodsrods..

IncreasingIncreasing thethe areaarea ofof thethe gridgrid coveragecoverage cancanalsoalso significantlysignificantly reducereduce thethe groundgroundresistanceresistance..


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TheThe sideside ofof gridgrid conductorsconductors requiredrequired toto

avoidavoid fusingfusing underunder thethe faultfault currentcurrent II isisestimatedestimated asas

)}234/()234ln{(

76I

21

++=

am TT

ta

aaisis thethe coppercopper crosscross--sectionsection (circular(circular mils),mils), tt

isis thethe faultfault durationduration s,s, TmTm isis thethemaximummaximum allowableallowable temperature,temperature, andandTaTaisisthethe ambientambient temperaturetemperature..

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AA gridgrid effectivelyeffectively groundsgrounds thethe equipmentequipmentandand controlscontrols thethe voltagevoltage gradientsgradients atat thethe

surfacesurface ofof thethe groundground toto valuesvalues safesafe forforhumanhuman contactcontact..

GroundGround rodsrods maymay bebe connectedconnected toto thethe gridgridforfor furtherfurther reductionreduction inin thethe groundground

resistanceresistance..


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METHODS OF DECREASING GROUNDMETHODS OF DECREASING GROUNDRESISTANCERESISTANCE

DecreasingDecreasing thethe groundground resistanceresistance ofof aagroundinggrounding systemsystem inin highhigh resistivityresistivity soilsoil isisoftenoften aa formidableformidable tasktask..

Recently,Recently, somesome newnew methodsmethods havehave beenbeen

proposedproposed toto decreasedecrease groundground resistanceresistance..TheseThese methodsmethods areare listedlisted inin thethe followingfollowingsectionsection..


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11--Chemical RodsChemical Rods

ChemicalChemical rodsrods areare electrodeselectrodes withwith holesholesalongalong theirtheir length,length, filledfilled withwith mineralmineral saltssalts..

TheThe rodrod absorbsabsorbs moisturemoisture fromfrom bothboth airair andandsoilsoil..

ContinuousContinuous conditioningconditioning ofof aa largelarge areaareainsuresinsures anan ultraultra--lowlow--resistanceresistance groundground

whichwhich isis moremore effectiveeffective thanthan aa conventionalconventionalelectrodeelectrode..

IfIf thethe conductiveconductive saltssalts areare runningrunning low,low, thetherodrod cancan bebe rechargedrecharged withwith aa refillrefill kitkit..

70

TheseThese rodsrods areare availableavailable inin verticalvertical andandhorizontalhorizontal configurationsconfigurations..

TheyThey maymay bebe usedused inin rockyrocky soils,soils, freezingfreezing

climates,climates, drydry deserts,deserts, oror tropicaltropical rainrain forestsforests..

TheyThey provideprovide stablestable protectionprotection forfor manymanyyearsyears..


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Chemical rod

72


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74

Horizontal and vertical

models in standard 8 and

10 lengths


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Disadvantages are:

Chemicals concentrated around electrodeswill cause corrosion

Chemicals leach through the soil anddissipate

Scheduled replenishment may be required

May be prohibited because they maycontaminate the water table

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22-- Grounding Augmentation Fill (GAF)Grounding Augmentation Fill (GAF)

AboutAbout 9595%% ofof thethe groundinggrounding resistanceresistance ofof aa givengiven

electrodeelectrode isis determineddetermined byby thethe charactercharacter ofof thethe soilsoil

withinwithin aa hemispherehemisphere whosewhose radiusradius isis 11..11 timestimes thethe

lengthlength ofof thethe rodrod..

ReplacingReplacing allall oror partpart ofof thatthat soilsoil withwith aa highlyhighly

conductiveconductive backfillbackfill willwill facilitatefacilitate thethe achievementachievement

ofof aa lowlow--resistanceresistance groundground connectionconnection..

TheThe greatergreater thethe percentagepercentage ofof soilsoil replaced,replaced, thethe

lowerlower thethe ultimateultimate groundinggrounding resistanceresistance..


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80

The critical soil cylinder within an

interfacing hemisphere


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33-- Cracks with Low Resistivity Materials (LRMCracks with Low Resistivity Materials (LRM))

This method requires 3 steps:This method requires 3 steps: DrillingDrilling deepdeep holesholes inin thethe ground,ground, developingdeveloping crackscracks

inin thethe soilsoil byby meansmeans ofof explosionsexplosions inin thethe holes,holes,

fillingfilling thethe holesholes withwith lowlow resistivityresistivity materialsmaterials (LRM)(LRM)

underunder pressurepressure..

MostMost ofof thethe crackscracks aroundaround thethe verticalvertical conductorsconductors

willwill bebe filledfilled withwith LRM,LRM, andand aa complexcomplex networknetwork ofof

lowlow resistivityresistivity treetree likelike crackscracks linkedlinked toto thethe

substationsubstation gridgrid isis formedformed..

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FieldField teststests showshow thatthat thethe optimumoptimum spanspanbetweenbetween verticalvertical conductorsconductors isis inin thethe rangerange

ofof 11..55--22 timestimes thethe lengthlength ofof thethe verticalverticalconductorconductor..

ThisThis methodmethod isis effectiveeffective inin reducingreducing groundgroundresistancesresistances inin rockyrocky areasareas..


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Conductive CementConductive Cement

Concrete has a resistivity range of 30 to 90 Concrete has a resistivity range of 30 to 90 .m..ItIt isis hygroscopichygroscopic byby nature,nature, itit tendstends toto absorbabsorb

moisturemoisture whenwhen availableavailable andand keepkeep itit upup toto 3030days,days, thusthus maintainingmaintaining aa resistivityresistivity lowerlower thanthan

thethe surroundingsurrounding soilsoil..

DuringDuring aa longlong drydry seasonseason concreteconcrete willwill drydry outout

withwith aa subsequentsubsequent riserise inin resistivityresistivity..

84


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InstallingInstalling anan EARTHLINKEARTHLINK 101101 earthlingearthling stripstrip isis

simplesimple::

Dig a trench and lay in the wire.

86

Pour EARTHLINK 101 conductive cement, using the handyapplicator bag, and shovel in a thin protective layer of soil.


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Backfill the remaining soil using a front-endloader and restore the surface to grade.

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10 years in ground


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10 years in ground

90

10 years in ground


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92

NEUTRAL GROUNDINGNEUTRAL GROUNDING


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Power System GroundingPower System Grounding

SystemSystem groundinggrounding meansmeans thethe connectionconnection

ofof groundground toto thethe neutralneutral pointspoints ofofcurrentcurrent carryingcarrying conductorsconductors suchsuch asasthethe neutralneutral pointpoint ofof aa circuit,circuit, aatransformer,transformer, rotatingrotating machinery,machinery, oror aasystem,system, eithereither solidlysolidly oror withwith aa currentcurrentlimitinglimiting devicedevice..

UngroundedUngrounded systemsystem..

SolidSolid groundinggrounding

ImpedanceImpedance groundinggrounding (R(R andand X)X)ResonantResonant groundinggrounding

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96

Ungrounded SystemsUngrounded Systems

TheThe ungroundedungrounded systemsystem isis oneone thatthat hashas nonointentionalintentional connectionconnection betweenbetween thethe neutralneutraloror anyany phasephase andand groundground..

AnAn ungroundedungrounded systemsystem isis groundedgrounded throughthroughthethe conceptconcept ofof capacitivecapacitive couplingcoupling..

TheThe neutralneutral potentialpotential ofof anan ungroundedungrounded

system,system, withwith balancedbalanced loadingloading willwill bebe closeclosetoto groundground potentialpotential duedue toto thethe capacitancecapacitancebetweenbetween eacheach phasephase conductorconductor andand groundground..


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LowLow groundground faultfault currentcurrent..

VeryVery highhigh voltagesvoltages toto groundground potentialpotential ononunfaultedunfaulted phasesphases..

SustainedSustained faultsfaults leadlead toto systemsystem lineline--toto--lineline

voltagesvoltages onon unfaultedunfaulted lineline..

InsulationInsulation failurefailure..

ArcingArcing GroundGround FaultsFaults..

FailureFailure duedue toto restrikerestrike groundground faultsfaults..

ContinuedContinued operationoperation ofof facilityfacility..

98

Isolated Neutral System


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100


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Grounded SystemGrounded System

TheThe intentionalintentional connectionconnection ofof thethe neutralneutralpointspoints ofof transformers,transformers, generatorsgenerators andand

rotatingrotating machinerymachinery toto thethe earthearth groundgroundnetworknetwork providesprovides aa referencereference pointpoint ofof zerozero

voltsvolts..

AllAll powerpower systemssystems ofof todaytoday operateoperate withwith

groundedgrounded neutralsneutrals..

102

ThisThis protectiveprotective measuremeasure offersoffers manymanyadvantagesadvantages overover anan ungroundedungrounded system,system,includingincluding::

ReducedReduced magnitudemagnitude ofof transienttransient overvoltagesovervoltages

SimplifiedSimplified groundground faultfault locationlocation

ImprovedImproved systemsystem andand equipmentequipment faultfaultprotectionprotection

ReducedReduced maintenancemaintenance timetime andand expenseexpense

GreaterGreater safetysafety forfor personnelpersonnel

ImprovedImproved lightninglightning protectionprotection

ReductionReduction inin frequencyfrequency ofof faultsfaults..


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Solidly Grounded SystemSolidly Grounded System

The solidly grounded system is one that hasThe solidly grounded system is one that has

the neutral connected to ground without anthe neutral connected to ground without anintentional impedance.intentional impedance.

InIn contrastcontrast toto thethe ungroundedungrounded systemsystem thethe

solidlysolidly groundedgrounded systemsystem willwill resultresult inin aa largelargemagnitudemagnitude ofof currentcurrent toto flowflow (Aids(Aids inin

coordination),coordination), butbut hashas nono increaseincrease inin voltagevoltage

onon unfaultedunfaulted phasesphases..

104

LowLow initialinitial costcost toto installinstall andand implement,implement, butbut

straystray currentscurrents thenthen becomebecome aa possiblepossible

consequenceconsequence..

CommonCommon inin lowlow--voltagevoltage distributiondistribution systems,systems,

suchsuch asas overheadoverhead lineslines..

TypicallyTypically feedsfeeds toto transformertransformer primaryprimary withwith highhigh

sideside fusefuse protectionprotection..

NotNot thethe preferredpreferred groundinggrounding schemescheme forfor

industrialindustrial oror commercialcommercial facilitiesfacilities duedue toto highhigh

magnitudemagnitude faultfault currentscurrents


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Solidly Grounded Systems(a) Three-wire grounded system

(b) Four-wire grounded system

106

Solidly Grounded SystemsSolidly Grounded SystemsSolidly Grounded SystemsSolidly Grounded Systems

(c) Three(c) Three(c) Three(c) Three----wire multigrounded systemwire multigrounded systemwire multigrounded systemwire multigrounded system

(d) Phasor diagram for normal operation(d) Phasor diagram for normal operation(d) Phasor diagram for normal operation(d) Phasor diagram for normal operation

(e) Phasor diagram for a ground fault(e) Phasor diagram for a ground fault(e) Phasor diagram for a ground fault(e) Phasor diagram for a ground fault


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AdvantagesAdvantages

NeutralNeutral heldheld effectivelyeffectively atat groundground potentialpotential

PhasePhase--toto--groundground faultsfaults ofof samesame magnitudemagnitude asasphasephase--toto--phasephase faultsfaults soso nono needneed forfor specialspecialsensitivesensitive relaysrelays

CostCost ofof currentcurrent--limitinglimiting devicedevice isis eliminatedeliminated

SizeSize andand costcost ofof transformerstransformers areare reducedreduced bybygradinggrading insulationinsulation towardtoward neutralneutral pointpoint NN..

107

DisadvantagesDisadvantages

AsAs mostmost systemsystem faultsfaults areare phasephase--toto--ground,ground,severesevere shocksshocks areare moremore considerableconsiderable thanthan withwithresistanceresistance groundinggrounding

ThirdThird harmonicsharmonics tendtend toto circulatecirculate betweenbetween

neutralsneutrals..

108


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Resistive Grounded SystemsResistive Grounded Systems

ResistanceResistance groundinggrounding isis thethe mostmost

effectiveeffective andand preferredpreferred methodmethod..

ItIt solvessolves thethe problemproblem ofof transienttransient

overvoltages,overvoltages, therebythereby reducingreducing

equipmentequipment damagedamage..

ItIt allowsallows thethe magnitudemagnitude ofof thethe faultfault

currentcurrent toto bebe predeterminedpredetermined byby aasimplesimple ohmsohms lawlaw calculationcalculation..

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I=V/RI=V/R

WhereWhere:: II == LimitLimit ofof FaultFault CurrentCurrent..

VV == LineLine--toto--neutralneutral VoltageVoltage ofof SystemSystem

RR == OhmicOhmic ValueValue ofof NeutralNeutral groundinggroundingResistorResistor


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LimitingLimiting faultfault currentscurrents toto predeterminedpredetermined

maximummaximum valuesvalues permitspermits thethe designerdesigner toto

selectivelyselectively coco--ordinateordinate thethe operationoperation ofof protectiveprotectivedevicesdevices..

ThereThere areare twotwo broadbroad categoriescategories ofof resistanceresistance

groundinggrounding::

lowlow resistanceresistance andand highhigh resistanceresistance..

InIn bothboth typestypes ofof grounding,grounding, thethe resistorresistor isis

connectedconnected betweenbetween thethe neutralneutral ofof thethe transformertransformersecondarysecondary andand thethe earthearth groundground..

Resistance groundingResistance grounding

AA resistorresistor isis connectedconnected betweenbetween thethe transformertransformerneutralneutral andand earthearth..

MainlyMainly usedused fromfrom 66..66 toto 3333 kVkV

ValueValue isis suchsuch asas toto limitlimit anan earthearth faultfault currentcurrent toto

betweenbetween 11 andand 22 timestimes fullfull loadload ratingrating ofof thethetransformer,transformer, typicallytypically 400400 AA..

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AdvantagesAdvantages

LimitsLimits electricalelectrical andand mechanicalmechanical stressesstresses ononsystemsystem whenwhen anan earthearth faultfault occurs,occurs, butbut atat thethesamesame timetime currentcurrent isis sufficientsufficient toto operateoperate normalnormalprotectionprotection equipmentequipment..

DisadvantagesDisadvantages

FullFull lineline--toto--lineline insulationinsulation requiredrequired betweenbetweenphasephase andand earthearth..

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Low Resistance Grounded SystemLow Resistance Grounded System

TheThe lowlow resistanceresistance groundedgrounded systemsystem isis oneone

thatthat hashas thethe neutralneutral connectedconnected toto groundgroundthroughthrough aa smallsmall resistanceresistance thatthat limitslimits thethe

faultfault currentcurrent..

TheThe sizesize ofof thethe groundinggrounding resistorresistor isis selectedselectedtoto detectdetect andand clearclear thethe faultedfaulted circuitcircuit..


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TheThe resistorresistor cancan limitlimit groundground currentscurrents toto aa

desireddesired levellevel basedbased onon coordinationcoordination requirementrequirementoror relayrelay limitationslimitations..

LimitsLimits transienttransient overvoltagesovervoltages duringduring groundground

faultsfaults..

LowLow resistanceresistance groundinggrounding isis notnot recommendedrecommended

forfor lowlow voltagevoltage systemssystems duedue toto thethe limitedlimited

groundground faultfault currentcurrent.. ThisThis reducedreduced faultfault currentcurrent

cancan bebe insufficientinsufficient toto positivelypositively operateoperate fusesfuses

and/orand/or seriesseries triptrip unitsunits..

GroundGround faultfault currentcurrent typicallytypically inin thethe 100100 600600

AmpAmp rangerange..

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High Resistance Grounded SystemHigh Resistance Grounded System

TheThe highhigh resistanceresistance groundedgrounded systemsystem isis oneone

thatthat hashas thethe neutralneutral connectedconnected toto groundgroundthroughthrough aa resistiveresistive impedanceimpedance whosewhose

resistanceresistance isis selectedselected toto allowallow aa groundground faultfaultcurrentcurrent throughthrough thethe resistorresistor equalequal toto oror

slightlyslightly moremore thatthat thethe capacitivecapacitive chargingcharging

currentcurrent ofof thethe systemsystem..


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TheThe resistorresistor cancan limitlimit groundground currentscurrents toto aa

desireddesired levellevel basedbased onon coordinationcoordination requirementrequirement

oror relayrelay limitationslimitations..

LimitsLimits transienttransient overvoltagesovervoltages duringduring groundground

faultsfaults..

PhysicallyPhysically largelarge resistorresistor banksbanks..

VeryVery lowlow groundground faultfault current,current, typicallytypically underunder 1010

AmpsAmps..

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SpecialSpecial relayingrelaying methodsmethods utilizedutilized toto detectdetect andand

removeremove groundground faultsfaults..

HighHigh resistanceresistance groundinggrounding isis typicallytypically appliedapplied

toto situationssituations wherewhere itit isis essentialessential toto preventprevent

unplannedunplanned outagesoutages..

RecentRecent trendtrend hashas beenbeen toto utilizeutilize highhigh resistanceresistance

groundinggrounding methodsmethods onon 600600 voltvolt systemssystems andandlowerlower..


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TheThe reasonsreasons forfor limitinglimiting thethe currentcurrent byby

resistanceresistance groundinggrounding11)) toto reducereduce burningburning andand meltingmelting effectseffects inin

faultedfaulted electricelectric equipment,equipment, suchsuch asas switchgear,switchgear,

transformers,transformers, cablescables andand rotatingrotating machinesmachines..

22)) toto reducereduce mechanicalmechanical stressesstresses inin circuitscircuits andandapparatusapparatus carryingcarrying faultfault currentscurrents

33)) toto reducereduce electricelectric--shockshock hazardshazards toto personnelpersonnelcausedcaused byby straystray groundground faultfault currentscurrents inin thethe

groundground returnreturn pathpath

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44)) toto reducereduce arcarc blastblast oror flashflash hazardhazard toto personnelpersonnelwhowho maymay havehave accidentallyaccidentally causedcaused oror whowhohappenhappen toto bebe inin closeclose proximityproximity toto thethe faultfaultcurrentcurrent

55)) toto reducereduce thethe momentarymomentary lineline--voltagevoltage dipdipoccasionedoccasioned byby thethe occurrenceoccurrence andand clearingclearing ofof aagroundground faultfault

66)) toto securesecure controlcontrol ofof transienttransient overvoltagesovervoltages whilewhile

atat thethe samesame timetime avoidingavoiding thethe shutdownshutdown ofof aafaultyfaulty circuitcircuit onon thethe occurrenceoccurrence ofof thethe firstfirstgroundground faultfault


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StrategyStrategy

Ungrounded

System

Solidly

Grounded

System

Low Resistance

Grounded

System

High

Resistance

Grounded

System

Overvoltages Severe None Limited Limited

Overcurrent - Damage

at point of fault Unknown Severe Minimal None

Maintenance Costs High Reasonable Reasonable Low

Continuous Operation

with Ground Fault

Possible but not

recommended Not possible Not possible Ideal

Relay Co-ordination

(Appropriate Equipment

Tripped, Ease of fault

location) Difficult Difficult Good Excellent

Personnel Safety to Personnel Poor Good Reasonable Excellent

Equipment

Damage

Downtime

Productivity Impact

System Type

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Reactance grounding systemReactance grounding system

EliminationElimination ofof thethe faultfault currentcurrent thatthat couldcould causecausethethe arcingarcing groundground conditioncondition..

NormallyNormally itit doesdoes notnot carrycarry currentcurrent

DuringDuring faultfault:: reactivereactive componentcomponent ofof currentcurrent II ==capacitivecapacitive componentcomponent ofof currentcurrent II

NoNo currentcurrent atat thethe fault,fault, preventingpreventing restrikesrestrikes andandeliminateseliminates thethe causecause ofof voltagevoltage buildupbuildup


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123

Compensated System

124

Resonant groundingResonant grounding UsingUsing PetersonPeterson coilcoil whichwhich isis aa tunabletunable ironiron coredcored

reactorreactor connectedconnected betweenbetween thethe neutralneutral andand groundground..

TheThe groundground faultfault neutralizerneutralizer isis saidsaid toto bebe effectiveeffective inin

7070 toto 8080 %% ofof thethe faultsfaults..

c

V

X

VI

ph

c

ph

/1==

In an ungrounded system arcing grounds takesIn an ungrounded system arcing grounds takes

place (fault on phase C):place (fault on phase C):

cVII phC 33 ==c

V

X

VI

ph

c

ph

/1==


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126

If grounding through LfXL 2=

(to cancel arcing grounds)

L

V

X

VI

ph

L

phL

==

CL II =

cL

23

1

=


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Arc suppression coil (Petersen coil)Arc suppression coil (Petersen coil)

AA tuneabletuneable reactorreactor isis connectedconnected inin thethetransformertransformer neutralneutral toto earthearth..

ValueValue ofof reactancereactance isis chosenchosen suchsuch thatthat reactancereactancecurrentcurrent neutralizesneutralizes capacitancecapacitance currentcurrent..

TheThe currentcurrent atat thethe faultfault pointpoint isis thereforethereforetheoreticallytheoretically nilnil andand unableunable toto maintainmaintain thethe arc,arc,hencehence itsits namename..

VirtuallyVirtually fullyfully insulatedinsulated system,system, soso currentcurrentavailableavailable toto operateoperate protectiveprotective equipmentequipment isis sososmallsmall asas toto bebe negligiblenegligible..

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ToTo offsetoffset this,this, thethe faultyfaulty sectionsection cancan bebe leftleft ininserviceservice indefinitelyindefinitely withoutwithout damagedamage toto thethesystemsystem asas mostmost faultsfaults areare earthearth faultsfaults ofof aatransienttransient nature,nature, thethe initialinitial arcarc atat thethe faultfault pointpointisis extinguishedextinguished andand doesdoes notnot restrikerestrike..

SensitiveSensitive wattwatt metricalmetrical relaysrelays areare usedused toto detectdetectpermanentpermanent earthearth faultsfaults..

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Arc suppression coil (Petersen coil)

EarthingEarthing via neutral electromagneticvia neutral electromagnetic

coupler with resistorcoupler with resistor

ThisThis typetype ofof earthingearthing providesprovides anan earthearth pointpoint forforaa deltadelta systemsystem andand combinescombines thethe virtuesvirtues ofofresistanceresistance andand reactancereactance groundinggrounding inin limitinglimitingearthearth faultfault currentcurrent toto safesafe relayablerelayable valuesvalues..

TheThe currentcurrent shouldshould bebe resistiveresistive inin naturenature totocomplycomply withwith IEEEIEEE standardstandard 142142 andand SABSSABS02000200..

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EarthingEarthing via neutralvia neutral earthingearthing compensatorcompensator

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Advantages of Neutral GroundingAdvantages of Neutral Grounding ArcingArcing groundsgrounds areare reducedreduced oror eliminatedeliminated..

TheThe systemsystem isis notnot subjectedsubjected toto overvoltageovervoltage surgessurgesduedue toto arcingarcing groundsgrounds..

TheThe voltagesvoltages ofof healthyhealthy lineslines withwith respectrespect toto earthearthremainremain atat harmlessharmless valuevalue.. TheyThey dodo notnot increaseincrease toto33 timetime normalnormal valuevalue..

TheThe lifelife ofof insulationinsulation isis longlong.. TherebyThereby reducedreducedmaintenance,maintenance, repairs,repairs, breakdownsbreakdowns andand improvedimprovedcontinuitycontinuity..

TheThe earthearth faultfault relayingrelaying isis relativelyrelatively simplesimple..


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TheThe overvoltagesovervoltages duedue toto lightninglightning areare dischargeddischargedtoto earthearth..

TheThe earthearth faultfault currentcurrent cancan bebe controlledcontrolled..

ImprovedImproved serviceservice reliabilityreliability..

GreaterGreater safetysafety forfor personnelpersonnel oror equipmentequipment duedue totooperationoperation ofof fusesfuses oror relaysrelays onon earthearth faultfault andandlimitationlimitation ofof voltagesvoltages..

LifeLife ofof equipment,equipment, machinesmachines andand insulationinsulation isisimprovedimproved duedue toto limitationlimitation ofof voltagesvoltages..

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7-Grounding Systems.pdf

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