2 Grounding 1

8/8/2019 2 Grounding 1

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GROUNDING SYSTEMSGROUNDING SYSTEMS

PartPart 11

Professor Ahdab ElmorshedyProfessor Ahdab Elmorshedy


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Reference:Reference:

High Voltage Engineering Theory andHigh Voltage Engineering Theory andPractice, Text Book, Marcel Dekker Inc. NY,Practice, Text Book, Marcel Dekker Inc. NY,

USA,USA, 20002000..

Mazen AbdelMazen Abdel--Salam, Hussein Anis, AhdabSalam, Hussein Anis, Ahdab

Elmorshedy, Roshedy Radwan.Elmorshedy, Roshedy Radwan.

Grounding Systems,Grounding Systems, Chapter (Chapter (1313).).

Overvoltages on Power Systems , ChapterOvervoltages on Power Systems , Chapter

((1414).).

HighHigh--Voltage CablesVoltage Cables Chapter (Chapter (1212).).


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The objective of a grounding system are:

1. To provide safety to personnel during normal and fault

conditions by limiting step and touch potential.

2. To assure correct operation of electrical/electronic

devices.

3. To prevent damage to electrical/electronic apparatus.4. To dissipate lightning strokes.

5. To stabilize voltage during transient conditions and to

minimize the probability of flashover during transients.

6. To divert stray RF energy from sensitive audio, video,

control, and computer equipment.


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A safe grounding design has two objectives:

1. To provide means to carry electric currents

into the earth under normal and fault

conditions without exceeding any operating

and equipment limits or adversely affectingcontinuity of service.

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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The PRIMARY goal of the grounding

system throughout any facilities is

SAFETY.Why ground at all?

PERSONNEL SAFETY FIRST

EQUIPMENT PROTECTION SECOND


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The three main types are:

EQUIPMENT GROUNDING (SAFETY)

SYSTEM GROUNDING

LIGHTNING/SURGE GROUNDING

What are the three main types

of grounding?


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TheThe soilsoil resistivityresistivity isis thethe singlesingle mostmostimportantimportant factorfactor affectingaffecting thethe resistanceresistance ofof

thethe groundground systemsystem..

MostMost soilssoils behavebehave bothboth asas aa conductorconductor ofof

resistanceresistance R,R, andand asas aa dielectricdielectric..


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ForFor highhigh frequencyfrequency andand steepsteep--frontfrontwaveswaves penetratingpenetrating aa veryvery highhigh resistiveresistivesoil,soil, thethe earthearth maymay bebe representedrepresented byby aa

parallelparallel connectionconnection ofof resistanceresistance R,R,capacitancecapacitance C,C, andand aa gapgap..

ForFor lowlow frequenciesfrequencies andand dcdc thethe chargingcharging

currentcurrent isis negligiblenegligible comparingcomparing toto thetheleakageleakage current,current, andand thethe earthearth cancan beberepresentedrepresented byby aa purepure resistanceresistance RR..


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Soil CharacteristicsSoil Characteristics SoilSoil typetype.. SoilSoil resistivityresistivity variesvaries widelywidely

dependingdepending onon soilsoil type,type, fromfrom asas lowlow asas 11

OhmOhm--metermeter forfor moistmoist loamyloamy topsoiltopsoil toto almostalmost1010,,000000 OhmOhm--metersmeters forforsurfacesurface limestonelimestone..

MoistureMoisture contentcontent isis oneone ofof thethe controllingcontrolling

factorsfactors inin earthearth resistanceresistance becausebecause electricalelectricalconductionconduction inin soilsoil isis essentiallyessentially electrolyticelectrolytic..


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TheThe resistivityresistivity ofof mostmost soilssoils risesrises abruptlyabruptlywhenwhen moisturemoisture contentcontent isis lessless thanthan 1515 toto

2020 percentpercent byby weight,weight, butbut isis affectedaffected veryvery

littlelittle aboveabove 2020 percentpercent.. TheThe moisturemoisture alonealone isis notnot thethe predominantpredominant

factorfactor influencinginfluencing thethe soilsoil resistivityresistivity..

IfIf thethe waterwater isis relativelyrelatively pure,pure, itit willwill bebe ofofhighhigh resistivityresistivity andand maymay notnot provideprovide thethe

soilsoil withwith adequateadequate conductivityconductivity..


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The influence of moisture content


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The influence of temperature


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Typical variations in soil resistivity as a function of

moisture, temperature and soil content


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Resistivities of different solutions


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TheThe solublesoluble salts,salts, acidsacids oror alkalialkali

presentedpresented inin soilsoil influenceinfluence considerablyconsiderablythethe soilsoil resistivityresistivity..

TheThe mostmost commonlycommonly usedused saltingsalting materialsmaterials

areare sodiumsodium chloridechloride (common(common salt),salt), coppercopper

sulfatesulfate andand magnesiummagnesium sulfatesulfate..

DifferentDifferent typestypes ofof saltssalts havehave varyingvarying

depletiondepletion ratesrates;; consequently,consequently, differentdifferent

typestypes maymay bebe combinedcombined toto produceproduce thetheoptimumoptimum depletiondepletion andand conditioningconditioning

characteristicscharacteristics..


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TheThe temperaturetemperature effecteffect onon soilsoil resistivityresistivity

isis almostalmost negligiblenegligible forfor temperaturestemperatures

aboveabove thethe freezingfreezing pointspoints..

WhenWhen temperaturetemperature dropsdrops belowbelow waterwater

freezingfreezing point,point, thethe resistivityresistivity increasesincreasesrapidlyrapidly..

CompactnessCompactness andand granularitygranularity affectsaffects

soilsoil resistivityresistivity inin thatthat denserdenser soilssoils generallygenerallyhavehave lowerlower resistivityresistivity..


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TheThe twotwo factorsfactors -- moisturemoisture andand saltsaltcontentcontent --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 andand

inin somesome casescases thethe onlyonly economicallyeconomically

soundsound solutionsolution inin obtainingobtaining lowlow impedanceimpedance

ofof thethe groundground systemsystem..


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TYPES OF GROUND ELECTRODESTYPES OF GROUND ELECTRODESGroundGround electrodeselectrodes mustmust penetratepenetrate intointo thethe

moisturemoisture levellevel belowbelow thethe groundground levellevel..

TheyThey consistconsist ofof aa metalmetal (or(or combinationcombination ofofmetals)metals) whichwhich dodo notnot corrodecorrode excessivelyexcessively

forfor thethe periodperiod ofof timetime theythey areare expectedexpected toto

serveserve..


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BecauseBecause ofof itsits highhigh conductivityconductivity andandresistanceresistance toto corrosion,corrosion, coppercopperisis thethe mostmost

commonlycommonly usedused materialmaterial forfor groundground

electrodeselectrodes..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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ThreeThree typestypes ofof coppercopper rodsrods areare availableavailable..

SolidSolid CopperCopper--CopperCopper --cladclad steelsteel rodrod (copper(coppershrunkshrunk ontoonto thethe core)core)--CopperCopper bondedbonded steelsteel

corecore (copper(copper isis molecularlymolecularly bondedbonded toto nickelnickel

platedplated steelsteel rod)rod) SolidSolid coppercopper rodsrods notnot proneprone toto corrosion,corrosion, butbut

areare expensiveexpensive andand difficultdifficult toto drivedrive intointo hardhard

groundground withoutwithout bendingbending..


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AA steelsteel coredcored coppercopper rodrod isis usedused forfor thisthis

reason,reason, howeverhowever thosethose rodsrods thatthat arearesimplysimply cladclad areare proneprone toto thethe claddingcladding

tearingtearing awayaway fromfrom thethe corecore whenwhen drivendriven inin

rockyrocky ground,ground, oror whenwhen bentbent.. ThisThis exposesexposes thethe internalinternal steelsteel corecore toto

corrosioncorrosion..

TheThe mostmost costcost effectiveeffective solutionsolution isis thethemolecularlymolecularly bondedbonded steelsteel coredcored coppercopper

rodrod..


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ThereThere areare threethree componentscomponents affectingaffecting

groundinggrounding electrodeelectrode resistanceresistance::

((11)) TheThe resistanceresistance ofof thethe electrodeelectrode whichwhich isis

negligiblenegligible..

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

interfaceinterface areaarea whichwhich isis negligiblenegligible

((33)) thethe resistanceresistance ofof thethe bodybody ofof earthearth

immediatelyimmediately surroundingsurrounding thethe electrodeelectrode..

TheThe mainmain partpart ofof anyany electrodeelectrode resistanceresistance isis

thatthat ofof thethe bodybody ofof earthearth surroundingsurrounding thethe

electrodeelectrode..


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GROUNDGROUND RESISTANCERESISTANCE OFOF ANAN ELECTRODEELECTRODE

GroundingGrounding pointpoint electrodeelectrode

TheThe equationsequations forfor thethe resistanceresistance ofof anyany complexcomplexsystemsystem ofof groundground electrodeselectrodes cancan bebe developeddevelopedfromfrom thethe fundamentalfundamental principlesprinciples..

TheThe startingstarting pointpoint forfor suchsuch aa developmentdevelopment isis thetheuseuse ofof aa buriedburied metallicmetallic electrodeelectrode withwith aahemisphericalhemispherical basebase ofof radiusradius rr..

ItIt isis assumedassumed thatthat thethe hemisphericalhemispherical basebase isiscompletelycompletely buriedburied inin thethe soilsoil..


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WhenWhen aa currentcurrent II entersenters thethe groundground

throughthrough suchsuch anan electrode,electrode, duedue toto itsits

hemisphericalhemispherical base,base, thethe currentcurrent flowsflowsradialradial outwardoutward asas shownshown inin thethe sketchsketch

belowbelow..

A Hemispherical

electrode

22 x

dxdR

TV!


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Current flow from aCurrent flow from a

hemisphere in a uniform earthhemisphere in a uniform earth


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IfIf isis thethe resistivityresistivity ofof thethe soil,soil, thethe

resistanceresistance offeredoffered byby aa hemisphericalhemispherical shellshellofof 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 rr11isis

22

1

xdxR

r

aTV!

!

1

11

2 raT

V


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IfIf rr11 isis mademade ,, thethe totaltotal resistanceresistance ofof thethegroundground electrodeelectrode willwill bebe

ThisThis isis thethe maximummaximum resistanceresistance ofof thethe

groundground electrodeelectrode.. TheThe generalgeneral equationequation ofof thethe resistanceresistance

shownshown aboveabove cancan bebe modifiedmodified toto aa suitablesuitableformform..

IfIf aa currentcurrent II entersenters thethe groundground electrode,electrode,thethe potentialpotential dropdrop upup toto thethe shellshell radiusradius ofofrr11willwill bebe givengiven byby

gp1ra

RTV2

!g


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g

1

1r

aIIV

!

1

12 r

aa

RTV

!

1

112 ra

RTV


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TheThe generalgeneral equationequation forfor thethe electrodeelectrode

resistanceresistance::

WhereWhere CC isis thethe electrostaticelectrostatic capacitancecapacitance ofof

thethe electrodeelectrode andand itsits imageimage aboveabove earthearth..

rr inin cmcm == CC ofof aa spheresphere inin cgscgs unitsunits..

CR

TV2!

r

Qsphere

04TI

!

rCVQ

04TI!!


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At At veryvery highhigh valuesvalues ofof rr11,, thethe resistanceresistance

valuevalue willwill approachapproach thethe valuevalue RR ,, beyondbeyondwhichwhich truetrue groundground cancan bebe assumedassumed toto bebe

presentpresent..

TheThe figurefigure belowbelow showsshows thethe plotsplots ofofR,R, VV

andand VabsVabs,, thethe absoluteabsolute potentialpotential withwith

respectrespect toto thethe truetrue groundground againstagainst thetheradialradial distancedistance rr11 fromfrom thethe electrodeelectrode..


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Step and touch voltages


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Step and touch voltagesStep and touch voltages


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Step and Touch voltages near a groundedStep and Touch voltages near a grounded

structure:structure:E electrostatic stress (voltage gradient)E electrostatic stress (voltage gradient)

22 x

IiEx T

V

V !!

!! 12

11

22

1

1 ra

Idx

x

IV

r

a

r TV

T

V


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Step potentialStep potential

StepStep potentialpotential isis thethe voltagevoltagebetweenbetween thethe feetfeet ofof aa personperson standingstanding

nearnear anan energizedenergized groundedgrounded objectobject..

ItIt isis equalequal toto thethe differencedifference inin voltage,voltage,givengiven byby thethe voltagevoltage distributiondistribution curve,curve,

betweenbetween twotwo pointspoints atat differentdifferent

distancesdistances fromfrom thethe electrodeelectrode..

AA personperson couldcould bebe atat riskrisk ofof injuryinjury

duringduring aa faultfault simplysimply byby standingstanding nearnear

thethe groundinggrounding pointpoint..


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Touch potentialTouch potential

TouchTouch potentialpotential isis thethe voltagevoltagebetweenbetween thethe energizedenergized objectobject andand thethefeetfeet ofof aa personperson inin contactcontact withwith thetheobjectobject..

ItIt isis equalequal toto thethe differencedifference inin voltagevoltagebetweenbetween thethe energizedenergized objectobject andand aa pointpointsomesome distancedistance awayaway..

TheThe touchtouch potentialpotential couldcould bebe nearlynearly thethefullfull voltagevoltage acrossacross thethe groundedgrounded objectobject ififthatthat objectobject isis groundedgrounded atat aa pointpoint remoteremotefromfrom thethe placeplace wherewhere thethe personperson isis inin

contactcontact withwith itit..


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Driven rodsDriven rods

R i t f d i dR i t f d i d


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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 (l)(l) drivendriven verticallyvertically intointo thethe soilsoil ofofresistivityresistivity (),(), cancan bebe calculatedcalculated asas followsfollows::

wherewhere:: SoilSoil ResistivityResistivity inin ohmohm--mm

ll BuriedBuried LengthLength ofof thethe electrodeelectrode inin mm

dd Diameter Diameter ofof thethe electrodeelectrode inin mm

TheThe rodrod isis assumedassumed asas carryingcarrying currentcurrent uniformlyuniformly alongalong itsits

rodrod.. ExamplesExamples

(a)(a)2020mmmm rodrod ofof 33mm lengthlength andand SoilSoil resistivityresistivity 5050 --mm ....R=R=1616..11

(b)(b)2525mmmm rodrod ofof 22mm lengthlength andand SoilSoil resistivityresistivity 3030 --mm ....R=R=1313..00

-

! 1

8ln

2 d

l

lR

T

V


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Earth resistance shells surrounding aEarth resistance shells surrounding a

vertical earth electrodevertical earth electrode


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TheThe resistanceresistance ofof aa singlesingle rodrod isis notnot sufficientlysufficientlylowlow..

AA numbernumber ofof rodsrods areare connectedconnected inin parallelparallel..

TheyThey shouldshould bebe drivendriven farfar apartapart asas possiblepossible toto

minimizeminimize thethe overlapoverlap amongamong theirtheir areasareas ofof

influenceinfluence..


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ItIt isis necessarynecessary toto determinedetermine thethe netnet

reductionreduction inin thethe totaltotal resistanceresistance byby

connectingconnecting 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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Equivalent hemisphereEquivalent hemisphere

R for large power station =R for large power station = 00..55 ohmohm

R for small power station =R for small power station = 55 ohmohm R for towers =R for towers = 1010--3030 ohmohm

R for labs =R for labs = 22 ohmohm

-

! 18

ln22 d

l

lreq T

V

T

V

-

!18ln

dl

lreq


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Rod Electrodes in ParallelRod Electrodes in Parallel

IfIf thethe desireddesired groundground resistanceresistance cannotcannotbebe achievedachieved withwith oneone groundground electrode,electrode,

thethe overalloverall resistanceresistance cancan bebe reducedreduced byby

connectingconnecting aa numbernumber ofof electrodeselectrodes ininparallelparallel..

TheseThese areare calledcalled arraysarrays ofof rodrod

electrodeselectrodes..


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TheThe combinedcombined resistanceresistance isis aa functionfunctionofof thethe numbernumber andand configurationconfiguration ofof

electrodes,electrodes, thethe separationseparation betweenbetween

them,them, theirtheir dimensionsdimensions andand soilsoilresistivityresistivity..

RodsRods inin parallelparallel shouldshould bebe spacedspaced atat

leastleast twicetwice theirtheir lengthlength toto utilizeutilize thethefullfull benefitbenefit ofof thethe additionaladditional rodsrods..


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IfIf thethe separationseparation ofof thethe electrodeselectrodes isis muchmuchlargerlarger thanthan theirtheir lengthslengths andand onlyonly aa fewfew

electrodeselectrodes areare inin parallel,parallel, thenthen thethe resultantresultant

groundground resistanceresistance cancan bebe calculatedcalculated usingusing thethe

ordinaryordinary equationequation forfor resistancesresistances inin parallelparallel..

InIn practice,practice, thethe effectiveeffective groundground resistanceresistance willwill

usuallyusually bebe higherhigher thanthan thisthis..


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Typically,Typically, aa 44 rodrod arrayarray maymay provide provide anan

improvementimprovement ofof aboutabout 22..55 toto 33 timestimes..

AnAn 88 rodrod arrayarray willwill typicallytypically givegive ananimprovementimprovement ofof maymay bebe 55 toto 66 timestimes..


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The multiple driven rod electrodeThe multiple driven rod electrode

TheThe drivendriven rodrod isis anan economicaleconomical andand simplesimple

meansmeans ofof makingmaking anan earthearth connectionconnection butbut itsitsresistanceresistance isis notnot sufficientlysufficiently lowlow..

AA numbernumber ofof rodsrods areare connectedconnected inin parallelparallel..

TheyThey shouldshould bebe drivendriven farfar apartapart asas possiblepossible toto

minimizeminimize thethe overlapoverlap amongamong theirtheir areasareas ofofinfluenceinfluence..

ItIt isis necessarynecessary toto determinedetermine thethe netnet reductionreduction inin

thethe totaltotal resistanceresistance byby connectingconnecting rodsrods inin parallelparallel.. TheThe rodrod isis replacedreplaced byby aa hemisphericalhemispherical electrodeelectrode

havinghaving thethe samesame resistanceresistance..


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TheThe methodmethod consistsconsists ofof assumingassuming thatthat

eacheach equivalentequivalent hemispherehemisphere carriescarries thethesamesame chargecharge..

CalculateCalculate thethe averageaverage potentialpotential ofof thethegroupgroup ofof rodsrods..

FromFrom thisthis andand thethe totaltotal chargecharge thethe capacitycapacity

andand thethe resistanceresistance cancan bebe calculatedcalculated..


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Rods too close


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Two groundTwo ground electrodeselectrodes

EquivalentEquivalent

hemispherehemisphere


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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

T

V

T

V

T

V

-

!!

drI

Vqactual

11

4Re

TV

rqideal

TV4

e !

1Re

Ree!!

actualq

idealq

actual

idealL


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Three electrode systemThree electrode system

Four electrode systemFour electrode system


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2 Grounding 1