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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
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! 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
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! 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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