AEMC 3640 - Ground Resistance Values

Digital Ground Resistance Tester Model 3640 and 4610 21
4.2 Ground Resistance Values
NEC
®
2008article250.56regardingtheresistanceofrod,pipeandplate
electrodesstatesthatiftherod,pipe,orplatedoesnothavearesistance
of25Ωorlesstogroundshallbeaugmentedbyoneadditionalelectrodeof
anyofthetypesspeciedby250.52(A)(4)through(A)(8).Wheremultiple
rod,pipeorplateelectrodesareinstalledtomeettherequirementsofthe
section,theyshallnotbelessthan6feetapart.
FPN:Theparallelingefciencyofrodslongerthan8feetisimprovedby
spacinggreaterthan6feetapart.
TheNationalElectricalCode
®
(NEC
®
)statesthattheresistancetoground
shall not exceed 25Ω.This is an upper limit and guideline,since much
lowerresistanceisrequiredinmanyinstances.
“How low in resistance should a ground be?”
Anarbitraryanswertothisinohmsisdifcult.Thelowerthegroundresis-
tance,thesafer,andforpositiveprotectionofpersonnelandequipment,it
isworththeefforttoaimforlessthanoneohm.Itisgenerallyimpractical
toreachsuchalowresistancealongadistributionsystemoratransmis-
sionlineorinsmallsubstations.Insomeregions,resistancesof5Ω or less
maybeobtainedwithoutmuchtrouble.Inothers,itmaybedifculttobring
resistanceofdrivengroundsbelow100Ω.
Acceptedindustrystandardsstipulatethattransmissionsubstationsshould
bedesignednottoexceedoneohmresistance.Indistributionsubstations,
themaximumrecommendedresistanceis5Ω.Inmostcases,theburied
gridsystemofanysubstationwillprovidethedesiredresistance.
Inlightindustrialorintelecommunicationscentralofces,5Ωisoftenthe
acceptedvalue.Forlightningprotection,thearrestersshouldbecoupled
withamaximumgroundresistanceof1Ω.
Theseparameterscanusuallybemetwiththeproperapplicationofbasic
groundingtheory.Therewillalwaysexistcircumstanceswhichwillmake
it difcult to obtain the ground resistance required by the NEC
®
. When
thesesituationsdevelop,severalmethodsofloweringthegroundresis-
tancecanbeemployed.Theseincludeparallelrodsystems,deepdriven
rod systems utilizing sectional rods and chemical treatment of the soil.
Additionalmethods,discussedinotherpublisheddata,areburiedplates,
buried conductors (counterpoise), electrically connected building steel,
andelectricallyconnectedconcretereinforcedsteel.
Electricallyconnectingtoexistingwaterandgasdistributionsystemswas
oftenconsideredtoyieldlowgroundresistance;however,recentdesign