AEMC 3640 - Page 24

22 Digital Ground Resistance Tester Model 3640 and 4610
changesutilizingnon-metallicpipesandinsulatingjointshavemadethis
methodof obtaining alowresistance groundquestionableand in many
instancesunreliable.
Groundrods,ofcourse,willberequiredinhighvoltagetransmissionlines,
wheremaximumresistanceof15ohmsisrecommended;andindistribu-
tionlines,wheremaximumresistanceof25ohmsispreferred.Allelectri-
calsystemsconstructedinaccordancewiththeNationalElectricalCode
®
,
shouldnotexceed25ohms.
The measurement of ground resistances may only be accomplished
with specially designed test equipment. Most instruments use the Fall
of Potential principle of alternating current (AC) circulating between an
auxiliaryelectrodeandthegroundelectrodeundertest;thereadingwill
begiveninohmsandrepresentstheresistanceofthegroundelectrodeto
thesurroundingearth.AEMC
®
Instrumentshasalsorecentlyintroduceda
clamp-ongroundresistancetester.
Note: The National Electrical Code
®
and NEC
®
are registered trademarks of the National Fire
Protection Association.