AEMC 3640 - Ground Resistance Testing Principle

Digital Ground Resistance Tester Model 3640 and 4610 23
4.3 Ground Resistance Testing Principle
(Fall-of-Potential — 3-Point Measurement)
Three-point measurement is used to measure resistance to ground of
groundrodsandgrids.ThepotentialdifferencebetweenrodsXandYis
measuredbyavoltmeter,andthecurrentowbetweenrodsXandZis
measuredbyanammeter.
By Ohm’s LawE = RIor R =E/I, we may obtain theground electrode
resistance R.
IfE=20VandI=1A,then:
R===20ohms
E
I
20
1
Itisnotnecessarytocarryoutallthemeasurementswhenusingaground
tester.Thegroundtesterwillmeasuredirectlybygeneratingitsowncur-
rentanddisplayingtheresistanceofthegroundelectrode.
ZYX
VOLTMETER (E)
AUXILIARY
POTENTIAL
ELECTRODE
GROUND
ELECTRODE
UNDER TEST
AUXILIARY
CURRENT
ELECTRODE
AMMETER (I)
CURRENT
SUPPLY
R
EARTH
Figure 8
NOTE: Terminals X and Xv are shorted together in three-point measurement