Digital Ground Resistance Tester Model 4500
25
Z
Y
X
VOLTMETER (E)
AUXILIARY
POTENTIAL
ELECTRODE
GROUND
ELECTRODE
UNDER TEST
AUXILIARY
CURRENT
ELECTRODE
AMMETER (I)
CURRENT
SUPPLY
R
EARTH
Figure 9
NOTE:
Terminals X and Xv are shorted together in three-point measure-
ment.
4.3.1 Position of the Auxiliary Electrodes in Measurements
The goal in precisely measuring the resistance to ground is to place the
auxiliary current electrode Z far enough from the ground electrode under
test so that the auxiliary potential electrode Y will be outside of the effec
-
tive resistance areas of both the ground electrode and the auxiliary current
electrode. The best way to find out if the auxiliary potential rod Y is outside
the effective resistance areas is to move it between X and Z and to take
a reading at each location. If the auxiliary potential rod Y is in an effective
resistance area (or in both if they overlap), by displacing it, the readings
taken will vary noticeably in value. Under these conditions, no exact value
for the resistance to ground may be determined.
