Position Of The Auxiliary Electrodes In Measurements - AEMC 3640 User Manual

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.
On the other hand, if the auxiliary potential rod Y is located outside of
the effective resistance areas, as Y is moved back and forth the reading
variation is minimal. The readings taken should be relatively close to each
other, and are the best values for the resistance to ground of the ground X.
The readings should be plotted to ensure that they lie in a "plateau" region
as shown in Figure 10.
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X Y' Y Y''
X-Y Distance
X
X-Y Distance
Z
Reading Variation
Figure 9
Y Y Y''
Figure 10
Digital Ground Resistance Tester Model 3640 and 4610
Effective Resistance
Areas (Overlapping)
Z
Effective Resistance
Areas (No Overlap)
Reading Variation