AEMC 4620 User Manual page 27

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.
Digital Ground Resistance Tester Model 4620 and 4630
X Y' Y Y''
X-Y Distance
X
X-Y Distance
Z
Reading Variation
Figure 9
Y Y Y''
Figure 10
Effective Resistance
Areas (Overlapping)
Z
Effective Resistance
Areas (No Overlap)
Reading Variation
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