AEMC 4500 - Measurement Procedures and Techniques

Digital Ground Resistance Tester Model 4500 27
Z
Y
X
X
v
TEST RANGETEST CURRENT
2
10
50
2
20
200
2
20
Alligator Clips
Ground Rod
Y Electrode
Z Electrode
Ground
Strip
Figure 12
Consider Figure 13, which shows the effective resistance areas
(concentricshells)ofthegroundelectrodeXandoftheauxiliarycurrent
electrode Z. The resistance areas overlap. If readings were taken by
moving the auxiliary potential electrode Y towards either X or Z, the
reading differentials would be great and one could not obtain a reading
within a reasonable band of tolerance. The sensitive areas overlap and act
constantly to increase resistance as Y is moved away from X.
Distance from Y to Ground Electrode
Resistance
Overlapping Effective
Resistance Areas
XY
Z
Ground
Electrode
Under Test
Auxiliary
Potential
Electrode
Auxiliary
Current
Electrode
Figure 13
Now consider Figure 14, where the X and Z electrodes are sufciently
spaced so that the areas of effective resistance do not overlap. If we plot
theresistance,measuredwendthatthemeasurementsleveloffwhen
Y is placed at 62% of the distance from X to Z, and that the readings on