Ground Resistance Testers Models 6422 & 6424
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If readings were taken by moving the auxiliary potential electrode S towards
either E or H, the reading differentials would be great and we could not obtain a
reading within a reasonable band of tolerance. The sensitive areas overlap and
act constantly to increase resistance as S is moved away from E.
Now consider the illustration below, where the E and H electrodes are sufficiently
spaced so that the areas of effective resistance do not overlap. If we plot the
resistance, measured we find that the measurements level off when S is placed
at 62 % of the distance from E to H, and that the readings on either side of the
initial Y setting are most likely to be within the established tolerance band. This
tolerance band is defined by the user and expressed as a percent of the initial
reading: ±2 %, ±5 %, ±10 %, etc.
2.3.1.3 Auxiliary Electrode Spacing
No definite distance between X and Z can be given, since this distance is
relative to the diameter of the electrode tested, its length, the homogeneity of
the soil tested, and particularly, the effective resistance areas. However, an
approximate distance may be determined from the following chart which is given
for a homogeneous soil and an electrode of 1 in in diameter. (For a diameter of
1/2 in, reduce the distance by 10 %; for a diameter of 2 in increase the distance
by 10 %.)
