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Digital Ground Resistance Tester Model 4500
4.5.1 Over-range Indication
Over-range is indicated when the display reads 1, or when the display is
blinking and the indicator is lit.
4.5.2 Incorrect Measurements
The incorrect measurement indicator shows excessive electrode resis
-
tance and excessive transient noise and stray current; it also indicates
when the selected test current is too high.
In the event of an incorrect measurement indication:
• Select the next lowest test current.
• Improve the quality of the auxiliary ground electrodes Y and Z; Z
is the most likely source of problems with excessive resistance.
• Check connections for continuity between leads and electrodes.
• Be sure that electrodes are properly inserted; they should be
completely buried, if possible.
• If stray currents are suspected, one solution to reduce their influ
-
ence is to move both Y and Z electrodes in and arc relative to the
X electrode (e.g. try a 90° shift), and test again.
4.6
2-Point Measurement
(Simplified Measurement)
This is an alternative method to three-point measurement when an excel
-
lent ground is already available.
In congested areas where finding room to drive the two auxiliary rods may
be a problem, the two-point measurement method may be applied. The
reading obtained will be that of the two grounds in series. Therefore, the
water pipe or other ground must be very low in resistance so that it will
be negligible in the final measurement. The lead resistances will also be
measured and should be deducted from the final measurement.
This method is not as accurate as three-point methods (62% method), as
it is particularly affected by the distance between the tested electrode and
the dead ground or water pipe. This method should not be used as a stan-
dard procedure, but rather as a backup in tight areas. See Figure 16.
