AEMC 4500 - Over-Range Indication; Incorrect Measurements; 2-Point Measurement (Simplified Measurement)

30 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inu-
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nding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nalmeasurement.Theleadresistanceswillalsobe
measuredandshouldbedeductedfromthenal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.