Amprobe GP-1 Earth-Tester-Ground-Probe - Manual (Page 10)

The following instructions outline the typical approach to perform a test using the “fall of
potential” or 62% method.
1) Switch the GP-1 to Test C “Earth Resistance”
2) Noting color codes, connect the four test cables to the input jacks of the instrument.
3) Connect the BLACK (C1) and GREEN (P1) test cables to the isolated electrode under
test.
4) To determine the distances from the electrode in which to drive the tet rods, use the
formula on Page 11.
Electrode or Grid System
Resistance:Test C & D
Page 10
Warning: The power must be turned OFF, the electrode
must be isolated from the utility ground, building steel, or any
other ground, in order for the GP-1 to correctly measure the
resistance value of an earth ground electrode (Test C).
Before isolating the electrode under test, verify that power is
OFF by:
• Measuring the voltage from the grounding conductor to a
reliable earth ground. Use the voltmeter measurement
feature on the GP-1 (see page 14).
• Measuring the leakage current through the electrode using
an AMPROBE Model DLC-100, or equivalent, Digital
Leakage Current Clamp.
Warning: If more than 50ma of current is flowing through
the electrode, do not isolate the ground electrode until the
source of current is located and turned OFF.