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FALL-OF-POTENTIAL METHOD TESTING PROCEDURE APPENDIX D: GROUNDING (EARTHING) ELECTRODE SYSTEM TESTING/VERIFICATION
D.3 FALL-OF-POTENTIAL METHOD TESTING PROCEDURE
D.3.1 TEST DESCRIPTION
The fall-of-potential test is the most widely accepted and recommended test method. This procedure is
documented in ANSI/IEEE STD 81, BS 7430:1998, NFPA 7800-2004, and MIL-HDBK-419A and
should be referred to for more details.
In the fall-of-potential test, two test rods are driven into the soil. These rods are placed in a straight line
from an electrode in the grounding electrode system to be tested (referred to here as connection 'X').
One of the test rods, referred to here as rod 'Z', is placed at a known distance from the X connection. The
other test rod (referred to here as rod 'Y') is placed at various distances between the X connection and
rod Z. If a known current is applied between the X connecting point and rod Z, a potential difference
(voltage drop) will exist between the X connection and rod Y. The fall-of-potential tester measures the
voltage drop between the X connection and rod Y, and converts the measurement to a resistance reading
using Ohm's Law (R= E/I). In this manner, the resistance at any point between the X connection and rod
Z can be measured. The multiple readings obtained during this test are entered and plotted as data points
on a graph. From the graph, the resistance of the grounding electrode system can be determined.
GROUND
ELECTRODE
UNDER TEST
CURRENT
SUPPLY
VOLTMETER (E)
X Y
AUXILIARY
POTENTIAL
ELECTRODE
R
AMMETER (I)
AUXILIARY
CURRENT
ELECTRODE
EARTH
FIGURE D-2 MEASURING POTENTIAL DIFFERENCE BETWEEN RODS
D.3.2 REQUIRED TEST EQUIPMENT AND SUPPLIES
• Fall-of-potential ground resistance tester (with supplied operator's manual, test leads, test clips, and
probes)
• Small sledgehammer
• Tape measure
• Safety glasses
• Insulated electrician's gloves
D-4 68P81089E50-B 9/1/05
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