The three-point or fall of potential method is one of the techniques carried out to determine ground resistance. This method uses three poles where the potential difference is measured on points X and Y while current is applied in points X and Z. Using Ohm’s Law, E=RI or R=E/I, you can obtain the ground stake resistance.
The proper spacing of the electrodes is the key in precise ground resistance measurement. The auxiliary electrode Z should be far enough from the ground pole (X) under test. This is to ensure that the potential probe Y is outside the effective areas of resistance of both the auxiliary current rod Y and ground rod X. To determine if the potential rod Y is outside an effective resistance area, move between X and Z and take a reading at each point. The values should not significantly vary and should be close to each other.
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Editor’s Note: This post was originally published on September 10, 2012 and has been updated freshness, accuracy and comprehensiveness.