Measurement of Earthing Resistance-Three Point method

3pointThe 3 Point Method or fall of Potential Test

 The 3 point method fall of potential ground resistance test requires complete isolation from the power utility. Not just power isolation, but also removal of any neutral or other such ground connections extending outside the grounding system. This test is the most suitable test for large grounding systems and is also suitable for small electrodes.

Fall of Potential Method:

In this method two auxiliary earth electrode, besides the test electrode, are placed at suitable distance from the test electrode. A measured current is passed between the electrode “A” to be tested and auxiliary current electrode “C” and the potential difference between the electrode “A” and auxiliary electrode “P” is measured. The resistance of the test electrode “A” is given by



R= Resistance of the test electrode in ohms.

V= Reading of voltmeter in volts and

I= Reading of ammeter in amperes.

There are three basic fall-of-potential test method.  The types of Fall-of-Potential are:

  • Full fall-of-Potential: A number of tests are made at different spaces of “P” and the resistance curve is plotted.
  • Simplified Fall-of-Potential: Three measurements are made at defined distance of”P” and mathematical calculations are used to determine the resistance.
  • 61.8% Rule: A single measurement is made with “P” at a distance 61.8% (62%) of the distance between the electrode under test and “C”.

The 3 Point Method or fall of Potential Test

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