Modelling a Fall of Potential Earthing Grid Impedance Test

Testing (A)

The fall of potential (FOP) method is a popular measurement method and the 61.8 % rule (both outlined by IEEE Std. 81) is used for establishing the resistance of an electrical earthing grid. While this rule can provide a quick estimation of the resistance of an electrical earthing grid, it has drawbacks that must be considered and understood.

In this presentation, we demonstrate through modelling how the soil resistivity variation and earth grid configuration cause inaccuracies in the earth grid impedance tests, and we provide practical recommendations that will improve the accuracy of earth grid impedance tests.