Fall of Potential (FOP) testing for measuring the ground impedance of station grids is described in IEEE Std 81. It requires the remote current probe C2 to be placed at least five times the largest grid dimension from the centre of the station. The potential probe P2 may be located in line with C or in the opposite direction. Locating P2 at 61.8 % of the distance to C2 in the same direction provides the correct ground impedance when (a) soil resistivity is uniform, (b) there are no external interconnections and (c) the grid is assumed to be hemispherical. Otherwise the user is simply advised to plot the measured impedances, taking the station ground impedance where this curve levels out. The new proximity correction method interprets the FOP test results in a more rigorous way. This project (a) investigated this proximity correction method by comparing its efficiency and accuracy to the existing method proposed by IEEE Std 81, (b) determined how the distance to the remote current probe needs to be adjusted when the soil resistivity contains anomalies, (c) studied the accuracy needed when measuring resistivity and current splits in support of the proximity correction method and (d) developed a way to estimate the current splits for stations where they cannot be easily measured.
Fall of Potential, Proximity correction method, Ground impedance