Project: T173700 #3722
 

Fall of Potential method for ground impedance measurement of station grids, described in IEEE Std. 81, requires the remote current probe “C2” be placed at a distance of at least five times the largest grid dimension from the centre of the station. The potential probe “P2” may be located in line with the current probe “C2” or in the opposite direction, with P2 located at 61.8% of the distance to C2, in the same direction under specific conditions. For two-layer soil, IEEE graphs are available to modify this “sweet” spot for “P2,” assuming the grid is hemispherical and there are no external interconnections. If these conditions are not present, the user plots the impedance for a number oflocations for P2 and looks for the curve to “level out”. Current split measurements described in IEEE Std. 81 standard indicates that the measurements are useful for evaluating the benefit of interconnections through overhead ground wires or multi-grounded distribution and help validate the modelled screening effect of overhead ground wires. Current split testing turns out to have great utility when test probe C2 cannot be located five times the largest dimension of the station. For some years, METSCO and Kinectrics have been using a modified Fall of Potential method with proximity correction that needs this current split information. Tested and modelled impedances have been in good agreement at most of the 200 stations measured in this way over the past 20 years. However, there are a number of possibilities for errors, which can be reduced when the distance to probe C2 is increased towards the above factor of five.