Safe Earthing Systems - Part IV: Earthing

The use of empirical formulae in calculating the behaviour of earthing grids under fault is a common practice in the design of earthing systems. However, the increasing availability of powerful personal computers has allowed for more accurate analytical analysis methods. A computer program has been developed by the authors for calculating earth grid behaviour under fault conditions. With this computer program, detailed analysis of two basic earthing grids has been performed for homogeneous and two layer soil structures. The results have been compared against those produced by the IEEE 80 recommended empirical formulae. Recommendations are given regarding the applicability, and limitations, of the IEEE 80 empirical formulae based on the comparison with the analytical approach. This will enable the selection of an analysis method, such that the apparent grid resistance and voltages associated with an earthing grid can be assessed with confidence.