Alternative earthing calculations for grids and rods

With no international standard for the method of calculating earthing grid and rod resistances, engineers are faced with a variety of national standards incorporating apparently differing formulae; without critical examination the effects of those differences are unclear. This paper collects many of the basic formulae for comparison of their differences and their deficiencies. To overcome such deficiencies as exist, new formulae are presented that are suitable for use with a ‘two-layer ground’ model that also is presented. A method of expressing the effective resistance of rods in heterogeneous ground, to determine a combined grid and rod resistance, is set out. The results of a rigorous survey using programs incorporating these features are presented in chart form to show trends and precise differences. A suggested list of preferred formulae is presented as an adequate basis for design calculations.