A comparative study of expressions for reduction factor for ground resistance of foot

A good estimate of the ground resistance of foot is required for calculation of permissible values of step and touch voltages at a substation during an earth fault. The ground resistance of foot is taken as that of a circular conducting disk of 0.08-m radius. A thin layer of crushed rock is usually spread on surface of soil in the switchyard area. The effect of the layer of crushed rock on foot resistance can be computed analytically by modeling it as a metallic disk lying on a two-layer soil. A reduction factor, which accounts for the finite depth of crushed rock layer, is thus determined. A number of empirical expressions are available in literature to determine value of the reduction factor. In this paper, a comparative study of the available expressions is made. Based on the study, two expressions, which are the most suitable for determining reduction factor, for practical values of parameters involved, are selected. The selected expressions are further improved upon by optimizing their numerical coefficients.