Use of Software for Calculations of Earth Electrode by Empirical Formulas and by an Analytical Method

Very often a computer program is used to compute earth resistance and step and touch voltages of a grid earth electrode; the programs used for such calculations are computer coded formulas given in IEEE Std 80. To analyze the performance of an earth electrode, another type of software is used. Such a software uses a network representation of an earth electrode. A basic dc network consists of self and mutual resistances. This paper brings out the principle aspects of the two methods.

When a grid earth electrode is designed by using the equations given in IEEE Std 80-2000, the input data consists of (a) soil resistivity model of earth, at the substation site (b) the maximum earth fault current and grid current (c) geometry and size of area of the station (d) shape and material of earth conductors (e) duration of fault current and shock duration and (f) location and length of vertical rods. Spacing of horizontal earth conductors of grid is chosen and earth resistance of the electrode is determined as also the step voltage and touch voltage. Generally, spacing of horizontal earth conductors of grid electrode has to be such that the step voltage and touch voltage are less than the respective permissible values. This may require the computations to be repeated. To reduce the labour involved, a computer code is often used. A record of the results of all steps of the calculations is obtained as the output. So even though a computer is used for making the calculations, the computer is used to reduce the labour in using the expressions given in IEEE Std 80-2000 to obtain the parameters of interest.

An alternate method analyses an earth electrode by using a computer software that is based on network representation of the earth electrode. For this purpose, each conductor or its segments that make up the electrode are represented as network elements. The simplest representation is dc network. To obtain dc network of the earth electrode, each conductor segment is represented by its self-resistance and mutual resistances between it and all other conductor segments. The fault current is the input to this network. The network equations are solved to determine the current flowing from each segment. The required parameters of the earth electrode can then be determined.