ASKARIAN - Amirkabir Univ. of Tech. Hafez Ave., Tehran, IRAN
EFATNEJAD - Amirkabir Univ. of Tech. Hafez Ave., Tehran, IRAN

P, Power plants and substations are extremely vulnerable to hazards of lighting strikes, of electrical and mechanical equipment malfuctionings, and of course, of human errors in which a surge current of the order of kiloamperes is impressed on the plant or is generated from within. An earthing system must be designed so that the touch and step potentials should always be contained below predetermined values that is would cause no severe damage to human being.Over the years, a number of techniques had been proposed to reduce the touch and step potentials.In this paper, the way of calculation of grounding system for H,V substation is given. In themethod dimensions of each mesh for grounding system is estimated first. Then for the designed system step and touch voltages are caiculated. If the values for step and touch voltages is greater than predetermined voltages, numbers of meshes are increased. In other word each become smaller. Computer program which is called SSG (substation systemgrounding) is repeated until all the necessary conditions fulfilled. The second part of the paper analysis of grounding system is made by using finite element technique the method has high accuracy. Variation diagrams for step and touch voltages for each vade and for all locations of the earthling system are shown in the final part of the paper. The results show the programs work satisfactory.