DESIGN OF EARTH MAT FOR A SUBSTATION

1) Soil resistivity

Before designing earth mat, it is necessary to determine the soil resistivity of the area in which substation is to be located. Resistivity of the earth varies considerably from 10 to 10,000 Ω-m depending on the types of the soil. Also resistivity varies at different depth depending upon the type of soil, moisture content and temperature etc., at various depths, which affects the flow of current due to the fact that the earth fault current is likely to take its path through various layers.

2) Tolerable limits of body current

Effect of current passing through vital organs of human body depends on magnitude, duration and frequency of current. Current in the range of 1-8 mA are known as 'let go current' because these currents, though unpleasant, impair the ability of a person, holding an energised object to release it. Currents in the range of 9-25 mA may be painful and impair the ability to release energised object. Still higher currents make breathing difficult. However, if the current is less than about 60 mA, the effects are not permanent & disappear when current is interrupted.

Currents higher than 60 mA may lead to ventricular fibrillation, injury & death.

3) Fault current

As the earthing system has to carry the earth currents, the maximum earth fault current likely to flow in the system is considered for designing of earthing. A good earthing system for substation can be designed using an earth mat which is formed by a grid of horizontally buried conductors which serves to dissipate the earth fault currents to earth, also as an equipotential bonding conductor system, along with required number of vertical earth electrodes which are connected to the points of earthing of various equipments, structures and also interconnected with the horizontal earth mat. M.S. rods are generally used for the earthing of substation.

Total Earth resistance of the station system must be below 3 ohms for low voltage domestic system, below 0.5 ohms for low voltage and medium voltage sub- station, and below 0.1 ohm for 220 KV and 400 KV sub-station and power plants. If value of Earth Pit resistance is found high, then it is to be treated to bring back the value within the normal range.