Increasing of Ground Resistance of Human Foot in Substations Yards

Tolerable step and touch voltages for a person largely depend on the grounding resistance of the foot. In a substation switchyard a 0.08 to 0.15 m layer of crushed rock is often spread on the surface of the earth to increase the grounding resistance of the feet of the people in the substation area. However, if the thickness of the layer of crushed rock is small and the soil underneath is of low resistivity, a high value of the footing resistance is not obtained. Cement concrete pathways in a switchyard when wet may result in a low grounding resistance of the foot. For designing a safe grounding system a high value of footing resistance is desirable. A simple method that can be adopted to obtain high footing resistance is the use of a sheet of plastic material such as polyethylene, permalon, etc., immediately below the layer of crushed rock or the cement concrete pathway, at places in a substation where high footing resistance is needed. Plastic sheets having tear and puncture resistant qualities are available and if used properly should last for a long time. Analysis of the footing resistance when a plastic sheet is spread below the surface of earth is presented in this paper. Scale model tests have been conducted to obtain results and a simple method has been developed to obtain the grounding resistance of the foot when a plastic sheet is used below the surface of the soil.