Use of Gravel in Electrical Substation (EHV)

Generally gravel size of approx. 40 mm, bed thickness of 100mm to 150mm used in and around EHV substation increases the soil resistivity in the outer layer and thus helps in reducing step and touch potential also it protects from lizards and snakes and Oil spillage.

Gravel and asphalt are commonly used in and around substations. Gravel is one of the most commonly used surfacing material for substation yards, while asphalt is sometimes used for driveways and parking areas. These high resistivity materials are very useful in reducing the exposure currents to workers during fault conditions. Both the electrical characteristics of a surfacing material and its moisture condition substantially affect the exposure voltage and resulting current. The electrical resistivities of several gravel and asphalt types are listed in IEEE Std. 80.

 As per NEETRAC project 2010, the following are concluded:-

The protective characteristics of a surfacing material are highly dependent upon its moisture content. A surfacing material is less effective in protecting a worker when it is wet. Crushed stones mixed with their own dust are significantly less effective compared to washed stones even in wet conditions. Dusty gravel retains the moisture for a long time increasing the time duration during which it is less effective. In comparison, the washed gravel dries out fast recovering its insulating properties. Among the washed gravels, the gravel with larger sized stones performs somewhat better than their smaller sized counterparts. Between concrete and asphalt, protective characteristics of asphalt are significantly better in almost any environmental condition.