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.
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.
