Stones/Gravel is used in Electrical Switchyard because of following reasons
1. During Short circuit current Step and Touch potential increases.
(a) Step potential: It is the potential developed between the two feet on the ground of a man or animal when a short circuit occurs. This results in a flow of current in the body lead to electrical shock
(b) Touch potential: It is the potential that is developed between the ground and the body of the equipment when a person touches the body during a fault condition. When operating personnel touch an electrical equipment during short circuit condition, fault current flows through the human body. This is defined as touch potential.
Gravel/Stones in the substation is provided to reduce the step potential and touch potential when operators work on switchyard.
2. Power Transformers installed in the substations will have oil as cooling and insulating medium. Oil leakage takes place during operation or when changing the oil in the transformer. This oil spillage which can catch fire is dangerous to the switchyard operation. So Stones/Gravel is provided to protect from fire when oil spillage takes place.
3. Stones eliminate the growth of small weeds and plants inside the switchyard.
4. Improves yard working condition.
5. Provides protection from wildlife and snakes.