If you observe any substation, the ground of the substation yard is filled with crushed gravel stones. This is because of the following reasons pertain to safety from shock.
1) Although substation grounding is chosen to provide low resistance path, the entire ground is filled with crushed stones to provide a high resistance layer so that it can act as an insulation between our foot and the ground.
2) The stone layer on the substation ground provide high resistance so that the fault currents flow into the ground but not along the ground.
3) To minimise step potential and touch potential voltages.
4) It avoids pool of inflammable oil etc. on the substation ground in case of any spilling of insulation oil from the equipment. This also avoids spreading of fire from one equipment to the other in the substation.
5) The rocky outer layer slows down the evaporation of moisture in the earth’s upper layers.
6) It restricts entering of snakes and other reptiles as the surface would be inconvenient to crawl.
7) It avoids growth of plants and weeds in the substation yard to some extent.
8) Generally 20 to 25mm baby gravel stones are used instead big size stones to facilitate movement of persons and equipment in the substation yard.