In Electrical substations (switchyards) stones are used rather than sand or grass for the following reasons : 

• Stones are used in the substation to reduce the risk of steps and touch potential when operators work on a switchyard as step and touch potential increase during a short circuit. We can define step potential as the potential between the two feet of a man on ground or an animal if a short circuit occurs. This results in current flow in the body resulting in an electrical shock. Touch potential is created by a combination of ground and equipment when a person touches the body during a failure. In short circuit situations, as operator touch the electrical equipment, failure flows through the human body.
• Power transformers mounted at substations have oil as a cooling and insulating medium. Oil leakage happens during service, or when changing the oil of transformers. The oil spillage capable of catching fire is risky for switchyard activity. Stones are, therefore, one of the solutions given to protect the oil from burning. 
• Stones eliminate the growth of small weeds and plants inside the switchyard.
• The use of stone can also increase the towering footing resistance and improve the yards working conditions.
• It also absorbs moisture in plain grounds when the grass grows and cause damage to the transmission lines, and it will also form current leakage. Therefore, it prevents the growth of small weeds and plants in the switchyard.  
• It avoids water pools during rainy seasons as water flows under the stones.
• It protects against wildlife and other species such as snake, which find it difficult to crawl in such areas.