There are many reasons why Power Substations uses stones in the switch yard, here are some of the reasons;
- Stone increases resistance between our foot and the ground.
- Stones prevent the accumulation of rain water in switch yard.
- Stones reduces step potential and touch potential when operators work on switch yard.
- Stones prevents vegetation and growth of small weeds, plants and grass inside the switch yard.
- Stones prevents the entry of animals and wild life.
- Stones improves yard working condition.
- Stones protect from fire when oil spillage takes place.
Step and Touch Potential;
During Short Circuit current Step and Touch Potential increases.
Step Potential – It is the potential developed between the two feet on the ground of a man or animal when short circuit occurs. This results in flow of current in the body leads to electrical shock.
Touch Potential – It is the potential that is developed between the gorund and the body during fault condition. When operating touch an electrical equipment during short curcuit condition,fault current flows through the human body.
Stones increase resistance of soil, for this reason, step and touch potential limit decrease when people are working in substations/swicthyard it reduces the possibility of electric shock during work.
That is why substations uses gravel/stones to reduce the step potential and touch potential when operators work on switchyard.
If you see the table above gravel has a resistivity 1500 to 5000 ohm meter.
Earth Potential Rise/Ground Potential Rise.
In electrical engineering, earth potential rise (EPR) also called ground potential rise (GPR) occurs when a large current flows to earth through an earth grid impedance. The potential relative to a distant point on the Earth is highest at the point where current enters the ground, and declines with distance from the source. Ground potential rise is a concern in the design of electrical substations because the high potential may be a hazard to people or equipment.
The change of voltage over distance (potential gradient) may be so high that a person could be injured due to the voltage developed between two feet, or between the ground on which the person is standing and a metal object. Any conducting object connected to the substation earth ground, such as telephone wires, rails, fences, or metallic piping, may also be energized at the ground potential in the substation. This transferred potential is a hazard to people and equipment outside the substation.
Earth Potential Rise (EPR) is caused by electrical faults that occur at electrical substations, power plants, or high-voltage transmission lines. Short-circuit current flows through the plant structure and equipment and into the grounding electrode. The resistance of the Earth is non-zero, so current injected into the earth at the grounding electrode produces a potential rise with respect to a distant reference point. The resulting potential rise can cause hazardous voltage, many hundreds of metres away from the actual fault location. Many factors determine the level of hazard, including: available fault current, soil type, soil moisture, temperature, underlying rock layers, and clearing time to interrupt a fault.
Earth potential rise is a safety issue in the coordination of power and telecommunications services. An EPR event at a site such as an electrical distribution substation may expose personnel, users or structures to hazardous voltages.