Why are Stones Used in an Electrical Substation? . Electrical substations are critical components in the distribution and transmission of electrical power. They house transformers, switchgear, and other essential electrical equipment that help regulate voltage and maintain the efficient flow of electricity. However, when visiting or examining these substations, one may notice a consistent feature across most of them: the ground is covered with stones, typically granite or crushed rock.
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The presence of these stones is not coincidental or merely for aesthetic purposes. Instead, they serve vital functions that contribute to the safety, functionality, and longevity of electrical substations. This article explores why stones are used in electrical substations, their benefits, and the technical reasons behind their application.
Why are Stones, Pebbles, Grit and Gravel used in an a Switchyard?
In an electrical substation, there are multiple devices and equipment such as power and distribution transformers, transmission lines, Potential and Current transformers, Isolators etc. are grounded. Beside the grounding in a substation, we will discuss why grit and pebbles are used in a substation and what is the reason behind doing so?
There are multiple factors to use stones in the switchyard while designing of multiple earthing and grounding in a substation grid.
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The main reason to lay gravel in the substation yard is to reduce Ground Potential Rise (GPR) AKA Step Voltage and Touch Voltage which may be defined as follow:
- Ground Potential Rise (GPR): The maximum electrical potential that a substation grounding grid may attain relative to a distant grounding point assumed to be at the potential of remote earth. This voltage, GPR, is equal to the maximum grid current times the grid resistance.
- Step Voltage (Es) is defined as the maximum potential difference that exists between the feet when a fault current flows in the body.A particular case of step voltage is the Transferred voltage (Etransferred) where a voltage is transferred into or out of the substation from or to a remote point external to the substation site. It is usual to consider a distance of one meter between the metallic structure and the point on the ground.
- Touch Voltage (Et) can be defined as the maximum potential difference that exists between an earthed metallic structure capable of being touched by the hand and any point of the ground, when a fault current flows.
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As we know that step and touch voltage increases during the short circuit current and the resistivity of stones are considerably high as compared to other materials which can be found easily and everywhere. That’s why stones or gravel are laid in the substation to reduce the step potential and touch potential in case of maintenance or working on live equipment.
The following table shows the resistivity of different materials such as stones and sand etc.
| Material | Resistivity (Ωm) |
| Clay & Saturated Silt | <100 |
| Sandy Clay & Wet Silty Sand | 100-250 |
| Clayey Sand & Saturated Sand | 250-500 |
| Sand | 500-1500 |
| Weathered Rock | 1000-2000 |
| Gravel | 1500-5000 |
| Sound Rock | 1500-10000 |
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Reasons to Use Stones in a Substation and Electrical Switchyard
- Stone increases the resistance between our feet and the ground.
- Stones prevent the accumulation of rainwater in the switch yard.
- Stones reduce step potential and touch potential when operators work on the switch yard.
- Stones prevent vegetation and the growth of small weeds, plants, and grass inside the switch yard.
- Stones prevent the entry of animals and wildlife.
- Stones improve yard working conditions.
- 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 ground and the body during fault condition. When operating touch an electrical equipment during short circuit condition fault current flows through the human body.
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Stones increase resistance of soil, for this reason, step and touch potential limit decrease when people are working in substations/switchyard 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.
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.
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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 meters 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.
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FAQs
Why do substations use stones?
Stones are used in substations to improve safety, provide insulation, reduce step and touch voltages, and aid in moisture control.
What types of stones are commonly used?
The most commonly used stones are granite and crushed rock due to their non-conductivity and durability.
Do stones in substations need maintenance?
Yes, they need periodic cleaning and replacement depending on wear and tear, as well as the accumulation of debris.
Can using stones prevent fires in substations?
While stones do not actively prevent fires, they do help reduce the spread of fire by being non-combustible.
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