Why are Stones Used in an Electrical Substation?

Why are Stones, Pebbles, Grit, and Gravel Used in a Switchyard?

In an electrical substation, you’ll find many types of equipment, such as power transformers, distribution transformers, transmission lines, Potential Transformers (PTs), Current Transformers (CTs), and isolators. To ensure safe operation, all of these devices are properly grounded.

But grounding alone isn’t enough. Another key feature of switchyard safety is the use of stones, pebbles, grit, and gravel on the ground surface. If you’ve ever visited a substation, you’ve probably noticed that the entire yard is covered with these materials. Many people wonder why they are used and what purpose they actually serve.

The reason lies in both electrical safety and operational reliability. These insulating materials help control step potential and touch potential, reduce the flow of dangerous fault currents, and provide a safer environment for both workers and equipment. At the same time, they help with drainage, dust control, and overall substation maintenance. Simply put, stones and gravel act as a protective layer that improves both safety and efficiency inside the switchyard.

Watch the video below to learn why stones are used in electrical substations.

There are several important reasons why stones, gravel, and pebbles are spread across the surface of a switchyard, especially when designing a multiple earthing and grounding system in a substation grid. The most critical purpose is to reduce the effects of Ground Potential Rise (GPR), which directly influences Step Voltage and Touch Voltage. These concepts are explained below.

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Ground Potential Rise (GPR)

Ground Potential Rise refers to the highest electrical potential that a substation’s grounding grid can reach compared to a distant point in the earth, which is assumed to be at zero or remote earth potential. GPR is calculated as the product of the maximum grid current and the grid resistance. A high GPR can be extremely dangerous because it increases the risk of electrical shock inside the yard.

Step Voltage (Es)

Step Voltage is the maximum potential difference that occurs between a person’s two feet while standing on the ground when a fault current passes through the body. In simpler terms, if the ground under one foot is at a higher potential than under the other, current may flow through the body. A special case of this is called Transferred Voltage (Etransferred), which happens when a dangerous potential is carried into or out of the substation from an external point. For safety testing, it is usually measured at a one-meter distance between a metallic structure and a point on the ground.

Touch Voltage (Et)

Touch Voltage is the maximum potential difference between a grounded metallic structure (such as a transformer body or fence) that a person can touch with their hand and the ground surface under their feet during a fault condition. This represents one of the most dangerous risks in a substation, as it can cause severe electric shock if someone touches equipment when fault current is flowing through the grounding system.

By covering the switchyard with stones, gravel, and pebbles, these risks are significantly reduced. The layer of insulating material increases the resistance of the ground surface, which in turn lowers the step and touch voltages, making the substation safer for workers and equipment.

It is well known that both step voltage and touch voltage increase sharply during a short-circuit fault current. To reduce this risk, stones, gravel, or pebbles are spread across the entire substation yard. The key reason is that the electrical resistivity of gravel and stones is much higher compared to normal soil or common ground materials.

Because of their high resistivity, this insulating layer limits the amount of fault current that can pass through the human body in case of accidental contact. As a result, it effectively reduces both step potential and touch potential, making it safer for personnel working near live equipment or carrying out maintenance activities in the switchyard.

The effectiveness of stones, gravel, or sand in a substation grounding system can be better understood by comparing their electrical resistivity values. Materials with higher resistivity provide greater insulation against fault currents, thereby minimizing the risks of dangerous electric shock.

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Resistivity of Common Materials

From the table, it is clear that gravel and sound rock provide much higher resistivity compared to clay or silt. This makes gravel the most effective choice for switchyard flooring, as it creates a strong insulating layer that significantly lowers the chances of electric shock hazards during fault conditions.

Reasons to Use Stones in a Substation & Electrical Switchyard

There are several key reasons why stones, gravel, and pebbles are spread in a substation switchyard instead of other surface materials. These benefits improve safety, reliability, and performance of the entire electrical substation system.

No Grass or Weeds Allowed

Grass, weeds, and small plants create problems in rainy or humid weather because they hold moisture and can become conductive. Dry grass is also a fire hazard and, in extreme cases, may even lead to short circuits around high-voltage equipment. By covering the ground with stones, plant growth is prevented, keeping the switchyard safe and clear.

Protection Against Wildlife

A stone or gravel surface makes it difficult for snakes, lizards, rats, and other animals to enter or hide inside the switchyard. This extra layer of protection reduces risks for both the equipment and the maintenance staff, who often work near energized systems.

Prevents Water Accumulation

Stones improve drainage and prevent waterlogging or puddles from forming. Since standing water is extremely dangerous around high-voltage equipment, this feature helps keep the yard dry and safe even during heavy rainfall.

Shock and Vibration Resistance

Unlike grass or loose soil, gravel and pebbles are more resistant to mechanical shocks. This helps reduce vibrations in transformers caused by magnetostriction in the core, and also provides extra stability during rare events such as earthquakes.

Improves Soil Resistivity

A layer of stones increases the soil resistivity, which lowers both touch potential and step potential. This significantly reduces the risk of electric shock hazards during faults or maintenance work. At the same time, it prevents plant growth, which could otherwise lower soil resistance over time.

Better Working Conditions

By keeping the substation clean, dry, and electrically insulated, stones and gravel create a safer and more comfortable working environment. When combined with a proper grounding system, this ensures smoother operation of all electrical equipment and provides workers with better protection from electric shocks.