Why Do We Use Charcoal and Salt for Earthing and Grounding?
For safe and effective earthing and grounding, it is important to keep the ground resistance as low as possible. This helps electrical current flow easily into the earth during faults or surges. To do this, we must lower the soil resistivity around the earthing electrode. The earth wire connects the metal parts of machines to an earth plate buried in the ground. To improve the soil’s conductivity, a mixture of charcoal and salt—in a ratio of 3:1—is placed inside the earth pit. This combination helps maintain a moist, conductive path for electrical leakage or fault currents.
You might wonder why we use only charcoal and salt instead of other materials. The answer lies in their special properties that make them ideal for maintaining low resistance and moisture in the soil.
When wood is burned at a controlled temperature, all its moisture and volatile contents are removed. The remaining black carbon-rich material is called charcoal, which has a very low amount of ash.
Charcoal—also known as soot, coke, or carbon black—has a porous structure, much like a sponge. Because of this, it easily absorbs and retains moisture from the air and surrounding soil. Being an allotrope of carbon, charcoal can conduct electricity, similar to graphite. It also helps distribute the current evenly through the soil, keeping the earthing resistance low. For example, if the soil dries out during hot weather, the charcoal layer helps trap moisture, ensuring the earthing system still works properly.
Salt, on the other hand, is a crystalline compound known as sodium chloride (NaCl). It is ionic in nature, meaning it attracts polar water molecules. Salt is hygroscopic, so it can absorb moisture from the air, especially when humidity is above 75%. By doing this, it keeps the earth pit moist even during dry periods. The salt also increases the electrolytic conductivity of the soil, allowing electric current to flow more easily.
Together, charcoal and salt form the perfect combination for improving the performance of an earthing system. Charcoal provides a conductive path and moisture retention, while salt ensures continuous humidity and enhances the soil’s conductivity. This balance keeps the earthing system reliable and safe for both domestic and industrial installations.
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Why Are Charcoal and Salt Added in the Earth Pit?
The mixture of charcoal and salt is widely used in earthing and grounding systems because it helps keep the soil moist and conductive. When mixed together, these materials create ionic bonds with water in the soil, which improves the flow of electrical current from the grounding rod or earth plate buried in the earth pit. As soil moisture increases, its conductivity rises, allowing electricity to pass easily into the ground. This is why the charcoal and salt mixture is the most effective combination for maintaining low resistance in earthing systems.
To further enhance performance, layers of charcoal and salt are arranged alternately in the earth pit. This setup increases the effective contact area between the soil and the electrode, which in turn reduces the overall earth resistance. For instance, each layer of salt attracts and holds water, while charcoal retains that moisture for a longer period—creating a stable and efficient conductive environment around the electrode.
As explained earlier, when charcoal and salt are placed in alternating layers, they continuously absorb moisture from the surrounding soil and air. The salt forms a strong bond with water molecules, while charcoal helps maintain the dampness and spreads the conductivity evenly. This results in a low-resistance path for fault current to flow safely from the metallic body of electrical machines through the earth conductor, down to the earth plate or electrode in the pit.
A mixture of coal dust or powdered charcoal can also help retain moisture around the soil for a long time. This consistent dampness keeps the earth resistance low, providing a safe path for fault currents to discharge into the ground. In the event of a fault, this setup prevents electric shock to anyone touching a metal-cased appliance and protects electrical machines from damage.
Since soil resistance varies depending on the type of soil and local conditions, it’s essential to test the soil’s conductivity before installing an earth pit. The standard recommendation is that the earthing resistance should not exceed 1 Ω for ideal safety. If it measures higher—say between 1 to 5 ohms—you can improve it by increasing the size of the earthing lead or the earth continuity conductor. It’s also important to pour water regularly into the GI pipe connected to the earth pit to maintain proper moisture levels. By keeping the soil around the earth plate moist and conductive, the entire earthing and grounding system continues to work efficiently and reliably for electrical protection.
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Good to Know:
According to the NEC (National Electrical Code) Section 250.56, the impedance of the grounding system should be less than 25Ω to ensure safe dissipation of fault current into the earth.
As per the IEC (International Electrotechnical Commission) standards, the ideal range of earth resistance is between 1Ω and 10Ω, depending on soil conditions and system requirements.
Guidelines from IEEE (Institute of Electrical and Electronics Engineers) and NFPA (National Fire Protection Association) recommend that the grounding resistance should be 5Ω or less for reliable protection and equipment safety.
To improve soil conductivity and reduce grounding resistance, you can follow the IEEE 80–2013 standard (Clause 14.5 (a–d)), which outlines proper soil treatment methods—including the use of chemical compounds, conductive materials, and moisture-retaining layers—to achieve a low-resistance earthing system.
Maintenance of Earthing Pit
To ensure the earthing and grounding system works safely and efficiently, it is important to maintain the earth pit regularly. Proper maintenance helps keep the system’s resistance low and ensures a reliable path for fault current to flow into the ground. Follow these simple steps to keep your earthing system in good condition.
Start by adding a 30 cm (1 ft) layer of powdered charcoal and salt inside the earth pit. This mixture helps retain moisture around the grounding rod and earth plate, reducing soil resistance. The combination acts as a natural conductor and moisture stabilizer, allowing electricity to pass easily into the earth even during dry conditions.
Use an earth tester or ground resistance meter to measure the resistance of the earthing system, preferably on a dry or sunny day. The measured resistance should ideally be 1 Ω or less. If it exceeds this value, you can improve it by increasing the size of the grounding conductor (also called the earth continuity wire) and the earthing lead to enhance current-carrying capacity and lower the resistance.
Because both salt and charcoal are leaching materials, they gradually wash away with water over time. To maintain proper moisture levels and conductivity, pour water periodically into the earth pit. This keeps the soil damp around the earth plate, ensuring the earthing and grounding system continues to operate effectively and provides consistent protection against electrical faults and shocks.
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Ratio of Charcoal and Salt in Grounding and Earthing System
There is no fixed standard ratio for using charcoal and salt in an earthing system, as the ideal proportion depends on the type of soil, its resistivity, and the local environmental conditions. Soil with high moisture and low resistivity requires less treatment, while dry or rocky soil may need more.
As a general rule of thumb, a 1:3 ratio of salt to charcoal works well for plate earthing. This means 1 kg of salt should be mixed with 3 kg of charcoal to achieve good conductivity. In the case of pipe earthing, about 5 kg of salt and 5 kg or more of charcoal are commonly used to maintain sufficient soil conductivity around the earthing electrode. This balance helps create a low-resistance path for current flow and ensures effective system performance.
Demerits of Charcoal and Salt in the Earthing System
While charcoal and salt are cost-effective and easy to use, they also have some disadvantages. Because both materials are leaching compounds, they dissolve gradually in water and wash away over time. This means the earth pit requires regular maintenance and periodic refilling to maintain its performance.
According to IEEE 80–2013 (Clause 14.5(d)), modern permanent earthing compounds like TEREC+ or CUREC+ are better alternatives. These materials are moisture-independent and can last for 25 years or more without frequent maintenance. They provide consistent low resistance and stable grounding performance, making them ideal for long-term and critical electrical installations.
