Why We put Salt and Charcoal into Earth-pit while Earthing?

What is Earthing Resistance?

Earthing resistance is not the resistance of the electrode. The earthing resistance is not the same all over the area. The value of earth resistance depends upon the soil resistivity of the particular area. Actually, the measured earth resistance is not only the resistance of the electrode, but it is also the combined resistance of the conductor and the associated soil. During a fault, if the soil resistance is higher, the fault current cannot dissipate to the associated soil.

To improve the conductivity of poor soil during fault current, salt and charcoal around the conductor are used to dissipate. Regarding this, IEEE 80–2013 clauses 14.5(a), (b), (c), and (d) address soil treatment to improve conductivity.

Using Salt & Charcoal has its own demerits. Salt & Charcoal are leaching compounds, so it required frequent application. 

Charcoal is used to retain the moisture for a long duration because it is an adsorbent, and salt is added to increase the conductivity. Both these products help to pass the leakage current through the earth wire as early as possible to reduce the chances of shock.

We can practice permanent earthing compounds, which are moisture independent as per IEEE 80–2013 clause 14.5(d).

You may read about:

01. What is the Function of an earthing or Grounding Transformer?

02. Equipment Grounding for De-energized Construction and Maintenance.

How do salt and charcoal work to improve earthing resistance?

Coal or Charcoal is made of carbon, which is a good conductor that can minimize the earth's resistance. The salt is used as an electrolyte to form conductivity between the earth electrode (generally GI pipe or plate), coal, and the Earth with humidity. Sand is used to form porosity to cycle water & humidity around the mixture.

The whole purpose of providing earthing is to discharge the leakage current or fault current safely to the ground. Means better the conductivity of the soil or less its resistance. Adding charcoal and water to the earthing pit decreases soil resistivity. 

The layer of charcoal and salt helps to maintain low resistance for earth fault currents. Because of the ionic behavior of salt and charcoal, they will maintain moisture content around the earth pit. 

Salt and charcoal reduce earthing (grounding) resistance by directly attacking the biggest variable in the system: soil resistivity around the electrode, rather than the electrode itself.

Why the electrode isn't the bottleneck

The measured "earth resistance" of an installation isn't the resistance of the GI pipe or plate — metal is an excellent conductor. The actual earthing resistance is the combined resistance of the conductor and the surrounding soil, and it varies with local soil resistivity. So when resistance is too high, the fix has to target the soil, not the electrode.

Per IS 3043, when installations involve high-resistivity soils, the standard approach is to dissolve a highly conductive substance into the moisture already present in the soil around the electrode. IS 3043 lists sodium chloride, calcium chloride, sodium carbonate, copper sulphate, and salt-and-charcoal combinations as acceptable soil-treatment agents.

How charcoal contributes

Charcoal's role is almost entirely physical, not chemical:

• It's porous and carbon-based, giving it inherent conductivity plus a large internal surface area.
• Its porosity lets it absorb and retain moisture for an extended period compared to bare soil.
• That porous structure also resists compaction, maintaining better physical contact between the electrode and the surrounding fill material over time.

In effect, charcoal acts as a moisture reservoir that keeps the zone around the electrode from drying out between rainfalls or watering cycles.

How salt contributes

Salt is the electrolyte:

• As a natural electrolyte, salt increases the ion concentration in the soil, and once dissolved in water, it lowers bulk resistivity by making current flow easier.
• Practically, only about 5% moisture-to-salt content is needed to sharply cut soil resistivity — beyond that point, additional salt gives diminishing returns.

Why does the combination work better than either alone

Together, salt and charcoal form an ionic bridge with the moisture in the pit — as moisture content rises, so does the conductivity between the earth conductor and the buried electrode. Standard practice reflects this synergy: earth pits are typically filled with alternating layers of charcoal and salt, or with a proprietary earth-reactivation compound, around a GI pipe electrode in a pit roughly 1.5 m × 1.5 m × 3 m.

This treatment approach is also codified internationally — IEEE 80-2013, clause 14.5 (a)–(d), addresses soil treatment methods for improving conductivity around ground electrodes, which parallels the IS 3043 salt/charcoal practice used across South Asia.

The catch: it's not permanent

This is the detail that's often left out of surface-level explanations, and it's genuinely important for your readers:

• Salt and charcoal are leaching compounds — they wash out of the pit gradually over time, so periodic reapplication and resistance re-testing are required to maintain safety margins.
• Excess salt use can also corrode metallic electrodes and negatively affect nearby soil composition and groundwater over the long term.
• For very high-resistivity soils (rocky, sandy, arid), the salt-charcoal method alone may not be sufficient, and additional measures are needed — this is where modern low-maintenance alternatives like bentonite-based or marconite (conductive concrete) backfills are increasingly specified instead of, or alongside, salt-charcoal in current design practice.

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