Why To 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 same all over the area. The value of earthing resistance is depends upon the soil resistivity of the particular area. Actually the measured earth resistance is not only the resistance of the electrode, it is the combined resistance of conductor and the associated soil. During fault, if the soil resistance is higher the fault current cannot dissipate to associate soil.
To improve the conductivity of poor soil during fault current, salt and charcoal around the conductor is used to dissipate. Regarding this, IEEE 80–2013 clause 14.5 (a), (b), (c), (d) tells us about soil treatment for better 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 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 earth wire as early as possible to remove the chances of shock.
We can practice permanent earthing compounds which are moisture independent as per IEEE 80–2013 clause 14.5(d).

How salt and charcoal works to improve earthing resistance?

Coal or Charcoal is made of carbon which is good conductor that can minimize the earth resistant. The salt use as electrolyte to form conductivity between earth electrode (generally GI pipe or plate) Coal and Earth with humidity. Sand has 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 lesser its resistance. Adding charcoal and water in earthing pit decreases soil resistivity. 
The layer of charcoal and salt helps to maintain low resistance for earth fault currents. Because of ionic behavior of salt and charcoal they will maintain moisture content around the earth pit. 

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