Conventional pipe and plate earthing is used for earthing purpose. The alternate layers of salt and charcoal is made when the earth pit is filled. Why are charcoal and salt used ? why not other materials?
Let us first understand the properties of the salt and charcoal.
When wood, other plant and animal materials is strongly heated up to remove all water and volatile constituents, the black carbon residuals are left is known as Charcoal.
The charcoal is a porous material and it absorbs moisture from the air. The charcoal is hygroscopic, it absorbs moisture from the surrounding air.
Salt is an ionic compound and it has strong attractive force towards highly polar water molecules. In other words, we can say salt is hygroscopic in nature and, it absorbs liquid water and the water vaporous from the air.
Earthing charcoal and salt makes the ionic bonding with soil , and maintain the moisture, and thus it increase the conductivity of the pit.
The earthing resistance is not only the resistance of the earth pit,but it is the combined resistance of earth pit and the surrounding soil. The earth resistance of the soil and the earthing pit should be very low in order to have low earthing resistance.
Why are salt and charcoal used?
The charcoal and the salt both are good absorbent of the moisture. The salt has good ionic bonding with charcoal,soil and water. The earthing charcoal and salt have good ionic behavior which help in marinating the moisture.
In presence of the moisture, resistance of the conductive path decreases. During the fault, the fault current is supposed to find the least resistance path. The lesser resistance of the fault current path is must for quickly discharging the fault current in the earth. Watering of the earth pits at regular time period must be done to increase the conductivity.
" Before making the earth pit, the soil resistivity test must be done. The soil resistance is very important for an effective earthing system."
The earthing resistance is not the resistance of the electrode, but it is the resistance of the electrode and the soil. The charcoal and salt can retain the moisture for longer period of time and it increase the conductivity, thus it is possible to quickly pass the fault current in the ground through earth wire.
The earth pit resistance must be checked regularly at least once in a year on the most dry day and the earth pit resistance must be recorded. If the resistance of the earth pit is high, first it must be watered to lower its resistance value. If the resistance remains unchanged, the earth pit must be discarded and new earth pit must be made.
The resistance of the earth pit must be as low as possible to avoid building up of step or touch potential. The resistance of earth pit can be accurately measured with 3 or 4 wires electrode method. With the latest state of art earth resistance meter, the resistance of the earth pit can be measured even without disconnection of the earthing strip from the earth electrode.
To improve the resistance of the poor soil, the soil treatment is done as per IEEE80-2013 clause 14.5(a),(b),(c),(d).
Salt and charcoal has its own disadvantage of leaching property. The salt and charcoal washed away after some time and it needs replacement.
Quantity of Salt & Charcoal for Earthing
About 3:1 ratio of charcoal and salt is added in one earthing pit for better result.