Earthing is defined as the connection to the general mass of earth by means of an earth electrode.an equipment is said to be earthed when it is electrically connected to an earth electrode.
The purpose of earthing is to avoid the risk of fire hazards which may arise due to the leakage current flowing to earth through some accidental path which is not intended to carry such current, and to eliminate the danger of serious injury from electrical shock to persons who may come in contact with the metal frames or metal enclosure etc., which may have become live due to the insulation failure. To Protect equipment buildings from LIGHTNING.
EARTH ELECTRODE: A metal pipe or rod and metal plate or other conductor buried in the earth and electrically in contact with the general mass of used for wiring installation, viz, Pipe Earth, Rod Earth and Plate Earth.
According to I.S.S. the pipe electrode is to be of galvanised iron is not less than 38 mm. in diameter and 2.5 meter long. The minimum size of the road electrode is to be 16mm dia and 2.5m long. 19 m dia and 1.8m long are also commonly used, specially for pole earthing of OH line. The top of the pipe or road is to be buried to depth of not less than 1.25m under the ground level.
The plate electrode is to be of GI or copper plate. The size of the GI plate is not to be less than 60cm×60cm×6.30mm and the size of the copper plate is not to be less than 60cm×60cm×3.15mm. they are to be buried in the earth with there faces vertical and their tops are to be not less than 1.5m, below the ground level.