Earthing of Substations :
For the earthing of substations, we generally use the horizontal mesh of steel rods and the vertical electrodes welded to mesh.
The vertical risers and steel earthing strips etc. are connected between the grounding mesh and the part to be grounded.
The earthing system consists of the following components :
(a) Earthing mat: It consists of a large number of rods that are connected through the copper conductor.
(b) Earthing electrode: It can be any wire, rod, pipe, plate, etc. which is driven into the ground either vertically or horizontally.
(c) Earthing risers: These are generally the steel rods welded to the earthing mesh at one end and another end is brought up to the equipment foundation.
(d) Earthing connections: These are generally copper strips that are used for the final connection between the earth riser and the part to be earthed.
Purpose of Substation Earthing :
Substations earthing is required for the following reasons :
(a) It provides safety to the working staff against the electric shock.
(b) It also provides the return path to the flow of fault current and the discharge current from the lightning conductors, surge arresters, etc.
(c) The overhead shielding wire is connected to the earthing system to obtain protection against lighting strokes.
(d) Earthing system also avoids electromagnetic interference in communication and data processing equipment in the substation.
Substation Earthing Grid :
Therefore for substation earthing special type of mat is used. It is also called an earthing grid.
The mat is usually the most practical way of earthing at substation yards.
The mat contains a large number of meshes.
The typical size of mesh being used 4 m x 5 m of earthing conductor buried horizontally at a depth varying 0.5 meters to 1.5 meters depending upon the soil resistivity.
The mesh of the mat is connected to several earth electrodes of size 1.9 cm x 3m driven at intervals.
The total number of electrodes is equal to the maximum fault current by five hundred
All the metal structures and frames including fencing posts are securely connected to the earthing mat. However special care is being taken to run to connections to every metallic part.
The material for the grounding conductor should have high conductivity and low underground corrosion copper, steel and aluminum are used for the grounding system. Previously copper has been the most common metal used.
However, the modern trend is towards the use of steel in place of copper as it is economical and its use avoids galvanic action between grounding systems and other underground utilities which are mostly of steel.
The size of steel rod of 20 mm diameter and 2.4 m long is used.