The earthing is of great importance because it protects both the equipment as wells as the persons using them. As the name implies the earthing means that the equipments or installations connected to the earth. The earthing is also called grounding. The resistance of the earth is very low.
So when the equipment or installation develops some fault then the current will take the path through the earth and the equipment will remain safe. Therefore all power supply systems, consumer installations and electrical appliances are grounded as per Indian electricity rules 1972.
Suppose now that the insulation of the line conductor inside goes bad and v conductor comes in contact with the metal casing etc. If the appliance is itself t on a wooden table, the casing would then be at the same potential as the inductor i.e. it would now become live.
If now by chance some person touches the body of the equipment and that erosion is not properly insulated then the current will flow through his body and he may experience a shock, which may be fatal.
Now if the body of the equipment is properly earthed then there will be large amount of the current flowing through the line because of very low resistance of the earth and the fuse in series with the equipment will blow and the equipment will be isolated from the line.
However at this time any person incidentally touches the equipment, the current will not pass through his body because the resistance of the body is very much larger in comparison to the earth resistance. For earthing every equipment 3 pin plug is used. The third pin which is relatively larger m the diameter is used for earthing purpose.
Its value for the complete earthing system i.e. earth continuity conductor, earthing lead and earth electrode must not exceed one ohm.
How earthing is done?
Systems of VIR cables is metal conduits or of lead- sheathed cables must have the conduit or sheathing in good mechanical and electrical contact, bonded and maintained at earth potential, by being connected to the water main, separate earth electrode or the earth terminal on the terminal board of the power supply.
The earthing conductor must be as short and straight as possible in its run be of phosphor bronze or high conductivity lined copper. It should not be less than half the sectional area of the largest cable being protected provided that it need not be larger than 0.6 Sq. cm in sectional area. No joints are permitted in the earthing cable.
Methods of Earthing:
(A) Earthaing to Water Main:
First the cold water pipe is thoroughly cleared of all grease and dirt. An adjustable clip is fixed at the cleaned spot. One end of the earthing cable is fixed to a lug which is attached to the clip by a fixing bolt and nut, tightening the nut draws the clip tight to grip the pipe firmly. The other end of the earthing cable is similarly connected to the conduit with a clamp and lug.
(B) Pipe Earthing:
A galvanized iron pipe 38 mm in diameter and 2 meter long is driven into the ground or a copper plate of adequate size buried edge wise in the earth is used as the earth electrode. Its resistance must be kept below specified limit, packir % salts and powdered coke or charcoal to a depth of 30 cm, all around the pipe in low resistance, keeping the earth moist by pouring water in to the pipe.
According to the Indian Electricity Rules, the frame of all motors, transformers etc and the metal casings of all power consuming equipments shall be earthed by two separate and distinct connections with earth. On electrical appliance the manufacturer provides an earthing terminale. The foundation bolts used for hold down the machines are often used for eathing them.
(C) Plate Earthing:
The earth connection can be provided with the help of a copper/G.I. plate. When G.I. plate is used it should not be of less than 60 cm x 60 cm x 6.35 mm.
The plate is kept with its face vertical at a depth of 3 m and is so arranged that it is embedded in an alternate layer of coke and salt for a minimum thickness of about 15 cm in case earthing is done by copper plate and in coke layer of 15 cm if it is done with GL Plate- The earth wire is securely bolted t0 the earth plate with the help of bolt nut and washer.
(i) The earthing electrode should be situated at a place at least 11/2 metre away from (i.e. outside) the building whose installation system is being earthed.
(ii) The earth wire should be of same material as that of earth electrode.
(iii) The minimum sectional area of the earth lead wire should never be less than 0.02 sq. inch (No. 8 S.W.G.) and not more than 0.1 sq. inch (i.e. 7.036 S.W.G.).
(iv) The size of earth conductor as a general rule should not be less than half of the section of live line conductor.
(v) The size of the continuous earth wires used with cables should not be either less than 0.0045 sq. in (14 S.W.G.) or half of the installation conductor size in case of light wiring.