10 b] What is earthing? Why earthing is required? With the help of neat sketch explain plate earthing.
Solution :
Earthing :
Connection of the body of electric equipment to the general mass of the earth by wire of negligible resistance is called Earthing. It brings the body of the equipment to the zero potential during electric shock.
Necessity of Earthing
- To protect the human beings from danger of shock in case they come in contact with the charged frame due to defective insulation.
- It guarantees the safety of electrical appliances and devices from the excessive amount of electric current.
- It protects the appliances from high voltage surges and lightning discharge.
- It provides an alternative path for leakage of current hence protects the equipment.
- It keeps the voltage constant in the healthy phase
- It protects the Electric system and buildings from lightning.
- It avoids the risk of fire in the electrical installation system.
- To maintain the line voltage constant under unbalanced load condition.
Plate Earthing :
In this method a copper plate or GI plate of 60cmX60cmX3.18cm is placed vertically down inside the ground at a depth of 3m. The plate is surrounded by the alternate layers of salt and coal with a minimum thickness of about 15cm. The earth wires drawn through the GI pipe are bolted through the earth plate. The GI pipe is fitted with the funnel on a top in order to have an effective earthing by pouring the salt water periodically. The schematic arrangement is as shown below.
The earthing efficiency increases with the increase of the plate area and depth of the pit. The depth of the pit depends upon the resistivity of the soil.
The only disadvantage of this method is that discontinuity of earth wires from the earthing plate which is placed below the ground as it cannot be observed physically this may cause miss leading and result into heavy losses under fault condition
