Earthing:
- The function of Substation earthing system is to provide a uniformly zero potential with respect to ground and lowest earth resistance.
- As the earth's potential taken as Zero for all practical purpose, hence any electrical appliances, part, machine when connected to earth attains zero potential and is said to be " Earthed" and voltage of this earthed appliance will fail or increase to zero if voltage is higher or lower than the earth potential respectively .
- In AC supply system the neutral wire and the middle wire of the three wire D.C. distribution system is always earthed to maintain line pressure (voltage) always constant.
- The frames, bodies of machineries are earthed. In case of any ground or leakage the chances of energizing human life, fire are eliminated by allowing the leakage current to flow to the earth at once and circuit fuse blows off, thus disconnecting the machinery , appliances from supply.
Objective Of earthing :
Earthing is carried out to achieve the following objectives :
- To save human life from danger of shock death in case it comes in contact with charged frame due to any fault leakage current.
- To maintain the line voltage constant.
- To protect large building from atmospheric lightening.
- To protect all machines fed from over head lines from atmospheric lightening.
Understanding earthing & Necessity of Earthing in Power system:
- Earth contains tremendous amount of mixtures of metals and their compounds which work as large good conductor of electricity.
- Every generator and their metal body and frame through which no current flows are connected effectively to earth through metal conductor pf very low resistance which is called earthing.
- The resistance of this connection should be very much low so that the fuse blows off or earth fault relay operates and disconnects the equipment from the supply mains when the body of the equipment becomes live due to deterioration of the insulation of the winding.
- We shall study from one simple illustration that what happens when the metal body of the electrical equipment is not earthed and the earth fault occurs and see what happens when the metal body of the equipment is not earthed and earth fault occurs.
- In fig 1 an electrical iron is connected to the supply mains and the iron is not earthed.
- No shock is experienced when the metal body of the iron is touched so far as there is no earth fault.
- But if insulation of the heating element is punctured, it touches the sole plate and the sole plate becomes live and its potential become equal to that of supply potential.
- Now if a person touches the metal part of the iron by standing on the floor, electric current flows through his body and he will receive electrical shock.
- Now let us see what happens when the metal body of the equipment is earthed and a person touches the metal body when there is earth fault.
- Body of electric iron or electric electrical equipment is connected to earth through green wire connected from the body and through the earthing pin.
- As soon as the insulation of the heating element deteriorates, heavy current flows through the fuse and metal body of the iron to the earth and the fuse blows and the iron is disconnected from the supply mains and when person touches the body of the iron he will not receive electrical shock.
- In other large electric equipments other protective devices like earth fault relay, earth leakage circuit breaker are used in addition to the fuse.
- So when earth fault occurs, these operate and isolate the faulty equipment from the supply.
- Thus earthing ensures protection to the equipment and human beings.