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Interview Questions on earthing of Substation

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How would you earth the substation equipment and auxiliaries as per I.E. rules?

Ans : Substation earthing system comprises neutral earthing, earthing of substation equipment and auxiliaries and earthing of noncurrent carrying metal frame work. For earthing substation equipment and auxiliaries, a peripheral G.I. strip earthing ring is installed to which all earth electrodes (spaced at about 3 meters interval) are connected and securely bonded by cross-connected G.I. strip conductors thus forming an earthing grid. The body of equipment and the structural frame work are connected to the earthing grid by two earth connections each. The transformer neutral is connected directly to two separate electrodes by strip conductors. The bases of lightning arrestors are connected directly to the separate earth electrodes by conductors as short and as straight as practicable. These earth electrodes are interconnected with the main earthing system to obtain low impedance between the lightning arrestors and the equipment.

What should be the minimum size of the earth bus for substation earthing ?

Ans : A minimum size of 65 sq. mm with a minimum thickness of 1.6 mm for copper and 200 sq. mm with a minimum thickness of 4 mm for galvanised iron and steel is recommended.

What is an earthing switch and where is it used ?

Ans : An earthing switch is basically an isolating switch connected between the line conductor and earth which is mounted on the frame of the isolator with a high speed operating mechanism controlled by protective relays and used rapid automatic earthing of faulty sections of the electric circuit by the system of protection.

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