State the nature of protection provided for following types of faults; 1) Over voltage 2) Under voltage 3) High air pressure in air circuit 4) Low air pressure in air circuit

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1) Over voltage: Controlled through tap changing, only an alarm needs to be provided for attention of crew. 

2) Under voltage: voltage below 17.5 kV is not desirable. A no volt relay is provided which trips the CB and calls for resetting of power and auxiliary circuits. 

3) High air pressure in air circuit: Safety valve / PRV (pressure release valve) is fitted in the compressed air circuit to protect against high pressure. 

4) Low air pressure in air crcuit: Relay senses low pressure in air circuit and trips the relevant CB.
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