Ans: 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.