Relay usually installed with current transformer for current measurement.
-The Relay should not operate when the circuit is carrying the maximum load continuosly.
-When the equipment or circuit is allowed to overload in limited time -say by 30%bfor less than half an hour then the relay should be set not operate at the over loaded current in the slightly shorter than the maximum over load time.
-Under fault condition,the relay should operate and disconnect the circuit in the time before exceeding the short circuit rating of the protective equipment.
-Realys are normally classified into the time/current characteristics as:
*Instantaneous
*Definite-time
*Inverse definite Minimum Time(IDMT)
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