Definite Time Overcurrent Relays
In this type, two conditions must be satisfied for operation (tripping), current must exceed the setting value and the fault must be continuous at least a time equal to time setting of the relay.



vs. Instantaneous Overcurrent relay (Define Current)
Definite current relay operate instantaneously when the current reaches a predetermined value.

http://electrical-engineering-portal.com/types-and-applicati...