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 for a time equal to time setting of the relay. Modern relays may contain more than one stage of protection each stage includes each own current and time setting. Figure (11) shows a multi stage over current relay.
The connection diagram of definite time over current relays with internal and external timers are as shown in figure (12), and figure (13) respectively. Definite time over current relay is the most applied type of over current. It is used as :
- Backup protection of distance relay of transmission line with time delay equal to fourth stage of distance
- relay which is 2.5 second in 220 kV lines, and 1.5 second in 66 kV lines.
- Backup protection to differential relay of power transformer with time delay equal to 2.0 second in 220/66 kV transformers, and 1.1 second in the 66/11 kV transformers.
- Main protection to out going feeders and bus couplers with adjustable time delay setting.
