In FIG. 13, when the short circuit line fails, the KA1—KA3, short circuit current through the current transformer TA is greater than the current relay rectification value, the current relay starts.
Because the three current relay contacts are in parallel, as long as one current relay contacts close, the start time relay KT, follows the pre-set time limit, its contacts close, and start the outlet intermediate relay KOM. After KOM operation, open the trip circuit to trip the QF circuit breaker and signal the signal relay operation. Because the action time limit of protection is independent of the size of the short circuit current, it is fixed, solid is called the fixed time overcurrent.