1MAC309294-MB F Section 4Protection functionsRER620 241Technical Manual4.5.1.5 ApplicationThe n-1 criterion is often used in the design of a fault clearance system. This means thatthe fault is cleared even if some component in the fault clearance system is faulty. A circuitbreaker is a necessary component in the fault clearance system. For practical andeconomical reasons, it is not feasible to duplicate the circuit breaker for the protectedcomponent, but breaker failure protection is used instead.The breaker failure function issues a back-up trip command to adjacent circuit breakers incase the original circuit breaker fails to trip for the protected component. The detection ofa failure to break the current through the breaker is made by measuring the current or bydetecting the remaining trip signal (unconditional).50BFT can also retrip. This means that a second trip signal is sent to the protected circuitbreaker. The retrip function is used to increase the operational reliability of the breaker.The function can also be used to avoid back-up tripping of several breakers in casemistakes occur during relay maintenance and tests.50BFT is initiated by operating different protection functions or digital logics inside therelay. It is also possible to initiate the function externally through a binary input.50BFT can be blocked by using an internally assigned signal or an external signal from abinary input. This signal blocks the function of the breaker failure protection even whenthe timers have started or the timers are reset.The retrip timer is initiated after the pickup input is set to true. When the pre-defined timesetting is exceeded, 50BFT issues the retrip and sends a trip command, for example, to thecircuit breaker's second trip coil. Both a retrip with current check and an unconditionalretrip are available. When a retrip with current check is chosen, the retrip is performed onlyif there is a current flow through the circuit breaker.The backup trip timer is also initiated at the same time as the retrip timer. If 50BFT detectsa failure in tripping the fault within the set backup delay time, which is longer than theretrip time, it sends a backup trip signal to the chosen backup breakers. The circuit breakersare normally upstream breakers which feed fault current to a faulty feeder.The backup trip always includes a current check criterion. This means that the criterion fora breaker failure is that there is a current flow through the circuit breaker after the setbackup delay time.
