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Breaker failure protection (BFP)
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This function issues a back-up trip command to trip adjacent circuit breakers in case of
a tripping failure of the circuit breaker (CB), and clears the fault as requested by the ob-
ject protection.
The breaker-failure function is started by a protection trip command, from the line and
busbar protection through the breaker-related trip relays. A general START input is al-
ways available that starts the measurement in all appropriate phases. In some imple-
mentations also phase selective START signals are available. For retrip there is always
a general RETRIP output available and in applications with phase selective START sig-
nals also phase selective RETRIP output signals are available. Correct fault current
clearing or failure is detected by a current check in each phase. The current level can be
set at 0,05 to 2 times the rated current.
Retrip of the faulty CB can be done with or without current check. A delay, 0-60 s, can
be set for the retrip.
The use of retrip, limits the impact on the power system if the breaker-failure protection
function (BFP) is started by mistake during testing or other maintenance work.
A second time step is used for the back-up trip command. It should be connected to trip
the adjacent breakers, to clear the busbar section and intertrip the remote end, if so re-
quired. The time setting range is 0-60 s.
By using separate timers for each phase, correct operation at evolving faults is ensured.
The timer setting should be selected with a certain margin to allow variation in the nor-
mal fault clearing time. The properties of the BFP function allow the use of a small mar-
gin.
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