Protection
Breaker failure protection, device 50BF, trips the surrounding breakers when a breaker ordered to open does not clear the fault. Testing proves the current detector, the timer, the initiate inputs and the backup trip path as one scheme.
Breaker failure protection, device 50BF, is a backup scheme that acts when a breaker has been told to trip and has failed to interrupt the current. It is not a protective element in the usual sense, since it does not detect faults itself. It is started by the trip signal of another protective relay, and it then checks whether current is still flowing through the breaker after a deliberate wait. If it is, the scheme gives up on that breaker and trips every breaker that can feed the fault, usually through a lockout relay. In digital relays it is a function with a current detector, a timer, an initiate input and trip outputs. In older installations it is built from a fast current-sensing relay, an auxiliary timer and a lockout relay. The test is applied in substations, switchgear lineups and generator and transformer protection schemes.
A stuck mechanism, a failed trip coil, a lost control supply or an interrupter that does not clear can leave a fault energized with the primary protection having done everything right. Without a backup, the fault stays on until remote protection clears it, taking much more of the system out, damaging equipment and increasing the hazard to people. The scheme is rarely called on, which is exactly why it fails unnoticed: a mistaken wiring change, a disabled initiate input, a timer left at the wrong setting or a current detector that no longer picks up can sit unseen for years. Testing exercises the entire chain on purpose, including the parts that nothing else exercises, and gives confidence that the rare demand will be answered.
The scheme is built on three questions asked in order. First, was the breaker told to trip? That is the initiate signal, taken from the contacts of the protective relays that trip it. Second, did the current stop? That is the current detector, set low enough to see the smallest fault current the scheme must handle, and fast to reset once the current is interrupted, because its reset time is part of the margin of the whole scheme. Third, has enough time passed for a healthy breaker to finish? That is the timer, set longer than the breaker interrupting time plus the reset time of the detector and a safety margin. If the initiate is present, the current is still there and the timer expires, the scheme concludes the breaker has failed and issues the backup trip. Some schemes also retrip the same breaker first, as a cheap chance to clear a trip circuit fault, and some use breaker position contacts in addition to current for low-current faults.
The relay is isolated from the live current transformer circuits, with the secondaries shorted at the test switch before any connection is changed, and from the tripping circuits of the breakers it controls, with the trip outputs blocked or the trip links opened so that a test cannot open a live breaker. A relay test set is connected to the current inputs, and a contact or output of the set simulates the initiating trip from the protective relay. The current detector is tested like any overcurrent element by raising the current to find the pickup and lowering it to find the reset, in the same way for each phase and for the ground path where one is used. The scheme timer is tested with a timed sequence: the initiate is applied together with fault current, and the timer measures the interval to the backup trip output. The sequence is repeated with the current removed before the timer finishes, to show that the scheme resets and does not trip. The retrip output, the backup outputs, the lockout relay, the blocking of reclosing and any transfer trip to remote ends are then confirmed one by one, and the trip links are restored and recorded at the end.
The record states the scheme type and its settings, the current detector pickup and reset found for each phase, the measured timer interval from initiate to backup trip, and the response of each output, including retrip, lockout, reclose block and any transfer trip. It notes which trip links or blocks were applied during the test and that they were restored, the instrument and its calibration status, and the as-found and as-left settings together with any correction made.
The detector pickup is compared with the setting from the protection study, allowing the tolerance published for the relay, and the timer is judged against its setting and against the margin the study assumed between breaker clearing time and the backup trip. The reset behavior of the detector matters as much as its pickup, since a slow reset can turn a healthy clearing into a false backup trip. Every output must operate only for the correct combination of initiate, current and timer, and must stay silent when the current stops in time. ANSI/NETA ATS-2025 and ANSI/NETA MTS-2023 list the breaker failure function among the protective relay tests. For the tolerances and values, refer to your own licensed copy of those standards and to the relay manufacturer's instruction book.
The equipment classes and standard sections that call for this test, and the UltraDb forms that carry it.
No. It is started by the trip signal of another protective relay and then watches whether the breaker actually interrupted the current. Only if current persists after the timer does it trip the surrounding breakers.
After a good breaker opens, the detector must drop out quickly enough that the timer is not still running when the current has gone. A slow reset eats into the margin and can cause an unnecessary trip of the backup breakers.
The trip outputs are blocked or the trip links are opened, and the outputs are proven by watching test lamps, a contact monitor or the lockout relay. The links are put back afterward and the restoration is recorded.
It is a second attempt to open the same breaker, issued early in the scheme. It clears failures caused by a fault in the first trip path before the scheme moves on to tripping the neighboring breakers.
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