Breaker-failure protection (50BF) has one job: if a breaker has been told to trip and fault current keeps flowing, clear the fault some other way. It is a backup scheme that operates rarely — which is why its design needs to be right on paper before it is ever needed.
The anatomy of a 50BF scheme
- Initiation: protection trips to the breaker start the scheme.
- Current check: a fast, fast-resetting current element confirms the breaker is still carrying current. For low-current conditions, breaker auxiliary contacts may be used as an additional check.
- Retrip (t₁): an optional, short-delay re-trip of the same breaker — via a second trip coil where one is available — to cover a failed trip circuit.
- Backtrip (t₂): a delayed trip of the adjacent breakers needed to isolate the fault, which may include busbar, transformer and remote-end breakers.
Reasoning about the backtrip timer
The backtrip timer has to exceed the time a healthy breaker takes to interrupt, plus the reset time of the current check, plus a margin. Too short, and a healthy breaker that takes slightly longer than expected causes a trip of every adjacent breaker. Too long, and a failed breaker leaves the fault in place longer, increasing equipment stress and, in some systems, the risk to stability.
There is no universal value. The timer should be derived from the breaker data, the relay's current-check reset behaviour, the margin agreed for the project and any system requirements such as critical clearing times.
An insecure breaker-failure scheme can turn one breaker's failure into the loss of a whole busbar. Security matters as much as speed.
What changes with GOOSE
IEC 61850 GOOSE messages can carry breaker-failure initiation and backtrip signals between devices, replacing interposing relays and cross-wiring. IEC 61850-5 defines transfer-time performance classes for messages such as trips; whether an installation meets them depends on the network design, loading and devices.
GOOSE subscriptions can also be supervised, so that the loss of a message stream raises an alarm — provided supervision is configured and the alarm is acted on. That benefit depends on deliberate design: VLAN and priority settings, handling of quality and test bits in the receiving logic, redundancy for critical signals where justified, and a test-mode strategy that allows one bay to be isolated without disabling protection elsewhere.
See the timing relationships
Our breaker-failure demonstration runs a healthy clearance, a failed trip coil and a stuck breaker in a simplified model. Its timings are examples for illustration, not recommended settings.