In short. Protection can only work if the breaker opens. Breaker-failure protection detects that current still flows after a trip command and trips the other breakers that feed the fault. Backup protection, from remote or upstream relays, provides a second line if primary protection or its breaker fails.
Why it matters
If a feeder breaker sticks closed during a fault, the fault continues until some other device operates, perhaps the main incomer after a long delay. In the meantime, equipment is damaged and arc energy rises. A breaker-failure scheme reduces that delay.
How a breaker-failure scheme works
- A protection relay issues a trip command and starts a breaker-failure timer.
- A current detector checks whether current continues to flow in the breaker after the trip.
- If current persists when the timer expires, the scheme trips the upstream or adjacent breakers that feed the fault, and may block closing.
- Timer settings allow for normal breaker opening time plus margin.
Backup protection levels
| Level | Description |
|---|---|
| Local backup | A second relay or element covering the same zone |
| Remote backup | Upstream relays with longer delay that cover downstream faults |
| Breaker failure | Dedicated logic to clear through other breakers |
| Fuse backup | A fuse behind a breaker that may not interrupt the full fault level |
Generator applications
- If a generator breaker fails to open on an electrical trip, the scheme should stop the engine, remove excitation and trip the bus coupler or upstream breakers to isolate the set.
- A generator keeps feeding a fault while it runs, so removing fuel and field is part of the backup.
Design points
- Use a reliable current detector with a low drop-out threshold.
- Set timers with margin but short enough to limit damage.
- Provide separate DC supply or redundancy for the backup path. See DC and trip circuits.
- Test the scheme by simulating a stuck breaker.
Illustrative example
A generator breaker receives a differential trip but fails to open. The breaker-failure timer expires in a fraction of a second, the current detector still sees current, and the scheme trips the bus coupler and the utility incomer while shutting down the engine and exciter. The fault is isolated in a fraction of a second rather than waiting for a long upstream delay.
The problem breaker failure protection solves
A protection relay may correctly decide to trip, but the breaker can fail to open because of a mechanism fault, a trip coil failure or a loss of DC. Without backup, the fault persists until a remote upstream device clears it, which can be slow and can de-energise a larger part of the system.
How it works
- A protection trip signal also starts a breaker-failure timer.
- If current still flows after the breaker should have opened, the timer completes.
- The scheme trips the upstream breakers or adjacent sources, and may send a transfer-trip signal.
Timer setting ≈ breaker operating time + current-detector reset time + safety margin
Example (illustrative)
Breaker opening 50 ms, detector reset 20 ms, margin 50 ms: timer about 120 ms. Backup clears in perhaps 200 ms in total, instead of the seconds required for time-graded upstream protection.
Other backup protections
| Backup | Description |
|---|---|
| Time-graded overcurrent upstream | Slower, simple |
| Dual trip coils and DC supplies | Remove single points of failure in the trip path |
| Duplicate protection | Two relays of different principles |
Where it matters
Generator incomers on critical sites and main switchboards benefit most. Test the scheme by simulation during commissioning. See DC trip circuits.
Common mistakes
- No breaker-failure scheme on critical breakers.
- Timer set longer than needed.
- Backup relying on the same DC supply as the primary.
What to send us
Send the single-line diagram and protection list. Send your application details. See also protection failure modes.
Questions engineers ask
Is breaker failure needed on LV boards?
Often handled by upstream devices, but critical boards may benefit from a dedicated scheme.
What is the typical timer?
Breaker opening time plus margin, set from manufacturer data and tested.
Does backup protection replace maintenance?
No. Breakers still need regular operation tests and maintenance.
How can I test it safely?
Use a test mode that simulates a stuck breaker without real faults.
This guide gives general principles. Scheme design needs project data.