In short. Before a breaker closes, the control must know the state of the bus on both sides. Dead to dead needs arbitration so only one source closes. Live to dead is allowed under conditions. Live to live needs synchronism. Each case has its own permissive logic, and sensing faults must block, not allow, closing.
Closing cases
| Source side | Bus side | Action |
|---|---|---|
| Live | Dead | Close allowed if the source is acceptable and the bus has no fault or lockout |
| Dead | Live | Close is a reverse energisation; normally not allowed unless defined |
| Live | Live | Close only inside the sync-check window |
| Dead | Dead | No source to close; wait or start a source |
Dead-bus closing
- Use a bus-dead detector with a voltage threshold and a time delay.
- Allow only one breaker to close onto the dead bus. Arbitrate with a priority or a hard-wired interlock.
- Check there is no bus fault: protection not tripped, breaker not locked out, earth switch open.
- Limit inrush energisation, for example by ramping or sequencing transformers. See inrush studies.
Live-bus synchronising
See synchronising control. Require voltage, frequency and angle inside windows, with a minimum stable time before close.
Sensing faults
If a voltage transformer fuse blows, the control may see “dead” on a live bus and try to close a dead-bus breaker onto a live bus. Use VT fuse failure detection, compare multiple measurements and treat sensing faults as a block. See interlocking.
Keys and mechanical interlocks
For critical bus ties, a mechanical key interlock or hard-wired scheme backs up the software. See main-tie-main control.
Illustrative example
Two generators restart after a blackout. Both detect a dead bus and both try to close. The scheme gives set A priority; set B waits and then synchronises to the live bus. A test with a simulated VT fuse failure shows that the scheme blocks closing instead of treating the bus as dead.
Three bus states, three rules
| Bus state | Breaker action allowed | Safeguard |
|---|---|---|
| Dead bus (no voltage) | One source closes | Interlock so no two sources close together |
| Live bus, source in sync | Source closes | Sync-check confirms voltage, frequency, angle |
| Live bus, source not in sync | Blocked | Synchroniser adjusts first |
Dead-bus closing
- Confirm the bus is dead with voltage sensing on all phases, below a threshold such as 10 percent.
- Confirm no other breaker onto the bus is closing.
- Grant a permissive to one source based on priority.
- Close, then confirm the voltage appears.
Race condition
If two generators reach rated voltage at the same moment, they may both see a dead bus and close. Arbitration by a hardware interlock or a priority timer prevents this. For example, generator 1 closes at once, generator 2 waits 2 seconds, then checks again and finds a live bus.
Failure of the voltage sensing
A blown VT fuse can look like a dead bus. Supervision of VT healthy status stops a false dead-bus closing onto a live bus. This is a classic cause of out-of-phase closing.
Testing
Simulate simultaneous start, VT fuse failure and breaker fail-to-close. See automatic synchronising and interlocking.
The master engineering question
Could any single failed sensor, wire or contact lead the control to close a breaker onto a live or faulted bus?
What to send us
Send the one-line diagram and sensing arrangement. Send your application details.
Questions engineers ask
How low is “dead”?
Defined by a threshold below normal voltage, with a delay to avoid false decisions.
Why a time delay?
Residual voltage from motors and transformers can briefly look like a live bus.
Can software alone prevent double close?
For critical ties, add hard-wired or mechanical interlocks.
What about black start?
See black-start logic.
This guide gives general principles. Confirm with a project-specific design.