In short. Interlocking makes unsafe switching impossible rather than merely unlikely. Transfer logic decides which breaker closes when a source fails. Together they stop two sources being connected by mistake, protect people during maintenance and make transfers predictable.
Types of interlock
| Type | How it works | Strength |
|---|---|---|
| Mechanical | Linkages, rods or cables stop one breaker closing if another is closed | Independent of control power |
| Electrical | Auxiliary contacts in the closing circuit | Flexible, depends on wiring and supply |
| Key (trapped key) | A key released only when a breaker is open and isolated | Strong for safety and maintenance |
| Software / PLC | Logic prevents commands | Flexible, should be backed by hard interlocks |
For critical safety, use at least one hard-wired or mechanical interlock independent of software.
Common schemes
- Two sources, one load: utility and generator incomers interlocked so only one can close.
- Two incomers with a coupler: any two of three breakers can be closed, never all three, unless parallel operation is intended and synchronised.
- Generator to dead bus: permission only if the bus is dead and no other source is closing.
- Maintenance isolation: earthing switch can close only if the associated breaker is open and isolated.
Transfer logic
- Detect source failure with a time delay.
- Start the generator and wait for stable voltage and frequency.
- Open the failed source breaker and optionally shed load.
- Close the generator breaker after interlock permission.
- On utility return, wait for a stable period, then retransfer.
See also ATS engineering.
Failure modes to design out
- Lost control supply leaving breakers in an undefined state.
- A stuck auxiliary contact giving a false “open” signal.
- Both sources trying to close in the same scan.
- Operators bypassing interlocks.
Testing
Test every interlock by trying to defeat it: attempt to close each breaker in each illegal state. Record the results. Repeat after any change. See switchgear testing.
Illustrative example
A board has a utility incomer, a generator incomer and a bus coupler feeding two sections. The interlock allows any two to be closed. During a test, the technician tries to close the third; the mechanical interlock blocks it. The test is recorded as part of the commissioning file.
What interlocking prevents
| Hazard | Interlock |
|---|---|
| Utility and generator closing onto each other out of phase | Mechanical and electrical interlock between source breakers |
| Backfeed to a dead utility line | Mains breaker open before generator closes to the load |
| Closing onto earth or a fault | Earth switch and key interlocks |
| Wrong switching order | Key interlock sequence |
Layers of interlock
- Mechanical: physical blocks or key trapping. Independent of control power.
- Electrical: auxiliary contacts and relay logic.
- Software: PLC or relay logic. Flexible, but should not be the only layer for safety-critical locks.
Transfer logic checks
- Confirm the outgoing breaker is open before the incoming closes (open transition).
- Confirm voltage and frequency on the incoming source.
- Provide a lockout after a protection trip so a fault is not re-closed on automatically.
- Provide manual override with safe interlock.
Testing the logic
Test every sequence including abnormal ones: a breaker that fails to open, a lost control supply, simultaneous mains return and generator fault. Document the result.
See ATS switchgear and AMF control logic.
Common mistakes
- Software-only interlocks.
- Untested interlocks.
- Overrides left in place after maintenance.
What to send us
Send the single-line diagram and operating philosophy. Send your application details. See also AMF control.
Questions engineers ask
Why do I need both mechanical and electrical interlocks?
They protect against different failures. Mechanical works without power, electrical adds flexibility.
What is a trapped-key system?
Keys that can only be released in a safe sequence, linking operations across devices.
Can interlocks be bypassed?
They should not be, except under controlled procedure with authorisation.
How often should they be tested?
At commissioning, after changes and during periodic maintenance.
This guide gives general principles. Follow local safety rules and standards.