In short. Interlocks stop actions that must never happen, such as two sources closing together or closing onto an earthed bus. Permissives are conditions that must be true before an action. Critical interlocks must not depend on software alone, and each must be tested in the blocked direction.
Types
| Type | How it works | Use |
|---|---|---|
| Mechanical | Physical linkage prevents two breakers closing | ATS, bus ties |
| Key interlock | Trapped keys enforce sequence | Maintenance isolation, bypass |
| Electrical hard-wired | Contacts in the close circuit | Fast, simple, highly reliable |
| Software permissive | PLC or controller logic | Flexible, secondary protection |
What must be interlocked
- Utility and generator breakers never closed together except in a controlled paralleling mode.
- No closing onto an earthed or isolated section.
- No closing on a locked-out bus after a fault.
- Bypass operations in the correct order.
- Door and racking interlocks on switchgear for safe access.
- Generator start inhibits during maintenance.
Design rules
- Rank hazards. Use independent layers for the severe ones.
- Use positive feedback, such as auxiliary contacts that show true breaker position.
- Wire for fail-safe: wire break leads to a safe state.
- Keep interlocks independent of a single power supply. See control power.
- Document each interlock in a table with cause, effect and test.
- Control bypass keys under a permit process.
Permissive example
To close a generator breaker onto a live bus: generator voltage and frequency in window, sync check OK, bus not earthed, no lockout, mode permits, breaker ready. Any missing condition blocks the close and shows the reason on the HMI.
Testing
Test not only that the permitted action works but that each blocked action is truly blocked. Test with simulated failures, such as lost feedback and wrong breaker position. See acceptance framework.
Illustrative example
An ATS has a mechanical interlock between its two contactors, backed by an electrical interlock in the close circuit. On a test, removing the mechanical link in a workshop setting shows that the electrical interlock still blocks the second close. The two layers are independent.
Interlock versus permissive
| Term | Meaning | Example |
|---|---|---|
| Interlock | Prevents an action that is unsafe | Utility and generator breakers cannot both close |
| Permissive | A condition that must be true to allow an action | Close generator breaker only if voltage is in window |
| Trip / lockout | Forces a safe state | Protection trip locks out the breaker from reclosing |
Three layers, in order of trust
- Mechanical: key interlocks and physical blocks, independent of power.
- Hardwired electrical: contacts in series with the close coil.
- Software logic: flexible, but not the only guard for safety-critical actions.
Writing the permissive matrix
| Action | Permissives |
|---|---|
| Close generator breaker to dead bus | Generator voltage and frequency OK, bus dead, no other source closing, no trip active, automatic or manual mode OK |
| Close generator breaker to live bus | Sync-check OK, generator healthy |
| Close tie | Both sides healthy, no bus fault trip, capacity OK |
| Return to mains | Mains stable for the return delay |
Testing
Test each permissive by removing it and confirming the action is blocked. Test failure of each input: a lost signal should block, not allow.
Common error
Using a normally closed contact as a healthy signal can fail unsafe when a wire breaks. Design so that loss of signal blocks the action. See breaker interlocking and fail-safe architecture.
The master engineering question
Which single failure, whether of a sensor, a contact, a cable or a program, could allow the action this interlock exists to prevent?
What to send us
Send the one-line diagram and operating sequences. Send your application details.
Questions engineers ask
Are software interlocks acceptable?
As an extra layer, yes. Critical safety interlocks need hard-wired or mechanical backup.
What are key interlocks for?
They enforce operating sequence by trapping keys until each step is done.
Can interlocks be bypassed?
Only under a controlled procedure and with the risk understood.
How often to test interlocks?
At commissioning, after changes and periodically.
This guide gives general principles. Safety interlocks must follow site procedures and standards.