Glossary · Protective devices and interlocks
Hardwired interlock
In machine safety, a hardwired interlock is an interlock implemented primarily with dedicated wiring and components rather than programmable application logic, so that its behavior does not depend on software written for the machine.
- Machine safety
- Technical documentation
In one sentence
A hardwired interlock relies on dedicated wiring and safety components, not application software; it is simpler to verify but harder to change.
Example
The guard door switches of a small press are wired through a safety relay directly to the motor contactors, independent of the standard PLC that runs the process.
How it applies
- Functional safety: Hardwired does not automatically mean safe. The achieved Performance Level or SIL still depends on component reliability, architecture and diagnostics, typically provided by a safety relay.
- Integrated systems: Many machines combine hardwired interlocks with a safety PLC. The boundary between the two must be clear in the safety concept and the circuit diagrams.
- Evidence: Verification of a hardwired interlock focuses on wiring checks and fault simulation rather than software review. Record both in the verification report.
- Technical documentation: Mark hardwired safety circuits in the electrical diagrams, and warn against adding jumpers or bridging contacts during troubleshooting.
Hardwired interlock vs. programmable interlock
A programmable interlock is implemented in safety-rated software, for example on a safety PLC. It is more flexible, but its software needs its own verification and change control. A standard PLC alone is not a substitute for either.