A safety circuit does nothing for months and then has to work perfectly, once. That is exactly why it needs periodic verification: failures only surface at the moment of triggering — the worst possible moment.
Safety relay checks
Dual-channel inputs. E-stops and guard switches are usually wired as two channels. Verify that opening either channel alone puts the relay into its de-energised state; a single channel being ignored means the redundancy is already gone.
Cross-fault monitoring. The relay must lock out when the two channels close out of tolerance. Verify on site: short the channels together and operate the device — a healthy relay reports a cross-fault rather than resetting normally.
Reset type audit. Automatic reset is only acceptable in low-risk applications. Relay types like the PILZ PNOZ range cover e-stop, guard and curtain inputs with selectable reset behaviour; the modular myPNOZ can be configured to the channel count and reset mode required. During retrofits, pay particular attention to reset circuits that have been bridged — a common violation.
Feedback loop (EDM). A broken feedback loop from the contactor auxiliary contacts makes some relays refuse to start. Confirm the feedback contacts track the main contacts, and make sure nobody has shorted the loop to “keep it running”.
Light curtain checks
Beam-block test. At every verification, block the beams one by one with a test rod and confirm the machine stops within the response time. For through-beam curtains such as the SICK C4000, first confirm transmitter/receiver alignment and synchronisation status, then look for contaminated optics and vibration shift — with little beam margin, even minor movement causes nuisance trips or missed detection.
OSSD outputs. Both OSSD outputs must feed separate safety inputs on the relay — never parallel them into one. Measure that each OSSD switches independently.
Response time re-check. Recalculate the stopping distance. Braking performance drifts over years of maintenance, and the safe distance may no longer hold.
Common failure patterns
| Failure pattern | Consequence |
|---|---|
| Reset signal bridged or shorted | Safety function bypassed |
| Welded EDM contact unnoticed | Relay no longer protects after contactor welds |
| Only one OSSD wired | Redundancy gone; single fault removes protection |
| Stopping distance never re-verified | Worn brakes leave the safe distance short |
| No test records | No evidence the safety function is under control |
Replacement and spares
When replacing a safety relay or curtain, match channels, reset type and output configuration to the original, verify the wiring terminal by terminal, then re-run the full functional test and record it. Send us the type on the device and we will match the specification.
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