The Safety Relay That Wasn’t Enough: Performance Level and Category
A press guarding circuit with a standard safety relay. The customer needed PL d (performance level d) for the press application. The relay was a Category 1 device — it met PL c. The safety engineer said it wasn’t enough. This is about safety relay selection, not about the relay being defective.
The performance levels
ISO 13849 defines performance levels (PL a through PL e) for safety functions. The PL depends on the reliability of the safety circuit: how likely it is to fail safely.
| PL | Category | Mission reliability | Use for |
|---|---|---|---|
| PL a | B | Low | Minimal risk |
| PL b | 1 | Low-medium | Simple guards |
| PL c | 1 or 2 | Medium | Light duty |
| PL d | 3 | High | Presses, robots, point-of-op |
| PL e | 4 | Very high | High-risk, folding machines |
What Category 3 means
A Category 3 safety circuit has redundant channels. If one channel fails (welded contact, broken wire), the other channel still opens the circuit. The machine stops. The fault is detected (the relay’s LED indicates). Category 1 (a single relay) doesn’t have redundancy. If the contact welds, the safety function fails.
For PL d, you need a Category 3 (or Category 4) safety relay with dual-channel inputs, force-guided contacts, and internal fault detection. The standard relay we installed was Category 1 — single channel. It didn’t meet PL d.
What we changed
We replaced the Category 1 relay with a Category 3 dual-channel safety relay. It has two input channels (from the e-stop and light curtain), two output contact sets (redundant), and internal monitoring. If one contact welds, the relay detects it and locks out. The circuit now meets PL d / Cat 3.
The light curtain and e-stop wiring
For Category 3, the safety devices (light curtain, e-stop, guard switch) must be wired to both channels of the safety relay. The two channels are monitored. If the channels disagree (one says the gate is open, the other says closed), the relay faults. This detects a single fault in the wiring or the device.
The reset
A Category 3 relay requires a manual reset after a safety event. The operator presses reset, and only then can the machine restart. The relay doesn’t auto-reset. This forces an acknowledgment after any stop. We wired a reset button on the HMI.
The safety relay rule: Match the relay’s category to the required PL. For point-of-operation guarding (presses, robots), use Category 3 dual-channel safety relays. A single-channel relay meets PL c at best. The safety engineer who flagged it was right — a Category 1 relay on a press is a PL deficiency, not a cost-saving measure.
