The Safety Relay That Wasn’t Enough (PL)
We used a safety relay (article 93) on a machine with a guard (article 74). The machine moved (a robot, article 52). The guard opened (the operator reached in). The safety relay stopped the robot. But the risk assessment said the machine needed PL d (Performance Level d). The relay we used was PL c (lower). The problem: the safety level (PL) was too low for the risk. We either upgraded the relay (PL e, article 93) or added another safety function. We upgraded to PL e (a higher safety relay). The risk assessment passed. The mistake was picking a safety relay without matching the PL (the risk level).
Safety Performance Level (PL) is the safety level (per ISO 13849). This article covers the basics.
The PL Levels (a to e)
ISO 13849 defines 5 performance levels (PL):
- PL a: Low risk (light, no injury). Simple safety (a basic switch).
- PL b: Low-medium risk.
- PL c: Medium risk (minor injury). A safety relay (single channel).
- PL d: High risk (serious injury). A dual-channel safety relay (article 93), with monitoring.
- PL e: Very high risk (death). A dual-channel safety, with redundancy (two relays, cross-monitoring).
Step 1: The Risk Assessment
The machine’s risk (article 74) determines the PL. A moving robot (article 52) with a guard is high risk (PL d, or e). A light curtain (article 160) on a hand-feed is PL d. A simple clamp (no moving part) is PL b. Do a risk assessment (article 74) — it tells you the required PL.
The PL rule: The risk assessment sets the PL (a–e). The relay that was too low was PL c on a PL d risk. Match the safety relay (and the guard) to the required PL. Don’t use a PL c relay on a PL d machine.
Step 2: The Safety Relay (Article 93)
A safety relay (article 93) has a PL rating. A basic relay is PL c. A dual-channel (with feedback) is PL d. A redundant (two relays) is PL e. Pick the relay with the right PL (per the risk assessment).
Step 3: The Guard (Article 74)
The guard (a door, a light curtain) also has a PL. A simple switch (on the door) is PL c. A monitored switch (dual, with feedback) is PL d. A light curtain (article 160) is PL d (or e, depending on the resolution). The whole system (guard + relay) must meet the required PL.
Step 4: Documentation (Article 99)
Document the PL (the risk assessment, the relay’s PL, the verification). The CE file (article 99) shows the PL. An auditor (or a customer) checks it. Without the documentation, the PL claim is unproven.
| Risk | PL | Safety Function |
|---|---|---|
| Light (no injury) | a | Basic switch |
| Minor injury | b–c | Safety relay (single) |
| Serious injury | d | Dual-channel relay (article 93) |
| Death | e | Redundant (two relays) |
A PL Checklist
- Has a risk assessment been done? (Article 74?)
- What is the required PL? (a–e?)
- What is the relay’s PL? (Article 93?)
- Does the relay meet the required PL?
- Is the guard’s PL right? (Article 74?)
- Is it dual-channel? (For PL d?)
- Is it redundant? (For PL e?)
- Is the documentation done? (Article 99?)
- Is the verification done? (Test?)
- Does the auditor accept it?
The Bottom Line
Safety Performance Level (PL) matches the safety to the risk. The relay that was too low was PL c on a PL d risk. Do a risk assessment (article 74). Pick the relay (and guard) with the right PL. Document it (article 99). The machine that passed wasn’t the cheapest safety — it was the right PL.
