PL ↔ SIL — the mapping table every robot engineer needs

The most common follow-up question after "which standard applies?" is: "my robot is certified to PL d — what does that mean in SIL terms?"

ISO 13849 uses Performance Levels (PL a through PL e). IEC 61508 and IEC 62061 use Safety Integrity Levels (SIL 1 through SIL 4). They measure the same thing — the average probability of a dangerous failure per hour (PFH) — but they use different scales, different categories, and different terminology.

The mapping is well established in the standards themselves. Here it is.

PL a corresponds to a PFH of ≥10⁻⁵. There is no SIL equivalent — PL a falls below the threshold for SIL 1. PL b has a PFH range of ≥3×10⁻⁶ to <10⁻⁵, and similarly has no SIL equivalent.

PL c corresponds to SIL 1, with a PFH range of ≥10⁻⁶ to <3×10⁻⁶. PL d corresponds to SIL 2, with a PFH range of ≥10⁻⁷ to <10⁻⁶. PL e corresponds to SIL 3, with a PFH range of ≥10⁻⁸ to <10⁻⁷.

What this means in practice: if your robot's safety system is certified to PL d under ISO 13849, and a customer asks "what SIL level is this?" — the answer is that it meets the same probability range as SIL 2 under IEC 61508. The physics is the same. The failure rate is the same. The standard and the verification methodology are different.

This matters because different markets, different customers, and different regulatory frameworks reference different standards. European machinery builders tend to speak in PL. Process industry customers speak in SIL. Automotive speaks in ASIL (from ISO 26262). When your robot crosses sector boundaries — which increasingly happens as Physical AI expands from factories into logistics, agriculture, healthcare, and defense — you need to translate between these systems fluently.

One important nuance: the mapping is based on probability ranges, not on the full set of requirements. Achieving PL d does not mean you have met all of the requirements of IEC 61508 SIL 2. The quantitative reliability target is equivalent, but the systematic capability requirements, the software development process requirements, and the verification methodology differ between the standards. A PL d certification under ISO 13849 is not a SIL 2 certification under IEC 61508 — it's a PL d certification with an equivalent failure rate target.

For any team navigating this landscape, the practical advice is: know which standard your market expects, know which standard your certification body assesses against, and know how to translate when the customer speaks a different language.