Preparing the published article.
Performance Levels and SIL: Choosing the Right Safety Integrity
How PL and SIL targets are set, and why the number must come from the risk assessment.
Two frameworks, one idea
ISO 13849-1 uses performance levels (PL a to e) based on category, MTTFd, diagnostic coverage and common cause failure. IEC 62061 uses safety integrity levels (SIL 1 to 3) based on probability of dangerous failure per hour. Both describe how reliable a safety function must be.
| Term | Framework | Simplified meaning |
|---|---|---|
| PL d | ISO 13849-1 | Common target for robot cell safety functions |
| PL e | ISO 13849-1 | Highest level, used where risk demands it |
| SIL 2 | IEC 62061 / IEC 61508 | Broadly comparable to PL d in practice |
| Category 3/4 | ISO 13849-1 | Redundancy and monitoring structure |
Where the number comes from
The required PL or SIL is derived from the risk assessment: severity of injury, frequency of exposure, possibility of avoidance. Choosing a level because a competitor used it, or because a component catalogue lists it, inverts the logic and usually results in over-engineering or under-protection.
Validate, do not just calculate
- Use components with the declared PL or SIL and the correct architecture.
- Check that the wiring matches the assumed category.
- Measure the achieved stopping time and separation distances.
- Record the calculation, the components and the measurements together.