PL and SIL Without Tears: Selecting Safety Functions

Performance Level (PL) and Safety Integrity Level (SIL) are two ways of quantifying how reliable a safety function must be. The challenge for most automation engineers isn’t the math — it’s mapping real-world machinery risks to the right level without over- or under-engineering.

PL vs SIL: The Basics

• PL (ISO 13849): Used for machine safety, based on component categories, MTTFd, and diagnostic coverage.
• SIL (IEC 62061 / IEC 61508): Used for process and functional safety, based on PFHd and hardware fault tolerance.

Selecting the Right Standard

• For discrete manufacturing — use PL.
• For process and continuous systems — use SIL.
• For mixed applications — harmonize both via ISO/TR 23849 cross-referencing.

Step-by-Step Function Selection

1. Define the safety function (e.g., “stop motion when guard opens”).
2. Assess risk (frequency, severity, avoidance possibility).
3. Determine PLr or SIL target using risk graphs or lookup tables.
4. Select components (sensors, logic, actuators) that meet the requirement.

Example

A packaging machine’s guard interlock was classified as PL d / SIL 2, using redundant switches and a safety relay. A single-channel stop circuit (PL b) would not have met the required risk reduction.

Related Articles

• Validating Safe Motion: STO, SS1, SLS, and SSM
• Safety PLCs vs Relays: When Each Makes Sense
• Common Cause Failures: What Your FMEA Must Include

Conclusion

PL and SIL don’t need to be intimidating. With a structured approach and proper component data, safety integrity becomes an engineering outcome — not guesswork.