SIL vs Performance Level: Comparing Safety Ratings

The IDEC overview associates Performance Level with SIL for a stated operating-mode context. Its table does not prove that the labels are universally interchangeable or that a component meets a complete safety-function target.

TL;DR: The source says ISO 13849-1 evaluates a safety function using average probability of dangerous failure and expresses the result as a Performance Level. Its comparison table associates PL a with no SIL, PL b and c with SIL 1, PL d with SIL 2, and PL e with SIL 3 for frequent operation request or continuous mode. Use that table only within its stated context and verify the governing standard, mode, assumptions, and complete design evidence.

“PL d equals SIL 2” omits the conditions and source required for the project.

What does Performance Level mean in this source?

The IDEC overview of ISO 13849-1 says that ISO 13849-1 evaluates safety functions using average probability of dangerous failure and expresses the result as a Performance Level (PL).

The page also presents supporting tables and design concepts. This article does not reproduce the probability bands, category rules, MTTFd ranges, diagnostic-coverage ranges, common-cause scoring, or risk graph. Consult the current authoritative standards and project documentation before using those details.

For a project, record the required safety function and the authoritative basis for its evaluation. A label without its framework, assumptions, and calculation evidence cannot support a design decision.

How can PL be associated with SIL?

The IDEC source says the current ISO 13849-1 approach permits PL to be associated with SIL. It contrasts this approach with the 1999 edition, whose categories could not be correlated with SIL.

For frequent operation request or continuous mode, the page's comparison table associates PL a with no SIL, PL b and c with SIL 1, PL d with SIL 2, and PL e with SIL 3.

Use associated deliberately. A table entry does not make the labels universally equal, interchangeable, or automatically compliant for a particular component or safety function.

Why should you avoid a context-free conversion?

The comparison table does not replace the project's governing evidence. Before applying different rating labels, answer:

• Which standard and edition governs the requirement?
• Which operating mode and table conditions apply?
• What exactly is being rated: a component capability, subsystem, or complete safety function?
• What calculation, architecture, and assumptions support the claimed result?
• What validation evidence must accompany the design?

Keep every table entry within the context in which it was established.

What should you not infer from the comparison table?

The table alone does not establish:

• that a component carrying one label automatically satisfies a project requirement written with the other;
• which standard or edition governs a particular machine, process, or jurisdiction;
• ratings or ceilings for a selected sensor, light curtain, relay, PLC, valve, or transmitter;
• the architecture, calculations, validation, proof testing, or documentation required for a complete design; or
• the cost, maintenance, or safety-value consequences of choosing a higher target.

Each question requires the applicable authoritative standards, exact product evidence, and a project-specific assessment.

A defensible way to handle mixed labels

When a datasheet and project requirement use different labels:

1. Preserve the wording of each claim exactly; do not convert it mentally.
2. Identify the standard, edition, scope, and rated object behind each label.
3. Obtain an authoritative source for the proposed association.
4. Check the complete design evidence and assumptions required by that source.
5. Record the rationale rather than treating the labels as synonyms.

If the available sources cannot answer these questions, mark the comparison unresolved rather than bridge it with a context-free conversion.

Frequently Asked Questions

Does the IDEC table associate SIL 2 with PL d?

Yes. For frequent operation request or continuous mode, the table associates PL d with SIL 2. That entry does not by itself prove compliance for a particular component or safety function.

Does Performance Level use dangerous-failure probability?

Yes. The IDEC overview says ISO 13849-1 evaluates safety functions using average probability of dangerous failure and expresses the result as PL.

Can a component's PL label automatically satisfy a SIL requirement?

No. The comparison table does not replace the governing standard, applicable mode, product evidence, calculation assumptions, or validation for the complete project.

Which standard applies to my machine or process?

This article cannot answer that project-specific question. Consult the standards and authoritative sources governing the actual application and jurisdiction.

Does a higher SIL or PL always mean a better design?

This source set does not support a cost, safety-value, or maintenance comparison between targets. Select and justify the required target through the applicable safety process.

Conclusion

ISO 13849-1 expresses a safety function's average probability of dangerous failure as PL. The IDEC table associates PL with SIL for frequent operation request or continuous mode, but does not validate a component or complete function against a project requirement. Use the project's governing standard, assumptions, product evidence, and validation.