SIL versus Performance Level (PL) — which framework for which safety function

The iso13849-1-performance-levels guide covers the PL categories (a through e) and the architecture categories of ISO 13849-1 in detail. This article addresses a different, frequently asked follow-up question: when do you use SIL (Safety Integrity Level, from IEC 62061) instead, and how do the two frameworks relate to each other?

Two frameworks for the same machine-safety function

ISO 13849-1 (Performance Level, PL a–e): applies to safety-related parts of control systems regardless of the technology used — mechanical, hydraulic, pneumatic, electrical or electronic.
IEC 62061 (Safety Integrity Level, SIL 1–3): the machinery application of the broader IEC 61508 framework, specifically aimed at electrical, electronic and programmable electronic (E/E/PE) safety-related control systems — so it is not applicable to a purely mechanical or hydraulic safety function.

Both standards are harmonised under the European Machinery Directive (and its successor, the Machinery Regulation (EU) 2023/1230), so that a manufacturer can combine a subsystem assessed under one framework with a subsystem assessed under the other within the same machine.

The harmonised correspondence table

Performance Level (PL)	Safety Integrity Level (SIL)
PL e	SIL 3
PL d	SIL 2
PL c	SIL 1

Note: this is a harmonised correspondence, not an exact mathematical equivalence — PL is calculated via the average probability of a dangerous failure per hour (PFH) combined with the architecture category (B, 1–4), while SIL under IEC 62061 uses its own PFH range per level. The table is intended to allow two subsystems assessed under different frameworks to be practically combined — not to make the underlying calculation methods interchangeable.

Which framework to choose

A purely mechanical, hydraulic or pneumatic safety function (for example a mechanically interlocked safety valve) can only be assessed with PL/ISO 13849-1 — SIL/IEC 62061 does not apply here.
A complex, programmable electronic safety function (for example a safety PLC controlling multiple sensors and actuators) is in practice often assessed via SIL/IEC 62061, although an assessment via ISO 13849-1 is also permitted for E/E/PE systems.
A machine with mixed technology (for example a mechanical emergency-stop button in series with an electronic safety relay) often combines both frameworks: each subsystem is assessed using the framework that matches its own technology, and the results are combined via the correspondence table into an overall assessment of the complete safety function.

Practical relevance

When assessing a safety function (for example the emergency-stop chain from the emergency-stop categories guide), it must first be established which technology each subsystem uses before choosing the correct assessment framework (PL or SIL) — and, where both frameworks are combined within one safety function, whether the correspondence table has been applied correctly to arrive at a valid overall assessment.

Common mistakes

Applying SIL/IEC 62061 to a purely mechanical or hydraulic safety function — that framework is limited to E/E/PE systems; ISO 13849-1 is the only applicable framework here.
Treating PL and SIL as exactly the same scale — the correspondence table is a practical harmonisation, not a mathematically identical calculation method.
Failing to correctly apply either framework per subsystem in a mixed-technology machine — each subsystem must first be assessed individually with the correct framework before the results are combined.