Providing Safety
Ensuring safety with SIL and PL

PL & SIL machine standards

Safety standards ISO 13849-1 and IEC 62061 are important when it comes to the safety of machinery or installations. Both standards delve into how technical safety functions should be described and applied. They use different terms to measure safety performance: ISO 13849-1 uses Performance Level (PL), while IEC 62061 uses a Safety Integrity Level (SIL). This blog discusses further the differences between these two parameters and how they overlap.

What do you use PL & SIL for?

Differences and similarities between the two parameters

Why are there two parameters, Performance Level (PL) and Safety Integrity Level (SIL)?

Imagine there are two types of companies: one dedicated to the production of conventional machinery, and another specialising in plant construction, especially within the chemical and process industries.

For mechanical engineering, where we talk about small, controllable machines, people generally use ISO 13849-1. Pursuing these standards ensures that safety is ensured in both qualitative and quantitative ways. To classify different technical safety performances, five Performance Level (PL) a, b, c, d, e are defined, representing the average probability of a dangerous failure per hour (PFH). It is about how often something can go wrong and how well the system handles it.

For large and complex plants, such as those in the chemical industry, IEC 62061 is more often applied. Here we talk about the probability of the system reacting properly if something dangerous happens. Linked to this is the Safety Integrity Level (SIL), which runs from level 1 to level 4. It focuses on reliable reactions of safety systems in large plants.

So, we have two parameters because different companies have different needs for measuring safety, depending on the size and complexity of their operations.

When and in what way can PL and SIL overlap?

The two parameters; Performance Level (PL) and Safety Integrity Level (SIL) overlap in one place, namely at MTTF d and at PFH d. When this value is evaluated against the safety parameters, the various PL levels can be compared with the SIL levels, thus converting one parameter into the other.

Using the risk graph in ISO 13849-1, the engineer can assess the potential hazards and risks of the machine. Then, this chart helps determine the required, Performance Level (PL) for the safety-related parts and control systems (SRP/CS) of the machine. The engineer then performs the required calculations to verify that the SRP/CS meets the established PL.

An important fact is that even though safety standard IEC 61508 defines four levels, general machinery applications require a maximum of SIL 3. This means that for most machines, a SIL 3 level of safety integrity is sufficient to manage risks at an acceptable level. SIL 4 is particularly for highly critical applications where there is a very high risk of damage or injury, as for example in fly-by-wire or steer-by-wire systems in vehicles, where even the smallest failure can have serious consequences.

Conclusion

Both Performance Level (PL) and Safety Integrity Level (SIL) play a crucial role in ensuring the safety of machinery and facilities, and are essential building blocks for achieving functional safety. Essentially, when designing systems that include electrical, electronic and programmable electronic (E/E/PE) components, engineers can choose between using either the ISO 13849-1 or IEC 62061 approaches. This gives them the option of working with PL or SIL, depending on the requirements of their projects. SIL is particularly suitable for large-scale and complex facilities or plants with multiple machines, where interaction between different systems is involved. On the other hand, PL is more suitable for individual machines, where the focus is on the performance of individual safety-related components.

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