Which one, SIL or PL?

When a safety function is required for risk reduction a safety control system is engineered and implemented to perform that function. Standards and directives can be a jungle. So, if you are an engineer, integrator, or build machines, what do you have to follow?

TWO MACHINERY APPLICATION STANDARDS

There are two machinery application standards that engineers can utilize to design their safety control system. There is IEC 62061: 2005 which is used to determine the SIL for electrical, electronic, and programmable electronic systems, and ISO 13849-1: 2015 which is used to determine the PL of the safety-related parts of the control system e.g. mechanical, electrical, pneumatic, or hydraulic.

‘’Which one should I use?’’ is usually the question that we are frequently asked – which I will attempt to answer. Both IEC 62061 (SIL) and ISO 13849-1 (PL) specify requirements for the design and implementation of safety-related control systems of machinery. The methods developed in each standard are different but, when correctly applied, can achieve a comparable level of risk reduction.

COMPARISON OF THE STANDARDS

The two standards classify safety-related control systems that implement safety functions into levels that are defined in terms of their probability of dangerous failure per hour.

ISO 13849-1 has five Performance Levels (PL’s) a, b, c, d, and e; while IEC 62061 has three Safety Integrity Levels (SIL’s) 1, 2, and 3.

If we compare the technical requirements between IEC 62061 (SIL) and ISO 13849-1 (PL) and considering the following aspects:

• Terminology.
• Risk estimation and performance allocation
• Safety requirements specification
• Systematic integrity requirements.
• Diagnostic functions
• Software safety requirements

We can draw the following conclusion: Safety-related control systems can be designed to achieve acceptable levels of functional safety using either of the two standards. This is achieved by integrating a non-complex SRECS (safety-related electrical control system) subsystems or SRP/CS (safety-related parts of a control system) designed in accordance with IEC 62061 (SIL) and ISO 13849-1 (PL), respectively. 

Both standards can also be used to provide design solutions for complex SRECS and SRP/CS. This is achieved by integrating electrical, programmable, or electronic subsystems designed in accordance with IEC 61508.

SO WE CAN USE EITHER… WHICH ONE DO WE USE THEN?

The selection and use of either standard may still not be clear, therefore the following considerations may help in attempting to determine the use of either IEC 62061 (SIL) or ISO 13849-1 (PL).

• Previous experience | Previous knowledge and experience in the design of machinery safety-related control systems based upon the concept of categories described previously in ISO 13849-1:1999 can mean that the use of ISO 13849-1:2006 is more appropriate.
• Other media than electrical | Safety-related control systems based upon media other than electrical can mean that the use of ISO 13849-1 (PL) is possibly more appropriate.
• Customer requirements | Customer requirements to demonstrate the safety integrity of a machine safety-related control system in terms of a SIL can mean that the use of IEC 62061 is more appropriate.
• Existing machinery using SIL | Safety-related control systems of machinery used in, for example, the process industries, where other safety-related systems (such as safety instrumented systems in accordance with IEC 61511) are characterized in terms of SILs, can mean that the use of IEC 62061 is more appropriate.

THE MOST FREQUENTLY USED STANDARD TODAY

It is generally accepted that the majority of industry uses ISO 13849-1 (PL) because the machine builders were previously using EN 954-1, which have naturally migrated to ISO 13849-1 (PL). This is reinforced by a survey illustrated below that was carried out by the joint working group of ISO/TC 199 – IEC/TC 44, which at the time was considering the merging of IEC 62061 and ISO 13849-1.

WHAT NEXT?

There was a project to migrate the two standards into one, but due to the timeline to complete the project running over and significant disagreement the project was terminated. Both IEC 62061 (SIL) and ISO 13849-1 (PL) are currently in revision and are expected to be released sometime in 2021. It is likely that the selection criteria determining the use of either standard will become even harder to determine.

CONCLUSION

I can recommend you read the existing Technical Report (IEC/TR 62061-1: 2010) which, although dated, provides further information and will continue to provide further updates here on the proposed contents of IEC/TS 62061-1 (Guidelines on the application of IEC 62061) and further news on the contents of the revisions of IEC 62061 and ISO 13849-1.

It would be preferable if there was only one standard, but for the time being, you may have to use them both.

PS - WHY ARE STANDARDS IMPORTANT?

Standards not only help in providing a presumption of conformity, which came sharply into focus with the ‘’James Elliott ruling’’.
But also provide the catalyst for innovation, and contribute in excess of €40 billion to the economic growth for the EU, detailed in the image below from a recent European Union workshop: 

THE WRITER

David Andrew Main-Reade has been involved in the field of functional safety in excess of thirty years. He represents BSI as an expert on a variety of International and National standards (IEC, ISO, CENELEC, and BS) and is a TÜV Rheinland FS Expert and has spent over twenty years with Rockwell Automation as their Global standards specialist.