What does a “SIL” requirement really mean?

Safety instrumented systems are often assigned a “Safety Integrity Level”: This is an important concept for ensuring that automatic controls intended to maintain the safety of a technical safety actually bring the risk reduction that is necessary. In the reliability standards IEC 61508 and IEC 61511, there are 4 SILs:

  • SIL 1: a failure on demand in 1 out of 10 demands is acceptable
  • SIL 2: a failure on demand in 1 out of 100 demands is acceptable
  • SIL 3: a failure on demand in 1 out of 1 000 demands is acceptable
  • SIL 4: a failure on demand in 1 out of 10 000 demands is acceptable

This way of defining the probability¬†of failure applies to so-called “low-demand” systems. In practice that means that the safety function does not need to act more than once per year in order to stop an accident from occurring.

The SIL requirement does not only involve probability calculations (Probability for failure on demand = PFD). The SIL consists of four diffent types of requirements:

  • Quantitative requirement (PFD, defined as probability of failure when there is a demand for the function)
  • Semi-quantitative requirements (requirement for redundancy, for a certain number of possible failures of the system leading to a safe state – the socalled safe failure fraction)
  • Software requirements (a lot of the actual control functionality is implemented in software. For this a work process oriented take on things is required by the standards – implications increase in rigor with increasing SIL)
  • Qualitative requirements (avoidance of systematic errors, quality mangement, etc.)

Most people focus only on the quantitative part and do not think about the latter thre parts. In order for us to have trust in the probability assessment, it is necessary that issues that cannot be quanitifed are properly managed. Hence – to claim that you have achived a certain SIL for your safety function, you need to document that the redundancy is right, that most failures will lead to a safet state, that your software has been developed in accordance with required practices and using acceptable technologies, and that your organization and workflows ensure sufficient quality of your safety function product and the system it is a part of.

If people buying components for safety instrumented systems would keep this in mind – it would become much easier to actually create safety critical automation systems with can trust with a given level of integrity.

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