IEC 61508 and IEC 61511 functional safety standards allow Safety Integrity Levels (SIL) to be used four ways by end users:

  1. To establish risk reduction requirements - How much risk needs/can be removed from the process to achieve a tolerable risk level? (For more info see Risk Reduction.)
  2. Probabilistic limits for hardware random failure - This is the random probability of a failure. This is calculated by PFDavg for low demand mode of operation or PFH for high or continuous demand modes (See PFDavg or PFH for more info.)
  3. Architectural constraints - Sets rules for establishing the need of redundancy. (See Architectural Constraints for more info.)
  4. To establish Systematic Capability - Systematic Capability is achieved either by choosing a certified part with SC to the given SIL level or greater or by completing a prior use justification to the given SIL level or greater. The lowest SC for any device in the SIF determines the SIL level for the SIF with respect to SC. (See Systematic Capability for more info)

For each of the ways to use SIL, there are tables and calculations in the standard. Some of these are shown below. 

SIL - Safety Integrity Level

Related Items

Back to Basics 01 - Functional Safety

Back to Basics 02 - Safety Integrity Level (SIL)

Back to Basics 03 - Safety Instrumented Function (SIF)

Back to Basics 04 - Safety Instrumented System (SIS)

Back to Basics 05 - What is a Safety Function?

Back to Basics 06 – IEC 61508

Back to Basics 07– Safety Lifecycle – IEC 61508

Back to Basics 08 – IEC 61511

Back to Basics 09 – Safety Lifecycle – IEC 61511

Back to Basics 10 – How Does a Product Get a SIL?

Tagged as:     Systematic Capability     SIL     Risk Reduction     PFDavg     Loren Stewart     IEC 61511     IEC 61508     functional safety     Back to Basics     Architectural Constraints  

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