Example SIS architecture

SIS architecture and safety Instrumented function example with different devices shown 

NOTE 1 The higher the safety integrity level, the higher the probability that the required safety instrumented function 

NOTE 2 There are four levels of safety integrity for safety instrumented functions. 

NOTE 3 In determining safety integrity, all causes of failures (both random hardware failures and systematic failures) which lead to an unsafe state should be included for example, hardware failures, software induced failures and failures due to electrical interference. Some of these types of failure, in particular random hardware failures, may be quantified using such measures as the failure rate in the dangerous mode of failure or the probability of a safety instrumented function failing to operate on demand. However, the safety integrity of an SIF also depends on many factors, which cannot be accurately quantified but can only be considered qualitatively. 

NOTE 4 Safety integrity comprises hardware safety integrity and systematic safety integrity. 

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