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Article | REF: SE4058 V2

Probability of failure on demand of safety instrumented function

Author: Olivier IDDIR

Publication date: November 10, 2015

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6. Method for assessing the probability of failure of a RIS

6.1 LEGO approach

The simplified analytical formulas are all based on a "LEGO" approach, which involves estimating the PFD avg of the FIS by assembling (i.e. summing) the average PFD avg of each block.

As previously explained and shown in figure 18 , a RIS generally comprises three subsystems.

Decomposition of a RIS into sub-systems
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Method for assessing the probability of failure of a RIS
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