Analysis of a safety loop

The safety integrity level (SIL) of a function is partly defined by its probability of failure and its architectural constrains on hardware. 

There are a number of techniques to analyze the safety integrity of an E/E/PE safety-related systems. The two most used techniques are reliability block diagrams and Markov models. Both methods, if properly applied, give similar results however, for complex programmable electronic sub-systems (e g. when a logic 

To calculate the PFD/PFH of a safety instrumented function, we must process the sum of the PFD/PFH of each of its sub-systems (sensors, logic processing, acmators) and taking into account various parameters  

The general methodology for calculating the PFD/PFH of a safety function can be summarized as follows  

For certified components, the manufacturer provides the necessary values (SIL CL and PFH) alternatively, here is a method for obtaining these values. 

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