Basic reliability analysis applied to safety systems

We need to be able to carry out some basic analysis steps to get us reasonably close to a credible estimate of likely performance. We cannot expect very high accuracy since we are dealing with probabilities and we are using data that is sometimes very limited. But here are several good reasons for doing the best we can with a reliability analysis. 

This last point refers specifically to clause 7.4.S.2 of lEC 61508 part 2. It is worth noting here that part 2 of the standard sets out detailed requirements for procedures that are to be followed in the design and development of E/E/PES system. We shall look at these some more when we get to the detailed engineering steps in Chapter 8. 

Before embarking on the trail of reliability analysis it s a good idea to look at what we want to achieve  

Overall SIS probability of failure on demand (PFDavg) for evaluation of risk reduction factor and ability to meet SIL targets. 

PFDavgs for each section of the SIS (or fail to danger rates if the subject is in high demand rate mode). 

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Basic reliability analysis applied to safety systems... 

Abstract. Increasingly complex systems lead to an interweaving of security, safety, availability and reliability concerns. Most dependability analysis techniques do not include security aspects. In order to include security, a holistic risk model for systems is needed. In our novel approach, the basic failure cause, failure mode and failure effect model known from FMEA is used as a template for a vulnerability cause-effect chain, and an FMEA analysis technique extended with security is presented. This represents a unified model for safety and security cause-effect analysis. As an example the technique is then applied to a distributed industrial measurement system. 

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