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Reliability of Safety-Critical Systems; Theory and Applications

Submitted by anonymous » Wed 25-Jun-2014, 17:53

Subject Area: Systems Engineering

Keywords: Rausand, IEC 61508, IEC 61511, Functional Safety, Reliability

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Marvin Rausand, Reliability of Safety-Critical Systems; Theory and Applications, Wiley, 2014, ISBN 978-1-118-11272-4, 448 pages.

Marvin Rausand’s Reliability of Safety-Critical Systems; Theory and Applications is an amazing book. The author has managed to explain a seemingly tedious subject in a way that it makes you understand the topic and makes you aware of the pitfalls.

The book’s title is an accurate description of its contents: it covers the mathematics behind reliability calculations of safety-critical systems, mainly aimed at those used in the process industry, although it would be equally relevant for other industry domains. It thereby addresses parts of IEC 6108 and IEC 61511 aimed at calculating hardware reliability. The author humbly admits: the book does not cover other aspects of these standards, e.g., how to derive reliability targets, life-cycle issues, software reliability. So be aware: this book is mathematics only.

What makes this book so good? First of all, Rausand is very precise, terms all well-defined and consistently used throughout the book. Mathematical derivations are patiently explained, always in small steps (no “exercises to the reader”), and always in the same fashion. The explanations make it look so simple that you might start thinking that you could have done it yourself, but I know that this is not true. An extraordinary achievement is that Rausand fosters understanding of the formulas, which is also his explicit aim. Only sometimes Rausand ambitions are too high, an example being the explanation of the Laplace transform in half a page.

Rausand compares his own derivations with those in IEC 61508/61511 and the PDS method (a Norwegian developed method for reliability assessment of offshore safety-critical systems). These comparisons very much add to understanding by highlighting the differences in the approaches. Rausand openly explains the different interpretations, using them to create insights rather than to defend his own position. This objective stance is very pleasant.

I enjoyed the chapter on uncertainty. Rausand is very down to earth and treats the subject in a balanced way. Understanding this topic is necessary, because functional safety engineers will often be confronted by questions about the uncertainty in the results of the calculations. Rausand provides ammunition for sensible answers.

Only seldom I disagree with Rausand, an example being the following. Rausand explains the reliability of a 2oo3 voting group of fire detectors. There is nothing wrong with the mathematics, but I find using fire detectors for this purpose rather unfortunate. It is true that fire detectors normally are in voted groups, the aim being to create an alarm when one fire detector triggers and a shutdown when two of them trigger. But: the fire detectors are spread throughout a fire area and are not each other’s backup; the reliability requirements apply to each single fire detector, as is explicitly stated in Norwegian Oil and Gas’ Guideline 070, Table 7.1: “The SIL-requirement applies to the sub-function needed for fire detection, given exposure of one detector”. Voting is therefore not relevant for the reliability calculations. There may be applications where this is the case, but they are the exception. Given the rather frequent occurrence of this misconception, I would have appreciated if Rausand would have used other components in the example.

After all the hard work done, Rausand dryly concludes that the formulas for the different approaches give virtually the same results. This may be true, but this defies the strength of the book which is that it makes you understand the mathematics behind reliability of safety-critical systems. As such this book is compulsory reading for anyone in the field of functional safety. The patient explanations make the book also suitable as a textbook for courses in functional safety.

Meine van der Meulen

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