Use of Graphical Probabilistic Models to build SIL claims based on software safety standards such as IEC61508-3

摘要

Software reliability assessment is 'different' from traditional reliability techniques and requires a different process. The use of development standards is common in current good practice. Software safety standards recommend processes to design and assure the integrity of safety-related software. However the reasoning on the validity of these processes is complex and opaque. In this paper an attempt is made to use Graphical Probability Models (GPMs) to formalise the reasoning that underpins the construction of a Safety Integrity Level (SIL) claim based upon a safety standard such as IEC61508 Part 3. There are three major benefits: the reasoning becomes compact and easy to comprehend, facilitating its scrutiny, and making it easier for experts to develop a consensus using a common formal framework; the task of the regulator is supported because to some degree the subjective reasoning which underpins the expert consensus on compliance is captured in the structure of the GPM; the users will benefit from software tools that support implementation of IEC61508, such tools even have the potential to allow cost-benefit analysis of alternative safety assurance techniques. This report and the work it describes were funded by the Health and Safety Executive. The opinions or conclusions expressed are those of the authors alone and do not necessarily represent the views of the Health and Safety Executive.