A Proposal for Enhancing IEC 61508 Methodology for the β-Factor Estimation

The standard IEC 61508 provides a methodology to calculate β, a factor used to estimate the probability of common cause failures (CCF), i.e., failures that result from a single cause. This methodology consists of answering 37 checklist questions, each one providing a scored value that is accumulated in the final β-factor. Those questions cover 8 different defense measures, i.e., practices done to mitigate the CCF against system dependencies. Since the inception of the standard in 2010, there has been evolution regarding both new technologies with an impact on the system dependency factors, as well as new knowledge on how to address them. Hence, it is important to capture these aspects and update the methodology that can be used to reason about CCF’s causes. In this paper, we present an enhanced methodology for estimating the β-factor, which builds upon the core methodology provided by IEC 61508. In particular, we add 33 new questions and provide an estimation method for scoring the β-factor. We also illustrate our methodology by applying it to a realistic system and discuss the findings. Our proposed methodology permits the consideration of aspects not included in the core methodology, such as the level of defense support and safety culture. It also allows practitioners to consider more dependencies, leading to CCF reduction. The rationale is that the more defenses are addressed, the more protection can be achieved against CCF.