Programmable automation system safety assessment (PASSI): Reliability assessment of a software-based motor protection relay using Bayesian networks

Often to make justified reliability claim of a certain system different kinds of evidence needs to be combined. Some of the evidence supporting the claim may be of qualitative type, whereas some of the evidence may be of quantitative type. Combination of disparate evidence together is not always straightforward and the reasoning behind the conclusions obtained from the combination may be hard to explain. Bayesian networks provide a consistent and transparent method for the combination of the evidence and for the reasoning of one's beliefs on the relation of different pieces of evidence. In the special report we demonstrate the combination of disparate evidence with a case study on the reliability assessment of a software-based motor protection relay, where the combination of the reliability related evidence has been carried out using Bayesian networks. The reliability related evidence in the case study is the expert judgement on the development process and the operational experience estimated for the software-based motor protection relay.