Quantitative Aspects of Common Cause Failures and Review of Commonly Applied Models

摘要

Multiple failures of components due to shared causes, also known as Common Cause Failures (CCF), comprise an important class of failure types. These have to be taken into account in any serious assessment of safety critical systems deploying any redundancy concept. Whereas a qualitative assessment of CCF can be regarded as common practice, the exact numerical impact of CCF is usually less widely understood. Explicit representation of CCF is quite cumbersome and usually involves complex graphical representations within fault trees and RBDs. Focusing on a correct derivation of RAM-data rather than on a comprehensive common cause analysis, an implicit representation of CCF therefore is often preferred but involves in-depth knowledge of underlying mathematical models. This paper aims to enable safety experts to make a fast, simple but effective RAM-analysis including CCF. Popular CCF models are reviewed together with their advantages and disadvantages based on the long-term experience of a supplier in the context of a large European ATM project. The single-parameter beta-factor model is explained in detail and demonstrated to be most effective for typical ATM applications. Based on this model, results are given in terms of representative figures depicting the influence of CCF on various typical real world availability requirements.