Fault tolerant systems have been designed for safety critical applications including the protection of potentially dangerous industrial processes. These systems are typically evaluated and certified to functional safety standards with IEC 61508  by agencies like exida Certification or one of the TUV companies. Many factors are taken into account during the certification process including hardware diagnostic capability, level of hardware redundancy, design processes used, software diagnostics and general equipment strength. It has clearly become recognized that common cause failures can have a major negative impact on the safety and availability of a fault tolerant system . The whole value of redundancy may be ruined. Common cause is recognized as an important factor but there is disagreement regarding how to account for common cause in the quantitativemodeling. Part 7 of IEC 61508 at least provides a list of questions with a point scoring system .