The Improvement Proposal of Common Cause Factor Score Table on IEC61508 Edition 2.0
A Common cause factor has a large impact on the achievement of high result Safety Integrity Level (SIL) such as SIL3 and SIL4. Even if the dangerous undetected failure rate is very low, the result SIL will decrease in case the common cause factor (β) is high. In this study, the new classification method for common causes is proposed to establish more practical countermeasures to lower β. Common causes can be classified into hardware failure, systematic failure and human error. In order to determine β, it should be confirmed that the detailed countermeasures for preventing these three kinds of common causes are fully implemented on site. The most representative methodology for determining β is the β score table on latest IEC61508. However, there are several issues on IEC61508 β score table. In this study, the β score table on IEC61508 is modified to solve those issues. A new β score table is proposed based on advanced systematic classification of common cause including hardware failure, systematic failure and human error. This table based on latest technologies is able to provide practical rates and scores for various subtopics. A new β score table is well harmonized with the requirement of international association of oil & gas producers (IOGP) report. The new β score table contributes to prevent the accident by common cause failures more than the latest IEC 61508.