The Elusive Beta Factor - A Practical Method

摘要

When it comes to determining the common cause factor for the verification of instrumented protection functions (IPF), is it based on a consistent technical methodology or just what makes you feel good? The problem is that it is not practical to specifically identify the myriad of contributors to common mode failures. As a result, not all common mode failures can be modeled quantitatively at a fundamental level. To deal with this, beta factors are developed via a qualitative means using expert judgment for use in quantitative models. Increased redundancy does not necessarily result in improved system performance if potential common mode has not been adequately addressed. Beta factor analysis techniques have been established for control room electronic systems and documented in 1EC 61508 which has become the process industries' generally accepted good engineering practice. However, when we move from the control room to the field, the applicable adjustment factors effecting determination of the Beta factor need to be different. The major difference between control electronics and field equipment is the process interface. This paper introduces a method to estimate a common cause factor, Beta, for sensors and final elements employed in user defined fault tolerant architectures using a spreadsheet format.