Estimation and Evaluation of Common Cause Failures in Sis

ANSI/ISA S84.01-1996 and drafts IEC 61508/61511 are standards covering the design, operation, maintenance, and testing of safety instrumented systems (SIS). The standards stress the importance of minimizing potential faults in the SIS through good design and engineering practice. These faults include random hardware, systematic, and common cause faults. Common cause faults occur when a single fault results in the corresponding failure of multiple components, such as a miscalibration error on a bank of redundant transmitters. The frequency of common cause faults is difficult to estimate. The modeling techniques and available failure rate data make the predictive calculations of these failures cumbersome and, sometimes, the results obtained are questionable.