Extending IEC61508 reliability evaluation techniques to include common circuit designs used in industrial safety systems

Recent international standards such as IEC 61508 and ANSI/ISA84.01 cover the design and application of safety instrumented systems (SIS). These standards are “performance based” and involve establishing risk reduction targets followed by a reliability and safety evaluation to verify that the targets have been met by the design. The standards provide guidelines on how to do these reliability and safety calculations that are quite useful and provide a common evaluation framework for products used in safety instrumented systems. However, the reliability and safety evaluation methods require extension when SIS products include independent diagnostic circuitry or analog circuitry. An additional failure mode, diagnostic annunciation, must be considered. A definition of “fail-safe” versus “failure-on-demand” must be added for analog circuits. Markov models must include additional states. With extension, comparable results useful for standards-based product certification can be obtained