Systematic failures in safety systems: Some observations on the ISA-S84 standard

Emergency shut-down (ESD) systems are used in the process industry to perform safety functions by shutting the process down in a pre-determined way in case of an emergency. Complete ESD systems nowadays consist of sensors, (computer) logic and actuators. Many parameters can influence the safety of ESD systems like system layout, diagnostics, common cause effects and testing. Standards like IEC 1508 and ISA-S84.01 often require quantitative risk analyses. One of the techniques for calculating the probability of an unsafe failure of an ESD system is the Markov modeling technique especially if enhanced with uncertainty analysis to model effects of uncertain data. When a quantitative analysis is performed often systematic failures and common cause failures are dominant factors influencing safety. This means that the way these failures are modeled is very important for the actual predicted safety of the ESD system. Because the time intervals used for periodic testing are often derived from the safety level as function of time, the approach chosen for modeling systematic failures can have a major influence on intervals for periodic testing