Abstract
The knowledge about the possible influence of demands on the reliability of safety systems comes from a long way back in time. In IEC 61508 this has been taken care of by indicating two different ways to evaluate the safety integrity of SIS: low demand and high demand mode. The main focus of this paper lies in the intermediate and high demand modes. We investigate the effect of the demand on the system hazard rate when the failed states of individual components are detected during a demand. Component failures can then be repaired before the occurrence of the failure of the safety system. The results of this paper show that the system hazard rate exhibits an unexpected behaviour in the intermediate to high demand region. It is also shown that depending on the SIS repair scheme, the use of Probability Failure per Hour (PFH) based on the equations for a continuous demand as proposed in IEC 61508 leads to very conservative results in the high and continuous demand regions. New asymptotic equations are proposed to evaluate the PFH of kooN systems for all values of the demand rate, together with new associated rules for discrimination of the demand regions.
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