Does the mode of operation affect how the safety integrity level is determined?
Yes. First, it is helpful to use a common term hazard rate to examine the differences between low demand mode of operation and high demand or continuous mode of operation. This is the estimated rate at which specified hazards will take place unless other protective measures are in place (such as other safety-related systems). The fundamental aim is to design a safety-related system so that the resulting hazard rate is sufficiently low to meet the tolerable risk in the context of the specific application. For a safety function operating in low demand mode, the achieved hazard rate depends on the rate of demands on the E/E/PE safety-related system and the probability of failure on demand of the E/E/PE safety-related system in the context of a specified safety function. That is: Hazard rate (h) = Demand rate (d) x Average probability of failure on demand (PFDavg); or h = d x PFDavg In the context of a system architecture where the E/E/PE safety-related system is acting as a protection sys
