For energize-to-trip (ETT) systems to operate, power must be available to take the safety action at the time of demand. Consideration of the probability of failure on demand needs to consider this. Of course, its effect is very often overblown. From a risk perspective, the supply of power to the SIS equipment (or more generally, motive force, including pneumatics and hydraulics) is not an isolated independent system). As such, loss of the utility supply usually has a far reaching impact beyond just the SIF. If for instance, instrument air was lost to a shutoff valve, there is also a very good chance that instrument air was lost to ALL of the valves in the area. This will most likely result in the process moving to a safe state somewhat inherently, and making the failure of the SIS to be able to take action a moot point.
