Specific problems to be taken into account

Link between classic parameters and standards parameters 

Generally, a reactive or curative HIPS is a periodically tested stand-by safety system. For such a system, it does not matter whether it was faulty or not before a demand occurs, if it has been repaired in the meantime. It suffices that it be available at time t when the demand arises. The classical probabihstic parameter of interest is therefore its availability Aft). Conversely, the probability of failure of interest is its unavailability Uft). 

Generally, a preventive HIPS is a safety system operating continuously. For such a system, an incident occurs as soon as there is a failure completely inhibiting its safety function. It must operate without interruption during the period when it is exposed to the demand. The classical probabilistic parameter of interest is its reliability R(t). Conversely the probability of failnre of interest is its unreliability F(t). 

Unfortunately the standards have chosen to use the concepts of PFDavg (average probability of failure on derrtartd) and PFH (probability of failure per hour) instead of those of rrrravailability arrd rrrrrehabiUty to characterize the SIL. It follows that the first task is to establish the link between the traditional rigorous notiorrs and those of the standards. 

Note first that for the traditiorral reUability analysis, the term PFD, is reserved for failttres caused by the derrtartd itself (denoted g) and not for the failttres occurring in a pending period. 

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