Problems in designing and operating functional safety solutions of higher integrity levels

The aim of this article is to identify and discuss some problems that have been encountered in designing and operating the functional safety solutions of higher safety integrity levels (SIL 3 and SIL 4) in the light of analyses outlined in a new version of the international standard IEC 61508:2010. Examples of such solutions are the electric / electronic / programmable electronic systems (E/E/PESs) and the safety instrumented systems (SISs) being designed and operated respectively according to IEC 61508 and IEC 61511 in the system safety life cycle. The role of functional safety solutions is effective reducing and controlling the individual and/or societal risk with regard to tolerable levels defined. Some aspects of potential influence of danger failures of the E/E/PESs or SISs on the plant safety are considered. The influence of common cause failures (CCFs) and dependent failures in the context of the layer of protection analysis is also discussed.