The higher the SIL rating the higher the availability required. SIL 1 90% availale, SIL 2 99% available and SIL 3 99.9%...(notice the number of nines is equal to the SIL number).
You then the take credits for layers of protection, such that these levels of availablity are actually acomplished. One way to do that is a 2 out of 3 voting (typical of SIL 2) or 2 out of 4 voting to not only make sure that the protective function is available, but also to prevent false trips.
SIL 3
Re: SIL 3
