Availability is the probability that a device or a SIF is in a working state at a given instant, and unavailability is the flip side, 1 minus availability. In functional safety this is the quantity behind the headline number. PFDavg is the average unavailability of a SIF to respond to a demand, and safety availability is 1 minus PFDavg, so a SIL 2 function is somewhere between 99% and 99.9% available.
Key Points
- Availability is an instant-in-time measure of uptime. Reliability is the probability of running a whole period without failure. Different questions, often confused.
- PFDavg is the mean unavailability of the SIF over the TI; RRF is 1 over it and safety availability is 1 minus it.
- Availability of the process (few spurious trips) and availability of the SIF (ready for a demand) pull in opposite directions, which is a big reason 2oo3 is so common.
Example
A SIF with a PFDavg of 2E-3 has a safety availability of 99.8% and an unavailability of 0.2%. Averaged over the TI, it is unable to respond to a demand about two hours out of every thousand.
Cited Sources
- IEC 61508-4:2010, Clause 3.6.18