Safety integrity level SIL is defined as a discreet level (one of four levels, defined by a norm) for assessment of requirements on safety integrity. When discussing the safety functions assigned E/E/EP to systems connected with safety, then integrity level 4 is the highest and 1 the lowest.
| SIL | (PFDavg) | (PFH) |
|---|---|---|
| SIL 1 | ≥ 10-5 up to <10-4 | ≥ 10-9 up to <10-8 |
| SIL 2 | ≥ 10-4 up to <10-3 | ≥ 10-8 up to <10-7 |
| SIL 3 | ≥ 10-3 up to <10-2 | ≥ 10-7 up to <10-6 |
| SIL 4 | ≥ 10-2 up to <10-1 | ≥ 10-6 up to <10-5 |
SIL Safety Integrity Level
PFDavg Probability of failure on demand average, this is the probability that a system will fail dangerously, and not be able to perform its safety function when required. PFD can be determined as an average probability or maximum probability over a time period.
PFH Probability of safety function failure average per hour.
The system of analyses is elaborated in the set of norms ČSN EN 61508-X, 61511-X.