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Risk analysis SIL (Safety Integrity Level)

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.