SIL selection is the process of assiging a SIL (Safety Integrity Level) to a safety function in order to close the gap between residual risk (evaluated in HAZOP) and tolerable risk (as per Client Risk Matrix).

SIL (or its inverse – Risk Reduction Factor RRF) is defined in IEC 61508/61511 for low demand operation, as follows:

SILPFD (avg)PFD (power)RRF
10.1–0.0110−1 – 10−210–100
20.01–0.00110−2 – 10−3100–1000
30.001–0.000110−3 – 10−41000–10,000
40.0001–0.0000110−4 – 10−510,000–100,000
Safety Integrity Levels (IEC 61511 Table 2)