Safety Integrity Level (SIL)

This is a performance indicator that is the goal that safety-related parts must achieve, as stipulated in IEC 61508 (Functional safety of electrical, electronic & programmable electronic safety-related systems) and IEC 62061 (Safety of machinery and safety of safety-related electrical, electronic and programmable electronic control systems).

How to determine SIL (Safety Integrity Level)

SIL is a measure of the safety performance of a system in IEC 61508. When SIL is defined in terms of PFH avg (average frequency of dangerous failure per hour), the maximum SIL that a subsystem can achieve is constrained by its architecture’s Hardware Fault Tolerance (HFT) and Safe Failure Fraction (SFF).

IEC 61508-1:2010 Safety Integrity Level (High Demand Mode)

SIL	Average Frequency of Dangerous Failure / Time (PFH avg)
1	≥10⁻⁶ to <10⁻⁵
2	≥10⁻⁷ to <10⁻⁶
3	≥10⁻⁸ to <10⁻⁷
4	≥10⁻⁹ to <10⁻⁸

IEC 61508-2:2010 Maximum allowable safety integrity level

SFF (Safety Failure Fraction)	HFT (Hardware Fault Tolerance)
SFF (Safety Failure Fraction)	0	1	2
< 60 %	SIL 1	SIL 2	SIL 3
60 % to < 90 %	SIL 2	SIL 3	SIL 4
90 % to < 99 %	SIL 3	SIL 4	SIL 4
≧ 99 %	SIL 3	SIL 4	SIL 4