Requirement class referring to the functional safety of control systems according to IEC 61508. The Safety Integrity Level indicates the required degree of reliability at which a system (e.g. device or machine) fulfils its specified safety functions with regard to possible errors.
Control systems with high demand or continuous mode of operation are classified according to 'failures per hour':
- SIL 4: >10E-9 to <10E-8 failures per hour
- SIL 3: >10E-8 to <10E-7 failures per hour
- SIL 2: >10E-7 to <10E-6 failures per hour
- SIL 1: >10E-6 to <10E-5 failures per hour
Protection systems with low demand or mode of operation are classified according to 'failures per request':
- SIL 4: >10E-5 to <10E-4 failures per request
- SIL 3: >10E-4 to <10E-3 failures per request
- SIL 2: >10E-3 to <10E-2 failures per request
- SIL 1: >10E-2 to <10E-1 failures per request
