FAQs
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The Mission Time (MT) is the intended operational period of a Safety Instrumented Function (SIF) — the time after which all SIF components must be subject to a proof test with 100% coverage, equivalent to complete refurbishment or replacement to ‘as-new’ condition. MT is formally defined in VDI/VDE 2180-3 as parameter T2. It is not explicitly defined in IEC 61511 or IEC 61508, but is a critical input to the SIL Verification calculation.
Where proof test coverage (CPT) is less than 100% — which is almost always the case in practice — a longer MT results in a higher PFDavg and a correspondingly lower SIL. This effect is sometimes referred to as SIL-Degradation. MT should be defined in the Safety Requirements Specification (SRS) and should reflect realistic component replacement or overhaul schedules, taking into account the Useful Life of individual SIF components.
The Useful Life of a SIF component is the period within which the constant failure rate assumption — fundamental to all PFDavg calculations — remains valid. Per IEC 61508-2 (§7.4.9.5, Note 3), once the useful life limit is exceeded, the results of probabilistic SIL Verification calculations are no longer valid. This limit is derived from the underlying failure rate ‘bathtub curve‘, which describes the three phases of a component’s life: early failures, constant failure rate, and wear-out.
Manufacturers publish useful life limits in the component Safety Manual or SIL-certificate. Typical values for common SIF components are:
- Sensors: up to 50 years
- Barriers / isolators: ~10 years
- Logic Solvers (fail-safe PLC): ~15 years
- Solenoid valves: 8–12 years
- Actuator / valve assemblies: ~10 years
Where a component’s useful life is shorter than the SIF Mission Time, the component must be replaced or overhauled before the end of its useful life, or the Operator must justify an extension via a formal ‘Prior Use’ assessment per IEC 61511-1 (§11.5.3). See our blog Mission Time vs Useful Life for a worked example
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The Mission Time (MT) is the intended operational period of a Safety Instrumented Function (SIF) — the time after which all SIF components must be subject to a proof test with 100% coverage, equivalent to complete refurbishment or replacement to ‘as-new’ condition. MT is formally defined in VDI/VDE 2180-3 as parameter T2. It is not explicitly defined in IEC 61511 or IEC 61508, but is a critical input to the SIL Verification calculation.
Where proof test coverage (CPT) is less than 100% — which is almost always the case in practice — a longer MT results in a higher PFDavg and a correspondingly lower SIL. This effect is sometimes referred to as SIL-Degradation. MT should be defined in the Safety Requirements Specification (SRS) and should reflect realistic component replacement or overhaul schedules, taking into account the Useful Life of individual SIF components.
The Useful Life of a SIF component is the period within which the constant failure rate assumption — fundamental to all PFDavg calculations — remains valid. Per IEC 61508-2 (§7.4.9.5, Note 3), once the useful life limit is exceeded, the results of probabilistic SIL Verification calculations are no longer valid. This limit is derived from the underlying failure rate ‘bathtub curve‘, which describes the three phases of a component’s life: early failures, constant failure rate, and wear-out.
Manufacturers publish useful life limits in the component Safety Manual or SIL-certificate. Typical values for common SIF components are:
- Sensors: up to 50 years
- Barriers / isolators: ~10 years
- Logic Solvers (fail-safe PLC): ~15 years
- Solenoid valves: 8–12 years
- Actuator / valve assemblies: ~10 years
Where a component’s useful life is shorter than the SIF Mission Time, the component must be replaced or overhauled before the end of its useful life, or the Operator must justify an extension via a formal ‘Prior Use’ assessment per IEC 61511-1 (§11.5.3). See our blog Mission Time vs Useful Life for a worked example