There are three barriers to consider when designing a Safety Instrumented Function (SIF) to meet a target integrity level:

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• Architectural Constraints

• Probabilistic Performance, pfd

• Systematic Capability Constraint

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Architectural constraints also known as Hardware Fault Tolerance (HFT) were included in the IEC standards to protect against overly optimistic failure rates. HFT is guided by IEC61508 Route 1h (Safe Failure Fraction, SFF) or Route 2h (Proven in use argument). IEC 61511 Process Industry HFT is derived from Route 2h.

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The HFT for a SIF will also depend on the mode that the SIF is operating in (Low Demand, High Demand or Continuous Demand mode).

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Reference Table 6 Minimum HFT requirements according to SIL IEC61511.