Functional Safety: When to Verify a Safety Instrumented Function

A safety instrumented function (SIF) is only as good as the assumptions behind it. When those assumptions change, the probability of failure on demand (PFD) may no longer match reality.

Common triggers for re-verification

• Process or equipment changes that affect demand rate or consequence.
• Proof test intervals extended beyond the original design basis.
• Field devices replaced with different models or configurations.
• New hazards identified from incident investigations or HAZOP updates.

What verification should cover

Re-verify the SIL target, architecture, device failure data, proof test effectiveness, common-cause factors, and maintenance bypass policy. Update the safety requirements specification to match the current operating context.

S5 Consultants provides SIL selection, SIF verification, and lifecycle assurance aligned with IEC 61508 and IEC 61511. Get in touch to review your SIS portfolio.