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Risk and requirements · Core workflow

Determining the required SIL

Required SIL comes from the risk that remains after justified independent protection layers are credited. It should be determined before the SIF design is adjusted to meet the target.

Engineering guide10 min readUpdated 10 October 2026Published by SILProof Engineering

In brief

What to carry into the engineering work

  • Define one hazardous scenario before assigning frequencies or protection layers.
  • Credit an IPL only when its effectiveness, independence and lifecycle controls are justified.
  • Record required RRF and SIL separately from the capability of the proposed design.

Start with one hazardous scenario

State the initiating event, causes, consequence, exposed population or asset, operating mode and safeguards. Avoid combining unrelated scenarios into one number. The scenario definition controls which frequencies, consequences and protection layers are relevant.

LOPA workflow

  1. Define the unmitigated consequence and initiating-event frequency.
  2. Set the tolerable-event-frequency criterion used by the organisation.
  3. Credit only protection layers that are effective for the scenario, independent of the initiating cause and other credited layers, auditable and maintained.
  4. Calculate the mitigated frequency without the proposed SIF.
  5. Determine the additional risk-reduction factor required from the SIF and map it to the required SIL band.
Required RRF = mitigated frequency without the SIF ÷ tolerable frequency

Risk graph and company matrix

Risk graphs and calibrated company matrices can provide a structured qualitative or semi-quantitative route. Record every selected parameter, the matrix revision and the rationale. The result is only reproducible when another reviewer can follow the same path.

What the record should contain

  • Scenario and consequence description.
  • Initiating-event source and frequency basis.
  • Credited IPLs, PFD assumptions and independence rationale.
  • Tolerable-risk criterion and company procedure reference.
  • Required RRF and resulting required SIL.
  • Participants, date, review status and open actions.

Frequent errors

  • Crediting the same protection twice or crediting a safeguard that shares the initiating cause.
  • Using generic frequencies without checking plant conditions.
  • Rounding a borderline result into a lower requirement.
  • Changing the required SIL because the proposed design is difficult to achieve.

Primary references

  1. IEC 61511-1 — process-industry SIS framework and requirements
  2. UK HSE — Functional safety
  3. ISA — ISA-84 series of standards

Use the applicable edition and national adoption. Project specifications and controlled procedures may add requirements.

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Related guides

Functional safety fundamentalsUnderstand functional safety, SIS, SIF, SIL and the lifecycle thread that connects risk to operation.Designing a Safety Instrumented FunctionTranslate the safety requirement into sensors, logic solver interfaces, final elements and a testable safe action.Writing a SIF Safety Requirements SpecificationWrite one controlled SIF requirement set that can drive design, verification, testing and validation.