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Functional safety fundamentals

Functional safety is the part of overall safety that depends on a system or other measure responding correctly to a hazardous condition. In process plants, that usually means following a defined safety function from its risk basis through specification, design, verification, validation and operation.

Engineering guide8 min readUpdated 10 October 2026Published by SILProof Engineering

In brief

What to carry into the engineering work

  • SIL is a requirement and claim for a safety function, not a quality label for one device.
  • The risk basis, SRS, design, verification and validation should form one traceable engineering thread.
  • Quantitative performance, architecture and systematic capability all constrain the supported SIL claim.

The engineering thread

A useful starting point is a single traceable question: what hazardous event is being controlled, and what must happen before it becomes unavoidable?

  1. Identify the hazardous scenario and the tolerable-risk basis.
  2. Determine the risk reduction required from instrumented and non-instrumented protection layers.
  3. Define each Safety Instrumented Function (SIF), including its trigger, action and safe state.
  4. Write requirements that can be designed, tested and validated.
  5. Verify random hardware integrity, architectural constraints and systematic capability.
  6. Preserve the assumptions through testing, operation, modification and decommissioning.

SIS, SIF and SIL

SIS
A Safety Instrumented System implements one or more safety instrumented functions using sensors, a logic solver and final elements.
SIF
A specific safety action that detects a defined condition and takes the process to, or maintains it in, a safe state.
SIL
A discrete integrity level attached to the safety function and its required performance. It is not a quality label for one instrument.

Lifecycle decisions and evidence

Every lifecycle stage should leave a reviewable output. Risk analysis establishes the required integrity. The SRS states what the function must do. Design selects the architecture and devices. Verification checks whether the design can support the target. Validation checks the installed function against the SRS. Operation preserves the assumptions through proof testing, repair and change control.

Good practice: keep the scenario, required SIL, SIF identity, SRS, design assumptions and verification result on the same engineering thread.

Common misunderstandings

  • A low PFDavg does not complete a SIL claim; architecture and systematic capability still apply.
  • A certified device does not certify the complete SIF.
  • Verification and validation answer different questions.
  • A calculation cannot compensate for an undefined safe state, missing response-time requirement or unsuitable device application.

Primary references

  1. IEC — Overview of IEC 61508 and functional safety
  2. IEC 61511-1 — process-industry SIS framework and requirements
  3. UK HSE — Functional safety

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

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Related guides

Determining the required SILBuild the risk-reduction basis with a defined scenario, tolerable-risk criterion and reviewable assumptions.Designing a Safety Instrumented FunctionTranslate the safety requirement into sensors, logic solver interfaces, final elements and a testable safe action.Functional safety glossaryPlain engineering definitions for the terms used across SIL determination, SIF design and verification.