Understanding the IEC 61508 Functional Safety Lifecycle – Part 4

14 Sep 2026

Verification, Validation, and the Evidence Problem

By the time a functional safety project reaches verification and validation, a significant amount of engineering effort has already taken place. Hazards have been identified, risks have been assessed, safety functions have been defined, hardware and software architectures have been developed, and safety requirements have been allocated throughout the system design.

At this stage, many organisations believe the difficult part is complete. In reality, this is often where some of the biggest problems begin. Within IEC 61508, it is not enough to simply state that a system is safe or that a design “should work.” The lifecycle requires organisations to demonstrate through structured evidence that safety functions have been implemented correctly and are capable of achieving the required level of risk reduction under real operating conditions.

This is where verification and validation become critical. These activities are often discussed together, but within IEC 61508 they serve different purposes.

Verification asks: “Did we design the system correctly?”

Validation asks: “Did we design the correct system?”

Verification – Confirming the System was Implemented Correctly

Verification is carried out throughout the lifecycle and focuses on confirming that each phase has been completed correctly before moving to the next. This applies across hardware, software and system-level activities.

Within IEC 61508, verification is not treated as a single testing phase at the end of development. Instead, it is expected to occur continuously as the lifecycle progresses. Examples of verification activities may include:

• reviewing safety requirements for completeness and technical accuracy
• checking hardware architectures against SIL constraints
• verifying reliability calculations
• reviewing software design and code
• reviewing test specifications and procedures.

The objective is to identify problems early, before they propagate further into the lifecycle where they become significantly more expensive and difficult to correct.

Validation – Proving the Safety Function Works in Practice

Validation occurs later in the lifecycle and focuses on the completed safety-related system as a whole. The objective is to demonstrate that the final implemented system actually satisfies the original safety requirements derived from the hazard and risk assessment.

A system may pass every internal design review and verification activity while still failing to adequately control the real-world hazard it was intended to address. Validation therefore examines the integrated system within its intended operational environment. This may involve testing:

• real operating sequences
• fault conditions
• response times
• fail-safe behaviour
• operator interactions
• communication failures
• abnormal operating scenarios.

The lifecycle expects organisations to demonstrate not only that the safety function operates, but that it does so consistently, reliably and within the assumptions defined earlier in the lifecycle. Where possible, validation activities should be traceable directly back to the Safety Requirements Specification and ultimately to the original hazard analysis.

The Evidence Problem

One of the biggest challenges within functional safety projects is not necessarily carrying out engineering activities – it is proving they were carried out properly. This is often referred to informally as the “evidence problem.”

Many organisations have capable engineers, strong technical designs, and extensive testing activities, but struggle when asked a simple question during assessment or certification: “Show the evidence.”

Within IEC 61508, undocumented work is effectively treated as work that never happened. This is one of the areas where functional safety differs significantly from conventional engineering projects. Compliance is not based solely on the final product behaviour, it depends heavily on demonstrating that a structured and compliant lifecycle process was followed throughout development. As a result, evidence generation becomes a major activity across the lifecycle.

Traceability

One of the most important concepts within IEC 61508 verification and validation activities is traceability. The lifecycle expects organisations to maintain a clear engineering link between, hazards, safety requirements, design implementation, verification activities and final validation evidence.

This allows assessors, auditors and internal engineering teams to understand exactly how each identified hazard was addressed and how confidence in the safety function was established. Without traceability, even technically strong projects can become extremely difficult to assess.

Independence and Functional Safety Assessment (FSA)

IEC 61508 also places strong emphasis on independence during verification and assessment activities. The standard recognises that the people designing a system are not always best placed to objectively challenge their own assumptions, calculations or implementation decisions.

As the required SIL increases, expectations around independence also increase. This is one reason why Functional Safety Assessments (FSAs) play such an important role within IEC 61508 projects. FSAs are structured assessments carried out at defined stages of the lifecycle to evaluate whether the required functional safety activities have been completed appropriately and whether sufficient evidence exists to justify progressing further.

In many projects, FSAs become one of the key gateways preventing lifecycle weaknesses from remaining hidden until much later in deployment.

Final Thoughts

Verification and validation are where confidence in the functional safety lifecycle is ultimately established. This is the stage where organisations move beyond engineering intent and begin demonstrating - through evidence – that safety-related systems actually achieve the required level of risk reduction under realistic operating conditions.

IEC 61508 places heavy emphasis on these activities because functional safety is not based on assumption or good intention. It is based on demonstrable evidence, structured lifecycle management and objective confidence that hazards have been controlled appropriately.

For many organisations, the greatest challenge is not necessarily designing the safety system itself, but maintaining the discipline, traceability and evidence needed to support the final safety claim.

In the next blog in this series, we will explore functional safety management, competence and lifecycle governance - the organisational framework that underpins the entire IEC 61508 lifecycle and ensures functional safety can be maintained long after the original design phase has been completed.

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