Functional Safety Assessments: Why Independence Scales With SIL

26 May 2026

A Closer Look at Clause 8 of IEC 61508-1

During a recent ICFSP training course I delivered, one topic consistently sparked discussion – and uncertainty: Functional Safety Assessments (FSAs) under Clause 8 of IEC 61508-1, and in particular, what “independence” actually means in practice.

Most attendees were already familiar with the requirement to perform FSAs at defined stages of the safety lifecycle. However, when we explored how independence requirements evolve as target Safety Integrity Levels (SILs) increase, it became clear that this aspect of Clause 8 remains widely misunderstood.

The result is a common – and entirely avoidable – compliance gap: assessment approaches that may be acceptable at SIL 2 do not automatically translate to SIL 3.

Clause 8: Not Just a Final Checkpoint

Clause 8 defines the requirements for performing functional safety assessments, the objective is to confirm that:

• the functional safety lifecycle has been correctly applied, and
• the outputs of that lifecycle are appropriate for the claimed SIL

A common misconception is that the FSA is simply a final audit, performed once engineering activities are complete. In reality, the standard expects multiple assessments across the lifecycle, with increasing depth and rigour as the design matures and risks become more clearly defined.

Just as importantly, Clause 8 sets expectations on the independence of those performing the assessments. These requirements are not arbitrary – they are explicitly linked to risk and the target SIL.

Independence Is Not Binary – It Scales with Risk

IEC 61508 does not treat independence as a simple ‘Yes/No’ condition. Instead, it introduces graduated expectations that become more stringent as the consequences of failure increase. Information on this can be found in IEC 61508-1, cl. 8.2.18, tables 4 and 5.

At lower SILs, independence may be achieved through internal role separation, peer review, and appropriate governance controls. As the target SIL increases, this is often no longer sufficient. The standard increasingly expects organisational and managerial separation, and for higher SILs, assessment activities that are independent of the development organisation itself.

The intent is straightforward: as risk increases, the assessment process must provide stronger protection against self-confirmation bias – not just against technical error.

What This Means for Organisations Targeting Higher SILs

In practice, many organisations only discover independence gaps late in a project. By that stage, assessors may already be embedded within delivery teams, reporting lines may be fixed, and external involvement becomes reactive rather than planned. When this happens, the consequences are rarely limited to documentation rework.

Organisations that succeed at higher SILs tend to address Clause 8 early. They treat FSAs as an integral part of the functional safety plan, scale independence deliberately from the outset, and recognise when internal governance mechanisms are unlikely to be sufficient for higher-risk claims.

This proactive approach avoids last-minute rework, uncomfortable challenges to SIL claims, and delays caused by the need to retrofit independence.

How Intertek Supports Functional Safety Assessments

Intertek supports organisations across the full functional safety assessment lifecycle in line with IEC 61508-1 Clause 8, particularly where higher SIL claims demand demonstrable independent organisations.

Support typically begins at the planning stage, helping organisations define an assessment strategy aligned to the target SIL, identify appropriate assessment stages across the lifecycle, and clarify independence expectations early. This often includes highlighting potential organisational or governance gaps before they become project risks.

As projects progress, Intertek performs stage-based lifecycle reviews and readiness assessments, providing independent review of functional safety management, lifecycle application, safety requirements and traceability, and verification and validation activities. These reviews support confident progression between lifecycle phases.

For SIL 3 and above, Intertek can act as an independent assessment organisation, providing organisational and managerial independence, experienced functional safety assessors, and clearly scoped, well-documented assessments. The resulting reports are suitable for internal governance, regulatory confidence, and third-party assurance.

Final Thought

IEC 61508-1 Clause 8 makes one principle clear:

Independence is not fixed – it increases as risk increases.

Organisations that design their assessment approach to scale with SIL can support high-integrity claims with confidence. Those that do not often discover, too late, that independence has become their weakest safety control.

A simple question is often revealing:

If your target SIL increased tomorrow, would your functional safety assessment approach still be compliant – or would independence suddenly become a problem?