SIL Determination, Verification and Validation: Three Different Things People Keep Confusing

These three terms appear in the same specifications, get used interchangeably in conversation, and describe genuinely different exercises at different points in the safety lifecycle. The confusion is expensive, because operators regularly commission one believing it covers another and discover the gap at assessment.

The short version: determination sets the target, verification proves the design reaches it, validation proves the installed system actually works. Each is a separate deliverable requiring separate effort.

 

The Three at a Glance

 

	Determination	Verification	Validation
Question answered	How much risk reduction is needed	Does the design achieve it	Does the installed system perform it
Lifecycle stage	After hazard study, before design	During and after design	After installation, before startup
Method	LOPA or risk graph	PFDavg calculation and constraint check	Physical testing of the function
Output	SIL target per function	Verification calculation report	Validation records and test results
Evidence type	Analytical	Analytical	Empirical

 

Determination: Setting the Target

Determination assesses risk and produces a required SIL for each safety instrumented function. It starts from hazard scenarios, applies existing protection layers, and calculates the remaining gap against tolerable risk criteria. LOPA is the most common method, and IEC 61511 recognizes it explicitly.

It happens once the hazard study is complete and before detailed design, because the target shapes the architecture. Determining SIL after the system is built is the wrong order and routinely produces a target the installed system cannot meet.

 

Verification: Proving the Design Reaches It

Verification calculates whether the designed loop achieves the target. It covers three conditions: random hardware failure expressed as PFDavg, architectural constraints expressed as hardware fault tolerance, and systematic capability of each device. Failing any one means the target is not met.

Verification is analytical rather than physical. It proves the design should work, using failure data, architecture and proof test assumptions. It is the step most frequently missing when operators believe they are compliant.

 

Validation: Proving It Actually Works

Validation is physical. It tests the installed system end to end, confirming each safety instrumented function performs as the safety requirements specification says it should: the sensor detects the condition, the logic solver responds correctly, the final element moves to its safe state, within the required time.

It happens during commissioning, and this is the critical constraint: validation evidence has to be produced at the time. It cannot be reconstructed afterwards from memory or inference, which is why missing validation records are among the harder findings to close.

 

The sequence that causes the problem An operator commissions a determination study, receives SIL targets, and treats the report as proof of compliance. No verification calculation is produced because nobody asked for one, and validation is folded into general commissioning without specific records. At assessment, two of the three are missing. Closing that gap after the fact means recalculating against as-built data and, where validation records are absent, potentially repeating commissioning tests on a live plant.

 

What to Ask For When Scoping Work

• If you have hazard scenarios but no targets: you need determination.
• If you have targets but no calculations: you need verification.
• If you are commissioning a new or modified system: you need validation, planned before commissioning starts.
• If an assessment raised findings: establish which of the three is missing before scoping remediation. Our guide to closing assessment findings covers the sequence.

A proposal that says "SIL study" without specifying which of the three it covers is worth clarifying before signature. The word carries no fixed meaning and the deliverables differ substantially in effort and cost.

 

Why Choose Arista Cyber

Arista Cyber delivers all three as one connected sequence, which matters because each depends on the one before it.

A verification calculation is only defensible if it traces to a determination that traces to a hazard scenario, and functional safety assessment examines exactly that chain. We run determination and validation and verification against the same scenario set, so the traceability exists from the start rather than being assembled for an assessor. Our practitioners are TUV Rheinland certified, which the standard requires as competence evidence in its own right.

 

Next Steps

If a specification requires a SIL study and you are not sure which of the three it means, resolving that before scoping saves rework. Explore our functional safety services, read about SIL verification, or contact the Arista Cyber team.

 

Common Questions

 

Is a SIL study the same as a SIL assessment?

The terms are used loosely and neither has a fixed meaning. In practice SIL assessment usually refers to determination, but some organizations use it to mean the full package including verification. Always confirm which deliverables a proposal covers rather than relying on the term, because the effort differs substantially.

Can validation replace verification?

No, and they test different things. Validation proves the function operates correctly under test conditions on the day. Verification proves it will achieve its required reliability over the operating period, accounting for random failures between proof tests. A function can pass validation and still fail verification if the architecture or failure data cannot support the target.

Which one do we need first?

Determination, always. It sets the target that shapes the design, so it belongs before detailed engineering. Verification follows during and after design. Validation follows installation. Running them out of order, particularly determining SIL after the system is built, is a common cause of findings at functional safety assessment.

 

Not sure which SIL deliverable you actually need? Arista Cyber delivers determination, verification and validation as one traceable sequence. Tell us what you have and we will tell you what is missing. Book a Free Consultation