Does the PFD Determine the Proof Test Interval?

Many engineers assume the PFD calculation determines the proof test interval. It does not — at least, not on its own. The proof test interval is determined by a hierarchy of three factors, and the PFD calculation is only the third. This article explains the correct priority and why it matters.

Why proof testing is needed

A proof test reveals dangerous failures that cannot be detected by online diagnostics. Electronic devices with low diagnostic coverage and mechanical devices with zero diagnostic coverage depend on periodic proof testing to confirm they will still work when demanded.

Without proof testing, dangerous undetected failures accumulate over time — silently increasing the probability that the safety function will not respond when it is needed.

The key question is: how do you decide when to proof test — and in what order of priority?

The proof test interval hierarchy

The proof test interval is decided in this order of priority:

1. Local law
2. Device manufacturer recommendation
3. PFD calculation

You always apply the most stringent requirement. Let's walk through an example.

Worked example

Step 1 — Check local law

First, check whether there are laws concerning your safety functions in your country. Assume that in your country, the law requires safety functions to be proof tested at a minimum every two years. If such a law exists, compliance is non-negotiable — it is your legal baseline.

Step 2 — Check the manufacturer's Safety Manual

Next, check the safety manuals of the equipment that makes up your safety function. Manufacturers often require proof testing more frequently than the law — for example, once per year.

What if the law says every two years, but the manufacturer says every year? You go with the manufacturer's recommendation. It will not stand in court that you ignored what your supplier requires — especially when their recommendation is more stringent than the law.

Step 3 — Check the PFD calculation

Finally, perform your PFD calculation. If the calculation shows that the safety function exceeds the required PFD limit after six months, then there is no choice — you must proof test every six months. The calculation overrides both the manufacturer and the law because it reflects the actual integrity of your specific configuration.

On the other hand, if the calculation shows you could proof test every 1.5 years, you still proof test once per year — because the manufacturer requires it.

The practical reality

Although the hierarchy above is the legally correct approach, it creates real practical challenges. In some industries, proof testing means shutting down part of the process, mobilising specialist teams, or coordinating across multiple systems. It is not always as simple as "just do it more often."

Ultimately, you need to find the balance between being legally correct and what is practically achievable. But whatever decisions you make, make sure you can explain and justify them. Document the rationale. When an accident happens, you want the law and the evidence on your side.