How to Determine the Proof Test Frequency — IEC 61511

Deciding how often to perform proof testing is one of the most practical challenges in functional safety. Get it wrong, and you either increase operational costs or unknowingly accept higher risk.

Proof Testing: The More Realistic, The Better

Many end users struggle with the question of how often a proof test needs to be carried out. Especially in the process industry, where end users try to operate their plants continuously for many years.

Performing a full functional test on a safety function would often mean an undesired process interruption. This makes proof testing in practice a difficult task. If you carry out the proof test, you might interrupt the process and lose money. If you do not carry out the proof test, you will not know whether your safety function still works and thus take (unnecessary) risks.

Every end user struggles in the end with the same dilemma. How often do I need to carry out a proof test without getting legally into trouble? How to decide the proof test interval?

The key question is: should your proof test interval be driven by calculations, manufacturer recommendations, or legal requirements?

What Functional Safety Standards Say About Proof Testing

Take, for example, the IEC 61511:2003 for the process industry sector. This standard stated in clause 16.3.1.3:

"The frequency of the proof tests shall be as decided using the PFDavg calculation."

Even the latest IEC61511:2016 states it in a similar manner:

"The schedule for the proof tests shall be according to the SRS. The frequency of proof tests for a SIF shall be determined through PFDavg or PFH calculation in accordance with 11.9 for the SIS as installed in the operating environment."

Not only in IEC 61511 but also in the more generic IEC 61508 standard is written that the proof test interval can be determined based on the PFDavg value. So, if the end user follows the standards, then the proof test interval is based on the PFD calculations.

And What Do Product Manufacturers Say?

Product manufacturers of safety equipment write safety manuals. And in those manuals, you will often find statements like this “We recommend proof testing the device at least once per year” or whatever frequency they recommend.

Product manufacturers either have done tests to decide on the proof test frequency, or they protect themselves legally by making a statement like this. Either way, you need to follow their instructions.

Proof Testing – Legally, Who Is Right?

So what does an end user do if the PFD calculation of the complete loop results in a proof test frequency of once every two years, but there is equipment in the loop where the manufacturer states in their manual to proof test it once every year? Legally which one of the two wins?

Even though I have been personally a safety expert witness on several court cases related to IEC 61508, I don’t know the legal answer to this one. During those court cases, where I was a safety expert witness, it was never about safety but only about money related to IEC 61508 projects. I have never seen a court case where this question was the problem. I think it is a difficult question to answer legally, and only the judge can decide on a case-by-case basis.

How to Decide the Proof Test Frequency

If you think about it practically, an end user must consider three things when deciding on the proof test frequency.

1. Follow the Law

First of all, the end user needs to follow the law. If, in your country, there are any laws that, in one way or another, would make a statement about proof testing safety functions, then you will have to follow those laws.

2. Follow the Manufacturer Instructions

Second of all, end users need to do what manufacturers of safety products write in their safety manuals. You cannot buy a product, not follow the manual, and then complain that it did not work. Product suppliers will always point out that it was clearly written in the manual how to operate their product.

3. Use the PFD Calculation

And at last, follow the PFD calculation. But keep in mind two things. First of all, the PFD calculation is only as good as you made the model and the reliability data you are using. There is always a lot of uncertainty in the PFD calculation. Do you want to base your proof test frequency on uncertain results?

Second, if the PFD calculation results in a lower frequency than the product manual states, you should still follow the manual. The product manufacturer knows his product best.

Recommended Order of Decision

So, in summary, the order of how to decide the proof test frequency should be like this:

1. Do what the law says.
2. Follow the instructions of the product manufacturer.
3. Base it on the PFD calculation.

Keep in mind that the higher frequency always wins. And finally, try to make the proof test as realistic as possible. The more realistic, the better it is.