Does the PFD decide the proof test interval?

2022-11-09

Before we answer, we quickly discuss why a proof test is needed. We do a proof test to reveal failures that cannot be revealed via diagnostic testing. Electronic devices with low diagnostics coverage and primarily mechanical devices with 0 diagnostics coverage benefit from proof testing.

The big enchilada question is when should we carry out a proof test? How often? How do we decide that?

We decide on the proof test interval in this order of priority:

1. Local law
2. Recommendation device manufacturer
3. PFD calculation

Let us take a look at an example. First, you need to see if there are laws concerning your safety functions in your country. Let us assume that in your country, there is a law that says safety functions need to be proof tested at a minimum every two years. If they exist, then, of course, the first thing we need to do is comply with this law.

Next, you check the safety manuals of the equipment that make up your safety function. By default, the equipment manufacturers often state that their equipment needs to be proof tested once per year. What do you do if the law says once every two years, but your supplier says to proof test once yearly? It would be best if you went with the recommendations of the supplier. It will not stand in court that you ignore what your supplier recommends (if it is more strict than the law itself).

Lastly, you must calculate PFD to comply with most functional safety standards. You do your PFD calculation, and it turns out that the safety function is not compliant with the required PFD limit after six months. Then there is legally no other choice than to proof test every six months. You always choose the most stringent proof test interval.

If, on the other hand, your calculation showed you in the example above that according to the PFD, you can proof test every 1.5 years. You still need to proof test once per year because the equipment manufacturer requires this to guarantee the performance of their devices.

Although this is probably the correct way to do it legally, this is a real practical problem. In some industries, proof testing is more complex than just doing it. Ultimately, you need to find the balance between being legally correct and what is practically possible. Could you make sure you can explain the decisions you take? You want the law to be on your side when an accident happens.