As you may already know, when it comes to the certification of equipment according to IEC 61508, there are several 3rd party agencies from which to choose. It is obvious from reviewing different assessments on for example a solenoid, that the interpretation of IEC 61508 and thereby the assessment method, can be quite different from one agency to the next. Why is it that for a specific solenoid one 3rdparty comes up with a failure rate of 0.47 FIT and another 3rd party comes up with a failure rate of 300+ FIT? It’s the same product!!

A few months ago I was at a manufacturer who had an assessment done by exida, as well as another 3rd party. The failure rate that exida concluded from the predictive analytics method was the high one. The manufacturer was convinced that the exida failure rate was too high (manufacturers always think their failure rates should be much lower). However the field failure study that supported the predictive analytics results showed that we were spot on.
Now how can another 3rd party come up with a failure rate that is much lower than the field failure data suggests? Clearly, they must not understand the application or, even worse the intent of the IEC 61508 standard.

When going over this with some of my colleagues, I remembered a story from a decade ago where a high school student was asked questions about her own novel. Let me first explain that in the Netherlands the high school structure is a bit different than in the USA. There are different education levels, ranging from crafts level to pre-university level. As part of our final exams, oral exams are conducted in addition to written exams which are identical across the country. Any of the language finals require students to read a number of literary books and during the oral portion of the exam they should be able to answer questions about those books. The oral exam is usually conducted by a three- person committee and the questions range from general content of the book, classification of the book, and deeper meanings along the lines of “what do you think the author meant with …”

The aforementioned high school student had written a novel at age 17 and had listed her own novel as one of the books to be read for her finals. During the oral exam the committee asked the “usual” questions, but also used the opportunity to argue with the student about what the author really meant by a certain passage. “Don’t you think the author really was talking about something else when she wrote this?” Where the student answered: “Absolutely not, that was not what I was talking about.” Clearly the interpretation of what the author wrote by the committee was incorrect, only the author knows.

Referring back to IEC 61508 certification, who would you think has a better understanding of the standard and its goals? A 3rd party that is involved in the IEC 61508 committee including writing the standard; or a 3rd party that only reads the standard and interprets its meaning. If you are convinced the 3rd party should be a member of the IEC 61508 committee, you should begin by asking you suppliers “Do you have an exida certificate?”

Tagged as:     IEC 61508 Certification     IEC 61508     FIT     field failure study     failure rate  

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