Generally speaking, one should assume a very low probability of failure for PLC controllers. Most of you who are looking for PLC controller failure rate estimates like MTBF statistics, the data is to be an ends to a means. What you are ultimately looking for are answers to questions, like what parts and how many should you stock, what are the weak areas, how to make your equipment more reliable, is a PLC controller more reliable than another form of control, etc
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