First, let me say thanks to all who have taken the time and effort to help me with my problem. I am very impressed with the quality and quantity of advice on this website.
I am only one of several engineers at my company who are trying to solve the Drive failure dilema. Before I posted my question, I had my theory as to the source problem. Now after hearing from all of you, I would like to tell you all the conclusion that I have come to and why I have come to it.
I once saw a Sherlock Holmes (Basil Rathbone) movie in which Holmes is explaining to Dr. Watson how it was that he was able to solve the mystery. He explained to Watson that once you have eliminated all the things that it cannot be, that whatever you are left with, however seemily unlikely, has to be the answer.
In looking at the failures, I asked myself, what are the things that could cause current to bridge a large gap? The only two things that I could think of were, high enough voltage, or ... a conductive path.
Next, I thought, where could a high voltage come from? Only one of two places. From the line, where it should be blowing the heck out the MOV's and rectifiers (and other equipment being fed from the same source); or regenerative voltage from the motor, but that's easily ruled out from the cases where the motor isn't even running.
So, what am I left with? A conductive path! Even though we have taken some substantial steps (high efficiency filters) to try to keep the Drives contamination free, there has been some evidence that some "dirt" still gets through.
By eliminating what I conclude that it cannot be (voltage), I believe that it HAS to be contamination. Possibly very fine graphite dust that gets pulled into the air intake grilles (located at the bottom of the Drive door), which either slowly deposits itself on the Drive components, or which occasionaly is conductive enough to be the path directly through air.
That's my "Sherlock Holmes theory". If anyone thinks that my logic is flawed, please point it out to me.
Once again, allow me thanks all of you... Skogsqurra, Itsmoked, Jraef, Sreid, Drivesrock, DickDV, and Marke for your thoughtful and solid technical advice.
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