So close yet so far diagnosis proceedures

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  #1  
Old 05-29-02, 07:04 PM
W
Wgoodrich
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So close yet so far diagnosis proceedures

To start I would like to say this consultation was done in an area not in my jurisdiction to ensure no conflict of interest.

Years ago I wired a factory for a friend of mine from another state. From time to time this factory still calls me for consultation when they have a major line shut down and need technical advice on controllers etc. They called today and it reminded me of a comment that I made in a post yesterday on diagnosis advising never to take anything for granted. In the rules of diagnosis never skip a step thinking you know that step in your diagnosis is correct and working. This factory called today with the main assembly line shut down when they took out the main component machine as one unit and a component part of that assembly line including the entire machine as a whole part, wiring, controllers, etc.

No wiring was tampered with on that machine the main feed wires were disconnected and the machine was hauled out by a crane. Then reworked as needed mechanically but without touching the electrical. Then the machine was again lifted back into place, bolted down and the main feed wires were reconnected.

The the operator of that main assembly line reset the PLCs and controls for a new starting proceedure to fire up this machine early in the day. All this time the entire employee base stood idle but only planned on a short shut down for a short period of time until the machine was reset and starting proceedures and reprogramming was completed. NO BRAINER, or so they thought.

The machine did not start back up. The maintenance people, the electrical contractor and the operator worked hours trying to diagnose the reason this machine would not start all the time during which the entire assembly line of employees stood idle. The electrical contractor said uncle after hours of getting nowhere with him knowing of the pressure of getting that machine restarted and the factory back on line and his frustration of being unable to diagnose the problem.

After hours of no success in their diagnosis process the general manager called me at home around 5:00 for a consultation and a desparate plea for help with no other hope available. I decided that they were in deep enough that they need me to see what they were doing concerning diagnosis.

Upon arrival I found only the plant manager still working on the machine all others left. Assembly line employees very quietly standing back in the background just watching. I started questioning their proceedure of diagnosis. I found nothing wrong in what they discribed. I tested and found my tracking of hte problem to lead to safety switches. I asked the plant manager if they checked and rechecked all saftey switches on this assembly line. He said that it can not be the safety switches they were all checked and rechecked way too many times. He then suggested that the motor broke while moving the machine.

I read the ladder logic prints and made the needed tests and it still said safety switch. We restarted the tracking of safety switches.

2 feet from the PLC was a red mushroom emergency stop switch. I pulled it out and asked the manager to try the soft start proceedure and the line started up.

He is still talking to himself on how in the world they all missed this safety switch being turned open keeping the machine from running. Mumbling was all I heard as I walked out of the factory after about 10 minutes of diagnosis. Hearing the assembly line running full speed ahead.

Just goes to show it is often the simplest of problems and often right in front of your eyes the whole time. So simple but yet so hard if diagnostic proceedures are skipped or taken for granted that part is right.

YOu have no idea how many memories that experience brought back to mind over all my years of electrical work where I did exactly the same thing.

Just for fun

Wg
 
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  #2  
Old 05-29-02, 07:55 PM
M
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Wg,

Some of the conveyor systems I've worked on can have 50-60 stop buttons on them. You might suggest adding pilot lights to each stop station........I know it adds $$$ but so does spending a whole day looking for a startup problem
 
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