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Old 05-22-2016, 01:44 PM   #1
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Default Ground fault, ship

Two generators running in parallel, had several brownouts: some shut down circuits activated, lights in and out emergency switchboard changed over to E/G. It happened several times within an hour and was noticed that the main switchboard ground fault indicating lights showed a ground fault when it occurred. Later it was found an air compressor had an exposed wire intermittently touching the metal junction box. Also when one of the gen. was on line by itself only, the MSB ground fault indicating lights also indicated a very slight ground fault, almost unnoticeable when in parallel with others. Compressor ground fault corrected. It took a while to trouble shoot the one on the gen. Finally it was found that the gen. had a faulty suppressor (I don't have the detail yet).

Not like some other vessels that I had worked before, there are no drawings for the starter panels (MCC). I remember working on some ship that a similar ground fault did trip the breaker of the equipment which was not an essential one, to protect the upstream equipment and thus the electrical plant (selective timing?). I believe it is done by sensing the unbalanced voltages among the phases. It did not happen this time.

It seems there is a serious voltage drop, big enough for it to change the ESB over to the E/G.

Not sure if the generator's suppressor fault was a pre-existing one or it was caused by the air compressor ground fault. I would imagine there is some difference if the ground faults are of the same phase or on different phases. Is it possible that it is a pre-existing condition but by itself the plant is OK, only when there is another ground fault that compounds the situation causing brownout or even blackout?

Tired to google the above but not much help. Even got confused as one of the documents using the terms "insulated neutral" and "isolated system" in the same page. The good thing is that it jogs my memory that I had a short course on electrical ground protection 40 years ago (for 33kV, irrrelevant to ship's and it was beyond me), a pass time between shore leaves, but that is all I could remember.
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