60A LPJ to 600A LPJ to 1600A fuse blown.

[powersoff]

My problem started with a 600A LPJ fuse blown on phase B during a startup about a month ago. A loose connection was found on fused disconnet. It feeds 19 25hp DOL starting motors. Problem reappeared today with same 600A LPJ on phase B blown again. 1 of the motors had OL's tripped and 60A LPJ fuse on phase B blown. Motor was locked out and isolated and connections at 600A fused disconnect checked. 600A LPJ fuse on phase B replaced now 18 motors running.A couple hours later 600A fuse on phase B blows again. In the search for a solution(how fast can the customer make the 18 motors run again) the decision is made to bypass the 600A fused disconnect and feed directly from 600A breaker. 18 motors run for about 2 hours and now 1600A fuse from main that feeds 600A breaker blows. At this point power quality analyzer is installed and has been logging data . The motors have IEC style starters with aux contacts on the face of starter. After 1600A fuse blew 3 of the Aux contacts have failed in such a way as that a OL will not break the 480 power to motor essentially stuck in the closed position. Motors with failed aux have been locked out and now 15 motors running drawing aprox. 400 amps. As soon as practical I plan to replace all starters and aux contacts but as far as the root of the problem I would love to hear any ideas

 

[waross]

Try a megger on the "B" phase conductors and hardware. I suspect that you may have an intermitent fault somewhere on "B" phase. The fault may be temperature related or it may be as a result of slight overvoltages. It may also be the result of an overvoltage on "B" great enough to saturate the motor stators. This will cause the current to increase disproportionately on all motors. If the problem is a short or ground of some type, you should by now be able to see evidence of smoke and flames. An overvoltage saturating the motors may not leave any evidence but depending on your circuits the welded contacts may be related to an overvoltage problem. Are your transformer taps set higher on "B"? Are your voltages unbalanced? Is your plant load well balanced?
How is your plant fed, wye or delta? How is it grounded?

Bill
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"Why not the best?"
Jimmy Carter

 

[LionelHutz]

To me, it sounds like an intermittent short to ground. Megger the B phase wiring. Also, carefully check the insulation everywhere you can see it.