Need help with fuse problems on 4160V motor

[eeprom]

Hello,
Here are the symptoms of my fuse problem. I have a 4160V, 1500 HP induction motor driving a large fan. The motor was supposed to be wired in an ungrounded wye. The electrians mistakenly wired it as a delta. The system was protected by three A480R-1 18R goulds fuses. The motor ran for a few seconds and blew two fuses. The motor was then correctly wired as a wye. And upon starting, we blew another fuse. I can't figure out why the third fuse blew. The fuses are sized correctly. And I have taken into account the long start up time. The fuses are sized to handle locked rotor current for 20 seconds. The motor specs says that the motor can handle locked rotor current for 25 seconds. The fan/motor start up time is significant, maybe 12-15 seconds.

What I want to know is whether or not fuses can be damaged by "nearly blowing". If a fuse is rated at 1400A for 20 seconds, and then 1400A is run through the fuse for 19 seconds (assuming the fuse doesn't blow). Will the fuse be as good as new?



thanks.

 

[dpc]

Yes, fuses that see high current but don't melt can be damaged. Especially current-limiting fuses.

Basically, the fuse guys recommend replacing all three fuses after an "event".

 

[ePOWEReng]

The description of what happened makes sense. Two fuses cleared and the third one was damaged but didn't have enough time to clear. Remember that there is a minimum melt curve and a total clearing curve for the fuse. It sounds like the third fuse partially melted but didn't clear.

The minimum melt curve defines the time-current curve, which if you stay below, the fuse will not be damaged according to the manufacturer. If you exceed the minumum melt curve you may damage the fuse, hopefully if you exceed the minumum melt curve you will blow the fuse, but this is not always the case.

http://www.ferrazshawmutsales.com/pdfs/amptrapr.pdf

 

[davidbeach]

I'll third that; you should have replaced all three fuses to begin with. No surprise what so ever that the third one would have gone the next time the motor was started.

 

[eeprom]

This explains a lot. And yes, dpc, it was an Amp-Trap current limiting fuse. Thanks for your help.