Whoa...that was close!

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C

Cow

Senior Member
Location
Eastern Oregon
Occupation
Electrician
We have a customer with a 3000A service installed by us a few years ago. They have a 500HP compressor on a softstart fed from this gear that's been blowing fuses randomly. So one day when we're in the electrical room we noticed it made a terrible vibrating-rattle like noise on startup. Scary sounding enough, that my apprentice was halfway out the door when I looked around for him.....chicken!:D

Had the POCO shut them down today to check all the connections. All buss connections looked great, no heat marks anywhere, but we found six 3/8 x 1" bolts LOOSE where the bussing bolts to a long fiberglass insulator. The nuts on some of them were backed off about 2 turns and they were right over the Pringle main disconnect switch. I don't even want to think what would happen if one dropped...

But, a little loctite and now they're good to go!

Unfortunately, we turned the power back on and that damn vibrating/rattle is still there.:mad::mad: I really thought we had it. Now I'm thinking it's the sheetmetal at the back of the switchgear that I can't get to, that's causing the noise. Except for pulling the gear out to get to the back, I think the customer is going to have to live with it.:mad:
 
ptonsparky

ptonsparky

Tom
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
I would like to learn more about the random fuse blowing.

Had to change some verticle buss bars on a MCC and while we were in there discovered one sections back was loose and leaning against the wall. About a 6" gap at the top. Could have been open since the 70s. So a loose one doesn't suprise me.
 
C

Cow

Senior Member
Location
Eastern Oregon
Occupation
Electrician
Tom, we're trying to get that sorted out too. I've megged wiring from the fused bucket to the softstart and from softstart to the motor. Everything checks out. Time current curve on the 800 amp fuses says it should hold 4000 amps for at least 5 secs. This thing pulls about 1200 amps for about 5-8 secs on startup than drops down to 560 running amps.

We're going to ask the manufacturer if we can extend the ramp time anyways just to see if it makes a difference. This thing has only blown two fuses in 2 months. But the customer still doesn't like replacing 800 amp fuses and the downtime that comes with it....

We may also try a different brand fuse just in case these are faulty, but I've never had bad fuses right out of the box either...
 
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