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#10237 06/03/02 02:15 AM
Joined: Jun 2002
Posts: 13
Paxman Offline OP
Hi everybody - I've been checking in reguraly but this is the first time I jumped off the fence... so to speak.
Here is a situation I am dealing with as a maintenace electrician and I would like your opinion on it- maybe an inspector's prespective too - before I take it up to the engineer. There is this Crane bridge fed from 480 V ducto-bars through two sets of colectors on each phase. Each colector set has two shoes so we have a total of 4 shoes per phase. The wire attached by the factory to each individual shoe is # 6 and is only aobut 18 inch long, then is spliced together with a 3/0 wire. All four 3/0 wires are connected together in a J-box and from there one 3/0 wire feeds the line side of the Main breaker in the control cabinet on the Bridge (for simplicity I am only following one phase here). I found one of those # 6 wires completely bare with the insulation melted off. Could this be an illegal tap, where basically a small unprotected wire gets overloaded acting like a weak link in circut otherwise sized at 3/0? Theoretically there are situations when the small # 6 is called upon to carry the whole load (up to 200A) when for example the other 3 shoes are on a dead section of the ducto-bar. Even in a normal situation the 4 shoes are not evenly loaded given the fact that the current will follow the path of least resistence. I am thinking of replacing all these # 6 wires with 3/0 welding cable so everything will be the same size. I appreciate your comments on this and I will share them with the crew. Thank you.

#10238 06/03/02 01:50 PM
I am by no means that knowledgable about bridge cranes. However I did run into a situation like this just recently. This crane was a 480v 3 phase that used 2 shoes per line. The feeding cable from the shoes was only a #10. The wire had melted off and became brittle in other spots on the one phase. To make a long story short, I felt as if the wire was too small to handle the load. We replaced the #10 with a #6. (The bridge crane was protected with a 40A disconnect.) Oversized the wire for added comfort. The problem actually arose from the individual shoes and tracks being overly dirty and carbonized. Causing spiking currents to "pulse" through the wiring. After a good cleaning and new shoes the problem was fixed and the current was found to be well within the range of the #6 we installed. Hope this maybe helps.

#10239 06/03/02 05:48 PM
Joined: Apr 2002
Posts: 2,527
Inrush current from motor jogging can be rough on Ductobar/Insul8/FeedRail setups, but usually they are oversized for the duty at hand.

I’d check collector-shoe spring tension—there may be some weak ones, causing the strongest contact to hog current. If 3/0 per phase is needed, then sharing/splitting of four 6’s may be a lot to ask. 1/0 cable is supposed to be the lower limit on cable size for paralleling, but if the shoes are undersized then cable is probably immaterial.

Cooked insulation could be from eddy-current heating if the j-box does not use a nonferrous plate and individual non-magnetic cable glands where they enter the steel box. Alternately, you may find slots cut between KOs (cable openings) in a steel or iron j-box wall.

Regulars, in your hymnals, turn to 99NEC300-20. 'Induced Currents in Metal… “inductive effect” can be excess insulation heating and cable movement.'

Pax, send us a postcard from summer camp. For other readers—with cranes in production—everything {ehh, the ‘trouble’ spot} doesn’t always conveniently stay right in front of your face. Also, you may have the pleasure of working off the top of an extension ladder, or in the shop-made forklift basket piloted by a production guy that’s about as calm as two chihuahua’s in a burlap sack.

This may not be possible to use on each shoe, but maybe the engineer may buy into replacing the #6 with maybe #2 welding cable, {EPDM or TGAT lead even better} with each lead sleeved in oversized silicone-glass sleeving {motor repair shop—the stuff intended for 7200V rewinds is best} all the way from shoe to inside the j-box.

Sometimes maintenance electricians need a reason to spend as much time as possible hanging out in the engineer’s air-conditioned office. A good thought-out, detailed discussion with the guy can buy a little more time, especially if you give him a reason [not] to have to grab his hardhat and head out with his pager repeatedly squawking from a production complaint. If the old guy lets you sketch on the corners of his whiteboard, that’s gotta be good for at least another 20 minutes. If the discussion gets him interested enough to shut off that screaming walkie-talkie on his waist, so much the better!

[This message has been edited by Bjarney (edited 06-03-2002).]

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