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#27269 - 07/07/03 04:00 PM Hot Bus in the Motor City
MikeW Offline
Registered: 06/03/02
Posts: 35
Loc: Michigan
Hello everyone! First time I visited this site in a couple of months. My question is do any of you install swing-jacks into a hot 480 volt bus? I was told in our plant the policy is you can install up to 200 amp bus plug without shutting off bus. I do it and I always megger out the switch first. Am I being stupid. My boss told me if I felt uncomfortable with installing them hot then we would have the buss shut off. The problem with that is it is a big inconvenience to everyone else in a large factory and during production it is nearly impossible.

I would like to hear your opinions. Thanks.
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#27270 - 07/07/03 05:33 PM Re: Hot Bus in the Motor City
Bjarney Offline
Registered: 04/10/02
Posts: 2527
Loc: West-Southern Inner-Northeast ...
I wouldn't hesitate to check phase-to-phase and phase-to-ground...six tests. They should all be infinite. [The available short-circuit current on the bus-plug line terminals is not dependent on the rating of the bus plug.]

You might obtain the published installation literature from the duct manufacturer.
#27271 - 07/07/03 06:34 PM Re: Hot Bus in the Motor City
resqcapt19 Offline
Registered: 11/10/00
Posts: 2148
Loc: IL
If you install the switches into a hot bus, you must be wearing PPE rated for the arc-blast that could result if there was some type of problem.
#27272 - 07/07/03 07:31 PM Re: Hot Bus in the Motor City
Bjarney Offline
Registered: 04/10/02
Posts: 2527
Loc: West-Southern Inner-Northeast ...
Even though “many do it,” I don’t think live jacking in of circuit breakers, for example, in energized I-line panelboards is condoned.

In an IEEE journal, there is some commentary that seems to discourage intentional exposure of live parts in low-voltage industrial electrical systems.

UL standards for distribution equipment are written with the understanding that abnormal conditions will occur in electrical systems. Abnormal conditions may include explosive arcing and flaming of materials, either causing or resulting from heavy overcurrent conditions including arcing faults to ground and phase-to-phase short circuits.

It should be noted that enclosures for low-voltage electrical distribution equipment are also evaluated by the short-circuit testing of equipment they contain. Under these standard testing conditions, the enclosures are expected to retain general structural integrity and, in some cases, cotton indicators are used to show that nearby materials would not ignite. However, these tests in no way evaluate the ability of these enclosures to contain abnormal arcing or flaming from conditions other than interruption of a bolted fault. Further, equipment will not protect people working inside of it during the abnormality.

Anecdotally, there are reports of serious disasters {er, burndowns} from live removal/installation of plug-in "starter buckets" in MCCs. I don’t see bus-plug removal/installation being much different.

[This message has been edited by Bjarney (edited 07-07-2003).]
#27273 - 07/09/03 03:59 AM Re: Hot Bus in the Motor City
Redsy Offline
Registered: 03/28/01
Posts: 2056
Loc: Bucks County PA
Food for thought...

We had (at another location) an electrician modify an MCC bucket by installing an accessory on the back plate.
He used screws that were longer than the factory screws.
When he attempted to install the bucket, the screws contacted the buss, and an arc blast occured.
He lingered in the burn center for 4 days before succumbing.
And THAT was the impetus for our new Electrical Safety Program which borrows heavily from NFPA 70E.
Be aware, please

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