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 Post subject: Seeking input
PostPosted: Mon Dec 01, 2008 3:56 pm 
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Joined: Wed May 07, 2008 5:00 pm
Posts: 871
Location: Rutland, VT

I have a situation at a power plant that is 100 years old and the last update of equipment appears to be in the late 1940's or 1950's. The situation I'm concerned about is the 2 buses in the plant. They are constructed with steel pipe for framework with rubber insulation holding up the copper bus bars. There are 3 bars per phase, approx. 1/4" x 4 - 6 inches and are about 8 feet in the air, horizontal. There are vertical bus bars to the breakers, bolt in DB-75 and DB-100's. This bus work is fed by a three winding transformer via a DA-75 breaker on each secondary winding. This is 480V delta system with ~52kA bolted fault current. There is a marble panel board about 2 feet in front of each structure with the breaker control handles and associated meters and relays.

The breakers had the old trip devices replaced with AC Pro units to reduce the AFH from Extreme Danger to Category #2 a year or so ago.

My concern is that, regardless of reducing the AFH to Cat. #2, there is still a danger with the open bus work and an arc flash. There is no metal enclosure to contain any blast.

I'd be interested to hear other's opinions of this situation.

Thank you in advance for your comments.

Barry Donovan, P.E.

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PostPosted: Mon Dec 01, 2008 5:09 pm 
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Joined: Sun Dec 23, 2007 1:44 pm
Posts: 348
Location: Charlotte, NC
WOW....Thought I had seen some pretty old and antiquated stuff until now. Trying to picture exactly what it looks like from your description.

First off I would ask if there is a snowball's chance that the bus supports are mechanically rated for 52 kA? If not, then I would think it would have to be replaced once you recognize that it is exposed to faults in excess of its rating.

Second, sounds like a pretty tight place to work.....2 feet?

Third, if the bus structure is rated properly and no one works it hot, I hope, don't know that AFH is an issue since it is related to testing and maintenance. Where are the breakers located?

Obviously only "qualified" personnel allowed.

Sounds like it might violate most all of the codes today, but they are not retroactive (except for rating issues). Don't know if replacing the trip units would require additional code updates as well.....anyone got any thoughts?

I would do some checking on its suitability for continued use, and even if it is, I would lock the door and at least throw away my keys!


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PostPosted: Mon Dec 01, 2008 6:34 pm 
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Joined: Thu Jan 10, 2008 8:49 pm
Posts: 514
Location: New England
I have seen situations like this on old Merchant and Military ships. All open bus gear, but behind a locked cage so only trained personnel can enter.

One thing that I have learned over the years is NOT to try to out think the Code. So my comments are these:

1) Verify that only trained and knowledgable personnel would be permitted into this area.

2) Rate IE and AFB as per IEEE 1584. If that works out to be Cat 2 and 4 feet - so be it. Although in this case I would increase the AFB to something like 8 or 10 feet minimum.

3) Make sure there if proper signage to warn of the exposed voltage. I want to say the 'Danger - High Voltage signs' but this is low voltage. Of course to the unauthorized High Voltage is more frightening.

This situation would not be much different than a lineman working in an open air crib distribution center.

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