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 Post subject: Arc Flash Labels for Bus DuctsPosted: Tue Jan 13, 2009 12:49 pm

Joined: Thu Dec 11, 2008 9:45 am
Posts: 3
Since the incident energy for any switchboard/panel (that has a Main Breaker and many Feeder Breakers) is calculated on the main bus, which is the load side of the main breaker and the line side of feeder breakers.
What would you consider in case of bus duct which has many fused disconnect switches? Do you calculate the incident energy on the bus duct, or you calculate it on each disconnect switch individually after the fuse (on the load side of disconnect switch and fuse)?
Also, where are the warning labels supposed to be attached, on the bus duct itself, or on each disconnect switch?

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 Post subject: Posted: Wed Jan 14, 2009 7:22 am
 Sparks Level

Joined: Thu Jul 19, 2007 7:54 am
Posts: 201
Location: St. Louis, MO
I calculate the energy at the bus duct, and post the labels on the duct as well. About one every 20-30 feet, and on each end.

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 Post subject: Posted: Wed Jan 14, 2009 9:38 am
 Plasma Level

Joined: Wed May 07, 2008 5:00 pm
Posts: 712
Location: Rutland, VT
WDeanN wrote:
I calculate the energy at the bus duct, and post the labels on the duct as well. About one every 20-30 feet, and on each end.

Do you calculate the energy at worst case, the end of the duct and post that along the entire length? It is possible for the AFH to be different at the beginning of the bus duct and increase along the way, for long ducts.

_________________
Barry Donovan, P.E.
www.workplacesafetysolutions.com

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 Post subject: Posted: Wed Jan 14, 2009 10:09 am
 Sparks Level

Joined: Thu Jul 19, 2007 7:54 am
Posts: 201
Location: St. Louis, MO
wbd wrote:
Do you calculate the energy at worst case, the end of the duct and post that along the entire length? It is possible for the AFH to be different at the beginning of the bus duct and increase along the way, for long ducts.

Honestly, I had not considered that. This would be interesting to look at. I have only looked at the start of the duct.
I have tried to make an effort to change the bus gap variable in the equations to match the bus duct information, however, although I do not always have this information.

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 Post subject: Posted: Thu Jan 15, 2009 12:42 am
 Sparks Level

Joined: Mon Jul 09, 2007 11:43 pm
Posts: 51
Location: Sheffield, England
wbd wrote:
Do you calculate the energy at worst case, the end of the duct and post that along the entire length? It is possible for the AFH to be different at the beginning of the bus duct and increase along the way, for long ducts.

I've split the bus ducts into a number of buses and calculated each one. I have come across a significant variance which tends to worsen along the length of the bus duct and I would support labeling the whole system to the worst case. It may be worthwhile checking with the manufacturers of the bus duct if they support the practice of exchanging tap off units whilst the bus is live. You may be surprised that some of them discourage live tapping particularly on large current systems. You need to watch this point because they may be the "expert witness" called into any subsequent investigation or court case.

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 Post subject: Posted: Thu Jan 15, 2009 8:53 am

Joined: Thu Dec 11, 2008 9:45 am
Posts: 3
Thanks a lot for your opinions ... How about the tap off's of the bus duct? do we consider their fuses to calculate the energy at each tap off, or we donât consider them as the rating of each tap off will be the same as the rating of the bus duct itself regardless the fuse size/type .. ?

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