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 Post subject: 12 foot rule vs actual arc flash calculationsPosted: Wed Apr 11, 2012 6:15 am

Joined: Wed Apr 11, 2012 6:13 am
Posts: 2
The company I work for did not do the arc flash calculations for their power panels. Instead they said they used the 12 foot rule. Is this a vialble alternative to doing the calculations? I know that the labels are a lot simpler - basically saying to stay away.

Thank you

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 Post subject: Posted: Wed Apr 11, 2012 10:02 am
 Sparks Level

Joined: Mon Nov 23, 2009 6:37 am
Posts: 51
Location: Tampa, FL
David Rowe wrote:
The company I work for did not do the arc flash calculations for their power panels. Instead they said they used the 12 foot rule. Is this a vialble alternative to doing the calculations? I know that the labels are a lot simpler - basically saying to stay away.

Thank you

How do you operate the equipment in the panels? If you interact with the equipment, open/close, take readings you must know what ppe and other precautions are necessary.

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 Post subject: Posted: Wed Apr 11, 2012 10:25 am

Joined: Tue Jul 05, 2011 7:27 am
Posts: 2
Location: Graham, KY
That makes sense. If its a panel, more than likely its going to be opened. Many of them are not routinely opened and I am guessing that they required full body protection if it is opened.

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 Post subject: Posted: Mon Apr 16, 2012 8:45 am

Joined: Mon Nov 19, 2007 5:25 am
Posts: 33
Location: Titusville, Fl.
May I ask where the twelve foot rule came from? I realize in NFPA70E, there used to be a 4 foot rule for previously listed Article 130.3 (A) (1) Arc Flash Protection Boundary, assuming 50-600V panels. However, for the '12 NFPA70E, it is to be calculated as follows :
(A) Arc Flash Boundary.
[SIZE=2][font=Times-Roman]The arc flash boundary for systems[/font][/size]
50 volts and greater shall be the distance at which the
incident energy equals 5 J/cm2[SIZE=2][font=Times-Roman](1.2 cal/cm[/font][/size][SIZE=1][font=Times-Roman]2[/font][/size][SIZE=2][font=Times-Roman]).[/font][/size]

Informational Note: For information on estimating the arc flash boundary, see Annex D.
..

Otherwise, you can find the following from '12 NFPA 70E using Table 130.7 (C) (15)(a), based on the equipment rating:
Potential arc flash boundary with exposed energized conductors or circuit parts

using above parameters:
it varies based on the size of equipment.
Hope this info helps...

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