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 Post subject: Arc Flash Boundary at higher cal/cm2 than 1.2
PostPosted: Thu Jun 06, 2013 9:13 am 

Joined: Fri Apr 05, 2013 7:37 am
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I understand the arc flash boundary to be, at the outer edge of the boundary, the point where unprotected skin could receive a 2nd degree burn and is equal to 1.2cal/cm2. What I'm wondering if there's a way to find out what the boundary would be for higher incident energies. For example, if I have category 3 PPE that is rated 8 cal/cm2, how close can I get to the arcing point and still be protected?


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PostPosted: Thu Jun 06, 2013 12:44 pm 
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Using the equasions for normalized Incident Energy (En) and Incident Energy converted from normalized (E) in IEEE 1584, solve for working distance.


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PostPosted: Fri Jun 07, 2013 9:46 am 
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Annex D of the NFPA-70E has formulas for calculating the arc flash boundary. However, I don't think that's the answer you're looking for... and I'm not sure I completely understand your question.

How do you know the incident energy is 8 cal/cm^2? Has an arc flash analysis been done to calculate the incident energy? If so, equipment should be labeled with the corresponding arc flash boundary distance.

Or are you asking, if inside the arc flash boundary, how close can you get to the arcing point and still be protected by your PPE? That answer depends on the working distance that was used to calculated the incident energy (which should also be shown on the label). There are standard working distances typically used for calculations:
  • 18" - 600V and below MCC and panelboards
  • 24" - 600V and below switchgear
  • 36" - medium voltage switchgear
Instead of doing the calculations, are you using the table method to determine PPE and HRC? If so, I believe a minimum arc rating of 25 cal./cm^2 is required for HRC 3.


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PostPosted: Fri Jun 07, 2013 10:15 am 
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A King wrote:
Annex D of the NFPA-70E has formulas for calculating the arc flash boundary. However, I don't think that's the answer you're looking for... and I'm not sure I completely understand your question.


It sounded like to me that he was asking if you know the incident energy, how close can you get to the arc flash and be protected at PPE rated at HRC 2.

i.e. what is the (working) distance from a known incident energy to the point where the incident energy is reduced to 8 cal/cm^2, as opposed to being reduced to 1.2 cal/cm^2 which is why i suggested solving the equasion for working distance.

I actually made up a spreadsheet in excel where i could get the incident energy at 6", 12", 18", 24", 36" . . . so I could say that the incident energy may be 51.56 cal/cm^2 at 18" but at 24" it is 32.16 cal/cm^2, and at 36" it is 16.53 cal/cm^2, and so forth.

[modified HRC to 2 to agree with 8 cal/cm^2]


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PostPosted: Mon Jun 10, 2013 6:56 am 
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No HRC 3 is rated 8 cal/cm2. That is the minimum of HRC 2. Did you mean, you have HRC2 PPE and want to know the distance from a 25 cal hazard you must stay with the 8 cal/cm2 system? Please clarify if that is what you mean. The answers above should help but I didn't want anyone to think you could have have an 8 cal/cm2 HRC3 system.


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PostPosted: Mon Jun 10, 2013 10:24 am 
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Bennett Richey wrote:
I understand the arc flash boundary to be, at the outer edge of the boundary, the point where unprotected skin could receive a 2nd degree burn and is equal to 1.2cal/cm2. What I'm wondering if there's a way to find out what the boundary would be for higher incident energies. For example, if I have category 3 PPE that is rated 8 cal/cm2, how close can I get to the arcing point and still be protected?


yes indeed IEEE P1584 equations (5.8) and (5.9) for determining the arc flash boundary can also be solved with other incident energy levels. Arc Flash Analytic Version 5.0 software program calculates arc flash boundaries based on 1.2 cal/cm^2 (5 Joules/cm^2) onset energy to second degree burn for bare skin exposure per IEEE P1584 Guide, or other incident energy levels as well (such as the rating of proposed personal protective equipment), or the evaluated onset to second degree burn energy.

_________________
Michael Furtak, C.E.T.
http://arcadvisor.com


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