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 Post subject: 480V 60Amp or less Cat 0?
PostPosted: Tue Mar 22, 2011 7:54 am 
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Location: Ohio
We are in the process of having an arc flash hazard analysis performed and the company that we are using said that it will calculate any of the panels that are above 60amps (480 3phase) but anything 60 and lower will just have a sticker with Cat 0. I have never done a study and have never researched how you calculate the incident energy values but does this seem like an acceptible procedure to follow when doing the study? I just want to make sure that when I put the Cat 0 labels on the panels that I am protecting my employees to the level that they need to be protected.


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PostPosted: Tue Mar 22, 2011 8:01 am 
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Let me guess, they were low bidder :mad:

What did your scope of work say on the RFQ?


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PostPosted: Tue Mar 22, 2011 8:18 am 
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I would be very careful about that, in fact that would be unacceptable to me. What are they basing that cutoff point on? It certainly is not in IEEE 1584 or NFPA 70E. Makes me wonder what else they are cutting corners on, like anything below 480V? Even if fed from a 300 kVA 480 - 208/120V transformer would be Cat 0?

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PostPosted: Tue Mar 22, 2011 1:29 pm 
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70E states a detailed analysis is not required for equipment <240V fed by a single transformer <125kVA. Anything above that should be part of the analysis.

Even if you decide not to do the analysis below that cutoff point, you still would refer to the tables. And unless the label is task specific the highest HRC is that catagory should be on the label, which is HRC 1 for equipment 240V and below that fall within the limits of even using the tables.


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PostPosted: Tue Mar 22, 2011 2:24 pm 
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mvann81 wrote:
We are in the process of having an arc flash hazard analysis performed and the company that we are using said that it will calculate any of the panels that are above 60amps (480 3phase) but anything 60 and lower will just have a sticker with Cat 0. I have never done a study and have never researched how you calculate the incident energy values but does this seem like an acceptible procedure to follow when doing the study? I just want to make sure that when I put the Cat 0 labels on the panels that I am protecting my employees to the level that they need to be protected.


Buyer beware! The category / incident energy is based on short circuit current and clearing time not the size of the breaker. If the 60 A breaker is at a switchboard or panel board with signficant short circuit current, all bets are off about Cat 0. An approach like that could potentially lead to a dangerous situation. Zog's reference to the 70E tables is really your only fall back position in this case.

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PostPosted: Tue Mar 22, 2011 4:12 pm 
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Obviously any three-phase panel should be in the arc flash model but I am curious how far down everyone explicitly models loads from a panel. For example, you have a 200A panel, 480V, three phase, 4 wire. Inside this panel are 15 Square D FA breakers rated 15A and fixed instantaneous connected with 12 AWG to various 3-phase loads. Some of the loads are roof top units 200' away and some are exhaust fans 25' feet away. Do you find the footage to each load and model explicitly in your software or "cut corners" and check the AF values with one 15A breaker and vary the cable length in 10' increments to make sure they are all HRC 0? You are modeling the loads but reducing field time. This is sort of the CYA analysis we do anyway when cable lengths cannot be precisely defined due to inaccessibility.


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