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 Post subject: Effects of enclosures on incident energy
PostPosted: Tue Dec 21, 2010 10:27 am 
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The company I work for is just beginning to understand/take seriously arc flash and PPE, and I'm part of the group assigned to get our stuff straightened out. There are a couple of things that I haven't yet been able to get my head around, however. I understand the incident energy calculation, but am unsure of how this is affected by enclosures. For instance, I have an enclosed 480V 800A disconnect, and the calculated available incident energy at the disconnect is X. How is this mitigated (or is it at all) by the fact that the switch itself is in a steel enclosure? Any references to regulatory guidance would be appreciated as well. Secondly, I have been unable to find any comprehensive guidance on arc flash as it relates to large DC generators (2500HP 1800V). Any help is much appreciated!


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PostPosted: Wed Dec 22, 2010 7:14 am 
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Enclosures & Arc Flash

No, your enclosure is not rated to contain an arc and therefore does not do anything to reduce the incident energy or arc flash protection boundary. The AFPB is defined in the 2009 edition of the NFPA-70E as a distance from a perspective arc source.

If you look in article 100 of 70E at the definition of arc flash hazard, FPN No 1 says that an arc flash hazard may exist when energized electrical conductors or circuit parts are exposed or when they are within equipment in a guarded or enclosed condition, providing a person is interacting with the equipment in such a manner that could cause and electric arc. The handbook notes go on to explain that in certain conditions an arcing fault within the equipment could generate a pressure wave and destroy the integrity of the enclosure. The committee suggests that interacting with the equipment could mean opening or closing a disconnect, pushing a reset button or latching the enclosure door but that if the equipment is properly installed in accordance with the NEC, adequately maintained and operating normally the likelihood of these actions initiating a arc fault is remote.

In 130.7(C)(9) FPN No 1 says the collective experience of the task group is that in most cases closed doors do not provide enough protection to eliminate the need for PPE for instances where the state of the equipment is known to readily change and that the premise used by the task group is considered to be reasonable based on the consensus judgment of the full NFPA-70 Technical Committee.

If you search this forum there are several discussions on this topic; figuring out how to apply this in a practical manner has been fun.
See question #2 for an [url="http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=INTERPRETATIONS&p_id=25973"]OSHA interpretation[/url].

There are several products available on the market to reduce the hazard to personnel and limit the PPE required. We don't use any of these products where I work but they were helpful in convincing people there is a real hazard here.
  • [url="http://www.chickenswitch.com/"]Chicken Switch[/url]
  • [url="http://www.utilityrelay.com/QUICK-TRIP_Page.html"]Maintenance Switch[/url]
  • [url="http://www.remoterackingsolutions.com/"]Remote Breaker Racking[/url]
  • [url="http://www.geindustrial.com/cwc/Dispatcher?REQUEST=PRODUCTS&pnlid=5&famid=40&catid=83&id=mvarcresist"]Arc Resistant Switchgear[/url]
  • [url="http://www.geindustrial.com/products/static/switchgear/arc_vault.htm"]Arc Vault[/url]
  • [url="http://www.moeller.net/en/products_solutions/power_distribution/power_communication/arc_fault_protection/index.jsp"]Arc Fault Protection System ARCON[/url]
  • [url="http://www.irwindows.com/"]IR Windows[/url]

For info on DC please see this [url="http://www.arcflashforum.com/showthread.php?p=7123#post7123"]thread[/url] on the forum.


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PostPosted: Wed Dec 22, 2010 8:30 am 
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The IE is not mitigated, it is increased over the open air value. Make sure you use the arc in a box formula.


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PostPosted: Wed Dec 22, 2010 8:53 am 
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A King wrote:
See question #2 for an [url="http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=INTERPRETATIONS&p_id=25973"]OSHA interpretation[/url].



Please note that this interpretation appeared before the release of NFPA 70E - 2009. This document defines a potential arc flash is, "

Arc Flash Hazard. A dangerous condition associated with the possible release of energy caused by an electric arc.

FPN No. 1: An arc flash hazard may exist when energized electrical conductors or circuit parts are exposed or when they are within equipment in a guarded or enclosed condition, provided a person is interacting with the equipment in such a manner that could cause an electric arc. Under normal operating conditions, enclosed energized equipment that has been properly installed and maintained is not likely to pose an arc flash hazard.
FPN No. 2: See Table 130.7(C)(9) for examples of activities that could pose an arc flash hazard.

NFPA 70E added "interacting" which implies operating a breaker or switch.

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PostPosted: Wed Dec 22, 2010 6:04 pm 
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Dukman wrote:
The company I work for is just beginning to understand/take seriously arc flash and PPE, and I'm part of the group assigned to get our stuff straightened out. There are a couple of things that I haven't yet been able to get my head around, however. I understand the incident energy calculation, but am unsure of how this is affected by enclosures. For instance, I have an enclosed 480V 800A disconnect, and the calculated available incident energy at the disconnect is X. How is this mitigated (or is it at all) by the fact that the switch itself is in a steel enclosure? Any references to regulatory guidance would be appreciated as well. Secondly, I have been unable to find any comprehensive guidance on arc flash as it relates to large DC generators (2500HP 1800V). Any help is much appreciated!


Just another view. From some research/testing. In one case a door was launched at several hundereds miles an hour and in another case I believe there was a possible pressure build up due to the door closed and then it blew open with even more authority.

For a low level arc flash, perhaps the door might stay closed but there is no way to know for sure. Arc resistant equipment is designed so the arc flash is contained and the door remains closed. In non arc resistant equipment, if the door decides to "relocate itself" during an arc flash, it could become quite a deadly projectile.

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PostPosted: Thu Dec 23, 2010 1:55 pm 
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brainfiller makes a very good point about the doors. One thing to note, most Arc Flash Resistant Switchgear does not contain the arc flash energy. It is instead re-directed upwards through hatches and duct work. The energy is vented to the roof of the building.

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PostPosted: Thu Dec 23, 2010 3:40 pm 
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Robertefuhr wrote:
One thing to note, most Arc Flash Resistant Switchgear does not contain the arc flash energy. It is instead re-directed upwards through hatches and duct work. The energy is vented to the roof of the building.


Correct! I meant contained behind the door. Pressure has to go somewhere. One of the early designs of one company had a hinged hatch and then the manufacturer quickly realized there was not much point to a hinge since the gear will likley be quite damaged. They went to a perforated blow out hatch instead.

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