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PostPosted: Mon Nov 12, 2012 8:57 am 
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belangerje wrote:
Jim,

Can you clarify what you mean by "no upstream device" ? I had posted a question about the <240V, 125kVA transformer exception last week. The situation I'm describing is: 112.5kva trans secondary side ---> 240V Main Breaker --> (5) 208V MLO feed-through panels. As shown in my uploaded PDF (attached). I'm meeting with my client soon and want to get their blessing to employ the exception, but I'm unclear about this comment.

On another note, I know you had previously discussed going with a minimum level of PPE for all panels in which you apply the exception. For example, I've heard of people saying that if you have under 5k of fault current on a 208V system, perhaps you could label everything as a category 2. This was common at a utility that I worked for previously, long before I was neck-deap in this arc flash hazard stuff. Any thoughts on that? Thanks!

The "no upstream device i.e. from a transformer secondary" statement means the exception is usually important when there is a 208V panel on the secondary of a transformer. A typical arc flash model would not consider the main device (i.e. acting like it is not there) of the panel as being the limiting device for the arc flash. This is because of the possibility of the plasma reaches the line side of the device. The arc could continue in that case even if the main trips.

Testing for this exception usually involves a panel or enclosure with no main to simulate the main not operating. The arc extinction is then based solely on self extinguishing. That is what the original exception was about, low current at low voltage doesn't usually sustain so the duration is limited and the total incident energy is limited. However, some test suggest under specific (and in my opinion - not very likely) conditions, it is possible to sustain an arc to lower levels of short circuit current.

This issue is still not resolved. A few options were on the table with the one that was receiving the most support about a year ago being: use a 30 kVA or 2500 amp cut off for circuit less than 240 V and use a default incident energy of something like 4 cal/cm2 to prevent circuits using the exception from being classified as NO hazard. There is still a hazard, it is just as severe. We did consider 5 kA as a cut off too (my preference) but several people expressed concerns.

As far as the exception being deleted from the 2012 edition of NFPA 70E, what is happening is we are establishing scopes for NFPA 70E and IEEE 1584. NFPA 70E is about PPE and electrical safety. IEEE 1584 is about calculation methods. So NFPA 70E has been deleting anything regarding calculations and it goes in the annex. The next edition of IEEE 1584 will not make any references to PPE but will focus exclusively on the calculations and methods.

btw, Paul's comments are correct, based on the present format of the IEEE 1584 equations, the energy exposure of the sun over a long enough period of time could result in a high calculated overall incident energy exposure. ...but a 40 cal suit at the beach would not make a good fashion statement. :D

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PostPosted: Thu Feb 28, 2013 11:48 am 
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Location: Salt Lake City, Utah
Jim, so, to comply with the standards, can we use a table to perform our calculations? We don't typically work on equipment higher than about 480, and we were going to use the tables and get our labels marked using that. I would appreciate a response as soon as possible - Thanks!

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