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 Post subject: Arc Flash Boundry vs....
PostPosted: Wed Oct 27, 2010 2:06 pm 
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So my AFB is 233" in a particular double ended 480V 800A rated westinghouse metal enclosure @ 34.1cal/cm^2 @24".

Is there any definition of where the AFB is practical up too? Say i have a 10" thick concrete wall @ 5ft from the gear, say i have a hillside at 8ft from the gear. Does IEEE1584 acknowledge AFB and construction in front of the gear? Can remote operate the gear from the otherside of the conrete wall, but technically its still in the AFB right??

Seems like theres so much interpretation here, and no authority providing answers...

Please help!


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PostPosted: Thu Oct 28, 2010 5:34 am 
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I am sure I have seen something on the concrete wall, I just can't find it. Remote racking and switching can easily be done outside your AFB's.


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PostPosted: Fri Oct 29, 2010 8:35 am 
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Seems like theres so much interpretation here, and no authority providing answers...

Please help![/QUOTE]

Yes, it would be safe to operate a remote racking device from behind the concrete wall. The calculations are determined using air as the medium. Obvious, concrete does not transfer heat and energy as easily as air. You may also look at retrofiting the breaker trip units or relays and adding a maintenance switch to reduce the time delay of the upstream protective device. There are many ways to reduce the AF energy and AFB. Please see http://ecmweb.com/mag/electric_ways_reduce_arc/

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Robert Fuhr, P.E.; P.Eng.
PowerStudies


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PostPosted: Fri Oct 29, 2010 10:48 am 
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Robertefuhr wrote:
Yes, it would be safe to operate a remote racking device from behind the concrete wall. The calculations are determined using air as the medium. Obvious, concrete does not transfer heat and energy as easily as air. You may also look at retrofiting the breaker trip units or relays and adding a maintenance switch to reduce the time delay of the upstream protective device. There are many ways to reduce the AF energy and AFB. Please see http://ecmweb.com/mag/electric_ways_reduce_arc/



Robertefuhr, thanks for the suggestion. But what i'm getting at is when, if they dont already, will standards be set up for the type of barrier material, strength of rebar, thickness of conrete. I see nothing that says what is and what isn't an acceptable barrier. Is there going to be an effort to standardize such issues as these? NEC codes are for the most part very well addressed, Arc Flash seems to be IEEE vs NFPA with no hard and fast like NEC which in my mind leaves alot of open ended interpretation and room for error. With such a extreme type of event, i would expect an effort to standardize arc flash methods for the saftey of everyone..


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PostPosted: Sat Oct 30, 2010 5:31 am 
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harcflash wrote:
Robertefuhr, thanks for the suggestion. But what i'm getting at is when, if they dont already, will standards be set up for the type of barrier material, strength of rebar, thickness of conrete. I see nothing that says what is and what isn't an acceptable barrier. Is there going to be an effort to standardize such issues as these? NEC codes are for the most part very well addressed, Arc Flash seems to be IEEE vs NFPA with no hard and fast like NEC which in my mind leaves alot of open ended interpretation and room for error. With such a extreme type of event, i would expect an effort to standardize arc flash methods for the saftey of everyone..


First off, keep in mind that there are vast differences between IEEE, NFPA 70, and NFPA 70E.

IEEE is a professional engineering organization. Most often their standards documents are guides only. They often discuss various details and alternatives but give NO recommendations at all as to the right way to do things. Thus quite often you are left with a lot of information but no clear path at all.

NFPA 70 (aka NEC) is just the opposite. It is a mandatory standard in all 50 states in the U.S., and also used around the world in a similar way. NEC has the force of law and is written as a regulatory standard. Thus there is almost nothing in it left up to interpretation.

NFPA 70E sits somewhere in between. It is still a voluntary standard and it's a consensus safety standard. There are still a lot of things left open to interpretation to be sure. For example, it says that arc flash studies need not be done on circuits fed from a single transformer <125 kVA and <240 VAC, but it doesn't say why. This is in keeping with IEEE 1584 which gives similar statements. The key here is that it is difficult to actually have an arc flash or to sustain it in these conditions. There has been further research quantifying it (arc flashes might still be possible in certain circumstances) and even recommendations to get away from a transformer kva and instead base it on available fault currents, but so far no hard and fast recommendations.

But, in this case, it doesn't give you an arc flash rating. Is it zero, or "none" (no arc flash hazard)? NFPA 70E doesn't say.

In a similar vein, IEEE 1584 gives arc flash calculations for effectively, exposed conductors with the doors on the enclosure wide open. It doesn't model doors-closed scenarios. A related ANSI standard for medium voltage "arc resistant" equipment (metal clad/metal enclosed gear with the addition of a "blast door" on the back/top) gives a pass/fail test which demonstrates that in doors closed scenarios, the arc flash hazard is 0. There is no similar standard for low voltage. 70E recommends that in low voltage cases (according to the tables), the arc flash hazard is zero, and depending on the equipment, medium voltage can be 0 or 2.

What's the rule in this case? Unknown.

Now moving on to explosions, which is where you are going, when looking at "air bursts" as I understand it from Baker Risk, generally speaking unless you are creating a special case such as a fuel-air bomb, you need about 10,000 lbs. or more of combustible material to actually achieve a reasonably destructive explosion (the knock-down building variety) in a wide open field. However once you standard to add walls and generally confine the area, the amount of material needed drops pretty quickly. They are often left with an interpretation challenge as to how much "confinement" a particular building/room actually presents and whether or not an explosion could occur when combustible dusts/fuels/aerosols are present, and whether or not a structure is sufficient to withstand it.

The key thing to remember here also is that NFPA 70E is concerned with just two particular hazards: arc flash, and shock. Shock is particularly easy to mitigate in most cases, and arc flash reasonably easy. There really isn't anything though that you can do about arc blast, and 70E does not speak to this. That's why they stop there. They recognize that this is a real, serious hazard. There have been some estimates done to calculate the pressure of the arc blast. This number could be used to design a blast wall to take it.

To give you an example, you can easily calculate the thermal protection offered by various barriers. As I used to be a kiln engineer, I did this on a regular basis. To give you an example, the calculations are tedious, but if you had an 800-1000 degree radiant heat source and you create a double wall structure with both layers being a simple piece of tin siding spaced 1" apart with a piece of unistrut, this is sufficient to drop the temperatures down to only 10-20 degrees above ambient, more than enough to shield people and equipment from a kiln that is operating well above normal operating conditions (operating with brick missing). However, I wouldn't even consider this "protection" with an arc flash. It will eliminate the arc flash hazard but arc blast would trivially penetrate this simple shield as the molten copper and pieces come rocketing out of the bucket.

You may want to look at NFPA 498, NFPA 499, NFPA 220, or NFPA 251 for hints. I realize that these tend to have highly qualitative answers for you though. Generally most NFPA committees recognize that dealing with an explosion after the fact is extremely difficult to do and to quantify how strong a structure must be because there are so many variables so they tend towards preventing an explosion in the first place rather than trying to address surviving it post-incident because in most cases, prevention tends to be easier to achieve.


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