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 Post subject: Flash Boundary
PostPosted: Fri Jun 26, 2009 8:33 am 
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I would like some comments on how others have handled, in our opinion, the unrealistic flash boundaries the 1584 methodology can produce. We have many locations with both large fault currents and potentially long clearing times or the FCT is clamped at 2 seconds due to several factors. These locations often return a very very large flash boundary. We are having issues with putting numbers over 30’ on a label much less numbers that are over a hundred feet. Our goal is to make sure the site take the results seriously but when the flash boundary is outside the gate people tend to ask questions. We are considering using a maximum flash boundary but are unsure of what this distance should be. Has anyone else established a maximum flash boundary? If so, what number did you use and what was your justification? If not, how have you discussed the extreme distances with your site or clients? What was their reaction? Thank you.


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PostPosted: Fri Jun 26, 2009 8:50 am 
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SCGEng1 wrote:
I would like some comments on how others have handled, in our opinion, the unrealistic flash boundaries the 1584 methodology can produce. We have many locations with both large fault currents and potentially long clearing times or the FCT is clamped at 2 seconds due to several factors. These locations often return a very very large flash boundary. We are having issues with putting numbers over 30’ on a label much less numbers that are over a hundred feet. Our goal is to make sure the site take the results seriously but when the flash boundary is outside the gate people tend to ask questions. We are considering using a maximum flash boundary but are unsure of what this distance should be. Has anyone else established a maximum flash boundary? If so, what number did you use and what was your justification? If not, how have you discussed the extreme distances with your site or clients? What was their reaction? Thank you.


Wireless remote racking and operators.


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PostPosted: Fri Jun 26, 2009 9:25 am 
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That is one of the "holes" in IEEE 1584. As you already found out, lower short circuit currents can produce long clearing times and the equations treat this the same as a high short circuit current and low clearing time. Other than the 2 second cut off which can still produce very large results, there is not much else in writing.

The newer reseach is lookng at arc extinction times since the arc might extinguish long before a device trips.

I was just in the lab this week "blowin stuff up" :eek: fpr some independent testing and we had a hard time gettting a 30 kA bolted / 15 kA arcing current to sustain more than 1 or 2 cycles. It was phase-to-ground testing. Three phase sustained quite well - we even managed to throw a bus bar placed on top of the phases about 20 feet with that low of current.

There is still a possibility for some equipment that a phase-ground arc flash can go three phase quickly.

With all that said, there really is not a good answer yet. I understand the new IEEE / NFPA testing might finally be looking at energy per time which is what it will take to distinguish between incident energy from low current / long clearing time and high current / low clearing time.

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PostPosted: Tue Jun 30, 2009 6:40 am 
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Thank you for the response, we look forward to more research and testing not only on this subject but many of the other arc flash “gray areas” at well.


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