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 Post subject: Pressure wave created by arc faultPosted: Wed Dec 20, 2017 9:33 am

Joined: Thu Dec 07, 2017 2:29 pm
Posts: 1
I am looking for a way to calculate pressure created by an arc fault while inside a manhole. I have found an write up by Paul Cambell on 4/8/16 called "Arc Blast Hazard" that outlines various ways to calculate the pressure.

1. Lee's equation - only used for open air (I'm assuming not in a manhole)
3. Ideal Gas equation

I'm ruling out #1 since we are working inside a manhole and I would also need to know what the initial pressure is. There seems to be a significant amount of variation between #1 and #2. Also, It seems to be somewhat counter-intuitive to use the Ideal gas equation since as the volume of the enclosure increases then pressure created would continue to increase. Is that correct?

Any help would be appreciated.

Thanks,
Stephen

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 Post subject: Re: Pressure wave created by arc faultPosted: Fri Dec 22, 2017 11:47 am
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Joined: Tue Oct 26, 2010 9:08 am
Posts: 2174
Location: North Carolina
sdedmo2 wrote:
I am looking for a way to calculate pressure created by an arc fault while inside a manhole. I have found an write up by Paul Cambell on 4/8/16 called "Arc Blast Hazard" that outlines various ways to calculate the pressure.

1. Lee's equation - only used for open air (I'm assuming not in a manhole)
3. Ideal Gas equation

I'm ruling out #1 since we are working inside a manhole and I would also need to know what the initial pressure is. There seems to be a significant amount of variation between #1 and #2. Also, It seems to be somewhat counter-intuitive to use the Ideal gas equation since as the volume of the enclosure increases then pressure created would continue to increase. Is that correct?

Any help would be appreciated.

Thanks,
Stephen

The ideal gas law model makes perfect sense because we're storing potential energy inside the sealed vessel and then releasing it at a fixed maximum pressure. As the volume increases the total amount of stored potential energy increases thus we have an increased pressure wave afterwards. Granted there are all kinds of arguments against this kind of modelling but the conclusions aren't counter intuitive at all. Think of say a pressurized coffee can vs. a pressurized 55 gallon drum, both pressurized to the same pressure and then the pressure is suddenly released...which one is the bigger "threat"?

Getting to your specific situation, the problem is that you don't have a truly sealed vessel (manhole itself), and the walls would reflect pressure waves so it's not an "open air" (Lee style) condition either. So I would think that neither model applies in your situation. I'd suggest using the CIGRE report that I referenced that specifically deals with arc blasts where the enclosure releases an arc blast in a highly confined outer room (vault in this case). It's considerably more complicated but would be the closest to an accurate model for the general case. Depending on the size of the vault, this level of modelling may not be necessary but without actually running tests it's hard to say what the "correct" size is.

Finally actual testing at Kinetrics sponsored by E-Hazard is producing testing results on arc blasts that show that, to put it politely, we don't really know what is going on. Several of the tests showed anomalous results that almost seem to defy the physics as we understand it.

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