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 Post subject: Category Dangerous when very close to busbars
PostPosted: Mon Nov 05, 2012 4:50 pm 
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A category 0 switchboard is based on ones normal working distance (of course). And of course, if someone was really close (say within 50mm/2inch), then the calculations come up with category Dangerous (>40cal/cm2) for that same switchboard.

I suspect this is not likely to be a case where the pressure wave would do more damage than the burning effects. Would the burning effects cause third degree burns to personnel through the full cat 4 suit, if said personnel had their chest that close to the arc fault.

I realise this may not be normal practice, but our arc flash study details the working distances per category for each panel, including up to cat 4.


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PostPosted: Sat Nov 10, 2012 6:53 am 
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ScottPET wrote:
A category 0 switchboard is based on ones normal working distance (of course). And of course, if someone was really close (say within 50mm/2inch), then the calculations come up with category Dangerous (>40cal/cm2) for that same switchboard.

I suspect this is not likely to be a case where the pressure wave would do more damage than the burning effects. Would the burning effects cause third degree burns to personnel through the full cat 4 suit, if said personnel had their chest that close to the arc fault.


Simple physics. It's radiation in a spherical pattern. As you get further from the arc, the heat flux decreases considerably, roughly with the square of the distance. There are actually some issues where the distribution of energy is not strictly spherical but aside from some rough "calculation" rules, exact modeling right now is not possible due to lack of data (and exponentially complicated modeling).

Quote:
I realise this may not be normal practice, but our arc flash study details the working distances per category for each panel, including up to cat 4.


I see this as bad for a couple reasons. First, you can indeed open up every panel and check very carefully to measure distances. In SOME cases it may be appropriate to adjust the standard table given by IEEE 1584. However, then you open yourself up to a huge liability issue because you'd also have to present your case if it ever ends up in a legal arena as to why you deviated from the consensus safety standard. In effect you are making your own standard. There is plenty of documentation out there on the idea that as I just stated, the heat flux is not necessarily uniform and spherical or hemispherical depending on distance. As an expert witness for the other side, I could use that kind of evidence to ruin your arguments.

Second, this kind of information also means that someone working on your equipment has to make a lot of engineering judgments on how far away they are. In practice it is just going to lead to a lot of confusion. In my operation all distances are standardized with one and only one exception. We use IEEE 1584 exclusively for calculations. The one exception is that for outdoor overhead gear, the standard distances in the table are inappropriate, especially if the equipment is operated with a hot stick. For those cases we change the working distance to what is appropriate for the type of equipment and we have mandated minimum lengths for hot sticks.


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