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 Post subject: Ka parameter and Energy En
PostPosted: Fri Jul 24, 2009 7:46 am 
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Location: Turin, Italy
Hello, my name is Peter Cadili. I'm an italian electrical engeneering student. I'm preparing my degree thesis about IEEE1584. I study the arc flash hazard. Please, help me for comprise two questions!
One: Why Ka=-0.153 in the open air configuration in the Iarc calc?Why Ka=-0.097 in the box configuration??
The second one: Why the normalaized incident energy is calculated at t=0.2 [sec] and gap between electrodes =610 [mm]?
Thank you at all!!!!And escuse me for my bad english!!!


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PostPosted: Sat Jul 25, 2009 8:20 am 
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Hello Peter, I notice that you have written to me personally as well as posting 2 threads on this forum without reply. I’ll do my best to address your question on this forum in the hope that more learned forum members will set out to correct me if I get it wrong or add to my reply.
The K factors arise out of the fact that the equations are based upon the least squares method of obtaining the best fit to test data which was obtained in large power laboratories. (The least squares method can be described as a method of fitting data to a formula through regression analysis.) As you can appreciate, predicting electrical arcs is not an exact science particularly at low voltages. The K factors are actually used in the equation for predicting arcing current below 1000 volts and if you are preparing a thesis which is specifically about IEEE 1584 then you will be interested to know that there are imperfections with the formulae. You will do well to read a couple of articles that are linked below.

The first one was referred to me by WDeanN on this forum after I stumbled across one anomaly some time ago where I calculated a higher arcing current than the bolted figure which is impossible in real life but is possible using the equations.
http://www.arcflashforum.com/showthread.php?t=74&highlight=mike+frain

The other article is available through the Easypower.com website called “Practical Solution Guide to Arc Flash Hazardsâ€, written by Chet Davis, PE, Satish Shrestha, Conrad St. Pierre, and Robert Luo, Ph.D. This has a section on variation of arcing current.

The 0.2 seconds arc duration and the 600mm working distance are simply the parameters that are use in the above tests. Why these exact parameters were chosen is something that I can’t help you with but there may be somebody in the know.

I wish you well in your work, Mike


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PostPosted: Mon Jul 27, 2009 2:59 am 
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Thanks for your help

Dear Mike,
thanks for your help, but I must understand why Ka has those values. In the opinion of IEEE1584 Ka is assigns = -0.153 in air configuration and =-0.097 in the box configuration because of arc plasma??? :confused:
And finally, En is normalized at 0.2 sec and 610 mm for reason of laboratory convenience?


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PostPosted: Mon Jul 27, 2009 6:24 am 
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peter81 wrote:
Dear Mike,
thanks for your help, but I must understand why Ka has those values. In the opinion of IEEE1584 Ka is assigns = -0.153 in air configuration and =-0.097 in the box configuration because of arc plasma??? :confused:


There's nothing to "understand" in the logical manner, like why P=V*A or W=m*g. A mathematical operation (a least square regression) on a bunch of arcs with different parameters (over 300 arcs if I'm not mistaken) yielded those results for Ka. For a subset of those tests (open air), the best fit was with Ka=-0.153, and in another subset (arc-in-a-box), the best fit was with Ka=-0.097. A single Ka value would have given a worse fit than any of those two fits.

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And finally, En is normalized at 0.2 sec and 610 mm for reason of laboratory convenience?


That means they used the same time and distance for all tests, so the sensors and current source duration wouldn't change for the whole batch of tests (hence laboratory convenience).

I'm really not sure they actually used the same distance for all tests: if they did, how would they have had different X values?


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PostPosted: Mon Jul 27, 2009 7:47 am 
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Joined: Fri Jul 24, 2009 7:37 am
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Location: Turin, Italy
Attention Vincent

Hello Vincent,
Ka parameter depends by the configuration (open air or box configuration) because the arc's plasma can dissipates in the air else can't dissipates in the box..
In the box the plasma is concentrated, is it correctly?
Whereby the plasma influence the short circuits?
Therefore my question in my threads.
Someone can help me for comprise the Ka parameters?
I have comprise the Ka meaning in the correctly mode?
Thanks for your patience


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PostPosted: Mon Jul 27, 2009 2:50 pm 
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Hi Peter,

I believe the plamsa conversation came from my response to the email you sent me off line the other day.

Everyone here is on the right track with this. The Ka value is strictly a factor that was required in the development of the equations that are used to recreate the results of the tests in the lab. I wish I could say it is some amazingly wonderful new discovery but it really is just a number to differentiate between the results when the arc flash is in a box vs. air.

There are many factors that come into play such as conducting plasma but the Ka does not specifically represent any single factor. Instead it represents what is required for the equation to work which includes everything that is different between the box and air models.

I imagine on going tests will introduce other factors in the future that may represent individual characteristics but for now that is in the future.

Arc flash is still an emerging science where we have answered many questions but found as many new questions that remain unanswered. I was just reviewing slow motion video today of some tests I ran last month and was amazed at how the arc jumps around.

We all wish you well in your studies!

_________________
Jim Phillips, P.E.
Brainfiller.com


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