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 Post subject: Open air vs In-a-box
PostPosted: Tue Oct 31, 2017 6:28 am 
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Hello all,

I'm new to Arc Flash, therefore I decided to run a few simulations to help me understand what are the effects of the differents parameters (time, working distance, etc.) in the IEEE 1584 equation on the resulting IE level.

Something I discovered through all the simulation I did is that the IE level will be greater when the arc is enclosed in a box than in open air. I would
have instinctly think the opposite, because the flash is more ''free'' to expand in the open air than in a box.

1) Is that right to say that resulting IE will always be higher when the arc is in a box?
2) Why?

Thanks all !
Eric


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 Post subject: Re: Open air vs In-a-box
PostPosted: Tue Oct 31, 2017 11:24 am 
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The arc flash energy is focused/concentrated when it is directed out of the opening of a box. The energy for an arc flash in open air radiates spherically in many directions so think of it as being "diluted" and a smaller portion of it reaches the worker.

Welcome to the world of Arc Flash and the Arc Flash Forum. A very good group of knowledgeable people hang out here and are quite willing to answer questions and help out. Stop back with any questions or use the search since many have been addressed.

- Jim


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 Post subject: Re: Open air vs In-a-box
PostPosted: Tue Oct 31, 2017 11:28 am 
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Thanks for the quick answer !

It makes perfect sense.


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 Post subject: Re: Open air vs In-a-box
PostPosted: Wed Nov 01, 2017 3:32 am 
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Heat transfer via thermal radiation (aka infrared) is several times more efficient than convection/conduction. That is why for instance you can stand next to a fire when the temperature is below freezing and feel nice and warm...it's radiation vs. convection. Just like light (which is also radiation), it will bounce off walls like an enclosure. You can tell that this is what's going on because the incident energy is proportional to the square of the distance in the open air model...it's the surface area of a sphere. If it were convective it would be proportional to the volume and thus proportional to the cube of the distance and the incident energy would fall off MUCH faster than it does.

In the arc-in-a-box model because the thermal radiation bounces off the walls and gets more focused, the exponent drops down to less than 2 and since it is already more like a beam instead of a sphere, the radiation is much higher to start with. In a more refined model (IEEE 1584 is mostly based on data collected with distances under 6 feet) it would eventually become more spherical but as of right now this is not how it is modeled. This causes problems for instance with medium voltage switchgear with high currents where the predicted arc flash boundaries are completely unrealistic (hundreds of feet) but without a lot more data it's the best we have.


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 Post subject: Re: Open air vs In-a-box
PostPosted: Wed Nov 01, 2017 5:39 am 
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Thanks for the detailed information.

From the IEEE equations, there is a K factor for open air vs in a box when calculating the bolted short circuit current and another (different) K factor for open air vs in a box when calculating the Arc Flash current.

Will both current always be calculed using the same environnement factor or can we imagine a situation where the bolted current would have to be calculated in a box and the Arc flash current would have to be calculed in open air? And vice versa?

Thanks again

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 Post subject: Re: Open air vs In-a-box
PostPosted: Wed Nov 01, 2017 10:37 am 
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Quote:
Will both current always be calculed using the same environnement factor or can we imagine a situation where the bolted current would have to be calculated in a box and the Arc flash current would have to be calculed in open air? And vice versa?


It will always be one environment as I can't envision the fault starting as bolted fault in a box and then the box disappearing at the same time the arc becomes and arcing fault. Typically, the fault will start as an arcing fault not a bolted fault.

On the other side, I can't envision an arcing fault in open air having a box appear around it.

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 Post subject: Re: Open air vs In-a-box
PostPosted: Wed Nov 01, 2017 3:39 pm 
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ETcofamek wrote:

From the IEEE equations, there is a K factor for open air vs in a box when calculating the bolted short circuit current ...


I believe you are mistaken. The k adder comes into play when calculating arcing current given bolted current. The standard recommends using commercial short circuit programs or the IEEE spreadsheet to get the bolted current.

The box versus open air question comes up again later when calculating IE. I would not be switching at this point, since it is the arcing current that causes the exposure.


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 Post subject: Re: Open air vs In-a-box
PostPosted: Thu Nov 02, 2017 3:19 am 
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Don’t forget these are all empirical models...someone made up an equation either based on how we think reality works or based on the shape of the curve and used computer software to fine tune all the fudge factors. It is not a model based on physics.

Physically however every half cycle when the current passes through zero, the arc extinguishes. After that the air cools down. As the voltage increases during the next half cycle eventually the arc restrikes and the cycle repeats. Higher currents heat air more, higher voltages allow the arc to restrike sooner, and if the area around the arc is more confined the air cools less so it restrikes sooner (electrical conductivity is temperature dependent). There is a lot going on here and the models just curve fit because the theory hasn’t caught up yet. The result is that the arcing current average will be higher in arc-in-a-box and so will the incident energy as described earlier. The best curve fit on the current uses the K factor. The best curve fit on the normalized incident energy uses that and to model the focusing effect the exponent x is adjusted.

This is pretty typical modeling for a lot of thermal and hydrodynamic processes. Even where finite volume and/or radiosity models can be done, often you need a high horsepower computer or supercomputer and hours or days of calculation time with $50,000 of software and training involved to do one calculation. The investment usually makes lab testing less costly, and you couldn’t for instance practically do a study on a small plant with say 150 buses that takes 6 months to run the calculations even if the theory caught up to the empirical work. That kind of model just tends to get used to calidate the empirical one anyway. But as I said were not there yet. Robert Lang has developed a time based model that is closer but more computer intensive because it takes hundreds of calculations per data point, and a another simplified one that beats IEEE 1584-2002 that might show up in the next IEEE 1584 standard. The time series model is riddled with fudge factors too but gets closer to a physics model in terms of underlying equations.


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 Post subject: Re: Open air vs In-a-box
PostPosted: Mon Nov 13, 2017 2:25 pm 
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Real equipment configuration makes a difference too by deflecting energy and focusing energy more or less than the calculations.

Additionally, the equipment can constrain the arc length or perpetuate the extinguishing of the arc.

Having about 170 incident investigations under my belt and about 50 are published with a group in an IEEE paper, typically IEEE 1584 is still conservative.

Also, arc ratings are conservative if the current is >8000A and might be a little under kill if the current is lower but this is an over generalization. Longer time is worse on garments than higher current due to ablation changes in the system. Distance also changes the convection portion. We test garments at 12 inches while the body is often farther away. Lots of evidence that we are very conservative, I'm working on a paper but some of the preliminary work is in the Handbook of Fire Resistant Textiles, Woodhead Publishing. My chapter is on arc flash.

Hugh Hoagland


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 Post subject: Re: Open air vs In-a-box
PostPosted: Mon Nov 13, 2017 2:31 pm 
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A couple of question about your answer below:

KNOWN: The heat energy transmission via radiation is the absolute T to the fourth power. Whereas convection heat energy transfer is Delta T x Area x h, "h" being a coefficient based upon the medium and orientation.

Question: This is not a rebuttal I merely want to get an accurate grasp of this topic. Down below in you post you mention volume and and the exponent dropping to less than 2. The convection energy is based upon area per the above formula, not volume? The delta T for radiation is "T to the 4th", which is far greater than the "less than 2"?

Question: What are the proportions of energy, in other words, what percent is by convection and what percent is by radiation at a 18"-24" working distance.

PaulEngr wrote:
Heat transfer via thermal radiation (aka infrared) is several times more efficient than convection/conduction. That is why for instance you can stand next to a fire when the temperature is below freezing and feel nice and warm...it's radiation vs. convection. Just like light (which is also radiation), it will bounce off walls like an enclosure. You can tell that this is what's going on because the incident energy is proportional to the square of the distance in the open air model...it's the surface area of a sphere. If it were convective it would be proportional to the volume and thus proportional to the cube of the distance and the incident energy would fall off MUCH faster than it does.

In the arc-in-a-box model because the thermal radiation bounces off the walls and gets more focused, the exponent drops down to less than 2 and since it is already more like a beam instead of a sphere, the radiation is much higher to start with. In a more refined model (IEEE 1584 is mostly based on data collected with distances under 6 feet) it would eventually become more spherical but as of right now this is not how it is modeled. This causes problems for instance with medium voltage switchgear with high currents where the predicted arc flash boundaries are completely unrealistic (hundreds of feet) but without a lot more data it's the best we have.


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 Post subject: Re: Open air vs In-a-box
PostPosted: Mon Nov 13, 2017 2:39 pm 
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I'll have to check the data. One paper did 18 and we test at 12 but they just looked at total energy. We found at 12 inches the convection vs. IR is about 50/50 in the open arc test. I would expect a box to do more convection but our arc length is long so I'm not sure.

If you watch the high speed video, the "fireball," which would be one indicator of the convection front, does not typically extend beyond about 12-14 inches in an open arc test like ASTM F1959. We didn't know that when we picked 12 inches over 20 years ago but it might explain why the test works so well with IEEE 1584 and calculations.

We are working to do more work on measuring this due to a recent attack on the standard. The attack is trying to get research money and push a European test which has other issues but it helps us learn.

Hugh


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 Post subject: Re: Open air vs In-a-box
PostPosted: Tue Nov 14, 2017 3:59 am 
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ETcofamek wrote:
Thanks for the detailed information.

From the IEEE equations, there is a K factor for open air vs in a box when calculating the bolted short circuit current and another (different) K factor for open air vs in a box when calculating the Arc Flash current.

Will both current always be calculed using the same environnement factor or can we imagine a situation where the bolted current would have to be calculated in a box and the Arc flash current would have to be calculed in open air? And vice versa?

Thanks again


The configurations for the 2002 model of IEEE 1584 are only an arc flash in a box vs. air with the test electrodes in a vertical configuration i.e. pointed down. A few different box sizes were used depending on the “equipment”. The next edition of IEEE 1584 is moving closer to completion.

Myself with a few other IEEE 1584 officers are presently reviewing the many comments from the first balloting of the next edition of IEEE 1584. Once this is all completed and the next edition is finally approved and published, there will be many more options regarding the configurations supported by a few thousand new tests.

Btw, regarding the K factors. For the arcing current there is only one K factor for the arc flash in an enclosure vs. open air. This adjusts the effect of constraining the arc which affects the arcing current. The incident energy calculations has two K values. K1 is again for the enclosure vs. open air and K2 is for grounded vs. ungrounded.

The original test for the 2002 model show a slight variation between grounded and ungrounded. We found in the present research (and subsequent new model) that this effect is not as significant as first though (only during arc initiation).

Here is an article that I wrote a while ago about the upcoming next edition of IEEE 1584-201X

My usual required disclaimer: although I’m Vice-Chair of IEEE 1584 and a member of the Steering Committee for the IEEE/NFPA Arc Flash Collaborative Research Project, the above are my personal comments and may or may not reflect the views of any particular standards organization


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