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 Post subject: Table 130.7(C)(15)(A)(b) Arc Flash Hazard PPE Categories
PostPosted: Mon Nov 10, 2014 6:42 am 
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This table only goes up to Metal Clad Switchgear, 1 kV to 15 kV. Some facilities I have seen have voltages on site greater than 15 kV so therefore tables do not apply. I would suspect that greater than 15 kV would require an analysis and put one more into the realm of utility like equipment/voltages and under 1910.269.

Thoughts?

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 Post subject: Re: Table 130.7(C)(15)(A)(b) Arc Flash Hazard PPE Categories
PostPosted: Mon Nov 10, 2014 7:32 am 
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wbd wrote:
This table only goes up to Metal Clad Switchgear, 1 kV to 15 kV. Some facilities I have seen have voltages on site greater than 15 kV so therefore tables do not apply. I would suspect that greater than 15 kV would require an analysis and put one more into the realm of utility like equipment/voltages and under 1910.269.

Thoughts?


Yes, welcome to my world. My internal distribution voltage is 23 kV.

Whether it falls into 1910.269 or not is clear as mud for many plants. I have a cogen. OSHA is pretty clear that this is 1910.269. Where it gets messy is for instance the cogen is one of about 16 circuits terminating at the main substation. So is the ENTIRE 23 kV system including about 2 dozen 23 kV switch-houses and associated lines considered 1910.269 (distribution), or just the generator, step-up transformer, and the line running to the main sub? Or does it extend down to even the 480 V level where I have switchgear that does not feed any loads directly without hitting say a panelboard or an MCC? OSHA is not clear on this subject. In the interests of making it clean we somewhat arbitrarily split the middle and said that the 23 kV distribution system including switch houses is distribution. One it hits a lower voltage substation, it's utilization.

Then comes the question of arc flash. Prior to 70E-2015, we just used the tables for this equipment. It was somewhat of a "punt".

Alternatives:
Lee equation. The results are ridiculous though and well documented, based on invalid assumptions about arc physics.
IEEE 1584. Not valid. Basically suggests to just use Lee.
NESC. Tables just like NFPA 70E except that the tables say they are for open air equipment only so not enclosed switchgear.
Duke Heat Flux. Values are very low compared to test results (<15 kV).
ArcPro. Values agree closely with test results (<15 kV). It gives a multiplier of 3-6 for enclosed equipment.

At this time we're angling towards using Arc Pro and we're in the midst of the 5 year revalidation exercise.


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 Post subject: Re: Table 130.7(C)(15)(A)(b) Arc Flash Hazard PPE Categories
PostPosted: Mon Nov 10, 2014 7:57 am 
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PaulEngr wrote:
ArcPro. Values agree closely with test results (<15 kV). It gives a multiplier of 3-6 for enclosed equipment.

At this time we're angling towards using Arc Pro and we're in the midst of the 5 year revalidation exercise.


That is my thoughts but wanted to hear from other people. I have done ArcPro for 34.5kV equipment and then switched to EasyPower for the utilization voltages of 480 and 208.

The ArcPro multipliers for 3 phase in a box range from 3.7 to 6.5. Which could lead to a whole other discussion. How do you choose? I lean towards worst case for conservatism.

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 Post subject: Re: Table 130.7(C)(15)(A)(b) Arc Flash Hazard PPE Categories
PostPosted: Mon Nov 10, 2014 12:37 pm 
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wbd wrote:
The ArcPro multipliers for 3 phase in a box range from 3.7 to 6.5. Which could lead to a whole other discussion. How do you choose? I lean towards worst case for conservatism.


Yes, that's another issue. Best I can offer is these articles on the subject:
Wilkins:
http://renmarkusa.com/wp-content/upload ... aylsis.pdf
Fontaine:
http://www.battcon.com/PapersFinal2014/ ... 0Final.pdf

Of the two, Wilkins is far less usable because there are too many secret formulas and fudge factors. Fontaine just generalizes the equations in 1584 but doesn't really leave it with something that can be easily used, but I think the big thing is that the multiplier is:

M=4*pi*k/(1+(0.03937/D)^2) where D is the working distance in inches. k is a fudge factor.

For all practical purposes, M=1.6 for panelboards, levels off at 3.85 at around 5 feet away for low voltage switchgear, and continues to increase and levels off at 5.23 at around 500 feet away for large switchgear (presumably the case at hand). It's around 2.5 for the IEEE 1584 empirical calculation.

So I've never seen a "6.5" case and I'm not sure where they got that from, and the values given in the IEEE 1584 data range from 1.0 to 5.23 for large switchgear which at least brackets the 3.7-6.5 range given by ArcPro. I think that since the arc physics has not drastically changed you could make a case for applying the multiplier from the Fontaine paper to the data in ArcPro, or at least picking one on the very low end as reasonable since there isn't much justification given by ArcPro. As far as I know though the ArcPro folks have given much tighter hints about how to determine the multiplier than the range that you've given.


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