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 Post subject: Open Air 3 phase arcs less than 15kV
PostPosted: Wed Feb 25, 2015 12:01 pm 
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OSHA 1910.269 Appendix E contains Table 3 which lists the acceptable method for 3 phase open arc <15kV incident energy calculations being IEEE 1584b-2011. This is fine until one looks at what OSHA lists in Table 5 for selecting for a reasonable arc gap. For single conductors in air, <15kV, for multiphase (3 phase) arc, the phase conductor spacing is listed to be used.

The utility I am working with uses a conductor minimum spacing on crossarms of 28 inches (711 mm) for 12.47kV construction. However, IEEE 1584 states in Clause 5.1, that the model is only applicable to gap ranges up to 152 mm or 6 inches.

It appears that even though OSHA lists IEEE 1584 as a reasonable method for 3 phase open air arcs for <15kV, this method cannot be used for typical utility conductor spacings.

Am I missing something here?

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 Post subject: Re: Open Air 3 phase arcs less than 15kV
PostPosted: Fri Feb 27, 2015 6:27 am 
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You are not missing anything. IEEE 1584 is only tested out to 6". It is invalid above that point. Further although it claims accuracy up to 15 kV, the actual test data was only measured to 13.5 kV. EPRI is the only one doing testing beyond the 6" limit. You need to look at them. References:
http://www.epri.com/abstracts/Pages/Pro ... 0001022632
http://www.epri.com/abstracts/Pages/Pro ... 0001022633

Also, IEEE 1584 is focussed on enclosed equipment with 3-phase arcing whereas ArcPro is specifically to model outdoor equipment with single-phase arcing. In an outdoor condition with 28" bus gaps, IEEE 1584 is just plain invalid all the way around and you should be looking at either the models and equations developed by EPRI or ArcPro. The first reference I gave above has quite a lengthy discussion about this exact engineering problem.


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 Post subject: Re: Open Air 3 phase arcs less than 15kV
PostPosted: Fri Feb 27, 2015 7:40 am 
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Thanks Paul. I have downloaded will read the material.

I still fail to see how OSHA can recommend something that is clearly not applicable to finding "a reasonable estimate of incident energy"

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 Post subject: Re: Open Air 3 phase arcs less than 15kV
PostPosted: Fri Feb 27, 2015 12:27 pm 
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IEEE 1584 empirical model is perfectly valid if you are dealing with say underground or enclosed switchgear in a voltage range of about 300 V to 15 kV, where arc gaps are 6" or less. It's just not valid for outdoor, overhead lines except for something with a very narrow spacing such as triplex or quadraplex. Since it is valid for many cases experienced by utilities in substations and underground gear, I don't see where OSHA erred in recommending it. With all of these different models, you need to know how the model works and what the basis is behind it in order to apply the correct one.


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 Post subject: Re: Open Air 3 phase arcs less than 15kV
PostPosted: Mon Mar 02, 2015 1:02 pm 
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Throughout the revised 1910.269 you'll see penalties for three phase exposure even when IE is the same. I've been told the reason is to encourage cover up to keep exposures single phase.


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 Post subject: Re: Open Air 3 phase arcs less than 15kV
PostPosted: Tue Mar 17, 2015 6:08 am 

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I am facing the same questions. I work for an electric utility so I am looking at phase spacing (arc gaps) that are much larger than that of the IEEE empirical method. We have decide to use ArcPro results only for 1LG, because we have been told by an "OSHA expert" that the 3LG data is shaky at best.

We will use cover-ups on other phases to limit faults to 1LG. My question is how to address true deltas where there are no 1LG faults. I still need to perform a calc. Does it make since to use the IEEE method and use an arc gap that is within bounds even though actual phase spacing is much larger?


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 Post subject: Re: Open Air 3 phase arcs less than 15kV
PostPosted: Tue Mar 17, 2015 8:49 am 
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Note that the appendix we are discussing does not use wording that suggests the only single phase exposures are from ground faults. I would consider a line to line fault on your ungrounded delta system to be a single phase exposure, just as a load involving only two lines is considered single phase. Cover up at least one phase to limit exposure. Note that Tables 6 and 7 are not applicable, and can not be used.


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 Post subject: Re: Open Air 3 phase arcs less than 15kV
PostPosted: Fri May 12, 2017 1:22 pm 

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ARCPRO can be a solution for SLG overhead arc calculations but its a brutal program to use because its one calculation at a time. I've used autoit coding to speed up massive runs for ARCPRO but its complicated if you aren't familiar with command prompt coding... it basically does the clicking and typing for you so you can run 500 calculations through arcpro from a list. Without that itd be very difficult to do then entire arc flash study on a distribution system.

Regarding the initial question, I think the answer is to only consider single line to ground arcs for overhead (coverups on other phases) and use IEEE 1584 for the calculation. IEEE states you can use the equations for SLG calcs because the results are often more conservative. For underground or transformer terminals where the spacing is smaller, three phase calcs can be done with the conductor spacing.


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 Post subject: Re: Open Air 3 phase arcs less than 15kV
PostPosted: Sat May 13, 2017 8:38 pm 
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Remember that it's an annex...not a "regulation" per se, just as 70E is riddled with informatonal notes and several Annexes that aren't Code as such.

As to SLG vs. other types of arcs, there have been tests done and what was found is that when the conductors are all near to each other any type of fault rapidly escalates into a full blown 3 phase arc within 1-2 cycles. EPRI has tested and provided empirical equations up to arc lengths of 12". They've done research but nothing definitive yet above 12". Generally speaking we'd expect incident energy to be almost linear with arc length for obvious reasons (exposure) but as the distance increases, all bets are off. EPRI has done some preliminary studies As the bus gap increases, the arc starter to show a lot of different properlies.


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