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 Post subject: ArcFlash question - Cyme and arcgap
PostPosted: Tue Jul 21, 2015 4:57 am 
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I work for a utility company and i have been asked to do an ArcFlash analysis on our 13.8 kV distribution system. Since i have never done an ArcFlash analysis i plan on using Cyme (this is what we use for distribution studies) to do the calculations. I plan on looking at the highest and the lowest fault current for each feeder then use the results for my analysis. It looks like Cyme uses NESC Table 410-2 to do the calculations based on a 15-in separation distance and a 2-in arc gap.
My questions are:
1. Does anybody use Cyme? Are the result reliable
2. The arc gap is 2-in. Is this the correct value to use for 13.8 kV overhead distribution feeders?

I have signed up to take the ArcFlash seminar in Sept.

Thanks


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 Post subject: Re: ArcFlash question - Cyme and arcgap
PostPosted: Tue Jul 21, 2015 9:40 am 
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Hi,
Since Cyme uses the NESC table, I don't think that would be acceptable to OSHA. See OSHA 1910.269 Appendix E, Table 3 for methods OSHA considers reasonable for an incident energy calculation method. Basically for your 13.8kV system, you will need ArcPro for single phase, open air faults and then IEEE 1584 equations for 3 phase faults.

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 Post subject: Re: ArcFlash question - Cyme and arcgap
PostPosted: Tue Jul 21, 2015 10:12 am 
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OSHA doesn't mandate one or the other. It's an annex for a reason. Let's just say it's a very strong recommendation. One where a significant justification for doing it any other way would be required.

But in this case, NESC tables except 410-1 ARE ArcPro. NESC never comes right out and states that but that's how the tables over 1 kV were calculated. The under 1 kV stuff is all documented extensively in NESC, and is more because it is based on testing on actual equipment than say IEEE 1584. That is why for instance the table defers to IEEE 1584 for 600 V switchgear and MCC's, which is what IEEE 1584 is intended to cover.

And OSHA's recommendation is for using ArcPro above 10 kV, REGARDLESS of whether it's 3 phase or single phase, and ArcPro itself contains multipliers for "box" and three phase conditions. Note that IEEE 1584 does not contain "enclosed" cases above 1 kV either...it's all "open air", and IEEE 1584 stops at 15 kV...good enough in this case but not for say a 69 kV subtransmission line.

And lets be clear here that we are referring to the IEEE 1584 empirical calculation for circuit breakers. IEEE 1584 also includes the Lee theoretical model as well as some empirical models based on current limiting fuses from one manufacturer. The same OSHA annex accepts but in no way recommends Lee and doesn't even mention the empirical fuse models.


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 Post subject: Re: ArcFlash question - Cyme and arcgap
PostPosted: Wed Jul 22, 2015 3:41 am 
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While the tables in NESC are done by ArcPro, they used 18 inch working distance for gloving while OSHA states that a distance of 15 inches is to be used. Therefore the values will be higher at 15 inches.

And yes it is an appendix that OSHA provides that "...contains information to help employers estimate available heat energy as required by 1910.269(l)(8)(ii)." I would think one would have to have very strong justification to use methods outside of the ones recommended by OSHA.

Not sure where the using ArcPro above 10kV came from as OSHA Appendix E Table 3 has ArcPro for single phase up to 15kV, then for everything about 15kV.

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 Post subject: Re: ArcFlash question - Cyme and arcgap
PostPosted: Wed Jul 22, 2015 7:17 am 
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Did some more digging. The key is to look at the extremely long final rule text which shows all the justifications. TVA proposed using 18" based on the same anthropomorphic data that I would assume IEEE 1584 used and the same data I've looked at myself before. OSHA disagreed and the fundamental difference comes down to this. Everyone can agree more or less that if you are working in enclosed gear, there will be a screwdriver or finger tips involved. In overhead line work, the employee is grasping the cable directly in some cases, so OSHA felt that the distance between the finger tips and the crotch of the hand between the thumb and fingers (2.4") should be included which reduces the distance to 15". For enclosed equipment, it stays with the IEEE 1584 recommended distance of 18". Since OSHA 1910.269 is primarily focused on outdoor, overhead equipment, they chose to use 15" as the basis for comparison in all of their tests.

See:
http://www.dol.gov/find/20140401/2013-29579.pdf

Starting around page 608.

Note that you can still use CYME (NESC) data but you'd have to scale it for the shorter distance. This is easily done because the exponent will be 2 for open wire work. And also note that when using a hot stick, OSHA agrees that the 15" rule goes out the window. All my calculations with hot sticks drop the incident energy way down to almost a non-issue.

As far as model acceptance goes, Table 12, page 616, is a summary of what's in the text. Suffice to say that for 601-15 kV, OSHA accepts Lee, IEEE 1584, AND ArcPro for single phase arcs although obviously ArcPro is going to produce the lowest value. For 3 phase arcs, they only accept IEEE 1584 or Lee and IEEE 1584 will produce the lowest results. Above 15 kV, its ArcPro all the way.


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 Post subject: Re: ArcFlash question - Cyme and arcgap
PostPosted: Wed Jul 22, 2015 10:02 am 
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Thanks for the information. I am reading the articles at this time. In the meantime can someone tell me how to know, or calculate, the correct airgap? I used 2 in. Should it be larger?


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 Post subject: Re: ArcFlash question - Cyme and arcgap
PostPosted: Thu Jul 23, 2015 6:22 am 
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OSHA 1910.269 Appendix E gives information on how to calculate arc gap. For under 15kV 2" is shown, however I do know that some utilities have taken a more conservative approach to use insulator length, as increasing the gap increases incident energy

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 Post subject: Re: ArcFlash question - Cyme and arcgap
PostPosted: Thu Jul 23, 2015 7:57 pm 
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Probably not a bad idea. Realistically an arc across two lines will have a very long arc/air gap but will be propelled away from the work space. So mostly equipment gaps apply. ArcPro is valid up to 12". Above this range there are some EPRI papers but that's it.


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