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 Post subject: Smoak & Eblen Paper
PostPosted: Tue Dec 31, 2013 12:44 pm 
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I have come across a statement in a report the says:

"Testing conducted by Malcolm Smoak, PE of SWEPCO and Marcia Eblen, PE of PGE have shown that single phase voltages of less than 300 volts fail to sustain arcs longer than 1/2 cycle even at gaps as small as 1/10 inch."

I have tried searching the web but have been unable to find more about this paper. Has anyone seen it? Know where I may obtain a copy?

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PostPosted: Wed Jan 01, 2014 3:02 pm 
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It's pretty common to find this. However, subsequent research has shown that it is inaccurate. Here is a link to a paper quoting the PGE material in more detail:

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=6&ved=0CHAQFjAF&url=http%3A%2F%2Fwww.efcog.org%2Fwg%2Fesh_es%2Fevents%2FESSG_Fall_11_Meeting%2Fpresentations%2F10-208V%2520arc%2520flash%2520calcs%2520efcog%2520fianl%252010-12.pdf&ei=Do_EUpWLHIa6yQH39oHoDw&usg=AFQjCNFQn4Vjgij7LgtLDwkbFb-5wcnDZg&sig2=FYqTqRbNx7sjphWt1Xd2Dw&bvm=bv.58187178,d.aWc&cad=rja

It is possible to initiate an arc even down to 4 kA under the right conditions even at 208 V. The PGE tests declared this impossible. Some subsequent research as I understand it showed that the IEEE "standard test" did not consider wire "fuse" diameter as a test condition, nor did it consider potential barriers or enclosure size. As these are varied, it is possible to get arcs to initiate even at very low fault currents.


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PostPosted: Thu Jan 02, 2014 10:03 am 
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Like many others in the industry, the work of Marcia that is referenced in this thread was ground breaking at the time. She continues to move what we know about arc flash in a positive direction. However, we continue to learn more about arc flash with ongoing research, testing and investigations all the time. Sometimes, new research and test results may cause us to rethink previous assumptions such as the minimum amount of short circuit current where a three-phase arc flash can be sustained. There still is a very limited amount of research regarding single phase arc flash compared to three-phase.

As we continue development of the next edition of IEEE 1584, [url='http://arcflashforum.brainfiller.com/threads/3102/'][See IEEE 1584 Update][/url] the “Question of the week” posted on December 15 was posted to help us gauge the success of what we have so far. It appears that of those that have had an arc flash event, the use of IEEE 1584 for the selection of PPE works. Thanks for the responses to this question and if you have not responded, your votes and comments are still welcome. [url='http://arcflashforum.brainfiller.com/threads/3099/#post-14408'][Use of IEEE 1584 Question][/url]

Thanks!

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PostPosted: Thu Jan 02, 2014 12:35 pm 
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Here's the one cited by NESC: http://www.epri.com/abstracts/Pages/ProductAbstract.aspx?ProductId=000000000001022218


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PostPosted: Thu Jan 02, 2014 1:52 pm 
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stevenal wrote:

This is a great one and I highly recommend it. Thanks for posting the link stevenal. Marcia made a presentation about this to the IEEE 1584 committee several years ago - excellent work. What she was questioning is that based on the IEEE 1584 calculations, incident energy is very time dependent. At a meter, if an arc flash occurs there isn't any upstream protection (except sometimes a very slow fuse on a transformer primary) so the duration could be quite large which should result in a very large number of severe and even fatal outcomes if an arc flash occurs. But she was observing that this wasn't the case. As it turns out, her tests revealed that the conductor in the meter base vaporizes so quickly, it self extinguishes and the incident energy is limited. Her initial efforts led to a more expanded body of work that ultimately found its way in the NESC.

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PostPosted: Thu Jan 02, 2014 8:39 pm 
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A quote from the work of Marcia

Quote:
The most extreme case (Test 29) had approximately six cycles of arcing. The IEEE 1584 approach predicts 3.5 cal/cm2 at 18 inches for a 208-V fault with a 40-kA bolted available fault duty. We measured much less than that.


The question then arises of whos measurements are accurate, the IEEE 1584-2002 or Marcia's? In any case, a look on arc flash and fire ball snapshots as well as pictures of scorched equipment from the article do not convince that network protector and meter arcing faults hazards on 120/208V systems can be neglected (and implicitly faced without protection) due to short arc duration and low incident energy output (max 0.2cal/cm2 at 18 inches reported).

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PostPosted: Mon Jan 06, 2014 9:21 am 
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Here is Malcom and Keith's paper.

http://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=6509021&sortType%3Dasc_p_Sequence%26filter%3DAND%28p_IS_Number%3A6508997%29

It was at the Electrical Safety Workshop paper. The main outcome was of specific equipment. It was not an evaluation of all voltages or all configurations. There specific equipment self extinguished at the voltages, configuration and current. Higher current or higher voltage or different configurations might not work this way. They also had the equipment open which could change the plasma effects on going from single phase to 3-phase.

Hugh Hoagland
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PostPosted: Mon Jan 06, 2014 9:28 am 
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I fail to see any implied suggestion that no protection is required. The NESC has tabulated values of protections, based in part on Marcia's work. Secondary networks feeding multiple services are common in big cities, and are worked live to keep service from being interrupted. The hazard of moon suits (reduced visibility, thermal issues, etc.) can be avoided if faults are self clearing and less restrictive PPE will suffice.


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PostPosted: Mon Jan 06, 2014 7:28 pm 
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I believe the biggest mainstream misconception in arc flash industry is that minimum of 1.2 cal/cm2 are required to cause 2nd degree burn. The view is widely cited in arc flash literature and standards. As an example, NFPA 70E year 2012 edition states that "a second degree burn is possible by an exposure of unprotected skin to an electric arc flash above the incident energy level of 1.2 cal/cm2 ( 5.0 J/cm2 )". As a matter of fact, even less then 0.2cal/cm2 incident energy (such as observed by Malcom and Keith) can very well cause 2nd degree burn or worse when delivered in sufficiently short time interval (see [1], [2], [3] for more information).

I generally agree with the opinion expressed in Malcom and Keith's paper that "the test results [...] generally support using single-layer flame-resistant clothing for this equipment" but one would have to convince me that the clothing will not set on fire during the incident. I would confirm with PPE manufacturer that the 4cal/cm2 rated PPE does not set on fire when charged with the energy it is rated to within 0.01sec time interval (the arc flash duration minimum set in IEEE 1584) prior to buying the PPE.

1. A.Stoll, "Heat Transfer in Biotechnology", Advances in Heat Transfer, v.4. Academic Press. 1967
2. http://www.arcadvisor.com/pdf/arc-flash-article-IAEI-2012.pdf
3. A. Brownell, et al., “Ocular and skin hazards from C)2 laser radiation”. Joint AMRDC-AMC Laser Safety Team, Philadelphia, Pensylvania, 1972

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PostPosted: Tue Jan 14, 2014 4:53 pm 
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Just to be accurate, testing that we (PG&E) did at 120/208V three phase showed that with bare electrodes in a box with just varying arc gap, we could sustain an arc for up to 10 cycles (in only one case) only when the initiating arc gap was 1/2". All other events self extinguished in under 1 cycle. Similar testing by Shawmut showed that with 1/2" gap and parallel vertical flat electrodes facing, they sustained up to 30 cycles. However that said, sustainability is extremely dependent on exact equipment electrode configuration, enclosure dimensions, and enclosure openings. We could not sustain in three phase meter enclosures nor could we sustain in network protectors for longer than 1 cycle. That said, 1 cycle is plenty long enough to eject heat and molten metal. I would not recommend wearing natural fiber in these situation because it can be ignited by ejected metal (PG&E had one documented case proving this). However, I would agree with the NESC table and say that 4 cal/cm2 protection is sufficient for all 12/208V equipment. Thanks...Marcia Eblen


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PostPosted: Wed Jan 15, 2014 12:08 pm 
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Thank you Marcia.

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PostPosted: Wed Jan 15, 2014 2:22 pm 
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mls2

How do you explain glaring disparity (3.5cal/cm2 vs. 0.2cal/cm2) between IE predicted by the IEEE 1584 model and measurements reported in the article?

Quote:
The most extreme case (Test 29) had approximately six cycles of arcing. The IEEE 1584 approach predicts 3.5 cal/cm2 at 18 inches for a 208-V fault with a 40-kA bolted available fault duty. We measured much less than that.

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PostPosted: Wed Jan 15, 2014 4:48 pm 
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I would think the key would be "predicted" versus "measured".

Sorta like the weatherman predicting partly cloudy and I measure 3 inches of snow.

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PostPosted: Thu Jan 16, 2014 7:56 am 
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Quote:
I would think the key would be "predicted" versus "measured".
Sorta like the weatherman predicting partly cloudy and I measure 3 inches of snow.


Granted, it is not the weatherman's fault that sometimes their forecasts are wrong. The weather can change in a few moments based on the atmosphere and other influences. They make their best educated guess and that is all they can do.

Contrary to weatherman predictions, the IEEE incident energy equation was reportedly developed from extensive statistical analysis. The formula gives a predicted value with a statistical 95% confidence limit (see "IEEE 1584 Guide for Performing Arc Flash Hazard Calculations" section 9 "Development of Model" for detailed information.)

In a 95 percent confidence interval, there is a 95 percent chance that the value of the parameter lies between the upper and lower confidence limits of the interval.

The way I see it, Marcia's findings question reliability of the IEEE 1584 IE equation.

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PostPosted: Thu Jan 16, 2014 9:19 am 
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arcad wrote:
The way I see it, Marcia's findings question reliability of the IEEE 1584 IE equation.


Go back and see how many <250V data points were used when the IEEE 1584 equations were developed.
The text from IEEE 1584-2002 section 9.3.2 reads:
"It was difficult to sustain an arc at the lower voltages. An arc was sustained only once at 208 V in a 508 mm × 508 mm × 508 mm box.... An arc was sustained several times at 215 V in a device box (100 mm × 100 mm × 50 mm size)."

"While the accuracy of the model at 208 V is not in the same class with the accuracy at 250 V and higher, it will work and will yield conservative results."


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PostPosted: Thu Jan 16, 2014 7:21 pm 
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[QUOTE=JBD]Go back and see how many <250V data points were used when the IEEE 1584 equations were developed. The text from IEEE 1584-2002 section 9.3.2 reads: "It was difficult to sustain an arc at the lower voltages. An arc was sustained only once at 208 V in a 508 mm × 508 mm × 508 mm box.... An arc was sustained several times at 215 V in a device box (100 mm × 100 mm × 50 mm size)."
"While the accuracy of the model at 208 V is not in the same class with the accuracy at 250 V and higher, it will work and will yield conservative results."[/QUOTE]

Indeed, the more points available for statistical analysis the better. The way I interpret the cited text, the several points collected by IEEE group sufficed the group to use the dataset to evaluate the confidence limit and include the <250V systems into the model. Looking into actual <250V points utilized by the group for the analysis as well as comparing calorimeter design and calibration may help to troubleshoot the close to 2000% disparity between Marcia's measurements and the IEEE model. Also, the same section 9.3.2 from IEEE 1584-2002 reads:

Quote:
Arc faults can be sustained at 208 V and have caused severe injuries with very high short-circuit current applications in meter enclosures. A meter enclosure is small and tends to confine an arc more than laboratory test boxes with no door. Used equipment at 208 V was not tested, but it is recognized that many types of equipment have relatively small open spaces between components, such as the space in a panelboard between the circuit breakers and the wall of the enclosure.


Arc flash and fire ball snapshots as well as pictures of scorched equipment from Marcia's article just don't line up for me with the reported incident energy values of 0.2cal/cm2 and less.

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