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 Post subject: Are there any firms that will do DC AFSes?
PostPosted: Mon Mar 25, 2013 11:48 am 
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We are having a hard time finding any engineering firms that are willing to do a DC arc flash study for lower-level current. Are there any out there?


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PostPosted: Thu Mar 28, 2013 1:23 pm 
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I use SKM Power Tools for arc flash studies. The module used for DC analysis is a $5k adder for the software. And it's just not something that's done that often. In addition to the $5k up front cost, there's an addition $500-1k a year in maintenance fees. The frequency that the studies come up just doesn't justify the added annual and upfront costs of the software. That may be why you're having trouble finding a taker.


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PostPosted: Mon Apr 01, 2013 5:34 am 
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Many consulting firms are concerned with liability and are not willing to perform the study since there is no consensus standard that describes DC arc flash calculations. There are several papers that propose DC arc models but there has not been a lot of DC arc testing yet. My company performs field service work and system studies and we have used the avaialble DC arcing knowledge to determine how our workers need to protect themselves around DC systems, but we are not willing to perform DC studies for customers until IEEE 1584 includes a methodology for DC.


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PostPosted: Mon Apr 01, 2013 5:47 am 
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We've done several system studies involving dc levels ranging from 12Vdc to 600Vdc with several MW energy behind them. Some of these were for a nuclear facility where there is a PASSION for validating every step of the process. However, the tools, including SKM, don't integrate the ac and dc systems so you'd have to run the dc and ac separately to get the available fault current and system interface impedances, then iteratively to determine if clearing times on the ac side of a rectifier that might limit the dc fault faster than a device on the ac side. Of course if you change any configuration in the ac or dc systems, you need to do the whole manual thing over again. With stored dc energy such as batteries there is a new complexity and getting vendor data to support the manual calcs or analysis software can be tedious. There is a lot to be said for doing manual calcs by the IEC or ANSI method with a well organized spreadsheet. That way you can get a second set of eyes to review your work in detail and then comparing the results with the results of commercial analysis tools. If all this seems daunting, it is...like eating the elephant...one bite at a time, knowing that the individual problems are not that hard, get organized, and get going. If you don't have the time, or if the concern for liability is high, I can help to whatever level you need.


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PostPosted: Mon Apr 01, 2013 2:59 pm 
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The problem we face is that the DC table does not go low enough to cover our Solar test arrays which are on the magnitude of 10amps. The thoughts go like this: "You could make the assumption that any values of current below 1000A @ 250V is Category 0 but nowhere does it say this. Therefore, due to liability issues we feel the prudent thing for us to do is use worst case hazard of Category 1."


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PostPosted: Mon Apr 01, 2013 3:01 pm 
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This leaves me wearing a face shield to test a 1amp circuit which feels like overkill to me


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PostPosted: Wed Apr 03, 2013 4:55 pm 
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Our problems, as SunDog stated, is the liability issue. Even is the fault current is 6A and the system Voltage is 400VDC, no one is willing to sign off on a label for anything less than catagory 1. We need a third party to do an analysis.

Walt Mendenhall wrote:
We've done several system studies involving dc levels ranging from 12Vdc to 600Vdc with several MW energy behind them. Some of these were for a nuclear facility where there is a PASSION for validating every step of the process. However, the tools, including SKM, don't integrate the ac and dc systems so you'd have to run the dc and ac separately to get the available fault current and system interface impedances, then iteratively to determine if clearing times on the ac side of a rectifier that might limit the dc fault faster than a device on the ac side. Of course if you change any configuration in the ac or dc systems, you need to do the whole manual thing over again. With stored dc energy such as batteries there is a new complexity and getting vendor data to support the manual calcs or analysis software can be tedious. There is a lot to be said for doing manual calcs by the IEC or ANSI method with a well organized spreadsheet. That way you can get a second set of eyes to review your work in detail and then comparing the results with the results of commercial analysis tools. If all this seems daunting, it is...like eating the elephant...one bite at a time, knowing that the individual problems are not that hard, get organized, and get going. If you don't have the time, or if the concern for liability is high, I can help to whatever level you need.


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PostPosted: Wed Apr 03, 2013 5:06 pm 
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A lot of good discussion here. i will add my own bit about DC arc flash not being part of a consensus standard, I wrestled with this before I included DC arc flash calculations in my book on how to perform arc flash studies. The following is directly from the book and how I addressed this standards issue. (shown on this site under "Forum Sponsors" banner on the side)

[INDENT=1]The majority of ac arc flash calculations use the equations found in IEEE 1584 Standard — IEEE Guide for Performing Arc-Flash Hazard Calculations which was first published in 2002. Even before this standard was introduced, many companies were already performing arc flash studies. How could that be? Because prior to 2002, arc flash studies were based on the best available information at the time. This included various bodies of research and theory published in many well known technical papers.[/INDENT]
[INDENT=1] [/INDENT]
[INDENT=1]Perhaps you are familiar with the phrase: “History repeats itself ”. Well, history is repeating itself again. Similar to performing ac arc flash studies before 2002, dc arc flash studies are also relying on the best available information until a standard is developed.[/INDENT]
[INDENT=1] [/INDENT]
[INDENT=1]A technical paper that helped elevate the discussion of dc arc flash calculations is titled: Arc Flash Calculations for Exposures to DC Systems by Daniel R. Doan. It was presented at the IEEE Electrical Safety Workshop in 2007. This paper provides a theoretical approach to dc incident energy calculations based on the concept of the maximum possible power in a DC arc flash occurs when the arcing voltage is 50% of the system voltage. The equations from this paper will be included in Informative Annex D of the 2012 Edition of NFPA 70E.[/INDENT]
[INDENT=1] [/INDENT]
[INDENT=1]A subsequent paper titled: DC Arc Models and Incident Energy Calculations by Ravel F. Ammerman, Tammy Gammon, P.K. Sen and John P. Nelson (referred hereafter as DC Arc Models) was presented at the IEEE Petroleum and Chemical Industry Conference in 2009. This paper provides a comparison study of the existing body of research into dc arcs and arc flash modeling that has been conducted over the years. It also provides a series of calculation methods for determining the incident energy from a dc arc flash in open air as well as in a box. The DC Arc Models paper is the basis for the dc arc flash calculations outlined in the chapter.[/INDENT]

When I decided to add DC arc flash calculations to my training program in early 2010, the very first phone call after we announced it was from the President of SKM so they were early adapters in all this. Others have also adopted some of this. So the question is: Does one ignore DC arc flash and hope for the best since there is not an "official" calculation standard, or do they use the best known available information?

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Jim Phillips, P.E.
Brainfiller.com


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PostPosted: Wed Apr 03, 2013 7:49 pm 
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Great information Jim, thanks for that. I will check out Annex D tomorrow. We have presented our calculations in several different ways, but they don't have credence coming from us. We are engineering technicians and they think we are just trying to get out of wearing level 1 PPE (which we are, but only because it is overkill, not an effort to skirt true safety procedures). We need the evaluation to come from a third party as MrF mentioned.

I'm not sure I understand why many firms are willing to do analysis on a 1000A system, but not a 10A. Will no one do an arc flash analysis on a small system for us?


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PostPosted: Thu Apr 04, 2013 6:47 am 
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SunDog wrote:
I'm not sure I understand why many firms are willing to do analysis on a 1000A system, but not a 10A. Will no one do an arc flash analysis on a small system for us?


I am just guessing, but could it be that lower current potentials inherantly have longer trip times based on I2t?

Or possibly there just is not enough modelling for low-current systems?


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PostPosted: Thu Apr 04, 2013 8:21 am 
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Larry Stutts wrote:
I am just guessing, but could it be that lower current potentials inherantly have longer trip times based on I2t?

Or possibly there just is not enough modelling for low-current systems?


It is that IEEE 1584 does not include instructions for calculating incident energy, so no P.E. is willing to risk calculating it based on say the methods suggested by Jim. From what I have observed, a 2 second clearing time is usually assumed for an arc that has a very long exsistance, but there may be hesitance in useing that number too.


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PostPosted: Mon Apr 08, 2013 5:53 am 
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MrF wrote:
We are having a hard time finding any engineering firms that are willing to do a DC arc flash study for lower-level current. Are there any out there?

I am the Electrical Engineering Division Manager for AVO Training Institute. We are performing DC arc flash based on NFPA 70E-2012 annex D.8. We will be pleased to assist you. You may call me directly at 214-331-7347. As you may know IEEE 1584 equations are empirical and only valid for the test parameters. Other AC calculations must use other equations such as such as those identified in annex D of NFPA 70E as acceptable methods. The situation for DC is similar. Just as AC methodology and IEEE 1584 are evolving with new testing, DC arc flash will evolve as well.


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PostPosted: Thu Apr 11, 2013 9:46 am 
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Sounds great. We will call you next week.


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