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 Post subject: DC Calculations on Photovoltaic Systems (>600V)
PostPosted: Tue Sep 23, 2014 4:54 am 

Joined: Thu Mar 13, 2008 4:23 am
Posts: 25
Location: Canton, OH
My company has been asked to provide arc flash labels on the AC and DC side of a large scale, utility, solar array. We have no problems providing labels on the AC side of things. When it comes to providing calculations and labels for the DC equipment, we have concerns because of the lack of an industry wide consensus standard for calculating incident energy on DC systems. We realize NFPA 70E D.8.1.1 (Maximum Power Method) and D.8.1.2 provide two methods for estimating DC incident energy, however this is in the Informative Annex and not the code.

I'm curious as to if other PE's have had concerns of their own with regards to stamping and providing calculations and labels for DC systems (specifically large scale solar arrays). I'm also checking to see how our insurance company/professional liability insurance views us performing work where no recognized standards exist.

Any thoughts?


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 Post subject: Re: DC Calculations on Photovoltaic Systems (>600V)
PostPosted: Tue Sep 23, 2014 3:19 pm 
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Joined: Mon Sep 17, 2007 5:00 pm
Posts: 1499
Location: Scottsdale, Arizona
It can be difficult issue. I included DC arc flash in my training beginning early 2010. My rationale was that something needs done so I used the method in Dr. Ammerman et. al. paper I later included DC in the book I wrote and prefaced it with the following text extracted from the book:

----text from Chapter 12 - Complete Guide to Arc Flash Hazard Calculations Studies -
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.
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.

Since then, several software companies have now addressed DC arc flash, many with the same equations. So, I'm not sure what others are doing but programs are addressing DC based on the best info that we have at this point in time.


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 Post subject: Re: DC Calculations on Photovoltaic Systems (>600V)
PostPosted: Tue Sep 23, 2014 10:36 pm 
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Joined: Tue Oct 26, 2010 9:08 am
Posts: 2174
Location: North Carolina
Duke had Kinetics do some exploratory tests. Ammerman's equation grossly overestimated the incident energy. Ammerman's method modifies the maximum power transfer method, itself an adaptation to DC of the same theoretical assumptions underlying the AC Lee method. Ammerman's eliminates thing voltage=current issue in the Lee method but probably misses the fact that conversion to radiant heat is not 100% efficient and to date is empirical. The equation used in Ammermans method also underpins the AC time domain model which has been shown to fit the IEEE 1584 data set much better than the IEEE 1584 empirical model, which seems to validate the concept, if not the results. A lot of this information is published in Battcon proceedings which are publicly available and free. A google search for DC arc flash readily turns them up.


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 Post subject: Re: DC Calculations on Photovoltaic Systems (>600V)
PostPosted: Fri Sep 26, 2014 2:04 pm 
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Joined: Tue Jan 13, 2009 5:00 pm
Posts: 532
So how much fault current can a photovoltaic cell produce at high noon on a cloudless day during the equinox at the equator? Rated current? Is there really anything here to label?


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 Post subject: Re: DC Calculations on Photovoltaic Systems (>600V)
PostPosted: Mon Sep 29, 2014 6:53 am 

Joined: Thu Mar 13, 2008 4:23 am
Posts: 25
Location: Canton, OH
Thanks for the replies.

stevenal - Each string of modules produces 9.27A of fault current. There are combiner boxes in the yard which the owner has asked us to label. At the combiner boxes we are looking at 9.27A x 20 strings in parallel = 185.4A @ 874V DC. The combiner boxes and inverters are the only DC equipment they have asked us to label.

We are in the process of 1) doing research ourselves to determine the magnitude of exposure using the different methods PaulEngr mentioned below, and 2) educating the owner about these studies and what code requires and what the real dangers are.


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 Post subject: Re: DC Calculations on Photovoltaic Systems (>600V)
PostPosted: Mon Sep 29, 2014 9:48 am 
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Joined: Tue Jan 13, 2009 5:00 pm
Posts: 532
So even if you run it out to the two second cutoff, any method you choose will indicate no hazard.


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 Post subject: Re: DC Calculations on Photovoltaic Systems (>600V)
PostPosted: Mon Sep 29, 2014 6:57 pm 
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Joined: Tue Oct 26, 2010 9:08 am
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Location: North Carolina
Incident energy at 2 seconds should be very low, even on theoretical equations. The sticking point is that DC arcing is theoretically infinite. So similar to AC in a confined space, we have to decide whether the 2 second cutoff applies because a victim can escape and/or is propelled from the area (arc blast). With DC battery systems, some of the Battcon papers make the argument of "show me the victims" which should validate the presence/absence of a credible hazard. I think the same applies here. There are lots of examples of AC victims but even in the hey day of large DC motors and generators and concerns about flash overs, I don't recall arc flash victims. Biggest DC hazards I know of are muscle lockups (can't escape shock/burning) and the hazards of a commutator flying apart if it is allowed to wear down to the banding.


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