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 Post subject: How to model DC Arc Flash in an UPS?
PostPosted: Thu Mar 24, 2016 9:21 am 

Joined: Wed Mar 23, 2016 4:07 pm
Posts: 5
Can someone who has done it help me with a DC Arc Flash calculation for UPS units?
I'm modeling in SKM as:
-battery (do I model all 24 batteries as one, or individually?)
-followed by a DC cable &
-DC breaker,
-followed by a DC cable &
-inverter.

So I'm trying to get the arc flash just before the inverter. Trip time should be determined by DC breaker.

I’m not familiar with some of the terminology used in the battery component in SKM. (I've highlighted these fields in attached PDF)
• ‘Number of Cells’. Is this number of batteries in a string?
• ‘Num Positive Plates’. Spec sheets that I found don’t have this info.
• ‘Rated Voltage’. Is this the overall DC voltage of the UPS?
• ‘Open Circuit Voltage’.
• ‘Battery Rating’. Is this one battery or the total batteries?
• ‘Circuit Resistance’.
• ‘Circuit Inductance’. How do you look it up? For example, a #4 AWG wire?

Appreciate any feedback! :) :) :)


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 Post subject: Re: How to model DC Arc Flash in an UPS?
PostPosted: Mon Mar 28, 2016 4:54 am 
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Joined: Tue Oct 26, 2010 9:08 am
Posts: 2174
Location: North Carolina
Never used that. Cells have around 1.2-1.5 V per cell depending on chemistry used so what you are thinking of as a "cell" may be a battery itself and they generally are. For instance a125 V substation battery contains 60 lead acid cells in series. See Battcon papers though on actual measured DC arc flash vs. theory. There are two popular formulas. The one is just Lee for DC (0.5*V*Ibolted). The other is from Ammerman et al ahat incident energy isn't a problem. Generally in a UPS either the arc gap is yoo wide to systain an arc or battery onternal resistanxe prevents enough ennd looks at the power arc using the dozen or so measured DC arc papers and is an iterative nonlinear calculation.. Both do not model arc extinguishment. Both over predict by 200%+ on stable arcs and totally fail on unstable ones. You can used Ayrtons equation easily to estimate if an arc is even possible. The Battcon papers have data showing DC arcs under 130 VDC are usually impossible or so short lived that it's below 1.2 Cal/cm2. Arc gaps are often a limiting factor as well. Even in a large 10 MW synchronous condenser I looked at a few years ago, incident energy was under 1.2 Cal/cm2 but SKM does not model it.


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 Post subject: Re: How to model DC Arc Flash in an UPS?
PostPosted: Mon Mar 28, 2016 2:57 pm 
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Joined: Tue Jan 19, 2010 2:35 pm
Posts: 138
I don't use SKM, but I'd model all the batteries as one battery.

Here's the deal with AGM UPS batteries (I'm assuming these are absorbed glass mat batteries). You have to assume a lot of stuff. First, that they're all in perfect condition and will produce maximum current during a fault. That's a big assumption. Then there are the inter-cell connections. Those are often okay, but if they are not in excellent shape, that slight increase in resistance can have a major effect on the max possible current. Then there's the state of charge. Was the chain just equalized? If so, the voltage is likely higher than the float voltage and that has an effect. The big unknown is the battery's internal resistance. If you have one battery in the chain that's had a big increase in internal resistance, there are lots of things that can happen. Here's what I've seen. UPS battery chargers generally just measure the entire chain voltage and not each individual battery's voltage. Suppose you have one battery that's going south, it can drop much more than it's rated voltage across it's terminals. If you had two 12 volt batteries failing, the voltage drop across those batteries, as a percentage of the entire chain's voltage, could be large. In turn, that would effect the whole chain's state of charge. So the remaining batteries would not get their full charging voltage. That lower charge means a lower fault current. There are so many factors to consider when trying to calculate the DC side of this.


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 Post subject: Re: How to model DC Arc Flash in an UPS?
PostPosted: Mon Mar 28, 2016 3:14 pm 

Joined: Wed Mar 23, 2016 4:07 pm
Posts: 5
Sorry I should've specified that I'm using lead acid battery.

How do I find 'Cell Interconnect Resistance/Inductance'? I couldn't find it on spec sheet.

If manufacturer's spec sheet gives 8-hr rating of 70A, SKM seems to be able to calculate the short circuit current.
Is there a formula somewhere? (SKM is using DC Maximum Power Method, so arc fault current is half of the short circuit current.)


* Battery
* Enersys Datasafe 12HX300-FR
* http://www.enersysdatasafe.com/wp-conte ... s-2011.pdf
* TOTAL OF 24 BATTERIES
* Open circuit voltage is 1.300SG + .84 = 2.14vdc
* ‘Number of Cells’. 6
* ‘Num Positive Plates’. 6 per cell / 36 per Jar
* ‘Rated Voltage’. 12v Nominal

Thank you! :D


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 Post subject: Re: How to model DC Arc Flash in an UPS?
PostPosted: Tue Mar 29, 2016 6:24 am 
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Joined: Wed May 07, 2008 5:00 pm
Posts: 822
Location: Rutland, VT
Hi,

The intercell connector resistance is usually determined when bank is installed and resistance measurements are taken of each intercell connector.

One way of doing the arc flash for a battery bank/system is to use NFPA 70E-2015 Table 130.7(C)(15)(B). The spec sheet that you referenced has the Short Circuit Current listed as 3175 A. The above table puts that at a Arc Flash PPE Category of 1.

You can also use the formulas in Annex D of NFPA 70E-2015 for the dc. Using the short circuit value above, 125V, an arcing time of 2 sec and distance of 18 in results in approx 2 cal/cm2. If in an enclosure or box, it is stated to multiply by 3.

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Barry Donovan, P.E.
www.workplacesafetysolutions.com


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