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 Post subject: Arc Flash with Battery Power?
PostPosted: Thu Mar 01, 2012 6:40 am 

Joined: Thu Mar 01, 2012 6:20 am
Posts: 1
I'm not going to pretend I'm someone or something I'm not -- I'm just an amatuer building an electric car and I want to ensure I don't die. Hundreds have done this before me, and none (that I know of) have died from it.... but in doing my research it's clear that assuming anything is a bad idea. So I wanted to run it by those that understand more than me.... you! I hope one of you will take pity and help me out. (for what it's worth, I'm confident on home-wiring... where one can work with the power disconnected. But not so confident that I think I can just have-at always-on battery packs)

I've got 48 LiFePO4 100Ah batteries (I'd link but the forum thinks I'm spam ... search for 100ah calib power cells) that I plan to wire in series for a total nominal voltage of around 150V, total pack energy of around 15kwh. From the reseach I've done to this point, I see most people mention that at less than 120VAC you really don't need to worry too badly about 'explosive' electrical faults.... but the info I've dug into on DC seems to be 'preliminary' at best.

So, really, I'm just looking to see if I'm dealing with enough power that going to kill myself due to some crazy explosion like I see on the various videos of commercial and industrial stuff (I saw a pretty impressive/scary one at 480V)... or if I can just proceed ahead with basic electrical safety (remove jewlery, check pack to ground to ensure infinite resistance before touching the pack at all, cover all connections/terminals when not currently working on them, avoid touching two parts of the pack at the same time -- especially with both hands, etc)

Any help you can give would be great....

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PostPosted: Thu Mar 01, 2012 6:20 pm 
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Joined: Tue Oct 26, 2010 9:08 am
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Location: North Carolina
DC arcing faults tend to be much more mild. The major difference is that DC doesn't tend to self extinguish as well. To do the calculation, you need to know what the available bolted fault current is, and the voltage. That is, what is the internal series resistance of your batteries? All battery manufacturers have this information if you ask. This should tell you the voltage and current. Then you need to know the maximum fault clearing time for the overcurrent protection that you are using.

Then there is a formula in 70E for DC arcing faults that determines the incident energy. They give a table in 70E for the required arc flash protection but as an example at 125 VDC it appears that you need to get to around 1000 A before arc flash becomes a serious concern. If you have a very low resistance system this could be an issue but it's not at least in the substation battery arrays I've checked on.

One more thing...below 50 volts, it is not possible to generate a self-sustaining arc no matter how high the current is. Hertha Ayrton's formula and data for a copper electrode indicates the minimum arcing voltage. Above 50 volts it is current dependent. Again, the resistance of the system matters greatly. So if you can live with a 48 VDC system then you really can just focus on whether or not you are going to get shocked into the next life. There's a formula for that too. Dalziel's equation relates current and time to the risk of stopping your heart.

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PostPosted: Tue Mar 06, 2012 5:29 am 

Joined: Fri Aug 26, 2011 7:38 am
Posts: 29
Location: Baltimore, MD
With an AC arc, 120 times per second (every half-cycle) the current falls to zero. At those "zero crossing points" an arc can extinguish itself AND NOT HAVE ENOUGH ENERGY TO RESTRIKE if the voltage is low enough (i.e. 208 volts). At 480 volts an AC fault will restrike, which is why 480 volts is qualitatively more dangerous than 208.

DC arcs will NOT self-extinguish, which makes DC more dangerous than AC vis a vis arc flash. That may be why automakers are standardizing on 48 volts- safety. I recommend you stay below 50 volts as well.

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