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 Post subject: Arc Flash Analysis for 28V DC equipment
PostPosted: Fri Dec 27, 2013 1:33 pm 
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Is it necessary to perform an DC Arc Flash analysis for 28V DC equipment. I am looking at an application that has a 36kW 28V DC generator which is used to charge a DC battery bank which in turn provides DC power to various pieces of equipment, panels, etc....

NFPA 70E states that an analysis is required for any equipment operating 50V or greater. Since this equipment in question is less than 50V, is a detailed analayis required, or can generic results be provided?

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PostPosted: Sun Dec 29, 2013 3:54 pm 
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An arc cannot be initiated at ANY current at that voltage. As the voltage increases it becomes possible at ridiculous currents above around 28-35 volts (depends on the electrode material). See pretty much any text book on neon lighting.


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PostPosted: Mon Dec 30, 2013 9:54 am 
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A detailed Arc Flash analysis would not be required and would likely give questionable results.

But should you still label the equipment with some level of PPE requirements?

I would think you would still be able to initiate an arc flash at that voltage especially with the current available from the batteries (welders or car batteries will arc when shorted) but it would not sustain long enough to create a blast.

Could you then have a scenario where you have to identify PPE based on the flash and not the blast, you could still have molten metal, high UV but not the high heat and pressure wave. (This assumes high currents available with the battery etc)

Safety Glasses , possibly face shield and gloves to protect from fall out of the sparks

Interesting to think about


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PostPosted: Mon Dec 30, 2013 8:10 pm 
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No. An AC shock fatality has not been recorded below 48 volts. Only one has been recorded by MSHA at 48 volts under extreme circumstances, and never for OSHA. For DC, there is a large body of research that was submitted as a public input to the 2015 edition of 70E which is going to raise the DC shock cutoff to 100 volts. So no shock hazard label needed. For arc flash, Hertha Ayrton developed the formula relating arc current, voltage, and arc length about 100 years ago for carbon. It was later extended to other materials. The key is that there is a constant, m0, below whjch an arc cannot occur no matter how short the gap or high the current gets. It is 36 volts for carbon and goes down a few volts for metals. it is barely possible to initiate an arc at 50 volts but arc gaps of around 0.1 mm and very high (10kA or so) currents are needed. You are approaching this with a large DC generator but the voltage is still toolow. Do not equate this with DC welders. Those have an open circuit voltage typically around 100 volts but sometimes even higher. As soon as the arc strikes, the voltage is current limited to a much lower value. The arc is sustained due to heating of the air which allows the arc voltage to rapidly decrease. See the very old uncopyrighted book Electric Arc Phenomena by Ewald Rasch as the much mkre uo to date reference (1913!) or The Electric Arc by Hertha Ayrton herself around 1895.


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