The 2012 Edition of NFPA 70E now addresses DC arc flash. Do you have any DC equipment where protection from the arc flash hazard needs to be considered?

• Yes
• No

Jim:

Yes.

Based on the information provided and available, DC Battery Work >100VDC may be able to sustain an arcing fault and create an arc flash. A DC arcing fault can expose the hands to thermal energy and a related burn. We need to consider other DC buses (e.g. locomotives, DC motor starters, drives, etc.) and the related voltage.

Regards;
Terry Becker, P.Eng.
ESPS Electrical Safety Program Solutions INC.
See my LinkedIn Profile

It's not as bad as what I would have thought once we put it in. I have several MD600 and MD800 series motors and equivalent generators in Ward Leonard loops (the 1960's version of servos at >1000 HP). Stall currents are up there around 4000+ A, but only at 40 V or so. Trying other points such as around 2000 A at 450 V results in nothing major to worry about.

One thing I've incorporated here locally is to consider Hertha Ayrton's formulas. There are more advanced formulas these days for DC arcs but this basic equation at least provides answers for determining if an arc is even possible or not. This eliminates the vast majority of the low voltage cases such as 6/12/24/48 V systems. Arcs can indeed occur in substation systems (125 VDC typically) but the available fault current is still not that great when you consider internal resistance of the batteries so the arc flash values are pretty small.

The tables in 70E in contrast consider both voltages are currents that are so high that they don't seem to be very common in practical applications even with large servo systems in the neighborhood of 5000-10,000 HP.

My question about DC arc flash has to do with hybrid bus drives using 700-900 VDC potential. What is the arc flash exposure and what level of protective gear is needed for disconnect and connect.

Please put your question in the appropriate forum. The answer requires some additional effort on your part.