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1. DC has not been nearly as rigorously tested as AC. And there are several variables that are very difficult to quantify. The state of the art for DC is somewhere around the same point that AC was in the 90's with regards to arc flash studies, and the existing studies are quick to point out that DC estimates are problematic at best. As of right now the equation in Annex D.8 is similar. Two examples of problematic areas:
A. Arcing time. In many situations such as battery buswork failures, the arcing time is extremely long. Many practitioners are borrowing the 2 second rule from IEEE 1584 but there is no support for this approach from a technical point of view.
B. Short circuit current and voltage. Both are problematic to identify when the source is no longer an infinite source as can usually be assumed in AC. This leads to modelling the source as a voltage or current source plus an internal resistor in order to derive a current. And if the source is extremely time varying such as a capacitor or inductor the better modelling approach is to model the amount of energy in the system rather than the continuous rating. However as far as I know there are no published equations for this approach and the end user is on their own.
2. The values used in the tables are so far away from typical equipment that the table is junk. 99% of the DC equipment out there, even the stuff above 50 volts, doesn't even get to 1.2 cal/cm^2. Since the stable starts out at a fairly high H/RC level that is completely unrealistic for most cases, it is unrealistic. A better approach would have been to do the same thing as what's in the rest of the table and that is listing specific typical equipment with perhaps one or two "catch-alls" at the end. The typical items that exceed 50 volts are substation/UPS battery packs (most often 125 V), internal DC busses in drives, DC motors/drives and DC components of motor systems such as synchronous motor armature circuits and generator exciters, DC trolley systems, welders, and electrostatic precipitators. Internal resistance on DC batteries is very low but nearly always enough to make it impossible to reach 1.2 cal/cm^2 even in fairly large battery strings. Drives are similarly limited by their storage capacity and either the I^2*t rating of the semiconductor fuses or of the semiconductor valving, leaving only the outside chance of a DC motor circuit as a serious problem and only if the drive protection logic doesn't pick up on a failure quickly. Welders (equipment) can put out a significant amount of thermal energy and that's why welders (people) wear fire retardant PPE very similar to H/RC 1, if not identical materials. So it becomes immediately obvious with a welder that the appropriate PPE would be H/RC 1.
By way of example, I have several very large systems here spanning the extremes from 50 kV outputs (precipitators) to 90 V outputs at 6000 A. Despite these very large numbers, the arc flash rating using Annex D.8 ends up being less than 1.2 cal/cm^2 for these.
3. Due to all of the above, even with the published results, at this point it can hardly be said that there is a consensus opinion on quantifying DC arcing faults at all. The material in NFPA 70E in my opinion for arc flash should have stayed in an informative annex until such time as the results are rigorous enough to move to the body of the standard as far as arc flash. Shock is pretty well known and is addressed appropriately.
The problem is that if you require labels AND then keep the labelling requirement as it stands in 70E, there are major problems because the label requirement today acts as if arc flash can be reasonably quantified enough to use either the tables or an engineering calculation but as of today, those methods are suspect. The most appropriate (as in least liability) label today would be to use a generic label ("Warning! Arc flash hazard") with no quantification as to the hazard. This is problematic obviously in an of itself because it leaves the end user (electrician) with a warning but no guidance on what to do about it, a situation that is even worse than no label at all. So today, the 70E standard should support 2 possible options. DC arc flash hazards are in my opinion nearly at the same point today as arc blast...some preliminary work has been done about it, we have some engineering estimates and calculations, but nothing approaching definitive or consensus. It's just not ready to be addressed by any standard greater than putting details in an informative annex and addressing it via minimizing exposures. If an end user wants to take on the liability and responsibility of doing an engineering calculation, then they can take on the voluntary responsibility to label equipment. That same end user can also just as easily make a case for lack of consensus opinion about the size and scope of DC arc flashes as well, unless someone wanting to be "proactive" attempts to mandate requirements based on poorly understood information. This is akin to where we were with 70E in the 2000 edition (remember that thing?) when someone decided that it was a good idea to stick a requirement and a reference to it in NEC. It wasn't until the 2004 edition before 70E was at a level of usability that it is today, and even modelling software at that time really wasn't ready.
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