PaulEngr wrote:
The values in Table I are WAY off from typical DC systems. I've run through the calculations for a lot of different conditions and actually reaching a point exceeding 1.2 cal/cm2 is actually pretty rare with most practical DC systems. That is combined with the fact that most systems are <50 VDC and of the "high voltage" variety, the most common is 125 VDC. By way of example GE MD 600 and 800 series DC motors that go up to the MD824 as the largest have a steady state DC voltage of 90 VDC, albeit at a stall current of 6000 A and obviously the DC drives feeding it have an output limit of a little higher than that.
At the "high" end we get into for instance electrostatic precipitators (ESP's) that typically run at around 50 kV but the corresponding current is very low (milliamps) leading to very low arc flash almost without limit for shorting ot, even though it is designed to occasionally short itself, and runs very close to limits at times.
Most of our applications are mixed AC and DC. We do a lot of common-bus systems where we have the DC bus (typically around 650V) supplying the inverters in a system. We also do Grid Tie and power conditioning systems where we have the DC bus that may be as high as 1000V.
I always calculate both components and use the higher of the two on the label. It is typically the AC that is the higher of the two, but we do have some battery enclosures where the DC incident energy is the higher value (around 22 cal/cm^2)
