NFPA 70E Question MCCs

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mikecj

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Guys,

NFPA 70E Table 130.7(C)(15)(A)(b) provide arc flash PPE categories for various equipment. For motor control centers there are two entries

600-V class motor control centers (MCCs) - Parameters: Maximum of 65 kA short-circuit current available; maximum of 0.03 sec (2 cycles) fault clearing time; working distance 455 mm (18 in.) Arc flash PPE Category - 2, Arc flash boundary 5 feet

AND

600-V class motor control centers (MCCs) - Parameters: Maximum of 42 kA short-circuit current available; maximum of 0.33 sec (20 cycles) fault clearing time; working distance 455 mm (18 in.) Arc flash PPE Category - 4, Arc flash boundary 14 feet

So am I correct in assuming that the lower the short circuit current or the longer the clearing time, the more hazardous the situation?

Be gentle. I'm a chemical safety guy trying to figure electricity :happysad:

Thanks,

MikeCJ
 

GoldDigger

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Generally yes
Actually the higher the short circuit current the higher the danger as long as the clearing time remains the same.
There is a seeming paradox in that if the fault current (not the short circuit current) goes low enough it can get into a region where a decrease in the current can delay the clearing long enough that the total energy is higher.
 
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