NFPA 70E arc flash calcs. 2

[mpparent]

All,

Hopefully someone familiar w/ this subject can answer. Firstly, the arc flash calcs. in the NFPA are incomplete in my opinion, in that if you have a fault current level below 16kA(the equations are only good from 16kA to 50kA), how do you calculate quantities for values under 16kA?(I'm speaking specifically of the "distance for curable burn" calculation). Also, when calculating incident energy, it's only dependent upon the amount of short circuit current available. I would have thought that the system voltage would be a variable, since, for instance, 13.2kV fault can be sustained much easier than a 480V fault. I'm not really believing the numbers I'm getting. Thanks in advance,

Mike

 

[busbar]

Might try:

http://www.arcwear.com/archaz.htm

 

[MarkAthay]

There are a couple of computer programs out there that can do these calculations for you.

The incident energy is indeed almost exclusively driven by arc current. The arc generates a voltage dependant on the current being drawn. Typically 80 to 200 volts. Contrary to popular belief arcs are NOT dead shorts. Voltage X current = power. 208 and 480 volt arcs look very much like very heavy loads to transformers. For instance to determine the incident energy from a 480 volt arc you MUST have one of these computer programs, PLUS a copy of the TTC curves for the transformer protection fuse. You ust build an AC model with the transformer and system impedance. Run the fault, feed in the arc voltage and current. Re-calculate the fault current. Run it again. Eventually you'll hit the point where you can determine how long that fuse will hold in before it blows from a low-side fault. It's a VERY scary number for transformer secondary faults.

For higher system voltages, even a 200 volt arc looks like a dead short on a 12 kV system.

Mark in Utah

 

[jbartos]

Suggestion: Visit

http://www.ieee-pcic.org/safety/safety.htm

for more info, related equations and a little spreadsheet in Excel