I have calculated the arcing current for a 900 volt supply and came up with an unexpected result. The bolted fault current is 20kA and the current in the arc came back as 24.88kA. I checked this with the web arc analyser and came back with the same result. Is there an explanation? Surely arc current cannot be greater than the available short circuit current?

I am using the following parameters; 900volts, MCC panel, 32mm gap, 457mm working distance, grounded supply, 20kA bolted current.

Any ideas?

Mike

Congratulations! You've found one of the few anomolies with the 1584 equations! For more information, you should read http://us.ferrazshawmut.com/arcflash/getting_help/downloads/ICPS-wilkins011904_web.pdf

Hopefully these will be ironed out with the upcoming (someday, soon, sometime, almost here ) NFPA/IEEE effort.