Cutler Hammer has, or will shortly have, an AFCI that can be used for troubleshooting, but only for their CH series panelboards. LED's will indicate if the breaker tripped on arc fault, short circuit, overload. You install it temporarily to get an indication what is causing the trip. When you're finished, you pull it out & take it with you.
If the breaker trips immediately, I'd suspect a ground fault problem, which could be neutral to ground or line to ground.
If the AFCI in question is a Cutler Hammer CH or BR, Cutler Hammer has a toll free number you can call for troubleshooting assistance.
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