On 2008-08-25, TWayne wrote:

Most likely you have a ground fault in the motor. It may be solid
or it may be intermittent. Have you tried measuring the resistance
from hot to ground for the motor?

Since Neutral and Ground are tied together back in the breaker box,
that's not going to prove anything without disconnecting the wires
and even then without a voltage to break down the gap it likely
wouldn't show anything but the resistance part of the fan motor.

That's why there is a (fairly expensive) device called a megaohmmeter
or "megger" which will apply a selectable DC voltage (e.g. 300V or
600V) and measure the resistance. Using it between the hot and frame
of a motor is a good way to test for insulation degradation in the
motor windings. (Never use the megger between the hot and neutral of
an appliance or load, they aren't designed to run on 300V DC!)

Cheers, Wayne