The GFI is not so much sensing ground currents as such, but senses the sum of the currents flowing thru the line wires and neutral, and will presume any unballance, or unaccounted for current (sum <> 0) will be leakage current.
A little more info would be helpful. I presume these 2500a breakers are not new, and that the GFI trip settings have been calibrated? We've had problems in the past in new installations when started up. The GFI in the 480V main is factory set to minimum time delay, meaning instantaneous, and minimum current. So sometimes the starting of a large motor would cause it to trip. Having the unit calibrated to specified settings completely eliminated the problem.
Radar
