A 15Hp ,460 volt, 17.7 amp ABB drive, is tripping out on over current. The motor is running a fan in an AHU. I didn't start the diagnoses, but I got pulled in, because right now nobody can find the problem. Originally, the motor megged greater 2.2GΩs. So they replaced the drive, same problem. The motor was then replaced, same problem. Replaced the wire between drive and the motor, same problem. Replaced the drive again, same problem. The drive does not trip right away, but seems like there is a 7-10 day interval, between trips. Could dirty power do this? All lugs at the drive, and in the peckerhead are tight.Any other ideas as to what to look for?