Both ac and dc motors operate with all three phases ungrounded. This allows for a single short to ground (ground fault) without stopping the equipment. Of course, if the first ground fault is present, it should be corrected as soon as possible before a second ground fault shuts down the equipment. For several decades, electrical codes have required the use of Equipment Ground Fault Protection (EGFP) on three-phase circuits above certain current levels.
Even if the code does not require EGFP, an early indication of a ground fault is useful for maintaining service to a continuous process. Most modern drives have ground-fault indication built-in. This can be used to alarm or trip the drive.
When a ground fault is indicated, the next job is to find the problem. If the detection is specific to one variable-speed drive, it is simpler to find. The problem can be with either the motor leads or a shorted motor. Stop the line. Removing the connections at the drive and meggering each of the three leads to the ground will prove the motor is faulty. Sometimes the problem is inside the drive. Removing the leads at the drive will isolate problems in the drive. Check bus work, capacitors and the power electronics.
Often ground faults are intermittent. These are truly difficult to track down. The advance notice provided by ground-fault indication will give you the time you need to find the fault over several scheduled shutdowns.