It's not that they have ground faults...
It's that their wave forms are electronically perceived by GFCI circuitry as being faulted.
It's inherent in the design of a GFCI's 'balance' circuit that the hot is compared to the return.
VFDs create zany harmonics that rise above the threshold of instantaneous imbalance -- triggering a 'trip.'
Hence, one must install an Equipment Utillization Receptacle rated at a higher threshold.
"In the case of Heat Trace Cables, instead of using the Class A, 6ma Trip Threshold GFCI Devices for controlling excessive "Leakage", Ground Fault Protection for Equipment ("GFPE") Devices are used. The standard Trip Threshold for these GFPE Devices = 30ma (0.03A)."
Per Scott ^^^^ down at the end of the GFI Tripping thread.
