The issue would seem to be the 2 poles of the power switch not contacting exactly simultaneously on switch-on, hence there's an imbalanced current surge going into the filter caps. With all the switch mode supplies in my control box I reckon there were probably 4 sets of EMI filters in my control cabinet, not just the VFD, even with the VFD isolated it would still trip out 2 times out of 3. Apart from the ball-ache of trying to bypass all the filters I'd rather install the correct RCB and keep the filters in as I have other EMI sensitive gear in my workshop, and like to listen to the radio as well!