I had a similar problem when I commissioned my machine (Sunfar VFD driving JGH-85 spindle); the RCB tripped out about 40% of the time when switching the control box on with the VFD isolated and 100% of the time with the VFD switched on. I did a lot of prodding and poking and the issue seemed to be the EMI filters and the capacitors to ground therein. If you're switching the mains with a mechanical double pole switch, the 2 poles don't make at exactly the same time, so for a short instance you're connecting a cap from one leg (L or N) to ground = current imbalance = trip out. The only sure-fire way I found to solve it was changing the RCD for a type F device which also handles HF current draw to boot which should solve any problems when running. Strange that your RCD is straight after the meter, surely there's a master fuse in circuit? If so a qualified sparky would be able to legally pull that to change the RCD thankfully I have a good mate who is one :-)