Thanks Doddy,

1) That makes sense. I think I just have a pathological need to make sure everything is grounded as much as possible..!!

4) That's my plan - it looks like that connection is the only place on the VFD to earth it, so I may end up connecting that to both the spindle body via pin 4 and the mains gnd.

Not sure, haven't spec'd a contactor yet, but will look into it. At the moment I don't think I'm asking the UB1 to drive it directly? Its driven by the 24V PSU via the PNOZ.

Re Back EMF, from the VFD? If so I've added a question above about noise suppression above. I guess I'll also get back EMF in the 60V PSU, but was expecting the capacitor bank to help out with that, will that cause issues upstream?