Quote Originally Posted by Jonathan View Post
It also adds a bit of redundancy to the system.
Understand your thinking and would agree if this was a just a normal motor and not run near stepper wires but because it's high frequency not so simple has just using the spare wire and running back to VFD earth without potential for creating ground loops if not done correctly.

Seen has often the spindle cable and Z & Y stepper cables often run in the same cable trunk then noise from the VFD could get introduced across to the stepper wires so If this is going to be done then make sure that earth goes to the main Star earth point in your control box other wise the potential for ground loops increase.
This then becomes a bit more awkward using the 4th wire has often the VFD is not in control box and that is why I prefer not to do this.!
I've never had a problem with shocks and even when my cable broke and dead shorted the spindle the fuse and trip worked, unfortunately the damage was done by then.!!

Not saying don't do it but make sure you do it correctly other wise you'll be pulling your hair out chasing ghost issues, like false E-stops or limit trips, missing steps etc which you won't automaticly realise are coming from this.?