Quote Originally Posted by JAZZCNC View Post
The 4th wire won't be connected. Personally don't bother with an earth onto spindle has the VFD and main machine frame are earthed
I thought the same until I got a shock off it. It's not good practice to rely on bearings to transmit the fault current to earth so I would always advise using 4-core cable to include an earth connection just by the spindle. The cost it's negligible - just one tapped hole in the Z-axis plate to screw the wire on to and you're done. It also adds a bit of redundancy to the system.