Quote Originally Posted by RichR View Post
I'll check all connections and try the vfd with a long extension.
I've ordered some screened 3 core.Should I ground the screen at both ends?
Consensus changes from time to time. Current thinking supports providing equipotential bonding as much as possible and grounding both ends of the screen. (Incidentally, conventional consensus is to ground one end for susceptible cables, and both ends for emissive - the spindle (and steppers) are emissive).

This is about to move the conversation away from your question - I'm guessing that the electrical connections from the controller to the machine are only the spindle supply, stepper drives, and possibly limit switches. In which case, a single fault in the spindle could result in the whole machine presenting a dangerous voltage. This is one reason to ground both ends of the shield, to export an earth to the machine. Better still, provide export the earth via a heavy gauge wire from the controller to the machine - or use a 4-core CY/SY cable - you avoid the risk of earth loops then and you can ground both ends of the screen.