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Terminating a shielded cable with a VFD & spindle?


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Somewhat extinguished member
8 Nov 2014
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West Yorkshire
I would tend to agree. Calling out specifics I would say there is a current risk that there is no CPC protection as it appears that the incoming earth connection is currently connected to what looks like an isolated screw. The diagram provided would also bear this out. This potentially means it is possible that if a live conductor touches any metal part of the drive it could become live and present a hazard.

Having said that - if the diagram shown relates to the equipment being discussed then the earthing is fairly clear.

As far as electrical isolation, there is nothing shown that indicates any isolation of the outputs to the input supply. A rectifier does not provide isolation.
CPC (if I understand correctly) is the direction I'm heading at the moment.

Just to be clear (to all) the photos posted of the current wiring aren't mean to be "how it should be"; they're "how it is so far and I'm seeking advice".

Instead of connecting the incoming mains cable earth and outgoing spindle cable earth cores together to a terminal inside the VFD I'm thinking of bringing these out to a single ground bus; such that I can have the CNC controller enclosure grounded via the bus (ensuring all items are connected in a star pattern to a single ground wire that goes back to the mains supply).

With an earth core being present in the VFD-to-spindle cable (and a mod to the spindle to ensure the ground pin is appropriately connected) all metal parts of the spindle will be earthed. By virtue of the fact there's metal-on-metal contact between the spindle and the CNC machine body, the body will also be earthed. I could add a separate earth line between the body and the ground bus, but that would create a loop (ground bus to spindle, through body, and back to ground bus). As there's no mains voltage going to the CNC machine (only 24VDC) I assume probably wouldn't make sense to do that then, as if for any reason there was not continuity between the spindle and CNC machine body (thus removing the ground connection from the CNC machine body) it would also mean there would be no chance of the body being live with mains voltage.


Established Member
19 Feb 2015
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North Cumbria

Go back to basic's,

The CPC / Earth has just a single purpose and that is to provide a path for a potential current to flow under a fault condition so that the protective device can disconnect within a specified time.

Bonding is used to ensure all conductive materials are connected to ground often through the MET to ensure that they cannot rise above ground potential or that any potential difference can occur between conductive materials during a fault condition.

Shielding is to prevent cables, components and systems from transmitting or being impacted by electromagnetic interference, done by grounding cable shielding and metal screening.

So now look at what you have and think in the above terms and apply to any fault conditions.