Ok so I have a question that I’m probably overthinking but I just thought I’d ask. I’m wiring a control panel for a cnc and the motor cables consist of 3 conductors, a ground, and are double shielded. Do I really have to run 2 ground wires to each cable (1 to actual ground wire and 1 to shield) or can I just twist the shield and ground together and run them both to the ground bar with a single wire ? Just trying to see if I can save time, wire, and room in the wire ducts and on the grounding bar. Thanks

 

Welcome to AAC!

On a CNC machine there will be high current signals to power the motors and low current signals from limit switches.
Hence you want to minimize stray signals and interference. Ground and shield should be kept separate.

Connect all shields to the machine frame and to earth ground.
Ground would be your 0V common to the power supply.

You can cheat by joining the two with one wire. The choice is yours but be prepared to face the consequences.

 

In a lot of cases where I have cables entering a panel, I will terminate them on a local terminal strip, which provides a separate terminal for shields and grounds, but are connected via shorting bars.

 

There are two schools of thought on the subject of keeping systems separate, as in CNC control, I have always been on the side of referencing everything to earth ground where possible, rather than keep every part separate.
The issue usually shows in DIY systems for e.g. where the system is made up of several pieces of equipment from different manufacturers, I hazard a guess they stress isolation as they do not know what other parts you are interfacing.
So far it has worked out for me.
There is also a paper from Siemens detailing equi-potential bonding and grounding of the M/C that includes a section on shielding.

 

On my CNC controller the limit switches are current loop and optically isolated. Hence you get two levels of protection. I still opted for shielded cable and kept it separate from COM.

 

Siemens Info.
Section 6 on shields.