I don't know the codes in MN, but it seems that using a grounding bushing is going to falter the reason why the CNC manufacturer wants an isolated ground, that is, an independent ground separate to the buildings common ground through its structure. When placed, levelled and secured, its likely your CNC's chassis will be making electrical contact somewhere to the buildings ground. If you route a dedicated isolated ground from ground rod/s through an EMT connector bodies ground bushing, you now have a parallel circuit (x2) of grounding conductors. Notwithstanding this, any existing "noise" that might be present on the buildings system ground will now also be figuratively "heard" at the CNC's CPU.
Regardless of all this, if your feeding AC to the CNC from the buildings existing service, then you are not installing a "service" to the CNC. Rather, it's just a branch circuit. Grounding bushings are only needed to ground the incoming metallic raceway AT THE SERVICE....not at branch circuits/feeders.
Circuits must be grounded and bonded in accordance with the code. Supplementary grounds may be added to the code grounds.
In the Canadian code, that isolated ground must be interconnected with the main system ground.
It is common on some very large sites to see a dedicated isolated ground bus for electronic instrumentation systems.
The isolated ground bus is then connected to the main system ground with a substantial jumper.
Grounding sensitive equipment in that manner prevents ground currents from power faults elsewhere on the main system from affecting the sensitive equipment.
Bill
-------------------- Ohm's law
Not just a good idea; It's the LAW!
