Re-post from last year on the same topic
I’m an electrical engineer and the round plug is called the ground and that short for bonding ground. It keeps you from getting shocked.
It connects to any metal box that holds electronics.
The
If you have this kind of J box you can just connect ground wire to the outside of it and you’ve got the same ground.
People generate a lot of static electricity, so if you make a static ground, you should put a small plate by your CNC and touch it when you come up to your CNC. Yes, you may get a little shock. You could also buy a static discharge wristband this would keep you from discharging straight into your CNC and possibly hurting it.
This is just scratching the surface of a very big topic.
Working for NASA. We have lost rockets because of static discharge (not lately). Have damaged lots of equipment It’s a big problem.