I use
The standard symbols on my drawings to represent what a ground is. The nomenclature is so varied I believe the first decision should be how to describe what is what. For me the EARTH symbol is the green (Green/Yellow) wire that goes back to the circuit breaker panel and from there into a ground rod or in my case a ground plate in the earth. At the machine, at only one point, this Earth is connected to the frame of the machine. At that point the symbol in my drawings changes to the GND symbol with the horizontal lines. Any AC powered drives, motors etc. also connect their green (green/yellow) to this single point. I believe the code here specifies that this mounting post must either be double bolted or bonded (welded) in such a way that it cannot vibrate loose. The green wires must use ring terminals, not spade terminals so a loose nut doesn't allow the wire to vibrate off. If you have something that has AC (single or multiple phase) in it that is metal and could have an AC line short to the metal box, it too must have that green wire connect in a non-removable fashion to the metal box, and return back to that single point earth point. Not to some other metal part of the machine with hope that it will eventually connect to the EARTH connection. Gene mentioned the ramifications if it's not. If you have an isolation transformer from one of the 3 phase legs (or two phase in North American 240VAC power) where 240VAC is changed to 110VAC with the transformer, the output side of that transformer must again be made into a proper neutral with a connection to EARTH to that single point EARTH point. Simply because once it becomes Black/White/Green headed out and wired to a standard AC outlet, that Neutral has to behave in the same way it would as if it were embedded in the wall. Just because it's isolated from the mains doesn't make it any less lethal. And if the system has been taken apart, cleaned up, painted and re-assembled, it's critical that DC continuity is checked between the bolted together metal parts. I've seen systems that worked perfectly and then were stripped down, painted, assembled and made ready for shipping but developed the most interesting problem with electrical noise. And the cause tracked down to what was a good electrical connection now insulated with paint. IMHO. My bible for wiring these systems is this: Electromagnetic Compatibility Engineering By Henry W. Ott published by Wiley. https://www.amazon.com/Electromagnetic-Compatibility-Engineering-Henry-2009-08-24/dp/B01FIX96A8/ref=sr_1_2?keywords=Henry+W+Ott <https://www.amazon.com/Electromagnetic-Compatibility-Engineering-Henry-2009-08-24/dp/B01FIX96A8/ref=sr_1_2?keywords=Henry+W+Ott&qid=1577644003&sr=8-2> &qid=1577644003&sr=8-2 Page 119 of my copy, Section 3.1.7 is titled Grounding Myths is pretty good. John Dammeyer
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