Article 100 has twelve definitions that start with the word “Ground”:
- Ground fault
- Ground stick
- Grounded conductor
- Grounded (grounding)
- Grounded solidly (solidly grounded)
- Ground-fault circuit interruptor (GFCI)
- Grounding conductor, equipment (EGC) (equipment grounding conductor)
- Grounding electrode
- Grounding electrode conductor
- Grounding, hard (low-z) (hard grounding)
- Grounding, soft (high-z) (soft grounding)
And finally, “Ground” itself. This appears first in the alphabetical list and its simple definition is the same one you find in NFPA 70 (NEC): the earth. Something to understand about the earth is its impedance is far higher than that of copper wire and it varies depending on the exact soil composition of the area under consideration (while copper wire is consistent).
For this reason, connections to ground do not and cannot provide an equipotential plane. They do not eliminate dangerous differences of potential.
Not all of the other 11 involve a connection to the earth. The grounded conductor, for example, is the neutral (in most systems). You never run a branch-circuit white wire to a ground rod. Nor do you run the grounding conductor (what we typically call the ground of a branch circuit) to a ground rod.
Most notably, the EGC doesn’t actually ground anything. While the EGC itself is ultimately connected to the earth, it is not an earth connection. It is metallic. You will not find the earth listed as an approved equipment grounding means in NFPA 70 250.118((A). In fact, all 14 types of EGC are metal.
If you connect a bonding jumper from a motor case to the EMT used to contain the motor’s supply conductors, the motor isn’t actually grounded — it is bonded (if that EMT is being used as part of the EGC). And the case of that motor is now at the same electrical potential as anything else that is bonded to the EGC. But if you take that same bonding jumper and connect it to a ground rod driven near the motor instead of to that raceway, the motor is merely grounded.
But the ground here and the ground at the service or separately derived source are not the same point electrically. So grounding the motor serves no electrical purpose. It’s just a waste of a ground rod an the associated materials. It also creates an additional tripping hazard and could give the impression to some that the motor case is at the same potential as a nearby enclosure when that is not at all the case.
To avoid confusion, always remember that ground connections are made at the source not at the load. If you ground (connect to earth) all over the place, it does not reduce noise or make the system safer. You end up with ground loops, which introduce noise. And you provide additional paths for lightning and other high voltage transients to enter the system.
