I can't figure out what a ground loop is in an electrical service (residential, commercial, industrial). All I can figure out is that it has something to do with different potentials in the system, but what is the exact issue and flow of electricity? Thank you.
"Ground loop" is a phrase from audio design. Ask an audio engineer.
It is practically not a thing in mains electrical, and let's think about why. Safety ground has two jobs:
- Return natural current (ESD, lightning) to source (being earth)
- Return human-generated fault current to source (being neutral).
The subject of a "ground loop" is all about managing and coordinating currents flowing on safety ground -- that is the logic, yes? Which currents are those? No currents. System design calls for no current whatsoever on safety ground, except during fault conditions (which should only be long enough to trip a breaker or GFCI).
From a mains system design perspective, safety ground is the one case where we want a "web" of connections. It's perfectly OK for grounds to criss-cross among different circuits and even services. We run a ground wire to an outbuilding which has a ground rod too (partly: we don't want lightning traversing the wire to the main building). The more's the merrier, the more's the safer.
And that's why we give audio engineers isolated grounds :)
Now, "ground loops" are concern in audio and computer networking; but understand in the electronics world, "GND" is a completely different animal: GND is the "common" or "zero-voltage reference" or what we in mains call "neutral". And this is often used as "signal ground", or the "zero point" in a referential signal (like RS-232, which has 1 common and many signal wires). As distinct from a "differential" signal like RS-422 (which has 2 signal wires per signal, and only the difference between them matters).
Particularly in networking/distance signaling, if this "common" is bridged to AC mains safety ground at two places, it becomes vulnerable to differences in voltage on that safety ground network - which shouldn't exist.
But it's possible to have an ongoing ground fault that is too little to trip a breaker and GFCIs are not installed... and that can introduce millivolts of gradient on the safety ground network, pulsing at 50/60Hz of course. Or a signal processing device can leak noise back through its power supply onto the AC mains, which can then feed it onto safety ground via capacitive coupling.
The lesson is that audio and network designers need to be careful about treating AC mains safety ground as some sort of signal zero reference. Also, they need to be careful about binding wires or shields in their cable to AC mains safety ground - lest that wire suddenly be asked to carry dozens or hundreds of amps during a bolted ground fault.
The term "ground loop" usually refers to a circuit in which various "end items" have a ground wire attached, and the ground wire from one item goes to the ground attachement on the next, and so on, with an earth ground connection somewhere in this continuous loop of wiring. The "end items" could be components on a circuit board or they could be the various consumer electrical items like outlets and stoves and lights in a house.
The problem with a loop like this is that any conducting loop is happy to act as an antenna, picking up pretty much any electromagnetic waves in the area (to within certain limits on the frequencies). This then dumps noise into your electrical system, because the ground wiring is everywhere.
The preferred diagram is called "star-grounding," in which each end item's ground wire goes directly back to a common earth ground point. In this way, the antenna gain is much less, and further because each ground wire dumps directly to earth, rather than passing by the other nodes, far less noise is transmitted into the devices of interest.