I was installing a TP-link HS210 3-way smart switch in my dinning room. On the side with the mains power I do have a neutral wire, but on the other switch I have no neutral wire from the wall (for that breaker). I do have a switch that's on the kitchen breaker right next to it, though, and that has a neutral and ground.
In my breaker box, both the neutral and grounds appear to be on the same row of lugs.
Running the neutral wire from the switch to the ground works, and I'm thinking it's because it's all going to the same place. This specific switch didn't explicitly say to do this, but other switches I've installed did.
Now, I could run the switch's neutral to the neutral on the kitchen circuit. I didn't at first because I had the other switch wired wrong, so I thought it was the no neutral switch causing issues.
Objectionable current propagates its hazards in two ways: 1) any part of the ground wire that is actively carrying the objectionable current back to the utility neutral, that's lava, and 2) any branches off of a part of the ground wire which is carrying objectionable current, that's also lava. In scenario 1, there must exist a voltage w.r.t. utility neutral, per Ohm's Law and due to copper still having a small resistance. Since utility neutral is connected to the site's ground rod, this means the generated voltage is a hazard to any person on site, because that person could accidentally conduct some of the objectionable current through their body, to the ground rod, and back to utility neutral.
Scenario 2 is a bit more subtle, but uses the same setup as scenario 1. If scenario 1's hazard was a dangerous voltage, then scenario 2 makes it worse by bringing that voltage -- unchanged -- to more destinations, amplifying the risk of accidentally touching that errant voltage.
For a typical USA domicile where all circuits and their ground wires are home-run back to a single main panel, the presence of objectionable current on Circuit A's ground wire will not create Scenario 1 on Circuit B's ground wire. This is because the flow of electricity is always back toward its source, in inverse proportion to resistance. From the perspective of Circuit A, no part of Circuit B's independent ground wire would help bring current back to the utility neutral. Of course, both circuits meet at the neutral/ground bar, but because of the ground rod, this point in the system is equal to earth. And so Scenario 2 will not appear either.
The equation drastically changes if -- unrequired but not prohibited by the NEC -- the separate circuit's ground wires are wired together in other parts of the house. For a common example, if two different circuits are ran into the same switch box -- such as by the front door for the porch light and the foyer light -- then it's quite common to tie both circuit's ground wires together.
Now we have a problem: if the porch light circuit develops objectionable current on its ground (eg rainwater in the lamp), then the foyer light circuit's ground is now a third path back towards the utility transformer, also carrying some objectionable current. And if the foyer circuit has an indoor receptacle with a TV attached, and that TV has a bunny-ear antenna attached via a coax connector, then there's a shock hazard when someone touches that antenna. This is totally unexpected, and that's precisely why it's so dangerous, irrespective of the actual amount of voltage on the ground wire. This is Scenario 2 in action, since although the antenna isn't actively carrying the objectionable current, the voltage from the original fault means a human could become an additional path for objectionable current to flow. Not good.
Because of this, it is absolutely verboten to intentionally put operating current to ground, and why GFCIs for outdoor circuits is basically mandatory in all regards now. That said, a lot of electronic appliances have "filtering capacitors" which block voltage transients from impacting their operation, but these work by shunting the transient to ground... This is not ideal, but transients by their nature are not part of normal operating current, so it's a reasonably acceptable practice. Whereas running a space heater 24/7 using live and ground would be a death wish.
More complexity occurs when subpanels are involved. Here, a fault causing objectionable current downstream of a sub panel can end up polluting all of its downstream ground wires, by way of Scenario 2. But not for other downstream ground wires in the main panel, due to it having the designated neutral/ground connection. The solution is still the same: GFCIs for active protection, and a vow to never abuse the ground wire. This is partially why NEC keeps adding GFCI requirements for circuits.
As for the term "objectionable current", I think it's solely a product of the NEC, which is published by NFPA, the fire insurance people. Yes, the electrical code is more about preventing fire and inherent safety, than it is about harnessing electricity. NEC is a floor for safety, but is not a design manual, so it mostly tells you what isn't allowed or what must be done. All else is fair game.
Duh, I wasn't thinking about the ground rod (see why I'm not an electrician?).
I appreciate your repitition of how voltage flows, always back to the source - this is the first I've heard this (again, not an electrician), I've always thought of it as flowing to the "easiest" path (I have more background in low voltage DC, , which is similar, but different. Learning DC first made learning AC harder, in my opinion). Seems this idea about voltage flow needs all your repitition - keep it up.
Yea, code is always about baseline safety - there's an electrician in my extended family (first licensed in the 80's), and we've occasionally talked about how much he's had to study over the years. I've seen how much new code has come in since about '85. Lessons learned from fires (or worse), I'm sure.
Thanks for your extensive explanations, I'm gonna have to read this a few times to fully grok the why's. Good stuff!
Phew! I thought I was beginning to sound like a broken record lol
I'm also not an electrician, but I did study some electrical engineering at uni. And for my job, I've spent a non-insignificant amount of time disabusing people of the awful stock phrase "electricity returns to ground"; it actively causes unbound confusion and the subsequent questions which somehow befall me.