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Let's say that a house connected to the power company in the USA has no ground rods (or pipes, plate, etc.) bonded to the neutral bar. Things should still work correctly because the current will be running from either hot wire (each hot should be at 120V) to the neutral wire, all contained within the lines from the power company transformer to the house. Even the ground wires at each outlet will work correctly because an accident from a grounded metal case will still trip the breaker at the panel.

However, this would be a "floating ground" situation. An issue with a floating ground, as I understand it, is that something can happen such that the voltage on the neutral wire is not actually 0V. For example, if you measure the voltage from an outlet in the house, it could be 480V at the hot wire and 360V at the neutral wire, which is still a 120V difference but maybe not what's connected to the outlet is expecting. I imagine this can be very dangerous because the ground and neutral wires are bonded at the panel, meaning a metal case to some appliance might be at 360V. Also, maybe there are some subtleties to how some appliances work and they cannot handle that magnitude of voltage. I do not know what other "bad" thing can happen without ground rods, short of a lightning strike.

So, what other "bad" things can happen without a bonded neutral? Also, in the above example where the voltages are incorrect, how can that even happen? If the power company has an issue with the voltages from the transformer, how does bonding your neutral to ground rods fix that?

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    Gee, I really hope this is actually purely hypothetical...
    – Ecnerwal
    Oct 19, 2021 at 16:20
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    haha, yes, it is purely abstract. this is now the 3rd time ive gotten grounding rods installed at different houses (due to repiping a house, just for good measure, etc.), and i realized it is something i dont satisfyingly understand. hopefully it is clear this is a conceptual question, not a concrete problem.
    – tau
    Oct 19, 2021 at 16:24
  • To clarify, as I understand the terminology, you would actually have a bonded ground in this scenario - i.e., ground and neutral connected together (typically "screw connects neutral bar to breaker panel metal case") and the issue is only "No proper connection from ground/neutral to the actual ground via grounding rods/metal cold water pipe/etc.". I believe if that's the case your hot/neutral voltage would be correct, as that is determined by the transformer tap, and your hot/ground voltage would be the same because it would be "bonded". The issue then is "what doesn't work due to Oct 19, 2021 at 16:28
  • there not being any ground rods/etc.". Right? Oct 19, 2021 at 16:28
  • Not going to close as duplicate, but pretty similar to a question I asked recently: diy.stackexchange.com/q/229683/43874
    – JPhi1618
    Oct 19, 2021 at 17:04

2 Answers 2

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You are at the crux of the problem. The whole shebang would float at some indeterminate voltage above natural earth ground.

There would be 240V of spread: neutral would be at X volts, phase L1 would be X-120, and phase L2 at X+120.

What's more, you're imagining a 480V offset, which might happen with leakage inside a 480V transformer found inside an industrial plant or mall. Except that typical pole-top supply transformers aren't 480V: they're 2400, 9600 or 12,000 volts.

So yeah. Gonna want that Grounding Electrode System tip top.

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  • thank you, but how does bonding to ground rods fix things? maybe the neutral wire in the house would be at 0V, but wouldnt the hot wires still be at the wrong voltage if the power company transformer had an issue?
    – tau
    Oct 19, 2021 at 17:54
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    @tau no, the hot wires should be coupled to the neutral voltage by the secondary of the transformer. Think of it as "pulling the earth around your house up to leakage voltage" if you like. Point is, if you touch your meter pan barefoot, you don't die. Oct 19, 2021 at 19:32
  • oh okay, but here is a really fundamental question: how can a rod in the dirt pull the voltage from the transformer up?
    – tau
    Oct 19, 2021 at 19:58
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    A proper grounding electrode system is supposed to influence the neutral to be at local earth potential, or somewhat vice versa. Earth is more like a vast network of resistors. There's an XKCD for that. Oct 19, 2021 at 21:16
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    @tau voltage gradients across the ground can happen and yes, they do create the potential for that to happen. Oct 20, 2021 at 18:21
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Actual example. Some idiot that somehow got his electrical license did some work on a house, and removed the connection to the grounding electrodes, putting a clamp onto a water pipe, ignoring the fact that the waterline into the house was plastic, even though the hunk near the box was copper.

A while later, a nearby lightning strike caused an occupant to get a nasty shock (fortunately non-fatal) as a fireball shot across the floor, leading to investigating and correcting that blatant error. No further shocks or fireballs.

If correctly bonded, everything rises kilovolts (together) as the lightning event affects the local electrical field. If not correctly bonded, multi-kilovolt differences arise between the stuff bonded to the neutral and "the actual ground," i.e. dirt and things connected to it. The neutral is typically grounded at the pole, but dirt is a terrible conductor, so the dirt voltage at the base of the pole and the dirt voltage hundreds of feet away at your house can be quite different.

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  • yes, i can understand that issue. do you also happen to have an explanation for the "something is wrong with the power company voltage" scenario in my question? thanks!
    – tau
    Oct 19, 2021 at 17:35
  • The power company voltages are not incorrect or wrong in your example. 240 line to line, 120 line to neutral. Not correctly referencing neutral to ground is the whole problem, as described.
    – Ecnerwal
    Oct 19, 2021 at 18:29
  • i mean about how if the power company makes a mistake and provides the incorrect voltage, somehow having ground rods can help fix that.
    – tau
    Oct 19, 2021 at 18:47
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    No, it cannot. In the extremely unlikely case that the power company provides the wrong voltage, the power company's insurance company buys you new appliances and wiring, or possibly an entire house, depending how large the error was. It's not a likely scenario at all, in point of fact. Having ground on "480V rather than 240V" supply is exactly the same as having ground on a 240V supply, and won't do a thing for all your overvolted appliances.
    – Ecnerwal
    Oct 19, 2021 at 23:39

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