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The accepted answer of this question mentions that appliances are grounded (to the ground wire) for safety, in case a live wire disconnects and makes contact with the chassis.

Thinking about it, it makes sense - live wire shorted to ground, short circuit flips the breaker.

But why not "ground" the chassis to Neutral? Wouldn't that do the same thing? Ground and neutral are even tied together, at the electrical panel.

I'm guessing that there could be the chance that, while the appliance is in use, the whole appliance is acting as a completion-path for the current, but I definitely would imagine that the potential voltage on the chassis is still ~0V relative to ground (since it's basically a short-circuit back to the panel), so no chance of electrical shock.

Tyler M
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  • This post is for a different question, but I think illustrates the point pretty well: https://diy.stackexchange.com/a/91405 – maples Aug 31 '22 at 13:35
  • Neutral can be at a different (higher or lower) voltage to ground. Many of the various Electrical Regulations will explain earthing systems (pne etc). – Solar Mike Aug 31 '22 at 13:38
  • @maples Thanks. Giving a look at the post, it looks like ground is there just to act as a back-up neutral. Is that really all there is to it? I guess it makes sense - a break in Hot, you're fine. A break in ground, still fine. A break in Neutral, you need a backup (the ground)... so basically adding a second layer of protection to the only wire which needs it? – Tyler M Aug 31 '22 at 14:14
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    @TylerM, no, quite the opposite actually. Notice that those illustrations have a "Code Man" and a "Rogue Man". Think of the subpanel/switch/light as the "appliance" in your question, and Rogue Man's bonding of neutral/ground as effectively having a single wire serving both. When both wires break in the second scenario (basically the same situation as if you had a single shared ground/neutral wire), notice that Rogue Man gets electrocuted. – maples Aug 31 '22 at 14:33
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    @TylerM Not quite. A break in the neutral (as long as we don't use your scheme to use the neutral as the ground)....that break will simply render the device inoperative. You probably have lots of ungrounded devices such as table lamps, cellphone chargers, tv/stereo equipment (although more and more of that is grounded now), a break in the neutral just means it doesn't work. If it were grounded (using your scheme), a break in the neutral would energize the entire device. – George Anderson Aug 31 '22 at 14:46
  • For decades (and you can still find them in place now, grandfathered, decades after installing one was illegal) USA Dryers and ranges did exactly this, with the 10-30 and 10-50 plugs. Enough people died for those to finally be forced out of new installations in preference to 14-30 and 14-50 plugs. – Ecnerwal Aug 31 '22 at 14:54
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    This is just a general comment to stress the importance of grounding: I grew up in a home with very poor wiring (old 4 fuse & 2 pull out fuse blocks panel). No grounding anywhere and a couple of add on switches for the WH and dryer. Sometimes my mom would get shocks when doing the dishes. I've been an electrical nerd since I was about 10 yo and at that age (bragging just a bit), I found that the WH had a ground fault and she'd get shocked when it was on. I connected (as a 10yo!) the water pipes under the sink to the metal drain pipe. No more shocks. Just saying, grounding is critical. – George Anderson Aug 31 '22 at 15:42

2 Answers2

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The neutral (exact term is "grounded neutral conductor") is expected to carry current, the same amount of current supplied by the hot. The ground ("equipment grounding conductor" in NEC parlance) is not expected to carry current unless there is a fault.

As you think thru this, if the neutral were also used as an equipment ground and something were to interrupt the neutral, like a bad connection or damaged wire, the entire piece of equipment or appliance would be hot at 120volts (USA). That's a very dangerous situation.

If you touch your dishwasher, garbage disposal or any other grounded appliance and a faucet at the same time, and the appliance is grounded via neutral, it'll light up your life if you are lucky, if not lucky, you're dead. It would be better to leave it ungrounded than use neutral as a ground.

Many appliances such as toasters, coffee makers, blenders, hair dryers, bathroom heaters, etc. are not grounded. That's why for decades it's been required to have GFCI breakers or GFCI outlets in kitchens and bathrooms (garages, basements too) due to the fact most sinks are defacto grounded via their pipes which would make a good return path for current from a defective appliance (less so these days due to PEX, still the water in them is a conductor), but it's still imperative to have GFCI protection in areas that you could be grounded and touch a piece of equipment or appliance that has a fault.

Radvylf Programs
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George Anderson
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    Drinkable water is actually a fairly bad conductor... – Ecnerwal Aug 31 '22 at 14:41
  • @Ecnerwal Yeah, I know, but it's still a conductor, I'll delete "good"! Still it is an issue. – George Anderson Aug 31 '22 at 14:48
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    Whoever gave me a DV, I'd like to know why so I can improve future answers. – George Anderson Aug 31 '22 at 22:23
  • I think this would be clearer if it said something like "If you were to touch your dishwasher, garbage disposal or any other grounded appliance and a faucet at the same time, and the appliance is grounded via neutral, ..." rather than what it currently says, since at first I read it as implying that those appliances typically _are_ grounded via neutral, rather than a continuation of the last paragraph about what would happen if they were. – Radvylf Programs Sep 01 '22 at 01:14
  • @RadvylfPrograms if you have an issue with my answer, please feel free to offer an edit. – George Anderson Sep 01 '22 at 01:33
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This is a settled matter in Code.

There already is a practical case which illustrates your question.

enter image description here

In 1966, when they finally put their foot down and grounded everything, range and dryer manufacturers asked for an exception to allow ungrounded 3-wire circuits (hot, hot, neutral) to continue to be installed. They said "We can ground the dryers and ranges to the neutral wire. And, that shouldn't be a problem because dryer and range receptacles are rarely ever disturbed". The exception was granted.

Thus, they gave your concept the Pepsi Challenge. They really did. For 30 years.

Now, the NFPA is a data-driven organization. They like hard numbers and accident reports. So in those 30 years they were not idle, they were collecting data, "seeing what happened".

And then in 1996, having gotten their answer, they outlawed the practice.

Feel free to delve into the record of the NEC code-making committee's discussion in 1996 as to why they changed it after 30 years. Or read the Handbook, which discusses the rationale behind many rules. (thanks Ed Beal for the suggestion).

I presume their reasons related to the body count!

Your thoughts

But why not "ground" the chassis to Neutral? Wouldn't that do the same thing? Ground and neutral are even tied together, at the electrical panel.

"At the main service panel" is not the same thing as "NOT at the main service panel". That's what you don't understand. It's because you are laying down a huge presumption that wires just get from one place to another magically, and this doesn't involve any actual stuff that could break.

Giving a look at the post, it looks like ground is there just to act as a back-up neutral. Is that really all there is to it?

"A back-up neutral so the light stays on when the neutral breaks" is a dramatic misreading of that post. The scenario where the neutral fails, and the service current re-routes through ground, is the BAD case. Look carefully, and you'll see it sets up a tragedy in the next slide, when ground also fails.

Indeed, this is the nut of why 3-wire dryer and range connections were banned.

Ground is absolutely not a backup service neutral. The desired thing, when neutral breaks alone, is for the device to stop working. That will motivate people to fix it properly.

Keep in mind also, on 35A or larger circuits, the ground wire is smaller than the neutral, and simply cannot do the neutral's job.

I guess it makes sense - a break in Hot, you're fine. A break in ground, still fine. A break in Neutral, you need a backup (the ground)... so basically adding a second layer of protection to the only wire which needs it?

One of the most dangerous things is a "sleeping menace" condition where there is a dire hazard, but it is not actually presenting as a problem. So to follow your comment:

  • A break in "hot" shuts down the appliance, so the person knows there's a broken appliance or circuit, resulting in immediate repair of the circuit and thus, no sleeping menace.
  • A break in "neutral" also shuts down the appliance, also resulting in immediate repair of the circuit and thus, no sleeping menace. But a break in neutral does not degrade the grounding protection so if the same casualty causes hot to get loose, it will still trip the breaker. And by the way, breaks in neutral are very common. We get "head scratcher" tier problems all the time that trace back to a disconnected or degraded neutral, and Lost Neutral is the most common whole-house failure.
  • A break in "ground" removes a layer of protection. Understand the system uses "defense in depth" or the "swiss cheese" model, there are several layers and all must fail in a particular way (the holes in the swiss cheese must line up) for there to be an accident. So it's not good, but a break in ground doesn't immediately cause an accident. This is the only one that "fails silently" creating the sleeping menace.
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