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First I want to mention that I've studied telecommunications (here in my country, is very similar to electrical engineering, I've studied circuit theory and electronics), although I've only dealt with small electronics and they're not my current line of work anymore (now I do more AI stuff, not that it matters). With that, I want to say that: please, don't shy away from giving a full fledged answer beyond the basics, but there's no need to explain the purpose of the earth wire, the live wire and the neutral wire in a household electrical system. I understand that the earth wire is there to provide a ground-fault path for defective devices whose chassis become live and triggering the RCD circuit breaker.

However, it seems to me that you could have the same ground-fault path just connecting a small bridge between the neutral and the earth end of electrical outlets, since earth and neutral are connected anyway at the box so they share the same reference. I think this post in Reddit sums up very well my thoughts. After VVillyD's excellent response, GasDoves asks:

I still am confused. Can you help me out?

Can you explain what the difference would be between the following situations:

A three prong outlet with neutral and ground ultimately connected in the breaker box

VS

A three prong outlet that really only has two conductors, but the ground has been jumped to neutral with a short piece of copper.

I don't think this is answered. In my mind, both of these setups would have exactly the same circuit. Perhaps I'm missing some non-ideal effect or there is a practical issue at hand?

[EDITED 2020/08/11] There was a mistake in the initial formulation of my question, as Ecnerwal pointed out. I meant "circuit breaker" instead of "RCD". I was thinking that, if the chassis becomes live, and it is connected to the neutral wire (or the earth wire, in a proper installation), then there would be a short-circuit and the breaker should trigger, signaling the home users that there's something wrong. The RCD, of course, would not play any role on this. My apologies, I'm not used to large electrical components! Also, I want to thank you for your responses! From what I could understand, the need of an earth wire essentially boils down to safety redundancy and to have a tidy role for each wire (and, of course, those are very good reasons!).

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    Understanding your background and you're still confused, it's no wonder rookies are really confused by this. It's a good question to ask and to point others to. – FreeMan Aug 10 at 18:31
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    "Non-ideal" is a good angle to look at here. If nothing goes wrong, you don't need a separate ground. When something does go wrong, sometimes a separate ground saves your life. For devices with an exposed metal case, an internal short to the case would shock the user -- we ground the case to prevent that. But if we used the neutral for this function, it would create a different lethal failure mode: disconnection of neutral at the outlet would make the case live, shocking the user. Separate neutral and ground fixes this: now TWO faults are required for the case to be live. – Glenn Willen Aug 10 at 20:38
  • (I had to think a bit to convince myself that I'm not cheating here, since in the end neutral and ground are still electrically connected. But notice, if you trace things all the way out: There are two completely independent return paths for current, via the ground wire or the neutral, to the grounding rod or the pole transformer. They are connected together in the main breaker panel, but in NO place are they reduced to a single point of failure. There is no single wire or connection anywhere which can sever both of them at the same time.) – Glenn Willen Aug 10 at 20:43
  • Missing in the discussion so far is the concept of a 'voltage drop'. Voltage running through a conductor will have an voltage difference (one end to the other) this is related to the resistance of the wire (ohm's law: voltage=currentresistance). SO, with *no current the neutral and ground are at the same potential (hopefully zero). When the circuit is under load, there is a voltage drop on the neutral (yes it is small, and yes, it does 'warm' the wire!) BUT the ground wire has no current on it so remains at ground potential. If they are tied together at the load end, BOTH will carry current; – Beekeeper Aug 10 at 22:32
  • @Beekeeper I've also thought about voltage drops as possible reason, although in not quite the same way. My question was more in the line of not having an earth wire at all, so the neutral would be the only return path to the source. Anyway, both the neutral wire and an earth wire would have roughly the same length, so they also should share, more or less, the same resistance (say, 0.05 Ohm). In the event of a fault in a device and its chassis becoming energized, wouldn't the voltage drop be the same for an earth wire and a neutral wire? – Asher Aug 11 at 8:28
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Good question. There are multiple sorts of faults we need to consider.

Disclaimer: I'm not an electrician. I have a similar background as you, some academic electrical engineering type stuff (I'm an experimental physicist). I've had similar confusion and done some self-learning on this safety stuff.

I will talk about the electrical supply for a building. There are three terminals to consider. Live, Neutral, and Ground. Note that the supply provides a voltage difference between its live and neutral terminals. This means that any complete circuit must have a path from the live terminal back to the neutral terminal.

It happens to be the case that the ground terminal is physically connected to the earth. This is not reasons having to do with clearing electrical faults but rather has to do with preventing objects within the building from floating too high above earth potential. This might happen due to static buildup, lightning, or other voltage surges. I only mention this because this one feature which distinguishes (electrically) the ground terminal from the neutral terminal. However, for the rest of this post we could COMPLETELY ignore this earth connection. However, what we can't ignore is the fact that metal surfaces in the building such as metal sinks, water pipes, etc. are electrically connected to the ground terminal.

First, a fact: at the supply for our electric circuits the neutral wire is physically bonded with the earth. This is done for reasons having to do with overvoltages that can occur due to static buildup, lightning, and other sorts of voltage surges. Basically they ensure that no circuit 'floats' too far from earth potential (note this is not for clearing circuit faults, I only mention it because we will need this fact later).

Another fact: At the supply the neutral terminal is connected to the ground terminal. We will see why this is useful and critical for safety shortly.

This first type of fault I'll consider you already understand. Suppose we had a two-prong device. There is a load which has a live connection on one side and a neutral connection on the other. Suppose there is a floating chassis. Under normal operation you can touch the chassis and a pipe in the building and have no problem. However, if there is short from the live wire to the chassis then the chassis is now at high voltage relative to a pipe in the building, so touching both could cause current to flow through you. DANGER. The way this is solved in modern circuits is that you run a wire from the ground terminal to the chassis of the device. Now, if such a short occurs then high current will from the chassis along the ground wire, through the ground to neutral jumper at the supply thus completing the circuit. This high current trips a breaker on the circuit turning everything off quickly to ensure safety until the fault is corrected.

However, forget the ground wire. If you just connected the chassis directly to neutral you would get the same effect. In this case current would flow from the chassis, through the chassis-to-neutral connection, then back along the neutral back to the neutral terminal at the supply completing the circuit. It would be high current which would trip the breaker.

What you suggest in your question is essentially the same thing. In the section you quote the suggestion is to have three wires coming from the device. A live, a neutral, and a "chassis". The live and neutral are wired to live and neutral on the outlet and the chassis is also wired to neutral on the outlet. There is no other connection on the backside of the outlet. The idea is to connect neutral directly to the chassis. Such a configuration is called a bootleg ground. It may be tempting to install a bootleg ground in a situation when, for example, an building only has two-prong outlets (because it is old) but you have modern three prong devices. You should not do this because it is dangerous as I will describe below.

In the event of a live-to-chassis short as I have described above this bootleg ground wiring WILL cause an overcurrent that will trip a breaker to clear the fault. This is good. However, equipment ground allows us to protect against another sort of fault.

Above the issue was a short circuit. The fault we consider now is an open circuit. See this diagram from the answer here: https://electronics.stackexchange.com/a/388134/180824

See this diagram. enter image description here

There are two possibilities with a bootleg ground. First, imagine it is a situation like in the diagram above where the two circuits on the right do not have any equipment ground wire running to them. Then, in the event that the neutral wire is cut as in the third device then, because of the neutral-to-chassis connection made at the bootleg ground, the chassis floats up to the live voltage. Now if someone touches the chassis and a pipe in the building they will complete a circuit and dangerous current will flow. Note that no current runs through the load after neutral wire is cut, however, if someone touches the chassis and a pipe then current does flow through the load.

The second possibility is the same as the diagram above except that the ground connections are actually completed from the supply to ALL three of the circuits. The two circuits on the right simply have the additional connection from neutral to ground. In this case, if the neutral wire is cut, the ground simply replaces the neutral wire. Current will flow through the load and return from the neutral side of the load, through the load side neutral-to-ground jumper, back along the ground wire, then along the supply side ground-to-neutral jumper back to the neutral terminal to complete the circuit. In this case current runs through the load even though the neutral wire was cut. Now what if you touch the chassis and a pipe? Well, now you are again completing a circuit providing a return path (through your body and the pipe) but you are in parallel with the direct ground wire. This isn't as bad as the previous case, in this case most of the current will run along the ground wire since it is lower resistance than you. However, if the current for the circuit is high then even the small fraction running through you could be dangerous. Also, if the ground wire was cut (or had increased resistance) in addition to the neutral wire then we would be in the previous situation as before with the bootleg ground with no true ground connection.

Very long story short:

  • It is important to have an equipment ground wire running to the chassis of devices to ensure live-to-chassis SHORTS are cleared by providing a path to supply neutral to trip the circuit breaker.
  • It is important that there is no connection between neutral and the chassis at the load side to ensure that the chassis can NEVER serve as part of the return path for currents. In particular such "bootleg ground" connections become especially dangerous when the resistance along the neutral/ground wires is increased due to breakages or damage to those wires.

edit: Another comment, note that under normal wiring (equipment ground bonded to neutral ONLY at the supply) that if there is a live-to-chassis short then the equipment ground line will clear the fault the circuit will trip. However, if there is a neutral-to-chassis short there will basically be no warning sign. This is somewhat acceptable because as long as the neutral and ground wires running from the outlet to the supply are in tact and the current through the load is moderate there is not substantial risk, but I still find it worth noting the lack of a warning.

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  • I'll add more diagrams to clarify – Jagerber48 Aug 11 at 8:28
  • Thank you! That's a very enlightening explanation. So, would I be right if I say that a three-wire system would, generally, require two faults to be dangerous (device fault+ground cut, or ground cut+neutral cut) while a bootleg ground would need only one (neutral cut)? – Asher Aug 11 at 9:47
  • I almost 100% agree with your two fault statement. However, even no excess faults, suppose the circuit draws an amp under normal operation and that the resistance along the neutral/ground wires is about an ohm and that the contact resistance for a person is 1000 ohms. Then even already touching the circuit would put 1 mA through the person. A little higher wire resistance or current and a little little lowers contact resistance leads to the excess current through the person exceeding 10 mA which can begin to be really problematic. This issue can be mitigated using a GFCI outlet which – Jagerber48 Aug 11 at 12:50
  • Would detect the lack of return current on the neutral wire and would trip the gfci circuit in the event of a fault current passing through the person. (Note circuit breakers trip at currents much to high to protect a person who is being electrocuted, they try to stop a fault before it causes a problem such as a fire or shock. GFCI outlets are designed to trip at fault currents which are dangerous for humans, I.e. a few mA. – Jagerber48 Aug 11 at 12:52
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The grounded (Neutral) conductor normally carries return current.

The grounding (Ground or Earth) conductor does not.

If a wiring fault opens the Neutral, the separate Earth/Ground conductor provides a return path for fault current. If you only had the Neutral, a neutral fault would lead to the entire thing becoming energized.

Exactly this "logic" was used for electric stoves and dryers in the USA years ago - they had (two Hot or Live, being the US system of 120V-0-120V with 240V between the lines) Hot, Hot, Neutral and no separate Ground. Enough actual problems resulted that code was changed to require Hot, Hot, Neutral AND Ground. There are still stoves and dryers connected that way (since the code system does not require replacing things installed to code when they were installed until you are changing the circuits, usually when remodeling.) But any new work is required to follow the safer standard.

Additionally, if fault current is returning on the Neutral, your RCD (which I understand as a "whole house ground fault circuit interrupter" - terminology differences) won't see a fault, because the current out on Hot/Live will match the current in on Neutral. Those devices look for a mismatch between current out on Hot/Live and current returned on Neutral (or for the US system Neutral or the other Hot) and trip when there is a difference between them, because current is going where it should not. We do use single circuit GFCI's to upgrade older circuits with no grounding conductor since they will trip if current passes out through a person or otherwise not in the circuit, but for the case of a fault that would go to Earth/Ground on the circuit, combining that with neutral would fool the RCD.

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  • Ecnerwal I believe this is the reason the US has stayed with branch circuit protection because a main GFCI or RCD can be fooled. Having smaller fault trip levels and almost every circuit requiring GFCI or AFCI I like the NEC approach even if it seams over done at times. – Ed Beal Aug 10 at 13:34
  • My apologies! I actually meant circuit breaker instead of RCD. As in: "if the chassis becomes energized, phase and neutral would short-circuit (either through the earth wire in a proper installation, or through the neutral in the shoddy one I'm proposing) and the breaker should trigger". I've edited my question to reflect that. Anyway, your answer's been very informative, thank you! – Asher Aug 11 at 8:37
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In a perfect world, both systems would be the same. The reason it's not done, not allowed by code (USA) is to keep the ground and neutral separated at the point of use. If your outlet lost it's neutral, the circuit would be completed through the ground; you now have an energized ground, not good. If you have a metal conduit installation, you run two wires, hot and neutral, to an outlet and the conduit is the ground... no need to jump it. If you ran 12/2 without a ground to an outlet, there's no ground in the box so nothing to jump the neutral to. If you ran 12/2 with a ground, then you have a ground and no reason to jump it.

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