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I know that the neutral point of a distribution transformer is connected to the grounding (earthing) wire so I thing they are the same.

Can I use the neutral wire for protection purpose and connect it to the metal case (cover) of the electric devices?

Can I use the grounding wire to close the circuit? For example: Can I connect a light bulb to live wire and ground wire?

I think the answer would be "yes, but It is bad to do this" but I don't know why it is bad?

Edit:

transistor, Thank you very much for your great answer and wonderful explanation. I appreciate that, and Thank you for giving me some of your time. I have one more thing that I still don't understand. I wonder if you would mind answering me and I hope I'm not bothering you by my questions.

If the bulb is connected this way:

schematic

simulate this circuit – Schematic created using CircuitLab

If the customer touches the lamp when it is switched on, I do not thing she will notice a slight tingle because the return path is the ground.

If the neutral wire fell off, the metal case is not live. If the ground wire fell off, the bulb will just turn off but there is no danger. Is that correct to wire devices this way?

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2  
If you would connect the bulb in my house, the rcd would trip. – PlasmaHH Jan 12 at 21:43
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Note this is one method of providing a protective earth (I think called TN-C-S) but it is not the only way. In some places your protective earth will literally be an earth spike. In these systems PE and neutral are very different. – David Jan 12 at 23:17
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Consider ground wire being cut or losing contact. Will you be happy with manis voltage on all grounded metal surfaces in your house? – Dmitry Grigoryev Jan 13 at 7:42
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This method of grounding was known in Germany as "Klassische Nullung" but forbidden more than 40 years ago for the reasons stated in the answers. de.wikipedia.org/wiki/… – Drunix Jan 13 at 12:10
up vote 58 down vote accepted

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1 and 2. showing the danger of earthing through neutral.

In the first example the Lunatic 'Lectrician has 'grounded' the lamp case by connecting it to the neutral wire. All appears OK although the customer notices a slight tingle when she touches the lamp when it is switched on. This is because there's a slight voltage drop caused by the current through the return wire. Customer lives this time.

In the second situation one of Lunatic 'Lectrician's wires fell off. Unfortunately it was the return wire and now the metal lamp is live. (The bulb resistance isn't high enough to protect the customer.) Lunatic 'Lectrician may lose this customer.

schematic

simulate this circuit

Figure 3. Phase reversal danger with 'neutral' connected to frame of lamp.

Now not all countries use polarised plugs and unfortunately Lunatic 'Lectrician operates in one of these countries. There's a 50/50 chance of the plug going in the 'safe' way. There's a 50% chance of it going in this way.

Do you need any more convincing?

schematic

simulate this circuit

Figure 4. Properly wired lamp.

Consider what happens in a correctly wired system. If the live wire falls off the bulb and touches the metal case a large current will flow to earth. If the fuse is correctly rated it will quickly blow disconnecting the live supply. If the neutral shorts to the case the fault may not be detected and the current may split between the neutral and earth wires. An RCD / ELCB will protect against this type of fault, but that's another question.


Update after question update.

schematic

simulate this circuit

Figure 5. Break in earth wire.

No. It is never safe to use the earth wire as a neutral. Consider Figure 5: the ground wire has broken and anything else connected to it will become life once S1 is switched on. It's just too dangerous. It will cause potentially fatal confusion to the next person to modify the system. Stick to good practice, local regulations and enjoy a long life.

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1  
Updated with correctly wired system. – transistor Jan 12 at 22:48
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+1 for grand storytelling – mcmiln Jan 12 at 22:54
3  
While everything you say is correct and the diagrams and the story are illuminating, I wonder whether diagram 1 or 2 depict what the OP meant. As I understand the question: there are, in a normal house wiring, two wires coming from the ground which are arbitrarily labeled "neutral" and "ground" although they both connect in the end, so are "the same wire" logically. Why should I not exchange "ground" and "neutral" when I wire a lamp etc.? (That's different from connecting the casing to the neutral end of the bulb, as in your dia. 1.: There would still 3 wires go to the lamp.) – Peter A. Schneider Jan 13 at 9:20
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To elaborate: A wire break like in your dia. 2 would not lead to voltage on the casing if the wriring would be such that in your "wonderful" diagram green would connect to the bulb and N to the casing. In order to demonstrate why that confusion is bad, one needs a second lamp and a break of the neutral wire (which then has life potential and is wired to the casing of the badly wired lamp). With correct wiring, neither a neutral break nor a ground break would cause the casing to have potential. – Peter A. Schneider Jan 13 at 9:26
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+1 for "Lunatic 'Lectrician" :-) – AndrewJacksonZA Jan 13 at 10:03

While usually neutral wire and earth wire are at the same potential, and can therefore be effectively swapped from a purely electrical point of view, the way they are connected makes them very, very different.

A domestic electrical system should always have two protection devices:

  • the RCD, aka residual current device
  • a circuit breaker

The RCD is connected to live and neutral and measure the current difference between the two wires: if the difference gets too high it trips disconnecting downstream wiring. The purpose of this device is to avoid that current coming from live is not coming back through the neutral: this can happen if such current is flowing on a load, e.g. a human, that is not connected to neutral but to some other similar potential, like earth.

The breaker usually includes two tripping mechanisms: magnetic and thermal. The magnetic mechanism intervenes if the current flowing in the breaker gets too high, even for a short time, while the thermal mechanism intervenes if a somewhat high current flows for much time. The purpose of this device is to avoid your wires catching fire, while still allow high, but not too much, current spikes that last for a brief moment.

If you just swap live and ground at a load the RCD would just trip and nothing interesting should happen, just forget it. If no RCD is present and you have small/long/bad wiring you can possibly rise earth from ground of several volts, and that is a Bad Thing.

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To my knowledge, in the US, RCDs (or "GFCI", as we say) are only required in bathrooms, kitchen sinks, and other wet areas, not generally. – mattdm Jan 13 at 15:15
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This migh be true, in the US the voltage is lower and somewhat less dangerous, but I'd protect the whole house with an RCD. – Vladimir Cravero Jan 13 at 16:01
    
Not saying it wouldn't be a good idea. We just don't. – mattdm Jan 13 at 16:04

Vladamir has the important answer: we treat the two conductors differently, even though they are nominally at the same voltage. He gives the example of a RCD, and the GFCIs you see in modern US construction operate very similarly. The expectation is that the current flow of the hot wire and neutral wire will be equal in magnitude. If they are not, the RCD/GFCI figures the current has to be getting back through another route (grounding wire, plumbing, etc.) so it will trip.

There is a second reason which he did not mention, which is that they are only nominally at the same voltage. Copper wires are not perfect. They have some resistance. This means that the current flowing through the neutral line can "lift" it to a voltage above ground. Typically, this is small enough that we don't notice, but stereo equipment notices big time. Audio equipment often needs to measure small voltages (i.e. less than 1V) against a common node, and they then amplify that signal to send it to the speakers. This measurement is often done off of the "ground" reference voltage (for many reasons). If you start putting noise on the "ground," it starts to show up in the sound rather quickly. That's why many stereo amplifiers have either a 3 prong connector or a dedicated ground port which you can connect to a ground if the nearby electrical noise has a large effect on your sound quality.

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Answering the question in the edit:

If the bulb is connected this way:

schematic

If the customer touches the lamp when it is switched on, I do not thing she >will notice a slight tingle because the return path is the ground.

If the neutral wire fell off, the metal case is not live. If the ground >wire fell off, the bulb will just turn off but there is no danger. Is that >correct to wire devices this way?

Electrons don't care about the color of wires or the arbitary labels we attach to them. If our supply cable had three conductors insulated with black white and green plastic (i'm assuming north american color codes here) and we connected all touchable metalwork to the white wire and all return paths to the green wire then there would be no immediate risk of shock. Even if a "single fault" developed.

However there are a couple of reasons why this is a bad idea.

  1. In many practical cables the ground wire is not merely another plastic covered conductor that differs only in color from the other wires. It may be insulated from people touching the cable only by the outer sheath while the other conductors are insultated by both their own insulation and the outer sheat. Sometimes it may not be insulated from people touching the cable at all.
  2. Conventions are conventions for a reason. Even if your system with weird wire colors is safe as it stands it only takes someone to come along and make modifications to it while assuming it follows the standard to render it unsafe.
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This used to be standard practice in house wiring- 3-prong outlets that had the N and PE bridged, and used 2 wires in the wall. Less risk of a wire in the wall breaking and causing the worst case scenario described above; still, damaged or poorly executed/modified wiring led to enough accidents to cause a phase out of such practices in most places.

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protected by W5VO Jan 13 at 19:18

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