I know that the neutral point of a distribution transformer is connected to the grounding (earthing) wire, so I think they are the same.
Can I use the neutral wire for protecting metal-cased electric devices by connecting it to the case (cover)?
Can I use the grounding wire to close the circuit? For example, can I connect a light bulb between the live wire and the 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 your time. I appreciate that. 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:
simulate this circuit – Schematic created using CircuitLab
If the customer touches the lamp when it is switched on, I do not think 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 it correct to wire devices this way?
Best Answer
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.
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?
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.
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.