Why don't we use neutral wire for to ground devices and earth wire for closing the circuit?
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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.







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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?







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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.







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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.