wiring - In any electrical equipment having single ph 230 v ac supply ,if neutral and earth wire exchanged then what will be the consequence

I am handling the supply in a locomotive coach and have noticed that the neutral and earth wires have been accidentally flipped,inside the appliance(hot case) is there a problem from the safety point of view?

Answer

There is a problem.

Generally the neutral and earth are connected at source. On fixed installations (e.g., your house) this may be the local transformer or at your meter-box, depending on local regulations. On your coach the generator / alternator most likely has its neutral connected to the chassis. The advantage of this is that we no longer need to fuse the neutral line as a short circuit between it and chassis will not cause high current to flow. On the other hand, a short from live to earth will cause a high current to flow and it will be detected when the fuse blows.

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Figure 1. Properly wired system.

Consider what happens in a correctly wired system.

• If the live wire falls off the load 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.



• The earth wire keeps the case at the same potential as the chassis.

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Figure 2. Swapped N and E on LAMP2.

Things get much more interesting and dangerous once we start mixing up N and E. In the example shown in Figure 2 all appears well to the user and even the electrician doing a voltage test.

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Figure 3. Broken neutral - live equipment case.

Figure 3 shows one potential scenario.

• The neutral wire has broken.


• SW1 is on so the N wire is connected to mains through LAMP1.


• LAMP2 is lit as it has a return path to the generator.


• LAMP2's case is connected to the now live part of 'neutral' circuit.


• Anyone touching LAMP2 is in danger of electrocution.

It's best to keep neutrals and earths properly connected.

See my answer to Why don't we use neutral wire for to ground devices and earth wire for closing the circuit? for answers to a similar question.