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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?

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  • \$\begingroup\$ Unless you are talking about a toy train running on safety extra low voltage (SELV), this question kind of worries me. On a more serious note, it's good you caught the problem and it's very good that you seem to be worried too, and are trying to fix this. \$\endgroup\$ – zebonaut Feb 28 '16 at 10:33
  • \$\begingroup\$ It should also be noted that deepening on the local codes internal ground wires don't have to be the same size as the neutral... ground is meant to last long enough to blow the fuse, not a replacement for the neutral. \$\endgroup\$ – Spoon Feb 28 '16 at 10:38
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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.

schematic

simulate this circuit – Schematic created using CircuitLab

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.

schematic

simulate this circuit

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.

schematic

simulate this circuit

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.

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Probably nothing. However, "neutral" may not be exactly at Earth potential. In most wiring codes, the neutral goes back to the transformer feeding the immediate locale. You take a risk doing this, as "neutral" can vary in voltage, depending on the type and number of loads present. In any case, wiring the coach's ground to the neutral wire almost certainly violates electrical code.

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High voltage wiring in ships, planes and trains using dedicated return (neutral) line. Earth in technically the chassis of the vehicles.

Although the neutral is also connected to the chassis around the transformer location, it doesn't mean the chassis can be considered as the neutral.

The very important reason why this is not allowed in wiring codes is because the chassis cannot be trusted as the good conductor.

When the chassis is used as neutral (return line), if the return current encounter high impedances, voltage drop will occur and resultant voltage differences in the chassis will be a very serious safety hazard.

People accidentally exposed to this voltage may died electrocuted. High current in poorly conducting joints will generate excessive heat and create fire hazard.

To put in general term, the answer for the question is, Don't Do It...!!

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Of course, it's a big safety problem. The earth is to protect the electric hazard if a device malfunctions, the current may not flow trough earth wire if everything is OK.
Now let's suppose that somehow the neutral wire is diconnected (in your specific case where the earth is connected instead the neutral) the entire case will have live on it, this of course is death dangerous.

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