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I was reading through some SE questions/answers on residential power transmission and came up with two scenarios I'm unsure about:

  1. Assume that your house is wired properly (with no degradation), but there is a fault on the neutral wire between the grounding post and the transformer on the corner of your street. Will your outlets work properly?

  2. Similarly, I'm familiar with the case where a fault in the neutral wire between the grounding post and a circuit in your house can pull it up to line voltage. I guess this means that the two 180° out of phase live circuits in your house NEVER share the same physical neutral wire? What would happen if they did and a fault occurred?

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If you have totally balanced loading on your two phases you will not be able to detect any change. EDIT: Note that this is pretty much never going to be the case.

  1. EDIT: If you have a poor ground connection or the utility provider has a poor ground connection you will have a floating ground. Unless the neutral is bonded to a good local ground point it will float. It will float in potential closer to the phase that has the heavier 115V loads. If you have a 10A load on one phase and a 1A load on the other phase you may find that the 1A 115V load sees 200V and the 10A 115V load sees only 30V. Electronic and reactive devices will behave unpredictably at 200V or 30V if they are designed for 115V. Resistive loads will dissipate more or less depending on the supply voltage, over voltage will easily burn out transformers, globes and motors, under voltage will also affect electronic power supplies and synchronous motors that fail to get to speed.
  2. They can will be sharing the utility neutral until the point where the neutrals is are bonded in the fuse cabinet. From this point it would be unwise to share it for reasons like you describe but I can see it happening when extra outlets are added with a mix of 115V and 230V without due supervision. EDIT: The consumer is not expected to cope with a floating utility side neutral or ground, it should never occur, in some wiring codes they are bonded in the fuze box, in others the ground is local and the neutral comes from the utility transformer common connected to the ground at the utilities transformer. Sharing an local neutral conductor between unbalanced loads is bad practice and not conformant to most wiring codes.

EDIT:

Below you can see how two dissimilar loads will cause the floating neutral point to be pulled from the earth reference if they share a neutral wire and it is compromised. In this example with the loads selected the larger load will be running at undervoltage (23V) and may survive if it is an incandescent lamp or heater but may fail if it is a motor or electronic load. The other load will see 207V and will likely suffer some failure no matter what sort of load it is.

It is also worth noting that the floating neutral point will be 92V above the protective earth, if any, in this example.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Your first sentence has me confused. Does this mean that in a perfectly balanced system, no current flows through neutral? How is the circuit a closed loop? Also, I get the general idea of the side with the higher load current pulling neutral closer to it, but how did you get those numbers? \$\endgroup\$ – KnightsValour Jul 30 '15 at 21:22
  • \$\begingroup\$ The current in a 230V circuit in the USA does not use the nutral line at all. The current in the two live lines is equal (opposite to each other by design). The numbers were illustrative. \$\endgroup\$ – KalleMP Aug 1 '15 at 7:39
  • \$\begingroup\$ Thanks for clarifying the numbers and posting the image! I disagree with your comment though. If there is only 1 load on one of the phases, there is no way current won't flow through neutral. I think the only time current doesn't flow through neutral is if you have two perfectly matched loads operating on both phases simultaneously. \$\endgroup\$ – KnightsValour Aug 5 '15 at 14:15
  • \$\begingroup\$ If you have 230V load(s) across the two live conductors the neutral is not even connected, hence no current. If you have any combination of 115V loads that are not perfectly balanced then you will obviously have the nett imbalance of current in the two live lines returned through the neutral. \$\endgroup\$ – KalleMP Aug 9 '15 at 9:10
  • \$\begingroup\$ Ah sorry - I misunderstood your earlier comment. You're right about the 230V loads not using neutral. Thanks for the explanation! \$\endgroup\$ – KnightsValour Aug 9 '15 at 18:29

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