I've read the Wiki section about borrowed neutrals which explains why they are a bad idea and i understand why it would be better to avoid them. Howver, do they comply with current regulations - ie while it is a good idea to avoid them is it actually against regulations to install such a situation?
Borrowed Neutrals
- Thread starter AndrewSchofield
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Borrowed neutrals are not permitted.
Reg 314.4. "Where an installation comprises more than one final circuit, each final circuit shall be connected to a separate way in a distribution board. The wiring of each final circuit shall be electrically separate from that of every other final circuit, so as to prevent the indirect energizing of a final circuit intended to be isolated."
Guidance Note 3. Inspection & Testing, says in 2.6.3 item 4, (Which is checking for compliance with the above regulation.)
"Circuits to be separate (no borrowed neutrals) (314.4)"
Reg 314.4. "Where an installation comprises more than one final circuit, each final circuit shall be connected to a separate way in a distribution board. The wiring of each final circuit shall be electrically separate from that of every other final circuit, so as to prevent the indirect energizing of a final circuit intended to be isolated."
Guidance Note 3. Inspection & Testing, says in 2.6.3 item 4, (Which is checking for compliance with the above regulation.)
"Circuits to be separate (no borrowed neutrals) (314.4)"
Cheers, thought that would be the case because it seemed odd that something that could potentially be so dangerous could be premitted by any regulations. Its just that the WikiSection just says that its a bad idea.
Suuget that the Wikisection is amended to ....."a very naughty, illegal, bad idea!" 
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521.8.2 is the main regulation where it says:- “The line and neutral conductors of each final circuit shall be electrically separate from that of every other final circuit, so as to prevent the indirect energizing of a final circuit intended to be isolated.”
Also 431.2.1 requires the neutral conductor to be protected against short circuit this is done by protecting the line conductor that feeds it where multi line conductors feed a single neutral it would need to be able to take the current from all the line supplies. 431.3 refers to disconnection and reconnection of the neutral and here we look at multi-pole switching so using the neutral from on consumer unit for a circuit supplied by another consumer unit would be against the rules as you could switch the neutral independent of the live. Also consideration of electromagnetic effects 521.5.2 can also cause problems where neutral wires are not fed with the line wire.
If a single lighting circuit for example a two way lighting circuits on a stairs borrows the line from another lamp so twin instead of triple wire strappers can be used this is not really breaking the regulations except for EMC considerations but if that lighting circuit is split and upstairs and downstairs lights are feed from a separate MCB then it would. Assuming that is that the individual cables are not feed through any ferrous material.
Since with a three phase supply a single neutral is feed by three line (Phase) conductors the regulations have to be worded to allow for this hence the allowing of the stairs two way switching but in real terms because of EMC and electromagnetic effect in real terms this method should never be used and it has not been used for some time now.
Eric
Also 431.2.1 requires the neutral conductor to be protected against short circuit this is done by protecting the line conductor that feeds it where multi line conductors feed a single neutral it would need to be able to take the current from all the line supplies. 431.3 refers to disconnection and reconnection of the neutral and here we look at multi-pole switching so using the neutral from on consumer unit for a circuit supplied by another consumer unit would be against the rules as you could switch the neutral independent of the live. Also consideration of electromagnetic effects 521.5.2 can also cause problems where neutral wires are not fed with the line wire.
If a single lighting circuit for example a two way lighting circuits on a stairs borrows the line from another lamp so twin instead of triple wire strappers can be used this is not really breaking the regulations except for EMC considerations but if that lighting circuit is split and upstairs and downstairs lights are feed from a separate MCB then it would. Assuming that is that the individual cables are not feed through any ferrous material.
Since with a three phase supply a single neutral is feed by three line (Phase) conductors the regulations have to be worded to allow for this hence the allowing of the stairs two way switching but in real terms because of EMC and electromagnetic effect in real terms this method should never be used and it has not been used for some time now.
Eric
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