Is this a common house wiring fault?

In an early eighties property, would it have been common wiring practice for a two-way switched landing light to use a live from the downstairs lighting circuit and a neutral from the upstairs lighting circuit?

Reason I ask is that it appears to be the reason why the downstairs and upstairs lighting circuits cannot be separated and connected, one circuit to each of the two RCDs in the consumer unit. In those days RCDs were not widely used, so I suspect it may not even have been considered a "fault" as such, though it was possibly not "best practice"?
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  • It has never been acceptable, or good practice, to borrow neutrals between circuits, but is fairly widespread amongst the lazier type of house-bashing electrician.
    A kind word lasts a minute, a skelped erse is sair for a day.
  • fwor
    fwor Posts: 6,806
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    Thanks - the house was built (apparently) in several goes, with one or possibly two builders starting and then going bust, so it doesn't surprise me that they got the cheapest least competent electricians they could find.

    Seems I need to do some more testing tomorrow. If true it's a pain, but I can't think of a simple solution that doesn't involve extra wiring from ground floor to loft (which is I guess why they took the short cut in the first place?).
  • ValHaller
    ValHaller Posts: 5,212
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    I doubt you are looking at RCD's. I think you mean MCB's
    You might as well ask the Wizard of Oz to give you a big number as pay a Credit Referencing Agency for a so-called 'credit-score'
  • System
    System Posts: 178,076
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    Don't forget that in the 'old' days before MCBs the neutral was a common point across all circuits with no separation between upstairs/downstairs/lighting so there was no need to ensure that the neutral came from the same area as the live feed

    It was only the advent of MCBs that brought in the concept of totally separate circuits and highlighted the problem of common neutral.
  • fwor
    fwor Posts: 6,806
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    ValHaller wrote: »
    I doubt you are looking at RCD's. I think you mean MCB's
    You doubt wrong. I meant RCDs.

    It works just fine split between two MCBs, but not between two RCDs - hence my suspicion that the two circuits share a neutral at some point.

    !!!!!!: agreed, I don't think it would have been considered an issue when the house was originally built. Oh well, some lateral thinking needed, then...
  • Dave_J_3
    Dave_J_3 Posts: 100 Forumite
    yes it was quite common and it has never been acceptable but people still done it to save running a 3rd core.
    You can either rewire that particular circuit
    or place both lighting circuits on the same RCD in the consumer unit.
  • fwor
    fwor Posts: 6,806
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    Dave_J wrote: »
    yes it was quite common and it has never been acceptable but people still done it to save running a 3rd core.
    You can either rewire that particular circuit
    or place both lighting circuits on the same RCD in the consumer unit.

    Must have saved them 2m of cable and 5 mins labour while the house was being built, but there you go...

    Thanks - as you say, I'll just have to have them put on the same RCD, and live with all lights out if that one trips (not really a big deal for me).
  • ValHaller
    ValHaller Posts: 5,212
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    fwor wrote: »
    You doubt wrong. I meant RCDs.

    It works just fine split between two MCBs, but not between two RCDs - hence my suspicion that the two circuits share a neutral at some point.

    !!!!!!: agreed, I don't think it would have been considered an issue when the house was originally built. Oh well, some lateral thinking needed, then...
    So you have individual RCD's for upstairs and downstairs lights? This would be unusual. Or do you mean RCBO's?
    You might as well ask the Wizard of Oz to give you a big number as pay a Credit Referencing Agency for a so-called 'credit-score'
  • fwor
    fwor Posts: 6,806
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    ValHaller wrote: »
    So you have individual RCD's for upstairs and downstairs lights? This would be unusual. Or do you mean RCBO's?

    No, a 17th Ed split load consumer unit with an RCD on each "half". I'm led to believe that where possible it's advisable to split lighting circuits across both "halves" so that if either trips you still have some lighting.

    But that can't be done in this situation.
  • zax47
    zax47 Posts: 1,263 Forumite
    edited 31 March 2013 at 9:27PM
    fwor wrote: »
    No, a 17th Ed split load consumer unit with an RCD on each "half". I'm led to believe that where possible it's advisable to split lighting circuits across both "halves" so that if either trips you still have some lighting.

    But that can't be done in this situation.

    Yep, that's the recommended way to split circuits across a dual RCD board ("17th Edition") - to minimise the effects of nuisance tripping. Upstairs lights and downstairs sockets on one RCD and vice-versa on the other. The commonly found "borrowed neutral" on the landing light always causes problems as it's got live from one RCD and neutral from the other - causing >30mA imbalance and thus RCD tripping.
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