1. Visiting from the US? Why not try DIYnot.US instead? Click here to continue to DIYnot.US.
    Dismiss Notice

Separate lighting circuits/borrowed neutrals

Discussion in 'Electrics UK' started by Alfie25, 1 Jun 2015.

  1. Alfie25

    Alfie25

    Joined:
    13 Apr 2012
    Messages:
    358
    Thanks Received:
    2
    Location:
    Durham
    Country:
    United Kingdom
    I’ve got an oldish fusebox with cartridge fuses and am looking to get it replaced with a consumer unit. At the moment it has separate upstairs and downstairs lighting circuits, each with a 5A cartridge fuse. I had a guy round to give a quote and he mentioned that there would probably be borrowed neutrals upstairs and that if this was the case then he would have to put the lights all on one circuit. I don’t doubt what he’s saying but wondered if someone could explain why. I’m just wondering why it’s ok to have separate upstairs and downstairs lighting circuits with borrowed neutrals on a cartridge fuse system but not on a rcbo or mcb consumer unit.

    Oh, and if replacing 2 x 5A circuits with 1 circuit, is he likely to make it 10A?
     
  2. Sponsored Links
  3. ban-all-sheds

    ban-all-sheds

    Joined:
    27 Aug 2003
    Messages:
    69,782
    Thanks Received:
    2,858
    Location:
    London
    Country:
    United Kingdom
    The traditional way of putting in 2-way switching for the landing light does indeed result in a borrowed neutral when the lighting circuit is subsequently split into separate upstairs and downstairs ones.

    It's quite possible that if he is local, and there are other houses like yours, he is familiar with the way they were wired and knows that if the lighting has been split it's likely to have created a borrowed neutral.


    He's being lazy - that's not the only way. If it is the jolly old landing light then he can either take a neutral to it from the downstairs circuit or rewire the switching so that the live comes from the upstairs one.


    It's not.

    http://www.diynot.com/wiki/Electrics:safety_information:sneutral


    Fuse/MCB is irrelevant.

    If the two circuits are on RCBOs, or different RCDs, then these will trip because the diverted neutral currents will cause an imbalance.


    No - it will still be a 6A one, unless you have a mad amount of lighting.
     
    • Thanks Thanks x 1
  4. securespark

    securespark

    Joined:
    11 Jan 2004
    Messages:
    37,154
    Thanks Received:
    1,290
    Country:
    United Kingdom
    At one time it was the done thing to wire ground and first floor lighting circuits in this way.

    The hall light fed a common line across to the downstairs landing switch. So in effect, the landing light is fed from the downstairs line and the upstairs neutral.

    It's more accurately a borrowed line, really.

    It is considered a danger today and the circuit is impossible to wire separately on the twin RCD boards as the RCD's trip out because of the imbalances between them.
     
    • Thanks Thanks x 1
  5. Taylortwocities

    Taylortwocities

    Joined:
    28 Jul 2006
    Messages:
    20,553
    Thanks Received:
    2,132
    Location:
    Oxfordshire
    Country:
    United Kingdom
    First of all, you only need to be concerned if there IS a borrowed neutral. This is not a usual problem, but it can happen.

    Secondly.
    If there is a borrowed neutral, this can be a problem with a dual RCD CU if the up and down light circuits are on different RCDs.
    It is not the best practice, but there is no issue if he puts both circuits on the same RCD (but then you lose all of your lights if the RCD trips.


    So thats sorted then.

    Can he join then up onto one MCB, yes, if the load is less than wiring can cope with. It will depend on the loading but it is likely to be limited to 6amp. He may be able to put a 10A MCB on the combined circuit but he would need to check if the wiring (and other factors) are OK first.
     
    • Thanks Thanks x 1
  6. ban-all-sheds

    ban-all-sheds

    Joined:
    27 Aug 2003
    Messages:
    69,782
    Thanks Received:
    2,858
    Location:
    London
    Country:
    United Kingdom
    Not without resolving any shared neutrals....
     
    • Thanks Thanks x 1
  7. JohnW2

    JohnW2

    Joined:
    28 Jan 2011
    Messages:
    48,646
    Thanks Received:
    3,191
    Location:
    Buckinghamshire
    Country:
    United Kingdom
    A 'borrowed neutral' is when one or more lights uses the neutral of another lighting circuit, rather than it's own. This was often done for convenience, particularly with landing/hall lights.

    The issue is with RCDs or RCBOs, which rely on identical currents flowing in the live and neutral wires of a circuit (if they differ, it trips). If two lighting circuits are connected to different RCDs/RCBOs, a 'borrowed neutral' will mean that some of the neutral current will go through the 'wrong' RCD/RCBO, causing the devices (probably both of them) to trip.

    If both lighting circuits were connected to the same RCD, the borrowed neutral would not be a problem (there would be no reason to combine them into a single circuit) - but that is considered bad practice (and may be non-compliant with regulations), since one RCD tripping would take out all the lights in the house. If you are going to have RCBOs (effectively a separate 'RCD' for each circuit), whilst there is a 'borrowed neutral' there would be no solution other than combining the two lighting circuits into one.

    The proper solution is attend to the wiring to get rid of the borrowed neutral, and then to have the two lighting circuits protected by different RCDs or RCBOs - which may or may not be easy to implement.

    By no means all old wiring has 'borrowed neutrals'. Unless the electrician did tests and confirmed that you do have the issue, it could be that the problem is not going to arise, anyway.

    Kind Regards, John
     
    • Thanks Thanks x 1
  8. ban-all-sheds

    ban-all-sheds

    Joined:
    27 Aug 2003
    Messages:
    69,782
    Thanks Received:
    2,858
    Location:
    London
    Country:
    United Kingdom
    When it was often done there was usually only one lighting circuit.


    That could be the case, and not checking would be another facet of the electrician's lazy nature.

    But I stand by what I said - if there are 10's or 100's of houses just like the OP's in the area, all built when 1 lighting circuit and a twin & earth connection between the hall and landing switches was de rigeur, and the CU tells him that the split of the circuit was likely done some years ago, and he's seen this many times, then he is right to say that there is probably a borrowed neutral.
     
  9. JohnW2

    JohnW2

    Joined:
    28 Jan 2011
    Messages:
    48,646
    Thanks Received:
    3,191
    Location:
    Buckinghamshire
    Country:
    United Kingdom
    Indeed - but it was at the point in time when that single lighting circuit was split into two that it was often 'convenient' (for the lazy) to allow that splitting to create a borrowed neutral, rather than do the splitting properly.

    Kind Regards, John.
     
    • Thanks Thanks x 1
  10. Sponsored Links
  11. Alfie25

    Alfie25

    Joined:
    13 Apr 2012
    Messages:
    358
    Thanks Received:
    2
    Location:
    Durham
    Country:
    United Kingdom
    Thanks for all the details there, it's helped me get my head round it (to some extent)

    ban-all-sheds - got it in one. It's a big estate and the electrician is well versed with my style of house so he's fully expecting my house to have the borrowed neutral on the landing light.

    I now see what you're saying around a better fix (i.e. either take a live from upstairs to the downstairs switch, or take a neutral from downstairs to the landing light) but I understand this would mean more disruption and work so I can see why most electricians (and householders) would prefer to avoid it.

    So... what's the better way of going on?
    1.Keep separate circuits but put them on the same RCD side of a split board.
    2. Combine the circuits (does this require much work?) and then put them on a RCD or RCBO.

    Having looked at other people's houses I've seen option 1, although a few of you say it's not good practice.
     
  12. JohnW2

    JohnW2

    Joined:
    28 Jan 2011
    Messages:
    48,646
    Thanks Received:
    3,191
    Location:
    Buckinghamshire
    Country:
    United Kingdom
    They may have all been built the same, with a single lighting circuit, but one images that a lot of different electricians were involved in subsequently splitting that one lighting circuit into two - so it's by no means certain that all houses would have ended up with borrowed neutrals (some electricians might have done the splitting 'properly').
    Both are very easy to implement, and, frankly, there's not a lot to choose between them. Neither is really 'good practice', for the reason I've already given (one fault could take out all the lights in the house) and, for the same reason, both are arguably non-compliant with regulations (which require that steps be taken to minimise the inconvenience/'danger' caused by a single fault). If you don't want the hassle and disruption of getting rid of the borrowed neutral (if it transpires you have one!), then you might as well go with whatever option your electrician suggestions.

    ... but, don't forget, there is at least a chance that you do not have a borrowed neutral - so don't let your electrician do either (1) or (2) unless/until he confirms that there is a borrowed neutral. If there isn't, he can put the two lighting circuits on different RCDs.

    Kind Regards, John
     
  13. Alfie25

    Alfie25

    Joined:
    13 Apr 2012
    Messages:
    358
    Thanks Received:
    2
    Location:
    Durham
    Country:
    United Kingdom
    The house is late 1980's and all the electrics are original, so whatever it's got, it's had from the start. If the old one circuit light system used to be the norm then I can't understand the change in trend to using 2 circuits, especially if many were done with this 'dodgy' borrowed neutral system.

    As for lighting load, I've 5 lights up and 5 lights downstairs. I believe a 5A fuse allows 1150 watts of total power and so at the moment I'm well under that per circuit, but if the 2 circuits are combined then I would be over if all lights were on at once (as some lights are 4 x 50W i.e 200W each)
     
  14. JohnW2

    JohnW2

    Joined:
    28 Jan 2011
    Messages:
    48,646
    Thanks Received:
    3,191
    Location:
    Buckinghamshire
    Country:
    United Kingdom
    I would have guessed that single lighting circuits would have been unusual by the late 80s, but it sounds as if I may have been wrong! As I've implied, I think that 'trend' to splitting the circuits into two was a result of wanting to comply with changing regulations - which for some time now have required the minimisation of inconvenience etc. which would result from a single fault.
    It's sounds as if it might be about time that you thought about changing to 'energy saving' bulbs/lamps (compact fluorescents or LEDs). Even 1150 watts for, say, an average of 6 hours per day, would be costing you about £400 per year to run. By changing your lamps/bulbs, you could probably get that done to about £50.

    For what it's worth, a modern CU would have 6A MCBs, rather than 5A fuses, giving you about 1380 watts per circuit. If you wanted to stick with energy-guzzling bulb/lamps, and wanted to have a single circuit (despite the regulations) you could have a 10A MCB, giving you 2300W.

    Kind Regards, John
     
    • Thanks Thanks x 1
  15. Taylortwocities

    Taylortwocities

    Joined:
    28 Jul 2006
    Messages:
    20,553
    Thanks Received:
    2,132
    Location:
    Oxfordshire
    Country:
    United Kingdom
    In my experience, a house built in the '80s will normally have had separate lighting circuits for up and down. There will not have been a shared neutral when it was installed.

    There may have been additions since then that may cause an issue. It can happen, but so could a Tsunami as a result of the Canary Islands slipping into the sea.

    So why are you worrying so much about something that might not be a problem???

    Your electrician should be testing the existing installation before installing the new CU. If he does his job properly then any issues will be flagged and discussed with you before the heart/lung transplant takes place.
     
  16. JohnW2

    JohnW2

    Joined:
    28 Jan 2011
    Messages:
    48,646
    Thanks Received:
    3,191
    Location:
    Buckinghamshire
    Country:
    United Kingdom
    As I said, that's what I would have guessed.
    Quite so (as I have said, several times!). BAS (and maybe the OP's electrician) is the only one who seems to think that a shared neutral is so likely that it's a bridge worth crossing before it is encountered!

    Kind Regards, John
     
  17. deadshort

    deadshort

    Joined:
    20 Sep 2014
    Messages:
    2,086
    Thanks Received:
    211
    Location:
    uk
    Country:
    United Kingdom
    The OP's electrician has only highlighted a potential additional cost as any competent electrical contractor would do.

    Regards,

    DS
     
    • Thanks Thanks x 1
Loading...

Share This Page