Shared neutral and RCD woes

Discussion in 'Electricians' Talk' started by JohnDeere8420, Mar 13, 2018.

  1. JohnDeere8420

    JohnDeere8420 Member

    I have both electric gates and outside LED lights powered from the house electrical supply via a single armoured multi core cable.

    Each of the three circuits (2 lighting & 1 gate) is powered by a separate a RCD in the house (although the lighting circuits are connected together here (at the start of the armoured cable in the house) so in essence act as one.

    However both RCDs trip when either the gate or the lighting circuit is turned on, when the other is already on (powered by the other RCD).

    However if both circuits are powered from the same RCD no problems occur.

    My thought is, due to the lack of available wires in the multi core cable, both the earth and more importantly neutral cables are common to all the circuits in question. I'm thinking this may be the reason why both RCDs trip when each circuit is powered through separately as there is an uneven load between the lives and neutral cables? It also explains why a single RCD does not trip with only one circuit live or both of them powered through the same RCD

    If this is the case I guess there is not much I can do about it apart from of course using the same trip or not using one at all (both of which I don't want to do).

    I read somewhere a relay might solve my woes. How and where would this be wired in?

    I welcome your views and any advice.

    Also just for the sake of completeness, I would like to test each circuit to see if there is any leak to earth. I have an Ohm meter, how would I go about this? Maybe take a reading between each live and an earth somewhere?

    If it makes any difference my electric supply to the house is overhead and carries only two conductor, live and neutral. It seems the earth for the house is bonded to the neutral incoming supply. Every few posts up the road seems to have the neutral grounded on it

    Excuse my ignorance on some of this, however I am definitely no electrician and have just gleaned some things through experience and off the net, so both could be incorrect !

    I welcome your views and any advice.


    Last edited: Mar 13, 2018
  2. Dr Bodgit

    Dr Bodgit Well-Known Member

    Yes you've got shared neutrals (and earth). Probably easiest to connect all the earth cables (3 of them?) into a single RCD.

    But this raises a bigger question, who the heck designed, installed and tested this lash up?

    Was it you?
  3. JohnDeere8420

    JohnDeere8420 Member

    I agree, it seems a lash up. However I bought the house like it and without digging the driveway up I have to live with it. In my view it is not unsafe (might be wrong) just badly designed in a very unorthodox manner.

    I would like to use RCD trips on both circuits for safety reasons and separate ones at that due to the fact say if a bulb should blow or dampness gets into a fitting which causes the RCD to trip, it then does not stop the gates from working (very important if everybody is out at the time as the gates are 8 foot high!).

    At present they are wired into one RCD with no problems
  4. Pollowick

    Pollowick Well-Known Member

    How many cores in the SWA?

    How is the cable terminated at each end?

    I have a couple of ideas ...
  5. JohnDeere8420

    JohnDeere8420 Member

    from memory there are....

    4 conductors which breakdown to 3 lives (2 for the two lighting circuits & 1 for the gate) and one common neutral. Earth is obtained from the armoured steel braiding.
    There is a junction box in the house at the start, one at the end at the gates which feeds the gates and the last lamps. (gates being on separate circuit to these lamps of course)
    I assume there are also two branches midway where there are 'T' offs for the lighting circuits (1 stand alone lighting circuit - the lampost) & the other lighting circuit that powers the step lights which uses the same live conductor as the one that goes to and powers the lights at the end of the cable near the gates, as mentioned above

    As mentioned previously the two lives that power the lighting circuits are connected together in the house junction box to become one and there is one common neutral shared between everything fed off the cable

    I want two rcds if possible, one for the lights and one for the gates. Or am I being OTT esp since nobody touches the lights, they are plastic, nobody touches the metal gates and they are earthed ?

    Does this make any sense ?
    Last edited: Mar 13, 2018
  6. Dr Bodgit

    Dr Bodgit Well-Known Member

    Just DIY, but I'm not sure if that earthing configuration is pukka, what size is the SWA/cores? Better, if the SWA size allows, is to use 3 cores for live, neutral and earth with appropriate bonding of the braiding. Mr P or other sparks are more qualified to advise though.

    Think its 4 core, right? 3 circuits/live and shared neutral.
  7. Pollowick

    Pollowick Well-Known Member

    There certainly is a mix and will be difficult to resolve. It can be, but will you have the same functionality?

    Where are the light switches?
  8. JohnDeere8420

    JohnDeere8420 Member

    there are no switches per-se Instead the gate is always on and the lights are on a timer at the mains plug.

    yes 4 core plus outer as earth
  9. JP.

    JP. Screwfix Select

    Right you have 4 cores and earth armour. At present 1 core feeds gate, 2 cores feed lights, and the other core is the common neutral - thus the transposition is as below

    1 core for live gate, and 1 core for neutral gate

    Lights made into one circuit thus 1 core lights, and 1 core neutral for lights

    You end up with two totally separate circuits in the one piece of SWA so I dont know what you are using db wise, but maybe one for one rcbo and one for t'other rcbo or whatever, the main thing is the neutrals are patent.

    Please flash cores accordingly and perform ir etc tests.. thank you :)
  10. Pollowick

    Pollowick Well-Known Member

    Have been thinking of something similar and just about to post! He will also need to consider the RCDs too. Is it worth putting the two circuits on individual RCBOs? Or filling a dual pole RCBO in IP 68 boxes at the gate or where the lights are? Also mention of a "timer at the plug" - is that in the house or outside? It really needs a local sparks to go and look and quote for the remedial work.
  11. JohnDeere8420

    JohnDeere8420 Member

    Many thanks for this.

    1st line as a summary is spot on
    2nd Yes the line is possible,
    3rd line.... Not sure that is possible, as I have one wire (neutral) going to the gates and both sets of lights. I would need to dig up the two junctions (after finding them) and rewire ?
  12. JP.

    JP. Screwfix Select

    True Mr P. Is it worth putting on two rcd's or rcbo's? Hmmmm well exterior lights can go dicky after H20 ingress etc so I would say yes, but as you say there are other things to consider like that timer affair and stuff.
  13. JP.

    JP. Screwfix Select

    Difficult for me to quantify further John..but hey good luck m8 and I hope you sort it.
  14. JohnDeere8420

    JohnDeere8420 Member

    I found this when I opened the main box in the house where the SWA terminates the house. It's the same as detailed above...

    4 cores SWA....
    3 cores used for lives. These are GREY for gate automation, BLACK to gate & step lights and BROWN to lampost.
    1 core BLUE for a common neutral
    Outer sheath as common earth

    To me I don't think there is a way out of it unless I rewire the junction box outside of the house. Not impossible to do, but difficult to access.

    Attached Files:

  15. Pollowick

    Pollowick Well-Known Member

    This is where is needs someone local to actually see it. If the junctions cannot be found easily then, will remote RCDs (of some sort) be the appropriate method?

    At least the information here will let the OP talk to a local spark and describe what he needs and understand what he is being told is possible.

    The drawing shows it is connected to a socket in the house which makes it more interesting. Definitely needs full on-site review and options to resolve.
    Last edited: Mar 13, 2018
  16. JP.

    JP. Screwfix Select

    In some installations the supply type might dictate 5 cores Mr P?
  17. JP.

    JP. Screwfix Select

    Hmmmm interesting..

    gate light wiring.jpg
  18. JP.

    JP. Screwfix Select

    Nice if the box under the flower bed could be exposed, the one on wall can be opened presumably..if this is done a bit of ducking and diving could then be started after answering the below question.

    What supply type have you got at intake position JD?
  19. Bob Rathbone

    Bob Rathbone Active Member

    The only way to resolve this easily will be to remove the RCD's from the origin of the circuits and re site them locally to the loads they protect. OR, put the whole lot on one RCD at the origin.
  20. JP.

    JP. Screwfix Select


Share This Page