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New Consumer Unit and Two Way and Intermediate Light Switches Problem.

Zoomup
Mornin' All,


Imagine this scenario. You have just installed a new R.C.B.O. consumer unit in a 1970s bungalow. The supply appears to be TN-S. But may be TNC-S in reality. There are two lighting circuits each supplied separately by an individual B6 30 mA R.C.B.O. The hallway is an "L" shaped design with two ceiling pendant lights. There are two two way switches at the ends of the hall way, but between them are two intermediate switches as well. Four switches in total control the two pendants together. All cables are in good condition and the insulation resistance is very high between L and N and all Lines. There is evidence of rodent infestation in the loft.


When just the front door lamp is fitted the light works and switches correctly from all four switches.



But when the second (remote) lamp is fitted as well and turned on by any switch, both R.C.B.Os trip off.


Why?


Z.




  • Former Community Member
    0 Former Community Member
    Morning Zoomm


    some sort of N cpc going at the the final switch ?
  • 0

    But when the second (remote) lamp is fitted as well and turned on by any switch, both R.C.B.Os trip off.



    My guess would be that 2nd lamp is connected to the "wrong" circuit's N. 1970s probably sheathed singles 'spider wiring' so very tempting to take a black to the nearest convenient N terminal rather than necessarily one on the same circuit which might be further away - usually seen in houses with the landing light fed from the downstairs L and upstairs N.


    Correct RCBO trips because the circuit has lost one lamp's worth of current - other RCBO trips because it's equally out of balance having mysteriously gained the same.


      - Andy,
  • 0
    A landing light scenario without any stairs.


    Andy B
  • 0
    Indeed A.J. has it in one. Wouldn't you have thought that a simple link using T&E cable between the two lighting points would have been so simple? The first light point was connected to the correct supply neutral, but the second lighting point was connected to the second lighting circuit's neutral. But the fault only showed when the second lamp was in place. The strange thing is that a twin and earth cable links the two lighting points and the black neutral conductor has been left disconnected at both ends. Why? I am not sure if any testing by me before the new consumer unit installation would have identified this fault. I did find the problem's cause pretty quickly though. That's the good thing about bungalows; all of the wiring is easily accessible in the loft space.


    Z.
  • 0

    dustydazzler:

    Morning Zoomm


    some sort of N cpc going at the the final switch ?




    Hello Dusty,

                               it is nice to hear from you.


    The problem confused me as the two lighting points caused some me some confusion. The first lighting point worked correctly. The second lighting point, linked to the first caused the problem. (Common line but different neutrals). I was lucky to be able to fault find on my own in a quiet environment with nobody there but me. It helped me to concentrate.


    Bye,


    Z.

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