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Another installation testing issue

Discussion in 'Electrics UK' started by Licklieder, 10 Jul 2021.

  1. Licklieder

    Licklieder

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    So, having completed the replacement of the mcb in my consumer unit with RCBO's I cracked on with testing and hit an issue with the lighting circuit.

    Everything tests out fine - with the exception of the Line-Neutral IR test, which tests out as 0MOhm at 250V. Doing a continuity measurement reveals a Line - Neutral resistance of 175 Ohm.

    I've managed to isolate the issue to the bathroom, but am at a loss to explain what's causing it. I've disconnect the switches, isolated and then disconnected the Fan but i can't get rid of this 175 Ohms. The only hypothesis i can come up with that makes sense is that the dimmable transformers must be permenantly wired in to Line & Neutral, with a seperate line from the dimmer switch to control the lights. Does that make sense ?

    I'd like to get the lights out from the bathroom to see what's up there but am struggling to get the out.
     
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  3. Swwils

    Swwils

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    How was the CPC continuity and polarity to each accessory point ?
     
  4. Licklieder

    Licklieder

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    Seemed to test out reasonably ok. When i linked out at the CU & tested at a wall sconce at the end of the run (dimmer linked out) I got continuitiy readings of:

    Line - Earth: 1.71 Ohm
    Line - Neutral : 1.46 Ohm
    Neutral - Earth : 1.59 Ohm​

    Bit more variance than i'd like, but the trip times for the PFC / PSC (>30A) are within spec for the 6A rating. I did find a Neutral / Line had been swapped at one point in the circuit and fixed that - .
     
  5. BS3036

    BS3036

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    Shaver socket?
     
  6. Adam_151

    Adam_151

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    I personally wouldn't even try to get a L-N insulation resistance test on an existing lighting cirucit with loads like fans and transformers connected. Stick L and N together and test to earth, L-N can go down as lim as its not resonably practicable to do so
     
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  7. Swwils

    Swwils

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    Possible neon on a hidden fcu?
     
  8. Licklieder

    Licklieder

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    There used to be one that was removed (I hope) when the bathroom was done & the walls tiled. I always assumed that was on the Final Ring Main anyway. Did make me think about the wall cabinet above the sink though - wonder if that's got an isolation transformer in it & if its wired into the lighting circuit. something else to investigate - it need to take it down anyway as a 'guest' snapped off the pull cord for the cabinet light
     
  9. Licklieder

    Licklieder

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    Completely forgot about tieing L-N together to do the IR test - got so focussed on trying to work out what the cause of the reading was it slipped my mind. I'll give that a go when i next power down
     
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  11. Licklieder

    Licklieder

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    I hope not - the only place for it be hidden would be above the lowered ceiling, and so inaccessible.
     
  12. EFLImpudence

    EFLImpudence

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    175Ω L - N is around 330W so will be quite warm if one item.
     
  13. AndyPRK

    AndyPRK

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    It's about 1.3A so more significant than a neon.

    If its real, worth finding it cus it could reduce your leccy bill!
     
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  14. BS3036

    BS3036

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    A neon will have a series resistor of at least 10k, more likely much higher.
     
  15. AdrianUK

    AdrianUK

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    Unless it is something transformer based. If the OP is measuring the DC resistance of a small transformer, its AC impedance will be much higher & hence pulling nothing like 330W at 50Hz
     
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  16. zebedee2001

    zebedee2001

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    Shower pump? TV aerial booster? Towel rail?
     
  17. Licklieder

    Licklieder

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    Thats my current thinking - just need to get the damned halogen lights out to check. I honestly don't know how they were fitted - the size of the cutout is smaller that the size of the lights at the edge of the springs, and there are no grooves in the plasterboard to indicate where they were inserted.
     
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