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voltage on back box

Discussion in 'Electrics UK' started by hodpodge, 17 Jan 2021.

  1. hodpodge

    hodpodge

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    Hi all
    when i put my meter on the permanent live on a light switch and the then the other probe on the back box or metal switch itself it is reading 230v but i dont get a shock when touching the switch to turn it on,
    so the fact im measuring between permanent live on the switch and the back box should it show 230v????
    any ideas please
     
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  3. EFLImpudence

    EFLImpudence

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    Yes, as it should be. The back box is earthed.
     
  4. terryplumb

    terryplumb

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    If the back box is earthed ,and the metal switchplate is earthed ,then 230 v would be expected.
     
  5. hodpodge

    hodpodge

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    there is an earth to the back box but when i remove it the reading is still 230v, also the metal switch has not been earthed and when measuring between permenant earth and the metal switch also show 230v, The RCBO tripped and will not go back on, i swapped the rcbo for a 6amp mcb and the lights are working. put the mcb in a position where its protected by a RCD and the rcd trips, (purely did this as fault finding) any ideas?????
     
  6. terryplumb

    terryplumb

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    Do you mean measuring between a permanent earth and a live terminal of the switch ? That would read 230v. But the metal switchplate itself should not be 230v to earth. What type of switch is it ?
     
  7. hodpodge

    hodpodge

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    on the switch they have use the loop in out method so there is a permanent live at the switch, when i measure between this and the unearthed face plate i get 230v, it is a complete mash up
     
  8. JohnW2

    JohnW2

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    You haven't said what you are measuring with but, with most digital voltmeters, you would quite probably get a reading of around 230V between a permanent live and almost anything else (like your own hand), due to 'stray coupling'.

    However, none of this alters the fact that the faceplate of a metal switch must be 'properly' earthed.

    Kind Regards, John
     
  9. hodpodge

    hodpodge

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    im measuring with my kewtech, KT1790, none of the face plates are earthed in the property as are none of the metal light fittings, im sure it's an earth fault causing the rcbo to trip as an insulation resistance on line to cpc measured 0.2
     
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  11. hodpodge

    hodpodge

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    love your video's John
    kind regards
    colin
     
  12. JohnW2

    JohnW2

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    Wrong John :) I presume you are referring to 'flameport'.

    Kind Regards, John
     
  13. Harry Bloomfield

    Harry Bloomfield

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    It will read 230/240v between
    0.2 what? mOhms, Ohms, Kohms, MOhms? and what sort of instrument are you measuring that value with?

    You suggest they are not earthed, what does that actually mean? Are they wired up for earth, but the earth doesn't go to earth?

    Connecting between a permanent live and anything with even a high resistance earth will show some voltage. You are saying 230v, more likely in the UK it should be nearer 240v.
     
  14. hodpodge

    hodpodge

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    sorry didn't explain properly, I measured at the board with all light switches on, board dead and loads removed, measured at 500dc with a megger mft1720, and it was 0.2 mOhms, yes i nearly fell over when i read it, R1 to Rn >999 on the meter, R1 to R2 0.2
     
  15. Taylortwocities

    Taylortwocities

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    There’s a load somewhere that you’ve forgotten about. Tv amp in the loft? Outside light? All sorts. I’d not be squirting 500v DC up R1&R2 before going a lot of other things first. If there is still a load somewhere it’s probably dead now.
     
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  16. JohnW2

    JohnW2

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    Whilst I agree with your comment in general, what's all this talk about "R1 & R2" - do you mean "L and E"?

    Kind Regards, John
     
  17. hodpodge

    hodpodge

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    yes, L and E
     
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