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Discuss weird lighting fault in the Electrical Wiring, Theories and Regulations area at ElectrciansForums.co.uk.

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  1. Dean Williams
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    Dean Williams Electrician's Arms

    Hi folks
    while testing this theatre, i attempted to get a Zs from an emergency exit sign. The steel conduit is being used as cpc.
    when i connect my three probes to l,n,earth i'm getting 110v on the screen and when i press test it comes up with about 160 ohms.
    i put a voltmeter across and found 110v between L and E but 230v between L and N.
    could this be a poor connection on the steel conduit cpc and if so, why does the voltage drop to approx 110v

    thanks...its a bit of a puzzle
     
  2. telectrix
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    telectrix Scouser and Proud of It Trusted Advisor

    Location:
    cheshire/staffordshire
    Business Name:
    Telectrix
    do a R2 test on the conduit end to end. then test to MET.
     
  3. Dean Williams
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    Dean Williams Electrician's Arms

    from the board to the fitting is about 300 metres...
     
  4. telectrix
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    telectrix Scouser and Proud of It Trusted Advisor

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    even if it's 3000m you've got to get it right, one way or another. do it in stages with a wander lead.
     
  5. Dean Williams
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    Dean Williams Electrician's Arms

    that will be fun, all the conduit is buried in walls and in ceilings to which there is no access. Thanks
     
  6. Dean Williams
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    Dean Williams Electrician's Arms

    assuming it is a bad earth, can someone please explain the science behind it...how does it become 110v???
     
  7. Guest 004
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    Guest 004 Guest

    sounds like a break in the conduit to the fitting, just code 1 it, and recommend rcbo
     
  8. telectrix
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    telectrix Scouser and Proud of It Trusted Advisor

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    code 1 it and FIX it. you don't put a band- aid on an arterial bleed.
     
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  9. Dean Williams
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    Dean Williams Electrician's Arms

    thanks. the conduit is 35 years old!
    just dont understand why its 110v
     
  10. Dean Williams
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    Dean Williams Electrician's Arms

    i will....i just want to get my head round the reason as to why the voltage drops to 110v
     
  11. Guest 004
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    Guest 004 Guest

    probably getting a reading through the damp walls, and forgot to mention, you should not walk away from a code 1, without isolating or rectifying.
     
  12. telectrix
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    telectrix Scouser and Proud of It Trusted Advisor

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    floating earth?
     
  13. Dean Williams
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    Dean Williams Electrician's Arms

    i'd be there forever and a day if i rectified all 76 code 1s that ive found so far!!!
     
  14. Guest 004
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    Guest 004 Guest

    There should be an agreement of what to do, as you find the code 1's
     
  15. telectrix
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    telectrix Scouser and Proud of It Trusted Advisor

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    only 76, must have been done on a good day.
     
  16. mogga
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    mogga Just me Electrician's Arms

    FFS I realy hope you have a contract saying code 1s to be repaired
     
  17. platty1005
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    platty1005 Guest

    I assume you are doing a PIR ?
    Conduit as CPC 35 years old
    76 code 1's so far I guess the recommendation is a full re-wire
    As for 110V L-E try testing much closer to the main DB
    I guess the conduit is badly corroded at various points and is a very high resistance compared to L-N insulated copper conductors
     
  18. WDMDL
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    WDMDL Electrician's Arms

    Location:
    Manchester
    If you've got good Zs figures elsewhere you can rule out the earthing conductor itself.
    So if we're dealing with a fault on just this one conduit run, here goes:

    Let's assume:
    The conduit is completely open circuit quite near the origin.
    The insulation resistance L-E is 0.5Meg and N-E is 0.5 Meg. (measured at end of circuit)
    Voltmeter input impedance is 10Meg.

    Think of a potential divider, L to N, with the conduit in the middle
    For a 230V supply, you'd get a theoretical 115V
    But with the 10 Meg of your voltmeter in parallel with whichever 0.5Meg you were measuring across, the voltage seen would reduce slightly.

    Now if you've got a measured Zs of 160 Ohm and 110V L-E, sounds like something is corroded AND full of water

    Unless someone's made up an IT system, centre tap earthed through an impedance....

    Simon.
     
  19. Knobhead
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    Knobhead Guest

    Try using an old analogue meter and then check the voltage. I hate DVM’s give me an AVO any time!
     
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  20. Dean Williams
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    Dean Williams Electrician's Arms

    Thanks for the explanation...i understand it more now.

    I'll try it with an analogue voltmeter tomorrow. Thanks
     
  21. telectrix
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    telectrix Scouser and Proud of It Trusted Advisor

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    one like this? i've got 2 of them

    140.jpg
     
  22. Des 56
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    Des 56 Trusted Advisor

    Location:
    Gliese 581C
    An Electrician and his Avometer
    It was at one time, like the Lone ranger and his trusted horse Trigger[​IMG] , couldn't get anywhere without the other


    Sadly my own passed away quite a few years ago,stolen it was,sob,I still miss her though,sob

    That picture has an uncanny resemblence to mine,where did you happen upon it tel
     
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  23. telectrix
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    telectrix Scouser and Proud of It Trusted Advisor

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    that particular one is at present on ebay. there's a few on. ranging from a few quid upwards. my 2 aran older model 8 ans a 7.

    edit: marston's pedigree is affecting my shpellinge
     
    Last edited: Sep 14, 2011
  24. Huw Matthews
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    Huw Matthews Guest

    Check your meter, Ive had allsorts of strange voltages.
    Once had 382v on what should have been a 230 circuit just because the batteries running flat.
     
  25. Knobhead
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    Knobhead Guest

    Mine was a model 8 mk 3 complete with 200A shunt and 5KV Aux transformer, I was gutted when the kit was nicked.

    Both the add on bits were from about the 50’s, beautiful varnished teak cases. Bloody death traps, but impressive when you got them out of their leather cases. Never had the bottle to use the transformer above 1.5KV even though I bench tested it to 5KV DC to earth and between windings. I might be daft, I’m not suicidal!
     
  26. Knobhead
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    Knobhead Guest

    Let us know how you go on please :smug2:
     
  27. sparkydude
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    sparkydude Electrician's Arms

    Another thought, seeing as its an emergency light in a theatre, there is not a sort of central battery system supplying it is there??? this used to be used a lot for cinemas,theatres etc and would account for the 110V being so perfect on your meter.

    Just an idea to throw into the pot

    Nick
     
  28. Dean Williams
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    Dean Williams Electrician's Arms

    Hi
    there's no central battery unit. its a dedicated DB just for em lights.
    i've been told to Code 1 it as possible bad earth and that a different company will carry out the fault finding/repairs.
    by the way ive finished testing the theatre now. total of 94 DBs, 643 section F observations including 93 code 1s.
     
  29. jay sean
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    jay sean Regular EF Member

    could it be a lose/missing neutral?
     
  30. Dean Williams
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    Dean Williams Electrician's Arms

    no because there's 230v across l and n
     
  31. rsmck
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    rsmck DIY

    Location:
    Dumfries
    That sounds like many theatres I've been in ;) - two MCBs being clamped to the fly rail (not in any form of enclosure) being one of the most amusing (electrically) things I've seen..
     
  32. Dean Williams
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    Dean Williams Electrician's Arms

    i found a 3 phase board that had been converted to single phase by linking across red yellow and blue.....fair enough, but the supply cable was 50mm red.......and they'd linked between with 16mm earth!!
     
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  33. Knobhead
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    Knobhead Guest

    One I came across a lot on old 3Ph 110V distribution was old EE SS type DP boards used for 3Ph. This was 25+ years ago. Quite ingenious, cut the bus-bars and put a link cable in. Bloody confusing for a wet behind the ears newbie.

    One apprentice was killed by one of these boards he stuck his fingers down the back of the fuses and found the bare bus-bars.

    View attachment 7963
     
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