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  1. DirtyDog368
    Offline

    DirtyDog368 Regular EF Member

    Hello All,

    After posting about a CU change and the one light circuit that kept on tripping the RCD on the new CU i had a lot of great advice. Armed with my Megger 1553 i approached the CU and condcuted some IR and continuity tests.

    At first a borrowed earth was suspected but from the following test results i think maybe not. I think there is a small IR fault and defiantely a fault to earth on the circuit 2. Seen by the resutls below. Can some one confirm that it is possible to have a small enough earth fault that will not trip a normal over current protective device but big enough to trip the more sensitive RCD protective devices.

    If indeed i do have an earth fault, how can i go about finding it without ripping up half the house to find it?

    cheers
    DD368

    [TABLE="width: 694"]
    [TR]
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    [/TR]
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    [TD="colspan: 2"]upstair lights = 1[/TD]
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    [TD][/TD]
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    [/TR]
    [TR]
    [TD="colspan: 2"]downstair lights = 2[/TD]
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    [/TR]
    [TR]
    [TD]IR test[/TD]
    [TD]P/N[/TD]
    [TD]P/E[/TD]
    [TD]N/E[/TD]
    [TD] [/TD]
    [TD][/TD]
    [TD][/TD]
    [TD][/TD]
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    [/TR]
    [TR]
    [TD]1[/TD]
    [TD]99.9[/TD]
    [TD]99.9[/TD]
    [TD]99.9[/TD]
    [TD](MΩ)[/TD]
    [TD][/TD]
    [TD][/TD]
    [TD][/TD]
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    [/TR]
    [TR]
    [TD]2[/TD]
    [TD]0[/TD]
    [TD]0[/TD]
    [TD]0[/TD]
    [TD](MΩ)[/TD]
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    [/TR]
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    [TD="colspan: 5"]nb switches off, 250v used with loads & Dimmers in[/TD]
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    [TD="colspan: 2"]Continuity[/TD]
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    [TD] [/TD]
    [TD]P1[/TD]
    [TD]N1[/TD]
    [TD]CPC1[/TD]
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    [TD]N2[/TD]
    [TD]CPC2[/TD]
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    [/TR]
    [TR]
    [TD]P1[/TD]
    [TD]_[/TD]
    [TD]open[/TD]
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    [/TR]
    [TR]
    [TD]N1[/TD]
    [TD]open[/TD]
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    [TD]open[/TD]
    [TD]open[/TD]
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    [TD]open[/TD]
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    [/TR]
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    [TD]CPC1[/TD]
    [TD]open[/TD]
    [TD]open[/TD]
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    [TD]P2[/TD]
    [TD]open[/TD]
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    [TD]69.3Ω[/TD]
    [TD]69.3Ω[/TD]
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    [/TR]
    [TR]
    [TD]N2[/TD]
    [TD]open[/TD]
    [TD]open[/TD]
    [TD]open[/TD]
    [TD]68.4[/TD]
    [TD]_[/TD]
    [TD]0.27Ω[/TD]
    [TD][/TD]
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    [/TR]
    [TR]
    [TD]CPC2[/TD]
    [TD]open[/TD]
    [TD]open[/TD]
    [TD]open[/TD]
    [TD]68.4[/TD]
    [TD]0.27Ω[/TD]
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    [TD="colspan: 8"]continuity is evident between live conductors on circuit 2, possibly due to switches in on position
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    [TD="colspan: 8"]however there should not be any continuity between live conductors and CPC.
    [/TD]
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    [/TABLE]
     
  2. ezzzekiel
    Offline

    ezzzekiel Guest

    loads and dimmers in circuit any reason why - join l/n to earth for quickness,
    a borrowed earth shouldnt cause any problems
    circuit 2 showing either a fault across all conductors or loads connected, have you checked borrowed neutral on landing.

    otherwise identify circuit with fault and all points on circuit then split in half and ir test untill fault clears identifying the part with fault.

    should you of changed the cu without being able to locate and repair faults?
     
  3. telectrix
    Offline

    telectrix Scouser and Proud of It Trusted Advisor

    Location:
    cheshire/staffordshire
    Business Name:
    Telectrix
    first remove all lamps/loads. then you can determine if it's a L/E or a N/E fault. then, as ezzzy says, split the circuit at approx. mid point and test each half of the circuit, then split the faulty section again. eventually, you will have narrowed it down to a fault either in a fitting, or on a single section of cable. however, a quick look for a nail through a switch cable might shorten the job.
     
  4. ezzzekiel
    Offline

    ezzzekiel Guest

    also its not the job of the mcb to interrupt earth faults thats why we use an rcd i assume you are either in training or newly qualifed and should really have someone with you to advise correctly.
     
  5. oldtimer
    Offline

    oldtimer Trusted Advisor

    Location:
    Edinburgh
    A borrowed earth does not do anything but as telectrix said a borrowed neutral best thing to do is switch all the lights off then reset the both RCDs now then switch on each light one at a time and check to see what light trips it. Youi may find that it is a light that sits between the 2 circuits. ie hall if a flat or stair landing if a house for a quick fix move both light circuits to either the same RCD row or CB.

    Also after 36 years never came across a borrowed neutral until 3 months ago funny old world
     
  6. ezzzekiel
    Offline

    ezzzekiel Guest

    fed up to back teeth of them at moment, every house on estate im working on.

    remove hall switch and see if there is a link between commons of the two gang switch this is usually a good indication of borrowed neutral.
     
  7. WDMDL
    Offline

    WDMDL Electrician's Arms

    Location:
    Manchester
    Not a borrowed neutral, but a resistance 0f 0.27 Ohm between N2 and CPC2 points to a direct short between N and cpc at some point in the circuit.
    With a reading so low, for 1.0T&E I'd look at the first ceiling rose / luminaire for starters.

    You could then check for N-E continuity at that and other accessible points until you found the culprit.
    Most likely a Neutral trapped in a metal fitting.

    Simon.
     
    Last edited: Oct 20, 2011
  8. telectrix
    Offline

    telectrix Scouser and Proud of It Trusted Advisor

    Location:
    cheshire/staffordshire
    Business Name:
    Telectrix
    or even a light connected L/E instead of L/N. that would only cause tripping when the light was switched on.
     
  9. DirtyDog368
    Offline

    DirtyDog368 Regular EF Member

    ezzzekiel, fancy popping round tomorrow then for a cuppa and a chin wag ;-) north london area N11?
     
  10. ezzzekiel
    Offline

    ezzzekiel Guest

    unfortunaely im busy all day, but im sure youll find it eventually
     
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