Very intermittent earth leakage/RCD tripping

Discussion in 'Electrics UK' started by grommett, 23 Jan 2018.

  1. grommett

    grommett

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    My Parents have a problem that several electricians have not been able to fix. Two or three times a year the RCD trips and turns the power off, usually when they are on holiday and the freezer defrosts. Now they are worried about going away. After lots of checking and testing, I think there is a short between earth and neutral that affects the whole house. So I guess there must be a nail or screw through a wire somewhere. My question is: is there a way to pinpoint where this short might be?
     
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  3. flameport

    flameport

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    Highly unlikely - if such a thing existed, the RCD would normally trip all the time, not just a few times per year.

    It's possible for such a fault to only cause tripping when there is a moderate/substantial load on the circuits, but that doesn't fit with it tripping when people are on holiday.
     
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  4. grommett

    grommett

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    A test meter shows continuity between the earth bus bar and the neutral bus bar in the consumer unit. Every socket shows continuity between earth and neutral. Very high resistance, so not enough to trip all the time. I think an electrician may have also put in a less sensitive trip at some point.
     
  5. JohnW2

    JohnW2

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    That proves nothing. Depending on the type of earthing system you have, the neutral and earth may well be joined together where the supply enters your house. To mean anything, tests would have to be done on individual circuits (cables) after their neutrals had been disconnected from the neutral bar.

    Kind Regards, John
     
  6. EFLImpudence

    EFLImpudence

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    Turning off the main switch should separate earth from the circuit neutral conductors.
     
  7. JohnW2

    JohnW2

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    Good point :oops:

    Having just re-read the post, I wonder if "Very high resistance, so not enough to trip all the time" applies to the test between neutral/earth bars as well as the sockets (I had previously assumed only the latter).

    grommet: how high were these very high resistances?

    Kind Regards, John
     
  8. JohnD

    JohnD

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    does it happen in rainy weather?

    or perhaps when the house is unoccupied it is colder and damper, or perhaps a water tank or cistern is overfilling so slowly that it takes a while to reach overflow.

    I had a washing machine where a fill valve was letting by, and only when it was unused for a week or more did it overfill and leak.
     
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  10. bernardgreen

    bernardgreen

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    A Neutral to Ground ( Ground not the CPC ) fault together with an incoming Neutral that has risen above Ground potential will create a current on the Neutral conductor through the sense coil of the RCD. This current may well be enough to trip the RCD if the Neutral in the district supply network bounces by more than a few volts above ground potentail. A fault from Neutral to CPC ( earth wire ) in a PME will not create that current as the CPC and the Neutral have the same potential ( they are connected at the cutout.
     
  11. Have any of the electricians done an insulation resistance test on all the circuits?
     
  12. John D v2.0

    John D v2.0

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    You could bypass it also, but either solution wouldn't comply with writing regulations, and given it's supposed to cut off in the event of a fault of this nature, it would be like shooting the messenger.
    If you really want to take that approach get an all RCBO board fitted. It'll cost around 500 pounds but may be worth it for a holiday!

    As above you need IR test on the whole installation with it switched off, and if it's ok then start to look at what appliances are connected.
     
  13. JohnW2

    JohnW2

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    Indeed - but it's hard to believe that this has not already been done by at least one of the 'several electricians' who have investigated the problem. If they didn't do IR tests as part of their 'investigations', I wonder what on earth (if anything!) they did do!

    It does sound as if the OP may have already partially done this himself, resulting in his statement that there is 'continuity' ("but very high resistance") between N and E bars in his CU. That's why I've asked "how high?" - since if he is getting a measurable resistance with a multimeter (at very low voltage), and IF everything were (DP) disconnected from all the the circuits at the time (it's difficult not to to 'miss' something that needs to be disconnected!), then that would presumably be very significant!

    Kind Regards, John
     
  14. grommett

    grommett

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    Hi John

    I will have to check again to see "how high" and will post back when I get a chance to do so. When you say: if everything was disconnected, do you mean make sure all plugs are removed from sockets? What about light fittings?

    Cheers

    Ian
     
  15. JohnW2

    JohnW2

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    Yes, certainly remove all plugs from sockets. The greater problem is with 'hard-wired' things (boilers, immersions etc. etc.) which really need to have neutral as well as live/line disconnected. If they are connected via a switched FCU, that will usually have a double-pole switch, which will do the job, but if the load is connected via an unswitched FCU (or some other means) then opening it up and physically disconnecting the neutral may be the only way to do it.

    Kind Regards, John
     
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