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RCD test button ineffective

Discussion in 'Electrics UK' started by Softus, 22 Sep 2007.

  1. Softus

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    Rented flat, about 6 months old (or less).

    Single lighting circuit - B6 MCB had tripped. I reset the MCB without any apparent problem, so I replaced three blown 240V halogen bulbs (lamps).

    I then checked the RCD (MEM 80A / 30ms) with the yellow test button - nothing happened. Switched RCD off - all protected circuits went off. On again - all circuits energised again.

    My advice to the letting agent:

    1. Either the RCD or the earth path is faulty.
    2. A qualified electrician should be called to replace the RDC and/or fix the earthing.
    3. Urgent action is needed, because there's a risk of electrocution if an ECP becomes live.

    Was my advice correct?
     
  2. ricicle

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    Yes, your advice is correct.The designer of the installation has chosen to fit an 80A/30mA RCD for whatever reason (TT?, Supplementary protection against direct contact etc)
    This device has failed its functional test.The mechanics of it have obviously failed which means it could also not trip in the event of an earth fault.
    Urgent rectification required.
     
  3. Softus

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    Thanks ricicle.

    And I realise I posted "30ms" whereas I meant "30mA". :oops:
     
  4. Adam_151

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    Most certainly the former (or at least its test function), the test button doesn't rely on an earth path (if it did, and the path didn't exist, then presisng the button could cause danger), but rather current is passed (via a resister) from the outgoing phase terminal to the incomming neutral terminal to set up an inbalence in the RCD

    Sensible advice indeed :)

    You have to strike the right balence between stressing the need to fix it, and scaremongering (and it looks like you've got it about right :) ), is this a ground floor flat, and does it have an electric shower?, it could be that there is no requirement to have an RCD, and while if there is one it should work... perhaps its not 'get electrician out of bed at midnight' territory yet :LOL:

    Was my advice correct?[/quote]
     
  5. Softus

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    Thanks Adam.

    It's a 3rd floor (in UK terms, i.e. 3 up from ground), and doesn't have an electric shower, but a Megaflo heated by two IHs.

    It happened yesterday, and the agent looked to be dragging his feet over getting an emergency electrician, so I didn't want to leave them in any doubt that it's potentially lethal. Since I carried out no tests, especially PAT tests, we don't know if there's an accident waiting to happen.
     
  6. Spark123

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    Could be just a broken test button, but has failed a functional test and should be replaced. How dangerous it is depends on what it is protecting and the reason for it being there i.e. is it a TT supply? It may have just been installed as it is considered good practice to protect sockets, possibly becoming a regulation in the 17th ed.
     
  7. Softus

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    It didn't make me comfortable that it was protecting the lighting circuit as well as most of the board. I didn't make notes, but I believe the non-RCD side comprised only the Megaflow related circuits.

    The feed came from the basement via SWA cable and appeared to have three conductors - there was a 3P isolator on the consumer side of the meter.
     
  8. Spark123

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    I can understand why you are not comfortable with that, it isn't a good idea to protect lighting circuits with the same RCD as sockets etc as a faulty appliance can cut all the lights. IIRC the only lights which currently require RCD protection are those installed in Z1 of a bathroom (bar on a TT system where an RCD will be required for protection against indirect contact on all circuits).
     
  9. Softus

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    Maybe it's TT. I don't know.

    I'll be going back at some point, so I could check. How would I tell?
     
  10. Spark123

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    If the earth is from the supply head it will be a TN system, the fact there are things fed from the non RCD side says it is very very unlikely to be TT.
     
  11. RF Lighting

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  12. Softus

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    Er, my question was a bit stupid, because I realise now that I knew the difference.

    The meter is one of a bank of about 24 in a cupboard in the basement, so I'll go down and take a look at the earthing next time. I was a bit surprised to find that the cupboard wasn't locked, but it looked as though it would take nothing more than a large Allen key to open it anyway.
     
  13. Steve

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    Take your camera ;) :LOL:
     
  14. Softus

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    I'm never without it. ;)
     
  15. JohnD

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    the test button has no reliance on an earth.

    might be the test button is faulty, might be the RCD is jammed. they do that sometimes, especially if not tested for a year or more (though your one sounds quite new)

    Edit: Oh, Adam_151 already covered it :oops:
     

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