Tripping RCBO

Discussion in 'Electrics UK' started by Knotty, 26 Sep 2011.

  1. JohnW2

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    Whilst that is true as a general statement, I find it hard to see how such a problem would go away when the 'suspect' ring was broken, in the manner described by the OP.

    Kind Regards, John.
     
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  2. Knotty

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    Another interesting, if daunting , possibility, but it explains why it is importsant that I should isolate more than just the ring in question.

    I am beginning to realise how much more I have to learn about the operation of RCBOs.

    Would the spill current go down the small white leads to earth, or just via the commom earth cables to the earth bus bar? If it is the forme, presumably I could disconnect the small wire and see it it still trips.
     
  3. JohnW2

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    What Bernard has described can happen when two circuits share a common RCD - such that the RCD can trip even when the fault is on a circuit whose (single pole) MCB is open. However, this cannot happen if there is a single circuit connected to an RCBO. The small white 'functional earth' wire should not carry any fault currents (unless, of course, teh RCBO itself is faulty).

    Kind Regards, John.
     
  4. JohnW2

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    I think subsequent discussions have sidetracked you away from this one serious fault that you have so far identified.

    That anticklockwise 'earth path' should have negligible resistance (well under 1Ω), just as the clockwise onr has. I think you should (with circuit isolated!) work you way back along the anticlockwise half of the ring, from your break (hence towards the CU), measuring the resistance from 'earth' wires to a water pipe (or whatever) at each socket (or other accessory/JB, if appropriate)in order to locate where the 100Ω resistance is arising.

    Kind Regards, John.
     
  5. Knotty

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    Thanks JohnW2 that seems a practical approach. I have just equipped my multimeter with a 10 metre lead so I should be able to tracck then akk back to the same point.
     
  6. JohnW2

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    You're welcome. I look forward to hearing what you discover! A poor earth connection will obviously not, in itself, cause an RCBO to trip, but when you've found the cause of the high resistance earth, it may give you a clue to what other problems may be present.

    Kind Regards, John.
     
  7. Knotty

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    I think this one's on hold for a whilel
    Bare wires sticking out from socket in kitchen has been deemed incompatible with a visit from my 3 year old granddaughter, especially as there is nothing not working. (Can't really argue with that.)

    Thanks for all the guidance, I will report back on what I find in due course.
     
  8. EFLImpudence

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    Just a thought while Knotty is on Granddad duty.

    As with another thread, I am surprised at such round numbers.

    Does anyone agree that the '100' may be the maximum (>100) on the meter and may just indicate an open circuit? (I know they are normally 200).

    Has anyone come across a multimeter with a 100 maximum?
     
  9. JohnW2

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    Good point. However, in terms of the display itself, 3-character displays are often actually 2½ character ones, with a maximum display value of 199, rather than 100. However, it's certainly possible that a meter might be designed to display ">100" for resistances >100Ω if it's on a 100Ω range, so maybe the ">" just didn't get reported to us. It's actually a bit confusing, since he mentioned a 'Megger' in that post, and also reported the L-L and N-N results as 'dead short' - so I'm not totally clear exactly what sort of meter we're talking about.

    Whatever, I think it's unimportant to the point at issue - since even 100Ω is essentially as bad as an 'open circuit' for a CPC!

    Kind Regards, John.
     
  10. EFLImpudence

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    Not really - An open circuit would indicate that there is a break in the cpc and the fault could simply be a reversed N & E.

    A 100Ω resistance to earth is much more problematic.
     
  11. JohnW2

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    Indeed, but 100Ω indicates a very-near-break or very iffy connection - as I said to him, something (in a CPC) which certainly needs to be investigated before he worries about anything else.

    Reversed where? Don't forget that there was 'dead short' L-L and N-N continuity between the broken ends of the OP's break.

    Of course, but we haven't got that!

    Kind Regards, John.
     
  12. EFLImpudence

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    There still would be. Once had same from a missed socket under a bed with extension lead and equipment plugged in. Knotty hasn't measured between N & E on faulty side at his break.
    There still has to be a complete break in the cpc as well.
    He says he measured 100Ω from cpc to water pipe.
     
  13. JohnW2

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    I'm struggling here. Can you perhaps explain in a slightly more spoon-feeding way exactly what scenario you are trying to describe. Thanks.
    Yes (or open circuit) - from the CPC on one side of the break. 'Dead short' to the pipe from the other side of the break and 100Ω (or open circuit) E-E across the break. Surely all that proves is that the water pipe is bonded and that the CPC on one side of his break has a resistance of 100Ω (or open circuit) between the break and his MET?

    Kind Regards, John.
     
  14. EFLImpudence

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    Continuity of L - L and N - N not affected.
    Knotty needs to test continuity between all the conductors at the test break to work it out.

    Precisely, but if it is open circuit rather than 100Ω that means faulty side is not connected to anything.

    If it is 100Ω that means something would be drawing 2.3A were it not for the rcbo tripping.
     
  15. JohnW2

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    Thanks, and apologies. You wrote "N-E reverse" and my eyes/brain saw/read "L-N reverse". I now understand your theory, but it does require a seemingly very unlikely combination of faults/errors - the N/E reverse in the first place (I would have thought quite difficult to do!) AND a load plugged in (and switched on) to the 'reversed' socket (despite the OP saying that no loads were connected) AND a break in the CPC. If I were a betting man, I wouldn't fancy my chances on getting a return from that 'treble':)

    Well, maybe - I guess that depends upon where the fault current is coming from (e.g. per your 'treble'above), in particularly where it is entering the circuit relative to the source of the resistance in the CPC. However, what we would have 'expected' would be that the resistance would be close to zero, not 100Ω, in which case the PFC would be much higher than 2.3A (if L is getting connected to the CPC on that side of the ring, on the non-CU side of the high resistance). I still don't really understand why you feel that 100Ω is worse than 'open circuit', given that the CPC is meant to provide a very low impedance path gto earth.

    Kind Regards, John.
     

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