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Consumer unit replacement cost

Discussion in 'Electrics UK' started by pdhan, 6 Apr 2021.

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  1. JohnW2

    JohnW2

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    Am I understanding you correctly? ....

    .... are you sailing that if an RCD 'failed' when tested 'in situ' (i.e. in the normal in-service situation), you would then test in 'in isolation' and if it 'passed' that test you would then you would 'pass' the RCD (and installation) - even though you would thereby be 'passing' an installation in which the RCD did not work satisfactorily when in-service?

    If so, that sounds a little 'odd' to me.

    Kind Regards, John
     
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  3. ericmark

    ericmark

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    No not quite, if in situ it fails and in isolation it passes then the question is why. I don't have a clamp on ammeter to measure 0.001 amps, so although insulation resistance may be OK, that is DC, and there may be some thing which I have failed to find.

    I have had new RCD's fail, simply due to strain on the terminals, I have also had RCD's pass, and trip all the time, but replace with another, and nearly same test results but did not trip.

    Modern RCD's tend to have electronics in them, the old RCD's were electro mechanical devices, and seemed to stick a lot more than the new ones. Also also trip with spikes on the supply.
     
  4. JohnW2

    JohnW2

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    True, but merely repeating the test with the RCD in isolation would not, in itself, help to answer that question.

    As I see it, the only point in repeating the test with the RCD in isolation is that, if it again fails, that would be an indication that the RCD needed to be replaced.

    However, if, having failed in situ, it passed 'in isolation' then one would have to do other things in an attempt to discover the 'why' - but, unless/until that could be ascertained and rectified, it would surely have to continue to be regarded as a 'fail', wouldn't it?

    Kind Regards, John
     
  5. pdhan

    pdhan

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    Late follow-up question: so if an RCD on an obsolete board (5486 in this case) fails, it can only be replaced with an old generation RCD or the whole board changed?

    Also wondering if it's wise to get RCBOs on all circuits with a new board on a 30+ year old installation, or if I risk having them trip for no obvious reason.
     
  6. AndyPRK

    AndyPRK

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    Yes

    With so few circuits, rcbo is the way to go

    Rcbos are less likely to false trip than one rcd.
     
  7. ericmark

    ericmark

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    I would agree I have 14 RCBO's.
     
  8. JohnD

    JohnD

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    I think that's more than me.
     
  9. pdhan

    pdhan

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    Thanks, and the only way of getting RCBOs on there would be to replace the board?
     
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  11. AndyPRK

    AndyPRK

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    yes
     
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  12. pdhan

    pdhan

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    Thanks. The electrician said the RCD tripped as soon as he plugged the meter in – before pressing the test button – hence "no result", but don't think he tried to test it in isolation.
     
  13. securespark

    securespark

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    No, but it would prove correct functioning of the RCD.
    The whole point of testing the unit in isolation is to remove the loads/appliances from the RCD that could skew the test results.

    If an RCD fails when tested via a socket outlet, then fails again when tested in isolation, it is most likely fubarred.
    I say most likely because once (and only once) in my career, I changed a Wylex board with an RCD incomer that failed the initial test via a socket outlet. In this situation, I would usually operate the RCD repeatedly and then retest: sometimes the mechanism becomes inactive through lack of regular testing. However, it failed again after repeated operation of the mechanism, so I tested it in isolation.
    It still failed, so I resigned myself to changing the board.

    When bench testing the failed RCD after disconnection, it was found to operate with spec.
    I can't speak for other test gear manufacturers, but the Tech guys at Metrel told me it is common for appliances to affect the RCD testing process, which is why testing in isolation is recommended.
    By excluding everything in circuit, you get a true picture of how the RCD is reacting to the test process.

    For me, if the RCD tests fine in isolation, it is a pass. You then know it will function as intended and, if there were to be a subsequent leakage fault, it would operate and open the circuit.
     
  14. flameport

    flameport

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    If something prevents an RCD test from completing, that same something is very likely to prevent the RCD working when a fault occurs.

    You know it will function as intended with no load connected, a situation which is mostly irrelevant.
    Not that it will work as intended with the installation connected to it.

    Exactly.
     
  15. JohnW2

    JohnW2

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    As I said, if an RCD 'fails' when tested in situ, it's useful to then test it in isolation - since if it fails again, then one knows that it needs replacing without wasting time looking for other problems. However ....
    Of course - but, unless it fails again when tested in isolation, one is back to having to investigate why it 'fails' when in situ.
    That's surely rubbish, and dangerous rubbish at that, isn't it?

    If (regardless of what happens when it is tested 'in isolation') the RCD does not operate (satisfactorily or at all) when tested in situ, then it's surely the case that it would NOT necessarily "operate and open the circuit if there were to be a subsequent leakage fault", isn't it?

    By testing 'in situ', you are simulating a "subsequent leakage fault", so if it fails that test, you surely would have a dangerous situation, and certainly a 'fail', wouldn't you?

    Kind Regards, John
     
  16. securespark

    securespark

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    You cannot say that testing the RCD with appliances in circuit is the way to go because there is no way you can account for what may be attached to the circuit at any one time.

    On my C&G courses, we were all taught only to test RCDs in isolation.

    Metrel were not suggesting any appliance would prevent an RCDs correct operation, just that they may affect the MFT's test procedure.

    What would you suggest the correct procedure would be if you found the RCD to fail to complete the test process?

    No, I don't believe it is.
     
  17. EFLImpudence

    EFLImpudence

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    I wouldn't think any connected circuits or loads will stop the RCD working when tested, but any permanent earth leakage will skew the test results and not be an accurate account of the RCD's characteristics.
     
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