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Earth Leakage - Boiler?

AncientMariner
Called at my friend's house where he is looking to run CAT6 underground today and he was telling me about his RCD tripping problem.


Virtually every day his RCD trips. Single phase overhead TT supply. Single 30mA RCD in Consumer Unit.

The problem started soon after a new oil fired CH boiler was fitted.

Putting a clamp meter around the meter tails, from 21 to 0mA unbalance step by step as the single pole MCBs were opened one by one.

Rather upsetting to find that with all MCBs in and 21mA unbalance there was only 2mA in the earth lead, could not find either the cable connection or the rod outside.

Hoping to find a convenient place on the boiler where I could access the L & N to check unbalance looked inside and nothing accessible within the time I had. Anyway out of "devilment" you could say, put the clamp meter around one of the CH pipes from the boiler and 21 mA flowing through the 22mm pipe! I'm guessing that assuming the cable and rod are ok, the earth leakage current will flow inversely proportional to the respective resistances.

He has been trying to get an Electrician in to sort out.

Besides ensuring a decent earth rod and cable to the MET I think the ideal would be a new Consumer Unit (metal of course) and Cobs.

To try and confirm the boiler being the main cause, since the CH system runs from a nearby 13a socket, have considered a short extension lead where the L, N & E are separated out for easy application of a clamp meter.

Any other checks to try before his Electrician gets in heavy style?


Clive



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  • 0
    21mA is high but quite credible for a full house with a lot of electronics , if it is the natural state, then it would benefit from 100mA delay at the front, and 30mA instant on the finals.

    But something is up - the leakage increases steadily with load, yes, but all of it is going outside via the plumbing, not the CPC.


    So the TT electrode is less well grounded than the plumbing - which suggests that the two are not connected, which they should be.


    Now it may well be that the new boiler is in effect, via plumbing and shiny new oil pipes a much better electrode than its predecessor.


    With an electrode of a few tens of ohms, you could have an NE short, and not realise, as the load dependant  NE offset voltage may not be enough to drive 30mA.

    (and an NE short is not isolated by MCB flipping.)


    So, there may have been a partial NE fault for years, and the new boiler is revealing it, or  the total leakage may just be that high.


    I'd flick all off at the double pole, and do an N+L to E resistance check, and expect megohms.

    IF not then investigate.

    If all OK, then arrange to split over many RCDs.

    And in either case, check the electrode resistance, and the bond to the plumbing.
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  • 0
    21mA is high but quite credible for a full house with a lot of electronics , if it is the natural state, then it would benefit from 100mA delay at the front, and 30mA instant on the finals.

    But something is up - the leakage increases steadily with load, yes, but all of it is going outside via the plumbing, not the CPC.


    So the TT electrode is less well grounded than the plumbing - which suggests that the two are not connected, which they should be.


    Now it may well be that the new boiler is in effect, via plumbing and shiny new oil pipes a much better electrode than its predecessor.


    With an electrode of a few tens of ohms, you could have an NE short, and not realise, as the load dependant  NE offset voltage may not be enough to drive 30mA.

    (and an NE short is not isolated by MCB flipping.)


    So, there may have been a partial NE fault for years, and the new boiler is revealing it, or  the total leakage may just be that high.


    I'd flick all off at the double pole, and do an N+L to E resistance check, and expect megohms.

    IF not then investigate.

    If all OK, then arrange to split over many RCDs.

    And in either case, check the electrode resistance, and the bond to the plumbing.
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