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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



  • 0
    pop the clamp meter over the flex to the boiler as it is... see what that reads? If the 21mA is coming down the pipes... it won't be returning thru the CPC in the flex, it may be that the pipes have a better connection to true earth than the electrical system: Which would raise 2 issues, a) the bonding would have to be missing, and b) the earth rod connection is missing or poor.


    That's just wild guesswork of course, but I've known pumps to cause issues before, and of course any leakage (of water) inside the boiler will likely induce leakage of electricity sooner or later!


    I'd be tempted to go with an S type RCD up front, (time delayed), or possibly as suggested in another thread, a 300mA rcd, then a new 18th edition compliant board with either split load, or preferably, individual RCBOs. The cost of those is coming down.


    Being overhead TT, an SPD may be needed also, but that's a whole other worm can
  • 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.
  • 0
    Could just be a failing central heating circulator sending the current down the pipe.


    Andy B
  • Former Community Member
    0 Former Community Member
    Hi Clive, I would go with an N-E insulation fault somewhere within the installation. It sounds very similar to a recent situation I found. It was an upfront 30ma RCD on a fairly sprawling 80s barn conversion with a recent boiler etc that the customer suspected was the cause but it turned out to be the tiniest knick on the neutral by a socket screw in a socket that had apparently been added a few years earlier... It could be nearby heating pipes are running hotter just giving a little more expansion to nearby metal parts?
  • 0
    Thanks for all the advice.
    MHRestorations
    Why I didn't measure the flex for unbalance I don't know. Likely I was watching the time since I needed to get home, change and take myself and my wife elsewhere.
    Also, that 21mA through the pipe, was just one of the outgoing pipes, but due a rather tight install, neat really considering that this was a replacement CH boiler, I could not find another suitable pipe.

    Whilst I understand the different earthling systems, our two house have both been TN-S although previously my parents farmhouse was TT with fuses - some likely in the Neutral and earthling via the lead water pipe with no ELCB or RCD…  My time at sea was with isolated systems with no neutral, the 220v domestic supply being across to phases.

    Once the fault at my friends house is found and fixed, I would like to see a new consumer unit with ideally RCBOs rather than one or two RCDs. Is there a requirement for 2-pole RCBOs due it being TT? However, one problem is going to be location for new equipment. It is an old farmhouse with the cut-out and meter in one triangular cupboard and the CU in one next to it. These being directly under the roof at a lowish height - I can just reach the bottom of the CU.  In fact until Scottish Power changed the supplied to overhead concentric, he could actually reach the bare overhead supply where it was secured to the porcelain insulator just above the kitchen door!  I would also like to see a REC2 between the meter and the CU, but no room in either cupboard. Yes, the earth rod, if there is one needs attention, just wondering whether it is the water supply pipe - will ask him to check.

    Mike
    Yes a N+L to E check may be revealing. I will dust off my megger.  Also a little bit here, a little bit there, it all adds up there must be some current due suppression capacitors - 0.4µF would give 30mA at 240 volt - quite a bit I know for suppression, but it all adds up!  Then again the RCD may have gone over sensitive. His electrician can check that.


    Andy
    The CH boiler and pump are fairly new (months) but could be a rogue one with low insulation resistance.

    A N-E fault brings back memories of some years back well before Part P when if you installed new circuits etc you could fill in an Electricity Board form and they would come out, test and connect up.

    I fitted a RCD or RCCB as it was called then and left it with new meter tails ready for connection. MANWEB's Engineer came out, pulled the cut-out fuse and connected it in. Problem now was that it would not set!  You have a problem he said as he left! He advised that I would soon find it - look in your sockets.  Sure enough in one back-box the earth wire had pressed its way through the black insulation of a neutral. A quick rearrangement of the cores and problem gone. It never tripped again for the remaining years that we were there. Chances are that fault had been there for some time just waiting to be revealed.
    Yes, a slight expansion of a water pipe may be all that is needed.


    Clive






  • 0
    I believe the requirement would be satisfied by single pole RCBOs provided the upfront RCD (S type or higher current) is double pole. But that is purely a belief based on 'what makes sense', not based on reading 7671!


    Interesting that at sea, the supply is 230 across phases, somewhat similar to the old continental system of 127/220 3ph (only they used a wye/star connected system with a neutral, but most loads were connected phase to phase.)


    It was only many years later that I thought '127v to neutral... THAT explains that weird voltage selector option on the family grundig reel to reel tape recorder'
  • 0
    Clive,

    your thoughts look sensible to me.

    Just one small mention ref RCD being oversensitive - in my (limited) experience RCDs usually work as per spec or not at all or are sluggish. I have not yet found any that are over sensitive per se. When one appears over sensitive it is often the standing leakage from other circuits/equipment that is causing this. Example, say you already have 11mA leakage and then you introduce a couple of items/circuits with 5mA each you are well within the permissible tripping zone of an RCD. Of course others may have actually found over sensitive RCDs when tested with no circuits connected - I`d love to hear of it.


    It`s also good to hear about different disciplines too. Mine is mostly domestic with a bit of commercial, yours is on board ships. People often expect me to know all about car electrics, well I do know a bit but an experienced auto electrician would run rings around me with fault finding (I am aware that poor chasis connections can cause strange happenings though)  but I`d probably trump them with a house electrics.
  • 0

    Is there a requirement for 2-pole RCBOs due it being TT?



    No absolute regs requirement as far as I recall, but certainly a good idea to have RCBOs with at least a switched N (as well as full protection & switching on L of course) if there's another RCD upstream. Otherwise on a fault N remains conneceted (and the L part of the faulty circuit downstream of the RCBO too if there are any loads connected) - which in effect can leave a N-PE fault on the tripped circuit still connected to the supply - and hence nuisance trip out the upstream RCD.


      - Andy.

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