Nuisance tripping - Earth Leakage Device on a TT system

Discussion in 'Electrics UK' started by mikgle, 19 May 2011.

  1. mikgle

    mikgle

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    Hi all you wise, experienced sparkies out there..... I'm only just qualified and my first job is to diagnose nuisance (usually happens in thunderstorms) tripping of the supply "Earth Leakage trip device" (100mA). I initially assumed this was an RCD but apparently I have since learned it could be a voltage device, not current. It is very old and this is an old problem that my friend would like sorted if poss. it is fitted between the main fuse and the meter, i.e. the whole supply.
    It is a TT system and has a Ze of around 125 ohms, so this seems ok? Western Power are coming to fit an isolator upstream of this - i had hoped they could just swap the device with a isolator switch but no can do, its up to me after they fit the isolater. There is a 30mA rcd in the CU but this one doesnt usually trip. I have been advised by my tutors that the rcd protection must be 100mA on a TT system, so after they fit the isol;ator I was planning to replace device with a 100A / 100mA rcd in the same place (Wylex price is reasonable from TLC). Does this sound ok? Cheers, Mike....
     
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  3. westie101

    westie101

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    If it is between the cut-out and the meter it is WPDs and their responsibility. If you fit a 100mA RCD get them to remove it
     
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  4. ELZ4742

    ELZ4742

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    whys that then?
     
  5. JohnW2

    JohnW2

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    Even though I'm not a practising electrician, I certainly wouldn't advise you to do that. Current regs require 30mA RCD protection on virtually all final circuits.

    The 'old' requirement for a 100mA RCD on TT systems is largely a left-over from the 'split load CU' days, when some final circuits were not RCD protected - and the requirement was that all circuits (even those not coming from an RCD in the CU) had to be protected by an RCD with a sensivity no worse than 100mA. 100mA ones were used to achieve discrimination from the 30mA one(s) protecting some of the final circuits.

    My understanding is that, provided all final circuits have (30mA) RCD protection, there is no need for an additional 100mA one upstream - but if it's a split load CU (and is staying that way) you do. Whatever, the RCD(s) within the CU should stay at 30mA.

    Kind Regards, John.
     
  6. mikgle

    mikgle

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    yes, its a split load at the CU - skt circuits only are protected by a 30mA rcd. Presumably a 100mA overall RCD would provide discrimination....regards and thnkx
     
  7. mikgle

    mikgle

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    don't know - they're the experts?
     
  8. mikgle

    mikgle

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    Will try - I'll be there when they come to fit the isolator, maybe I'll learn something from them?.... cheers
     
  9. JohnW2

    JohnW2

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    Having an overall 100mA overall RCD does not, in itself, guarantee discrimination. It would if a fault current to earth (detected by the RCD as a L-N imbalance) on one of the socket circuits gradually increased - the 30mA RCD would then trip first. However, if (as usually happens in practice) a high fault current 'suddenly' appears, there's no real telling which would trip first. For that reason, the usual/best practice for the 'overall' RCD in a split-load setup is to use one which is not only 100mA but is also time-delayed (Type S) - that more-or-less guarantees that the 30mA RCD will trip before the 100mA one (for the socket circuits).

    Where I think you need some input from the electricians here is in relation to the fact that you would be leaving it as a split-load installation. That would presumably not be compliant with current (17th ed.) regulations, not the least because the routing of the wiring of the 'non-RCD' circuits will almost certainly be such (depth of cables etc.) as to require 30mA RCD protection. I'm not sure what you, as an electrician, should do in that situation. You presumably would need to advise the client that it was not 17th ed-compliant, and I guess that if they then 'refused' to have the installation updated that would solve the problem. However, I'm sure that some of the helpful folk here will advise you.

    Kind Regards, John
     
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  11. baxpoti

    baxpoti

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    You need to measure Zs
    refer to table 41.5 page 50 17th edition and you can choose your RCD from the chart , observe the voltages!

    Personally I'd use a 100mA time delayed RCD

    Consumer unit split load board with 2 x 30mA RCD
     
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  12. JohnW2

    JohnW2

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    If you mean a split load board with all final circuits protrected by one or other of the two 30mA RCDs (i.e. no final circuits not protected by either), why would you also want a time-delayed 100mA RCD?

    Kind Regards, John.
     
  13. ban-all-sheds

    ban-all-sheds

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    Qualified is qualified is qualified.

    You lack practical experience, but that has absolutely no bearing on knowledge of the Wiring Regulations.

    Qualified is qualified is qualified - why don't you know what the regs say about RCDs?
     
  14. mikgle

    mikgle

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    Thanks John, a lot to take in here... I must admit i did consider a time-delay rcd but these are at least £60 more than the standard, not a great deal if its necessary but I rather was thinking (or maybe hoping?) that the main problem is the old type of voltage -controlled rcd that is currently installed; this being the case, its not surprising that there would be induced voltages in the earthing circuit when lightning strikes are around but you may well be right.....
    As regards the new system not complying with 17th edition, Brian Scadden (the doyen of electrical text books) actually recommends installing a split load system for TT using 100mA overall and 30mA sockets protection in his book "Electric Wiring: Domestic", page 22, although it was written in 2005. as I understand it, you do not have to retrospectivley bring the whole installation to comply with the latest edition regs, just the bit one is installing - there are plenty of rewirable fuse boxes around which are not illegal.... however, I am op[en to correction on this, as you say one can advise the client.....
    an interesting point is that my own consumer unit has no rcds at all, the whole system being under an overall 500mA device!!I have bought a new CU and intend to fit it this week! Many thanks, mike
     
  15. mikgle

    mikgle

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    I think its to ensure one achieves a satisfactory Ze - it may be ok in wet weather but not in dry apparently.......
     
  16. riveralt

    riveralt

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    If its in Scraddens book it must be right.
    Perhaps you should read the Scudden chapter on electric shock :evil:

    Don't you think it would be a good idea to conduct a PIR on the circuits first? You say you are qualified, just, does this mean qualified to complete your own building control notice through a competent persons scheme or are dealing direct with the LA building control.
     
  17. riveralt

    riveralt

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    I think you mean a satisfactory Zs ;)
     
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