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2 Ring Circuits combined in series

Discussion in 'Electrics UK' started by McKenna32, 23 Aug 2019.

  1. McKenna32

    McKenna32

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    Hi all,


    At some point in the past my two ring final circuits in my house (up/down) have been combined in to one, in series, so it’s effectively one ring doing two laps of the house and the R1+R2 reading is rather high – 0.519 ohms


    32A Type 2 breaker

    TN/C/S earth


    This ties up with a previous EIC that was done in 2011 so didn’t make me worry so much but it’s been nagging me as it seemed unduly high. Last night I found the smoking-gun: a choc block that somebody had gone to some efforts to hide in the mass of wires at the back of the CU, that confirmed there are two rings in series when I tested it.


    Firstly, I think it’s likely too high for the regs. for this setup. If I’m reading OSG correctly, Zs must be < 0.79ohm, and whilst I don’t know Ze (no tester) it could quite conceivably push me over that. I’d like to know if I’m reading the tables correctly, if somebody could please confirm? The reason it passed on the 2011 EIC appears to be mis-ID’d MCBs as type B, not Type 2. I'm guessing the Zs requirements must have been much higher for that setup back then (1.44 ohms Zs max recorded on the 2011 EIC, I can’t find that in 18th tables)


    So my plan is to just put them in parallel (4 conductors into one MCB) until I get an EICR done on the place (soon) at which point I expect it to be flagged up and I’ll get the sparky to add a new MCB and re-separate (along with every other problem they find). Otherwise the circuits tests up fine (isolation resistances fine and checked R1, Rn and R2 separately). I guess I might have to upgrade to Type B’s while I’m at it.


    Whilst it doesn’t nuisance trip at all at the moment, I have small daughters, so in about 5-10 years the upstairs hairdryer load is going to increase exponentially, so it makes sense to sort it now.


    So out of interest - and my main reason for posting - I cannot for the life of me figure out why it would it possess someone to add these two RFCs in series and make it do two laps of the fupping house?? Functionally (volt drop, cable heating, R’s) it seems obviously worse than just combining the two rings in parallel. I suppose when you’re testing you only have one ring to test, but that seems rather lazy...


    Am I missing something here? or is it just poor practice to combine ring circuits and so they were likely trying to hide it from a casual look-over? There are even three spare MCB positions in the CU, so somebody was probably trying to do this on the cheap/quiet...


    Oh I did post up in a confused state last night before I figured it out an hour or so later. I flagged it for deletion but it subsequently got locked so apologies if you wasted any time reading through that!


    Cheers


    Al
     
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  3. chivers67

    chivers67

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    How many wires are terminated in the Choc block from the sockets? Then how many wires from the choc block to the CU/MCB? All seems rather odd!
     
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  4. McKenna32

    McKenna32

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    Hi @chivers67

    Two - one end of the upstairs ring, and one end of the downstairs ring

    None directly, into the MCB is the other end of the upstairs ring and the other end of the downstairs ring
     
  5. JohnW2

    JohnW2

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    It's only 'rather high' in the context of the archaic (I hesitate to say 'obsolete') type of MCB ("Type 2) that you have. In fact, I am rather surprised that the R1+R2 for a 'whole house' ring final is that low.
    OK - to clarify .... a Type B MCB requires (in worst case) 5 times its rate current (In) to trip magnetically - hence 160A for a B32. Traditionally, the maximum Zs was therefore stated as 1.44Ω (230V/160A). However, Amendment 3 of the 17th edof BS7671 introduced a 'correction factor' (0.95), which they called Cmin, to take into the account that supply voltage might be less than 230V - the maximum Zs for a B32 therefore fell from 1.44Ω to 1.37Ω - which is where it remains (in BS7671) today (18th).

    This is one of those cases in which I do not understand where the OSG figures come from. The current OSG gives the max Zs for a B32 as 1.11 - very different from the 1.37 figure in the current BS7671 (18th). Neither the 17th or 18th eds of BS7671 even mention Type 2 MCBs, so I havent't got a clue as to where the 'maximise Zs' (0.79Ω) for a 32A Type B in the OSG comes from - it may be correct, but judging by the discrepancy between OSG and BS7671 for Type Bs, it wouldn't surprise me if it is lower than it needs to be!

    A type 2 MCB requires a current of 7 x In to trip magnetically (rather than 5 x In for a Type B) (hence 224A for a 32A Type 2) - so, if the max Zs for a B32 is (per BS7671) is 1.37Ω, then the max Zs for a Type 2 ought to be 0.98Ω.

    From what you've said, I don't think there's any reason to do all that. If you just change the MCB to a B32, the Zs of the present circuit should be well lower enough, shouldn't it?

    Kind Regards, John
     
    Last edited: 23 Aug 2019
  6. JohnW2

    JohnW2

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    Why this again? ....

    upload_2019-8-23_12-42-23.png

    Anyway, what I wrote was ...
    upload_2019-8-23_12-43-15.png
    upload_2019-8-23_12-43-47.png
     
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  7. EFLImpudence

    EFLImpudence

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    To avoid confusion -

    5 x 32 "hence 160A for a B32".
     
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  8. EFLImpudence

    EFLImpudence

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    And - I do not have permission to quote your post even though I have replied. ???

    upload_2019-8-23_12-46-20.png
     
  9. JohnW2

    JohnW2

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    Thanks - and apologies - the typo (missing "0") has been corrected.

    However, there's another confusion - did you actually see my original message (with the typo), even though it is allegedly ;'awaiting moderation' - or did you just see my screenshots of it?

    Kind Regards, John
     
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  11. JohnW2

    JohnW2

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    Mods, what is going on (I have reported this message) ?????????
     
  12. EFLImpudence

    EFLImpudence

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    Yes, it is visible, but when trying to 'quote' it I got the above message.
     
  13. JohnW2

    JohnW2

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    I think the forum software is having a 'senior moment' (although I understand that that phrase is no longer 'PC'!!).

    Kind Regards, John
     
  14. JohnW2

    JohnW2

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    I previously wrote ...
    However, I also wrote ...
    ... so, if that 0.98Ω figure is correct, then your present setup would be BS7671-compliant (goodness knows about the OSG!!), even with the Type 2 MCB!! (since your Zs would be 0.869Ω with a Ze of 0.35Ω).

    Kind Regards, John
     
  15. McKenna32

    McKenna32

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    I did "report" my own post last night, so could have inadvertently caused hell that way. will reply better in a bit cheers!
     
  16. EFLImpudence

    EFLImpudence

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    I have no idea why they did it - especially as you have spare ways - but perhaps they disagree with you and thought that was better than four wires in an MCB.


    I don't think it comes down to poor practice. It is just not necessary.

    People do all sorts of daft things.
     
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  17. AndyPRK

    AndyPRK

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    Didn’t have another 32A Mcb.
     
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