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Multiple FCUs

Discussion in 'Electrics UK' started by JohnW2, 18 Sep 2011.

  1. JohnW2

    JohnW2

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    I know the common sense and engineering answers to this question (at least, to my own satisfaction!), but I wondered what people thought about this, regs-wise...

    I have a switched FCU (3A fuse) spur from a 2.5mm² ring final circuit, supplying a small fixed load. The FCU is fed by about 1.2m of 2.5mm² cable from a socket on the ring. I intend to 'daisy-chain' two additional (physically adjacent) FCUs (again, both with 3A fuses) from the first one, using very short lengths of 2.5mm² cable.

    I can see absolutely nothing wrong with this, being 'not even as bad' as a 13A fused spur fed by 2.5mm² cable. However, it is not a scenario which (as far as I can see) is explicitly contemplated by the regs, so I was wondering whether anyone could think of any way in which a pedantic Jobsworth of a PIRer (or should that be EICRer) could have a problem with it?

    Kind Regards, John.
     
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  3. RB2004

    RB2004

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    ....
     
  4. JohnW2

    JohnW2

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    Could you perhaps expand on that a little? :)

    Kind Regards, John.
     
  5. PrenticeBoyofDerry

    PrenticeBoyofDerry

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    I can
    ..... ;)
     
  6. PrenticeBoyofDerry

    PrenticeBoyofDerry

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    Maybe an issue with selectivity (discrimination).
    As the FCU that would serve the daisy chained FCUs are rated at the same breaking capacity?
     
  7. JohnW2

    JohnW2

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    I'm talking about diasy-changing the inputs (supply side) to the FCUs, not putting them in series. There is no discrimination issue.

    Kind Regards, John.
     
  8. PrenticeBoyofDerry

    PrenticeBoyofDerry

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    I don't see a problem with that, I do that quite often in kitchens when dropping spurred outlets below worktop height, via 13A SFCUs for above worktop isolation to appliances and even cabinet lights and extractors.
    I don't see it any different to taking the feed off the protective device on the RFC(CU) than to taking it off a 3A FCU.
     
  9. JohnW2

    JohnW2

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    I have to say that I would see a problem with that, if (in analogy with the situation I described), you were feeding multiple FCUs with 13A fuses from a single 2.5mm² unfused spur from a ring final - the important thing about my setup is that they will all (all 3) be 3A fuses.

    With the kitchen arrangement such as you describe, the proper/normal way to do it is surely to have the several 13A FCUs all wired into the ring - not to have them all supplied by a single unfused spur from the ring?

    Kind Regards, John.
     
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  11. JohnW2

    JohnW2

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    Just found trhis edit ...
    The difference is that the several FCUs are then being fed by a single 2.5mm² cable. As I said, I see no problem in my cases, because of the 3A fuses in the FCUs (i.e 3 x 3A) - but I would see a problem if, as in the kitchen scenario you describe, multiple FCUs, each with 13A fuses, were being fed by a single 2.5mm² unfused sour.

    Kind Regards, John.
     
  12. PrenticeBoyofDerry

    PrenticeBoyofDerry

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    I think you have misunderstood!
    All the 13A S/FCU would still spurred of the RFC individually, not linked together and even if say they were, it sounds very much like what your set up is, but using 3A.
     
  13. Paul_C

    Paul_C

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    I agree that in practice it's no worse than a 13A socket spurred from a ring to have the three small loads you contemplate for this arrangement. It's conceptually little different from plugging a 3-way adapter into that 13A socket and then just plugging three appliances into that adapter, each with a 3A fuse in its plug.

    Strictly speaking it doesn't comply with BS7671 though, since the spur from the ring is feeding more than one fixed appliance. I would say that on a PIR it would warrant no more than a code 4, but judging by the crazy codes we see suggested for some trivial things, what anybody looking at it in the future would code it as is anybody's guess!

    The new reporting system doesn't contain a code which is equivalent to the PIR's code 4 though. Personally, I'd say that it means it doesn't warrant a code at all, but I have no doubt that some in the future will try to insist that something which does not comply in every last detail with BS7671 cannot go uncoded, and thus will put it down as at least C3 - "Improvement recommended."
     
  14. JohnW2

    JohnW2

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    There is indeed some misunderstanding going on here!

    The whole point of my question relates to the difference between spurring each FCU from the ring individually (which I don't want to do) - which is obvioulsy OK - and (as I'm talking about) having only one spur off the ring feeding several FCUs, hence the initial run of the spur cable (to the first FCU) carrying the total current of all three FCUs.

    Kind Regards, John.
     
  15. PrenticeBoyofDerry

    PrenticeBoyofDerry

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    If the the load only requires 3A and cable sizes is suitable, why the need for any additional FCU.
    I would just install some switches.
     
  16. holmslaw

    holmslaw

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  17. JohnW2

    JohnW2

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    The main 'Jobsworth' issue I was expecting was a suggestion (which I could not contest) that no-one can stop someone coming along subsequently, changing the 3A fuses to 13A ones and then connecting large loads to all three FCUs. That would cearly not be a desirable.acceptable situation if all of them shared a common 2.5mm² feed from the ring.

    I also considered that - but does BS7671 actually say explicitly that such an arrangement is non-compliant - and, if so, where? Each of the fixed loads has an appropriate, separate, fuse, the only issue being the short length of 2.5mm² cable common to the feed of all of them (and the supply path is obvioulsy common to all three FCUs once one gets upstream of that.

    Kind Regards, John.
     
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