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earthing a new double socket

Discussion in 'Electrics UK' started by jammiebodger, 22 May 2011.

  1. JohnD

    JohnD

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    surely the socket faceplate is the accessory

    not the backbox.

    I still do flyleads as a matter of good practice

    Of course, if the CPC is terminated at the backbox, as is sometimes done, a flylead is essential.
     
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  3. JohnW2

    JohnW2

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    I don't think that sentence has the meaning you suggest. I think it literally means terminated (e.g. connected to a terminal block), not necessarily a terminal on the metal box. We discussed this here fairly recently, in relation to the need to run a CPC to any accessory, even if it is a Class II accessory which does not require a CPC (just in case someone subsequently changes it for a Class I one).

    It is the first sentence/paragraph of 411.3.1.1 which requires that exposed-conductive-parts be connected to the CPC - and I had always assumed that a surface metal box (just like the casing of any other electrical equipment) counted as an exposed-conductive-part - or am I wrong?

    Kind Regards, John.
     
  4. jammiebodger

    jammiebodger

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    river,
    i think i follow. the rcd seems to cover several different circuits, not just the heating/socket, if that makes any difference?
    is it worth installing a fused connection unit?
    whats an OP?!!
    d
     
  5. Taylortwocities

    Taylortwocities

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  6. riveralt

    riveralt

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    Original Post(er) - you ;)
    Yes the RCD will cover several circuits but in 'broad terms' its purpose is to protect you while the MCB protects the circuit. So the RCD is not the issue here.
    If you have a 32Amp MCB protecting your 2.5mm T&E socket and 2.5mm heating circuit then you must reduce it to 20Amp or increase the size of the cable to 4mm T&E for both the socket and heating circuit.

    Even if you fit an FCU between the MCB and the socket you will still need to reduce the MCB size to 20Amps or increase the cable size from the MCB to the FCU to 4mm - you could also do the same for the heating circuit.

    But this is a lot of work when it would be easier to simply replace the current MCB with a 20amp equivalent.
    I assume you have a wylex consumer unit - these are widely available.

    But replacing the MCB will require you working on the consumer unit - if you are not TOTALLY confident in doing so safely then get an electrician in to do it for you.
     
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  7. jammiebodger

    jammiebodger

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    thanks very much for the advice!!
    d
     
  8. jammiebodger

    jammiebodger

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    just been back with the tape measure,
    the tricord from the CU is 4mm. shouldi still fit a FCU or just spur off the existing socket?
    d
     
  9. riveralt

    riveralt

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    Check here to be doubly sure you have the right size cable.

    http://www.diynot.com/wiki/electrics:cable_types:flatpvccables

    If you do have 4mm cable for BOTH your single socket and heating circuit then to add another socket you can either simply use 4mm cable and extend the socket circuit or interupt the 4mm between the MCB and the old socket and fit an FCU and then run 2.5 or 4mm T&E to the old socket then 2.5mm T&E to the next socket.
    There are other options involving junction boxes and FCU's after the first socket but I don't see the need for that.
    Seems to me the easiest solution is just to run 4mm T&E from the old socket to the new one.
     
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  11. DetlefSchmitz

    DetlefSchmitz

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    Mmm... you wouldn't normally measure the cross sectional area of one of the conductors in a twin and earth cable using a tape measure, I prefer a micrometer. So which dimension were you actually measuring?
     
  12. securespark

    securespark

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    If the cable is T&E then 4mm2 will be stranded, not solid like 2.5.

    As for the 2.5 on 32A argument, I would always change it BUT you could argue that there is no difference between that and a spur off an RF circuit at the MCB....
     
  13. jammiebodger

    jammiebodger

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    argh!
    if its meant to be 4mm squared then im not sure. tried to measure the widest part of the cable.
    im not about to mess about with the CU, have no intention of changing the cable supplying the original socket.
    i think i.m going to install a FCU before the existing socket then spur a second one after.
    Unless this seems very unsafe to anyone?
    d
     
  14. 1john

    1john

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    If it's stranded, check that the earth is a solid core, as if that is stranded too you could be looking at old cable 7/0.29 which is about 2.5mm equivalent.

    Now stop me if im wrong by the two CPC terminals on socket outlets are for use on high leakage circuits such as office computer circuits etc.
     
  15. riveralt

    riveralt

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    Yes it will be unsafe if the cable is 2.5mm from the CU and the MCB is 32Amp.

    1. As said above 4mm T&E has stranded cores - while 2.5mm has a solid core. Look at the cable connected to the socket and see if it is solid (single) core or stranded (several) cores. If it is stranded cores then it is likely that it is 4mm providing the installation is less than say 30 years old. Otherwise is could be imperial size equivalent 2.5mm.
    2. If it 4mm then just add the new socket to the existing socket using 4mm cable- no need for an FCU.
    3. If the cable is 2.5mm then you must change the MCB for 20Amp.
     
  16. davelx

    davelx

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  17. riveralt

    riveralt

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