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Garage RCD

Discussion in 'Electrics UK' started by morpheus83uk, 13 Apr 2020.

  1. morpheus83uk

    morpheus83uk

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    Hello,

    I have been wanting to sort out the electrics in the garage for a while now and since were on lock down what better time to do it. So at present the garage electrics come from a 16Amp fuse which is in the consumer unit with an RCD. The issue is every time the electrics trip in the garage the RCD trips in the house and everything goes off which means the other half is none to pleased. The smaller consumer unit in the garage has a main switch of 63Amps and a fuse for the sockets and a fuse for the lights. With no RCD apart from the one in the house.

    The setup of the wiring is that one 2.5mm cable goes from the 16Amp fuse labeled sockets goes to one socket and from there there are 8 more double sockets which come off this and it doesnt go back to the consumer unit.

    The lighting is 1.5mm cable which goes to 4 double LED tubes one after the other and again doesn't go back to the consumer unit. I'm not sure if it should or not go back to the consumer unit, as I have read that it should to create a ring circuit but also the opposite where it should just end of the last light fitting.

    First question is would this be OK with the layout of the sockets and the lighting? If not why not? I am assuming that the sockets should go back to the consumer unit to create a ring circuit given that lots of power tools are used in the sockets?

    The second thing is ideally I would want an RCD in the garage so that if it trips then it only trips the garage and not the whole house. What would be the best way of doing this? I am guessing I Would need to purchase a new consumer unit with an RCD in it as the current one has no space to put anything new.

    Thanks
     
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  3. terryplumb

    terryplumb

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    Fitting an RCD in the garage ,which is fed from an RCD protected circuit in the house consumer unit ,does not guarantee the RCD in garage would trip first .
     
  4. EFLImpudence

    EFLImpudence

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    Plus the cable to the garage might require an RCD (in the house) depending on where it is installed (if it is concealed somewhere), so there is no way out other than replace this cable with an armoured one.

    Also "the ring circuit" is only required if the size of the conductor(wire) is too small for the protective device (i.e. there needs to be two wires) - you say 16A fuse but it sounds like a circuit breaker so it is unlikely to be too small.
     
  5. ericmark

    ericmark

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    If you do things on the cheap, with a RCD powering many MCB's then you will get the problem, the answer is RCBO be it with a three neutral bar consumer unit or all RCBO's, this is not the time to be getting consumer units modified or changed.
     
  6. morpheus83uk

    morpheus83uk

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    Thank you for your very quick responses!

    I presume the lights are wired correctly then?

    I don't follow why this would be? If the trip comes from the garage why would the RCD not trip first if this would be the first place it would hit?

    So referencing the cable coming from the house to the garage, when would it need to have its own RCD and an armored cable?

    Regarding the 16Amp fuse in the consumer unit I have one of these: https://www.alamy.com/stock-photo-c...=1&vd=0&lb=&fi=2&edrf=&ispremium=1&flip=0&pl= which says B16 on it. Is that a circuit breaker?

    And based on that everything with the sockets is fine too?

    If it is not a job for me to do this I have no issue handing this off to an electrician when the lock down lifts, I just thought it would be a good time.

    Thanks

    James
     
  7. EFLImpudence

    EFLImpudence

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    They are both doing the same thing.
    Electricity goes really fast.

    Yes.
     
  8. morpheus83uk

    morpheus83uk

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    Brilliant Thanks it sounds like everything is setup correctly then and I don't need to make any changes.
     
  9. gman76

    gman76

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    What is it that you are doing in the garage to make the RCD in the house trip often enough to cause this reaction?
     
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  11. winston1

    winston1

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    If I read it correctly you have a 16 amp MCB in the house feeding the garage. In the garage you have another 16amp MCB feeding the sockets. No point in 2 similar trips in line. Most likely they will both go with a fault.
     
  12. SUNRAY

    SUNRAY

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    I believe the bigger issue here is imbalance and not overload.

    Probably most easily improved on by fitting an RCBO in the 'non-RCD' side of the house CU.
     
  13. Taylortwocities

    Taylortwocities

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    In fact it is very difficult to get discrimination between two MCBs in line. Take a look at the discrimination tables. The trip current difference has to be very large before there is adequate discrimination. The proper method is a fuse at the source feeder the second DB with its own MCBs.
     
  14. muffking

    muffking

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    I was just glancing through to see if I could add anything and that was my first thought too.
    I have to agree that if the OP has no spare ways then moving the MCBs and replacing the garage MCB with an RCBO on the non RCD side would be the simplest solution, although this depends on the type and layout of the board. In which case I would advise the OP to post a picture of the consumer unit.


    It's natural to assume that, but like EFLi says that's actually not the case. There may be enough milliamps or milliseconds reaction between the two RCD's for one to trip more quickly.
    Plus the impedance of the cable from house to garage will affect the load on the house RCD differently to the garage RCD.
     
  15. DetlefSchmitz

    DetlefSchmitz

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    I don't know, but I wonder if being locked up at home is making me more critical... But this makes no sense to me. How does the impedance come into it? The same current difference is seen in each RCD.
     
  16. SUNRAY

    SUNRAY

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    Isn't our job to be critical when it comes to electricity, especially safety?

    If the feed to the garage were long enough that the capacitance between L & N added an appreciable current prior to the house RCD, and the resistance were high enough then the differencial current at the two RCD's could be different enough.

    However we'd need a fairly big capacitance, something like 0.1μF, and I'll guess T&E is probably something in the order of 50pF per metre so it would take 2000m. These are all very estimated figures but give a very rough idea. At that distance a 30mA difference would be well and truly masked.

    Before anyone starts doing the calcs, yes I already know.
     
  17. EFLImpudence

    EFLImpudence

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    Not sure I understand that.

    Are you saying that the capacitance which might "mask" the 30mA difference could cause the RCD to trip on a circuit with no earth-leakage current,

    or

    are you making the mistake that an earth-leakage can be "cancelled out" - balanced - by the additional current or a similar leakage on the 'other' conductor?



    That is, of course, if the garage is two kilometres away.
     
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