Bathroom bulb popped, RCD FCU fitted, MCB still tripped?

Discussion in 'Electrics UK' started by Muzz, 22 Apr 2011.

  1. Muzz

    Muzz

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    Hi,

    The electrics for the ensuite i fitted are supplied off an RCD FCU which I wired into my upstairs ring main. This ring is protected by a non RCD consumer unit.

    This morning the wife switched the light on the ensuite and the bulb went, she shouts the bathroom is not working so i said to press the reset on the RCD, I then notice the whole upstairs circuit is off as the mcb has tripped.

    Can't tell if the rcd had tripped or not as the wife pressed the reset button before i saw it, but just wanted to check that it's normal behaviour for the rcd and mcb to go?

    really just want to make sure my ensuite is properly protected and that i've not inadvertantly installed something wrong.
     
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  3. bernardgreen

    bernardgreen

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    When a lamp goes pop ( bulbs grow in gardens ) it is likely to go short circuit for a short while.

    This short circuit results in an over current fault which the MCB will detect and react to. The short circuit is a plasma discharge in the vapourised metal from the filament.

    As there is no fault current to earth there is no un-balance in the currents in live and neutral even though they are too high for the MCB to allow so the RCD will not react to a blown lamp causing a short circuit between live and neutral.

    Edit put the missing L before amp
     
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  4. TicklyT

    TicklyT

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    It sounds reasonable for a MCB to pop without taking out the RCD when a lamp blows, as they each offer protection from different types of faults.

    A failing lamp can draw a very high current as it blows, enough to trip an MCB, which responds to what it sees as an overload.

    RCDs should only trip if current starts leaking to earth. They offer no overload current protection.
     
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  5. Taylortwocities

    Taylortwocities

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    OK, this ensuite that you fitted out.

    I assume that the RCD and FCU are there to feed just the lights in the bathroom?

    What size fuse is in the FCU?
    WHat rating is the MCB that popped?
     
  6. Muzz

    Muzz

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    cheers guys that makes sense.


    I think i see where you might be going with this, :) but i didn't go off half cocked when I installed it. I did my research and asked plenty questions (on here and elsewhere), obviously still open to advice if improvements need to be made.

    It's a combined RCD FCU, pretty sure it came with a 13amp fuse and I seem to remember putting a 5a in but i could be wrong, it was a while ago.

    The MCB is for my upstairs ring main, from memory I think it's 32A.

    The RCD FCU feeds the lights, fan and shaver socket. (no electric shower in this ensuite)

    The lights are the appropriate IP rating for the zone they are in and the fan is a SELV fan with transformer safely located in the attic space.
     
  7. Taylortwocities

    Taylortwocities

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    A 3amp fuse would be more than enough. If it's 13a then it's way too much.
     
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