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MCB tripping berofe RCD

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  • 19-05-2020 1:07pm
    #1
    Wearb
    Moderators, Home & Garden Moderators Posts: 6,289 Mod ✭✭✭✭


    I came across a live to earth fault (damaged insulation on the live in a t+e cable) and the mcb alone tripped. This happened more than once. I replaced the cable and everything ok now. I was worried about it and wired a 60w bulb L to E in a 13A plug to test that circuit. RCD alone tripped this time, which makes me less worried.



    Is that a normal sequence of events when there is a larger current, as in the first instance?

    Please follow site and charter rules. "Resistance is futile"



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    #2
    Conte..
    Closed Accounts Posts: 56 ✭✭Conte..


    I've seen the MCB trip or even both for a L-E short

    Sometimes the RCD can be stuck too when the fault occurs, switching it on and off and pressing the test button frees it up which is maybe what happened there


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    #3
    adrian92
    Registered Users Posts: 566 ✭✭✭adrian92


    Wearb wrote: »
    I came across a live to earth fault (damaged insulation on the live in a t+e cable) and the mcb alone tripped. This happened more than once. I replaced the cable and everything ok now. I was worried about it and wired a 60w bulb L to E in a 13A plug to test that circuit. RCD alone tripped this time, which makes me less worried.


    Is that a normal sequence of events when there is a larger current, as in the first instance?

    I think what you have done is ok, good.
    The short circuit was rectified by your actions
    Your test to simulate an earth fault confined that the RCD works.
    All ok


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    #4
    Wearb
    Moderators, Home & Garden Moderators Posts: 6,289 Mod ✭✭✭✭Wearb


    adrian92 wrote: »
    I think what you have done is ok, good.
    The short circuit was rectified by your actions
    Your test to simulate an earth fault confined that the RCD works.
    All ok

    Thanks for reply. I just thought it strange that only the mcb tripped in the first instance.

    Please follow site and charter rules. "Resistance is futile"



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    #5
    adrian92
    Registered Users Posts: 566 ✭✭✭adrian92


    Wearb wrote: »
    Thanks for reply. I just thought it strange that only the mcb tripped in the first instance.
    Yes. Generally an earth fault occurs, with a fast response that the fault is cleared before the MCB looks after it , but not always.

    It seems like a short circuit with no connections to earth.

    Your measures are good, I think.

    All good, in my view.

    Nothing more needed


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    #6
    Conte..
    Closed Accounts Posts: 56 ✭✭Conte..


    The lamp would give you an idea if it's operational same as the test button

    The RCD tester will check that it complies with regulations

    Wouldn't be recommended to connect test loads across the protective conducter, might be alright if u had checked the fault loop impedance on the final circuit


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    #7
    Retro.
    Closed Accounts Posts: 89 ✭✭Retro.


    The danger with connecting a 60 watt lamp like that is that the RCD may not be working and the earth may be broken on the circuit

    Anyone in contact with exposed metalwork would receive an electric shock


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