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RCD Fault - I'm dumbstruck

Discussion in 'General Electronics Discussion' started by Sadlercomfort, Nov 17, 2013.

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  1. Sadlercomfort

    Sadlercomfort Ash

    Feb 9, 2013
    Hi Guys,

    I have recently installed a new double socket, so I can put both my tumble dryer and chest freezer in a convenient space :) It's flush with the fuse board/consumer unit, and a direct spur from the ring main.

    However, when I use the double socket, I get an RCD fault. Note: that the ring main I took a spur from, was the same circuit I used the two appliances on before. I have a plug-in tester which works fine when I plug it into the double socket, the LED's indicate the correct wiring and I get a nice continous beep.

    I know that RCD's only trip when there is over 30mA earth leakage from neutral, or live. I also know these are common with appliances. Using the process of elimination, I switched off every circuit but left the double socket on, but still got an RCD fault when my appliances were plugged in. So i know its that circuit.

    I dont understand why the RCD doesn't trip when I plug it into the ring main? But trips from the double socket? Anyone got any ideas?

  2. duke37


    Jan 9, 2011
    I investigated a similar fault in my parents house. Everything normally worked all right but tripped the RCD on sunday morning. It turned out that there was a neutral/earth fault and this did not trip the RCD when the demand was low but when the demand was high, the drop along the neutral wire generated earth current.
    The fault was due to the cable being pulled around a sharp metal edge, cutting through the insulation.

    In your case, there may be an earth/neutral fault which provides an earth current when there is voltage drop along the neutral wire. You will need to disconnect the ring and check for an earth/neutral connection.
  3. Sadlercomfort

    Sadlercomfort Ash

    Feb 9, 2013
    Yeah I can understand why that would cause a fault, and I have had to go through a tight bend.. although its all thermoset plastic. No obvious signs of damage, I'll disconnect the spur and see what tests say.

    Attached Files:

  4. Sadlercomfort

    Sadlercomfort Ash

    Feb 9, 2013
    Hi Guys,

    I'm still haven't got round to testing this double socket. I can't remember how to carry out the tests, can someone briefly describe how to test it to jog my memory?

    I have a standard multimeter.
  5. shumifan50


    Jan 16, 2014
    At the risk of being offensive. Turning the trips off doesn't eliminate the other circuits from causing the problem, unless your trips isolate both live and neutral. If not, then the other circuits could still leak between neutral and earth tripping the RCD. It could even be an appliance on those circuits, so appliances also have to be unplugged.
    In your case I guess it would pay to check for a resistance between the different wires of the appliances on the new plug and earth. If the resistance is anything but unmeasurable(open circuit), the appliance has a problem.

    I had this problem with a washing machine that dripped water on its electronics.
    Last edited: Mar 12, 2014
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    I would be *extraordinarily* careful sticking my multimeter probes into a power socket. And that's true even if you think there's no voltage there.

    One probe at a time, one hand only, other one in your pocket. Suitably rated probes, and only on a voltage range.

    Measuring the voltage between neutral and earth is a safer way to check for earth/neutral faults -- beware that this may give misleading results if the earth is completely disconnected.

    The other (and perhaps the *best* way is to get a plug in tester)
  7. Sadlercomfort

    Sadlercomfort Ash

    Feb 9, 2013
    Finally fixed the problem. It seems there wasn't an RCD fault after all.

    I couldn't understand why the double socket spur would trip when my appliance was plugged in, but not trip when the appliance was plugged into the ring main.. Because the spur and the ring main are on the same circuit breaker? My appliance must be fine.

    So I decided to check the connections to the double socket spur again, and realised the spurs neutral was in the 3rd neutral block, not the 2nd block like the ring main. So i reconnected it to the correct neutral block and number.

    It seems to have fixed the problem? =D
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