Fuse Tripping When Changing Plug Socket

Discussion in 'Electricians' Talk' started by Danny-r, Nov 10, 2021.

  1. Danny-r

    Danny-r New Member

    So, switching out a regular socket for a smart socket. Double socket for double socket. Nothing difficult. Original had 6 wires, 2 of blue, brown and yellow/green.

    Socket originally had a plug in which lit up, which was handy to confirm power was cut. I switched one of two fuses labelled "sockets" and watched as the lights on the plug went out. Great, it's safe.

    Proceeded to replace the socket, just about to connect blue and the RCD trips.

    I don't understand how? There is no power to the socket? Socket is now installed and working fine, but it tripped the RCD twice despite having no power, before being wired up completely and without me switching the fuse for the socket back on.
  2. MGW

    MGW Screwfix Select

    Line and neutral are both considered as live, but a MCB only switches line, so sockets not isolated as still a neutral, so neutral to earth causes an unbalance so trips RCD.
    BiancoTheGiraffe and seneca like this.
  3. seneca

    seneca Screwfix Select

    There's your answer Danny !
    BiancoTheGiraffe likes this.
  4. Danny-r

    Danny-r New Member

    So even thought I'd switched the sockets off at the fuse box, there's still power to it?
  5. Teki

    Teki Screwfix Select

    When you switch off the MCB, the neutral is still connected. Only the live connection is disconnected.

    If you accidentally touch neutral to earth (metalwork on the socket or back box), the RCD will trip.
  6. Tony Goddard

    Tony Goddard Screwfix Select

    Its annoying and happens all the time, you just turn off the lighting to change a fitting, and accidently touch neutral and earth and pop, the RCD trips.

    The MCB only switches the live side, but the neutral is still in circuit, if it touches earth that results in an in-balance in the circuit that trips the RCD - normally its nothing to worry about, just irritating!
    BiancoTheGiraffe likes this.
  7. Comlec

    Comlec Screwfix Select

    And that is why you always warn the occupants that their zoom call may end sooner than they expected.
    Another argument for RCBOs
    BiancoTheGiraffe likes this.
  8. BiancoTheGiraffe

    BiancoTheGiraffe Screwfix Select

    It's one of those things I've never quite fully got my head around!

    The neutral and CPC are often connected together a few yards away at the meter anyway, so how is there enough of a difference in potential to trip a 30mA RCD?
  9. Teki

    Teki Screwfix Select

    It depends on the amount of current flowing in the other connected circuits.

    JW explains it well:
    BiancoTheGiraffe likes this.
  10. BiancoTheGiraffe

    BiancoTheGiraffe Screwfix Select

    It makes so much sense the way he explains it!

    It's embarrassing to admit that I've struggled to explain why it sometimes happens and sometimes doesn't, but now all is clear!
    Teki likes this.
  11. Comlec

    Comlec Screwfix Select

    JW is the antithesis of the typical Youtuber. With JW it is all ‘look at this and listen to what I know’, rather than ‘look at me and this what I think’. Refreshingly boring because it is actually informative.
    Sparkielev and BiancoTheGiraffe like this.
  12. MozzyMarr

    MozzyMarr Member

    I watch a lot of his videos, purely out of interest. Some of his videos on old electrical fittings are a real eye opener.
    Abbadon2001 likes this.
  13. Abbadon2001

    Abbadon2001 Member

    He reminds me so much of my dads friend, an expert in vintage radios and the like, they speak in very similar manner and with the same precise explanations.

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