I was sent this faulty Wylex circuit breaker to take a look at. It's a common brand used in consumer units (home distribution boards) in the UK. The contacts had suffered burn damage and were not making a reliable connection. After seeing what was wrong I thought it would be good to show how the mechanism in an MCB (Miniature Circuit Breaker) works. Especially the bit that lets it still trip while the toggle is being held in the on position.
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16 thoughts on “Inside a faulty uk wylex circuit breaker.”
  1. Avataaar/Circle Created with python_avatars Tobi Won Kanogy says:

    I know where to look for little fins to cool things off with . 😉 Thanks again Clive .

  2. Avataaar/Circle Created with python_avatars David Quirk says:

    I was told by the speaker at the International Arc Flash conference that the arc always travels away from the energy source irrespective of position of the electrodes.

  3. Avataaar/Circle Created with python_avatars Jos van Diepen says:

    Clive, one little remark. The short circuit current rating is 6000 amps in a rectangle. That means in home application 6000 amps and 10000 amps industrial. It means also that you can try to switch on the breaker whiteout destroying while there is stil a secondary problem. (for example a short circuit).

  4. Avataaar/Circle Created with python_avatars Yorkshire Rose says:

    Tres interesting, and a lot more complex than I might have thought. Somewhere hereabouts, we have one that will not switch back on. It tripped one day for no obvious reason, and the dolly will no longer reset nor latch into the on position – I wonder where it got put, and if it could be repaired, though by the sound of it, it's not worth repairing even if it were physically possible.

  5. Avataaar/Circle Created with python_avatars MrGoatflakes says:

    Would the magnetism from the solenoid or its components work to break up the arc?

  6. Avataaar/Circle Created with python_avatars Party Pillz says:

    I probably won't get a answer but if I use a circuit breaker with a higher amp than the device I'm using, would it only trip when it goes above that amperage?

  7. Avataaar/Circle Created with python_avatars Robert Putt says:

    Interesting stuff, any chance you could do an RCD also, I'd be mega curious to see inside on of those but don't have a spare knocking around or the BigCliveDotCom wisdom to understand wtf its doing, but I am sure it would also be some well engineered and interesting electromechanical goodness.Maybe if someone has an old non functional / spare RCD they'd like to donate it to the cause?

  8. Avataaar/Circle Created with python_avatars MetroidChild says:

    you know if I had a breaker I would honestly want it to vent out in the front so I could see if it was smoking and burning up

  9. Avataaar/Circle Created with python_avatars Chris Thomas says:

    Cool thanks for this, always wondered how a MCB worked inside, what causes it to trip etc. Great video as always 🙂

  10. Avataaar/Circle Created with python_avatars A jones says:

    Good advice as usual.interesting stuff

  11. Avataaar/Circle Created with python_avatars Matthew Obley says:

    It does seem weird that the arch would want to move away from where the contacts are close.  Perhaps the hot expanding gases carry the arch that way because its the only opening to atmosphere and the only direction to expand to.

  12. Avataaar/Circle Created with python_avatars sinephase says:

    I guess 6000 amps sounds like a lot until you look at an ampere graph for a dead short. I wonder how they (if at all?) standardize arc ratings?

  13. Avataaar/Circle Created with python_avatars ppdan says:

    Couldn't the solenoid blow the arc?

  14. Avataaar/Circle Created with python_avatars Ronnie K says:

    awesome

  15. Avataaar/Circle Created with python_avatars Hrnek Bezucha says:

    A broken circuit breaker?

  16. Avataaar/Circle Created with python_avatars Ted Haubrich says:

    Interesting, our US circuit breakers seem much simpler and more rudimentary, but same idea, thermal strip and the solenoid. Maddeningly unstandardized, different panels take different styles, Eaton, Square D, Siemens Cutler Hammer…..

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