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Electric underfloor heating tripping

Discussion in 'Electrics UK' started by kashcheema, 4 Jul 2021.

  1. securespark

    securespark

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    Is the one system fed via a FCU?

    Edit- must have been half asleep.

    Are you sure your figures are right?

    150W X 60 is just over 39A.Turning one stat up will just call for heat in one area of that total 60m².

    Something is not right somewhere.
     
    Last edited: 4 Jul 2021
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  3. kashcheema

    kashcheema

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    It is connected via a Double Pole Fused Connection Unit with a connector type device where all the mat wires feed into. The figures I do not know about to be honest. The entire 60m2 is warming up.
     
  4. JohnD

    JohnD

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    fuse cartridge 13A max.

    I wonder who designed, installed and tested it.

    I'd expect a stat per room.
     
  5. securespark

    securespark

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    There surely can't be 39A running though a 13A fuse?

    The mind boggles.
     
  6. Taylortwocities

    Taylortwocities

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    There can be, but not for long!
     
  7. fezster

    fezster

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    60 sqm! Wow. That is going to be very expensive to run.
     
  8. Taylortwocities

    Taylortwocities

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    It costs very little, if the MCB trips after a few seconds!:LOL:
     
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  10. flameport

    flameport

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    Having looked at the video again, it's evident that the heating mats are switched through a contactor, which can clearly be heard just after the thermostat is turned up. One thermostat for the whole lot most likely.

    From the rate of flashes on the meter, the heating load is approximately 8kW or 35 amps, far too much for a 20A MCB. Not exactly suitable for a 32A device either.
    For 60m² that's about 133W/m², close enough to the typical 150W/m²

    29 flashes in 4 seconds of video
    x15 = 435/minute
    x60= 26100/hour
    that meter is 3200/kWh
    26100/3200 = 8.15kW
    Less whatever other items were on at the time, likely virtually nothing based on there only being 2 meter flashes in 7 seconds before the heating was switched on.

    problems -
    If the wrong 20A MCB was installed and then just changed to a 32, is the circuit cable suitably sized for that load?
    8kW is overloading a 32A MCB
    With all heating switched from a single thermostat, there is no diversity
    8kW of heating will cost about £1.30 per hour to use, just 2 hours a day is £1000 per year.
    Tails from cutout to meter and out from meter look significantly undersized
    It's a looped supply, which is very likely limited to 60A, or even less.

    Contactor not visible, but may be the box visible at the end of the video on the right side with the collection of black wires dangling down.
    FCU probably just for the thermostat and contactor, not the full heating load.
     
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  11. JohnD

    JohnD

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    slightly modified....

     
  12. kashcheema

    kashcheema

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    Thanks for all the updates, like I said I am no electrician so will go buy whatever my electrician and you guys have said. I kept it on for 2 hours yesterday and it didn't trip and warmed up to 27 degrees. I have no intention on using this daily, even in winter. Even with the UFH off, the tiles are not exactly cold (we didn't have the best of weather in April and then it was completely bearable without the UFH).

    Also thank you for flameport to going to all the effort for doing the calculations for me, I appreciate it.
     
  13. Taylortwocities

    Taylortwocities

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    Indeed the MCB did not trip but you still have a problem:
    As well as increasing the MCB size to 32A, the cable that runs from the consumer unit to the contactor must also be increased in size. If you don’t then, as well as the floor heating up, the (undersized) cable will also heat up and this could have serious consequences. Flameport highlighted this in his post.
    The cable will need to be at least 4mm². Whoever changed the MCB must confirm the cable is suitable.
     
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  14. kashcheema

    kashcheema

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    Yep the cable will be changed to a 4mm2 this weekend.
     
  15. securespark

    securespark

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    So the spark knowingly added a 32A breaker to a radial circuit wired in 2.5mm² cable, then re-energised?
     
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