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9" grinder trips 16amp MCB

Discussion in 'Electrics UK' started by cdbe, 17 Sep 2019.

  1. cdbe

    cdbe

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    I recently fitted an outside socket outside my attached garage:
    15687156486888899037219591692643.jpg

    With this RCD on the inside:
    15687157232146650446207220187225.jpg

    Until now I've run the grinder off the kitchen ring via an extension cable out of the window with no problems. The garage is a 2.5mm radial on a 16amp MCB (only other things on the circuit are 2 small freezers). From the same MCB is another 2.5mm radial feeding my other garage at the top of the garden - I've never had any issues with this setup before.
    The grinder attempts to start but trips the main MCB (not the spur). If I plug it into the freezer socket it trips the main MCB. If I plug it in the kitchen it works fine.

    I know it's the start up draw causing the problem. Is it worth (I'll do it anyway) opening up and cleaning the grinder brushes etc?
    or
    Increasing the MCB to 20amp - doubt this will make much difference.
    or
    Convert circuit to ring (I have a spare 32Amp way), leave other garage radial on existing 16A. Though this doesn't help if I want to run the grinder from the second garage.

    or replace existing MCB type B with a type C?

    Thanks
     
  2. EFLImpudence

    EFLImpudence

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    In a word - yes.

    You seem to understand so any one of those options might do it.

    You say 20A MCB won't make much difference but it may be all that is needed.
    25A if they are available for the CU and the cable run is not derated for thermal insulation etc.

    Converting to ring is a bit silly. You may as well replace the cable with larger 4mm² or 6mm².

    C type - if circuit characteristics allow.
     
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  3. cdbe

    cdbe

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    You were right!

    Temporarily tried a B20 I had lying around and it works fine:
    IMG_20190917_153953861.jpg

    Unfortunately it's a Hager and doesn't fit my Wylex board but I'll get the correct one next time I see the family electrician.

    Thanks.
     
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  4. Risteard

    Risteard

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    You shouldn't have temporarily put the wrong device in. Hager must not be used in a Wylex DB.
     
  5. EFLImpudence

    EFLImpudence

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    Rubbish.
     
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  6. Risteard

    Risteard

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    It's absolutely not rubbish.
     
  7. AndyPRK

    AndyPRK

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    But does a b20 protect the cable to your top shed
     
  8. cdbe

    cdbe

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    Oh dear. Anyway I did it, obviously it didn't fit on the bus bar perfectly but it was fine for 2 minutes. Nephew will drop me round a Wylex next week.

    The top garage is run in 15m of buried 4mm SWA apart from the first metre into the CU which is 2.5mm. I don't know but I'll check with my nephew.
     
  9. conny

    conny

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    For what it's worth, I'm not an electrician but there is bare copper showing on those cables.
    Also I HATE seeing mcb's not in sequence. i.e. all 6's at the start, followed by 16's, 20's 32's etc
    I agree some circuits may be protected by the RCD whilst others may not but there is no excuse for having them installed in the manner of the picture.
    It looks so sloppy.

    (And yes I do have 'mild' OCD but I have pride in any work I do as well.)
     
  10. DetlefSchmitz

    DetlefSchmitz

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    Surely you would put the highest current value next to the switch to reduced voltage drops and minimise bus bar heating effects.
     
  11. Risteard

    Risteard

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    It's actually better to alternate between heavy and light loads to minimise heating effects. Although it's common practice to start with the highest rating and work your way down, it's actually not the best way.
     
  12. DetlefSchmitz

    DetlefSchmitz

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    I would have expected all MCBs at their max loads to have roughly the same rise in temperature. Prepared to be corrected though.
     
  13. flameport

    flameport

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    mcb_power_loss.png

    Other manufacturers will be different.

    Another important consideration is whether the MCB will be fully loaded during normal use.
     
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  14. Risteard

    Risteard

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    Sit in the corner of the room facing the wall with a dunce's hat on for the next 8 hours then, and then go to bed without any supper.
     
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  15. JohnW2

    JohnW2

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    Thanks. That's very interesting. I was initially rather surprised, since the figures imply a ~10:1 ratio of 'internal resistance' over the range of Ins from 6A to 32A and a ratio of ~100:1 over the range 2A to 32A. However, on reflection, I think it makes sense ...

    ... I would imagine that the resistance of (hence power dissipated in) the conductors and solenoid are probably fairly trivial, with the lion's share of the 'internal resistance' (hence power dissipated, and heat generated) being attributable to the thermal element (bimetallic strip). Given that there is relatively little scope for variation of materials or dimensions etc. of the bimetallic strip, it therefore seems to follow that, at currents just below the magnetic trip threshold (e.g. max of 5*In for a Type B) the amount of power dissipated in (hence temp rise of) the bimetallic strip will have to be similar, regardless of the In of the device.

    Does that make sense (it does to me!)?

    Kind Regards, John
     
    Last edited: 18 Sep 2019
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