What should trip first - plug top fuse, MCB or RCB?

Discussion in 'Electricians' Talk' started by diymostthings, Aug 11, 2016.

  1. diymostthings

    diymostthings Well-Known Member

    Mrs Diymostthings cutting hedge with electric hedgecutter - went through cable - twin no earth. (we've all done it). Normal plug top with 5A fuse, MCB and RCD protected circuit. Guess what tripped? (It surprised me).

    diymostthings
     
  2. PLPlanning

    PLPlanning New Member

    I'd guess and hope the RCD. You're going to say the 5 Amp fuse I Guess.:rolleyes:
     
  3. leesparkykent

    leesparkykent Well-Known Member

    Did the MCB trip?
     
    Last edited: Aug 11, 2016
  4. Bazza-spark

    Bazza-spark Screwfix Select

    Double insulated appliance so probably on a dead short wouldnt take out the RCD.

    You dont say what size the MCB is but I am guessing 32A and that it tripped before the 5A fuse blew.

    Kind regards
     
    leesparkykent likes this.
  5. PLPlanning

    PLPlanning New Member

    Can't stand the suspense......Ahhh.:eek:
    Is there a prize?:)
     
  6. diymostthings

    diymostthings Well-Known Member

    First prize: a weekend for two at the Ideal Home exhibition.
    Second prize: two weekends for two at the Ideal Home exhibition.

    diymostthings
     
  7. diymostthings

    diymostthings Well-Known Member

    The MCB was 32 amp on the ring main and the RCD was 80A capacity 30 mA trip
     
  8. Coloumb

    Coloumb Screwfix Select

    Whatever has the quickest fusing factor, so the plug top I would imagine.
     
  9. Coloumb

    Coloumb Screwfix Select

    Which I would imagine to be quicker than 30ms...
     
  10. tina lucinda lane

    tina lucinda lane Screwfix Select

    its the rcd (as a 30 miliamp trip rating is much lower than 5amp or 32amp mcb)
     
  11. philthespark

    philthespark Active Member

    I had a relative do this recently and nothing blew or tripped, I think it was due to the loop impedance being fairly high so no fuse or breaker taken out re and a 2 core flex so no rcd tripped
     
  12. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    Won't be the RCD as there is no cause for it to be activated. 5A fuse is most likely slower to blow than MCB.

    So MCB blows first.

    Mr. HandyAndy - Really
     
  13. nigel willson

    nigel willson Screwfix Select

    My bet is the 32A mcb, cos its a dead short.Wont be RCD as no earth leakage .
     
  14. Coloumb

    Coloumb Screwfix Select

    I was working on it being a dead short so around .01 = 10ms to pop the 1362...
     
  15. PLPlanning

    PLPlanning New Member

    Yep I think philthespark might be going to the ideal home expedition. :D

    That'll learn ya
    Glad no mention of third price.
     
  16. diymostthings

    diymostthings Well-Known Member

    Correct!! But isn't this a bit of a worry - Wired fuses are not meant to protect life - only equipment and to stop fires. If an RCd is impotent on double insulated appliances with a two core no earth flex - surely with a hedge trimmer this could be a life safey issue?
    PS Mrs Diymostthings is most indignant that no-one on the forum asked why she was cutting the hedge and not me!

    diymostthings
     
    Deleted member 33931 likes this.
  17. Coloumb

    Coloumb Screwfix Select

    In this case I think the plug top fuse wins as it's fusing factor is the highest in a fault between the mcb and the fuse, this being ~0.01 of a second, so the 30ma rcd woundn't even get a look in.

    NB, I would have to go a look at the various charts etc to do with trip speeds, bull rush diagrams etc, but I think my theory is correct.
     
  18. Mr. Handyandy

    Mr. Handyandy Screwfix Select


    Probably the fact that it's a double insulated appliance.....
    Mr. HandyAndy - Really
     
  19. Mr. Handyandy

    Mr. Handyandy Screwfix Select


    How would RCD work on non-earthed appliance?

    Mr. HandyAndy - Really
     
  20. Coloumb

    Coloumb Screwfix Select

    It's not really there to protect the appliance mate, just the fixed wiring.
     

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