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Fuses for showers.

Sparkingchip
From a electric shower installation manual:


Circuit Protection Table

Circuit Protection Device


Unit Rating (@ 240V)

8.5kW

MCB 40A

Cartridge Fuse 45A


9.5kW

MCB 40/45A

Cartridge Fuse 45A


Why a 45 amp cartridge fuse for a 8.5kW shower or for that matter even the 9.5 kW shower?


I presume it must be time related as the 8.5 kW shower will only load a 40 amp fuse to around 90% of its capacity.


How long can cartridge fuses be run at 90% capacity for?


 Andy B.
  • 0

    Why a 45 amp cartridge fuse for a 8.5kW shower or for that matter even the 9.5 kW shower?


    I presume it must be time related as the 8.5 kW shower will only load a 40 amp fuse to around 90% of its capacity.



    Isn't it just down to 40A not being a preferred size for BS 1361/BS88-3 fuses? - normally your choice would either be 30A (BS 1361)/32A (BS 88-3) or 45A.


      - Andy.
  • 0
    The 35, 40 and 45 amp fuses are all the same physical size, Wylex made the fuse holders for the rewirable fuse boards with the orange bases and marked them maximum 45 amps, but sold them with all three sizes of fuse in them.


    I was trying to decide if there is a technical issue, rather than preferred sizes.


    Andy B
  • 0
    I have a pre-used 35-amp cartridge fuse in a Wylex 45-amp fuse holder with a orange base on the side of the dashboard in my van. They were made for 7.5 kW electric showers as the showers were a bit too beefy for a 30-amp fuse or fuse wire.


    Andy B.
  • 0
    f138f86f3619143a1b5113d287d9ebe0-huge-20191123_132011.jpg


    A Wylex 45-amp fuse holder with a 35-amp cartridge fuse.


    Andy B
  • 0
    Apparently the local branch of Wickes has 35-amp fuses in stock.


    So not much of a rarity, they should be on the shelf with 40 and 45-amp cartridge fuses as well.



    Andy B.
  • 0
    Manufacturer's instructions are frequently wrong or unnecessary.
  • 0

    geoffsd:

    Manufacturer's instructions are frequently wrong or unnecessary.




    Given that if an existing fuse holder will take a choice of three available fuses, the 40-amp cartridge fuse seems the appropriate choice for a 35.5-amp 8.5 kW shower rather than either the 35 or 45-amp fuses as it will probably run around 36 to 37-amps.


     Andy B.

  • 0
    When I did the 16th edition course the lecturer said the authors of BS7671 like cartridge fuses as they are very predictable and there’s not any chance of mechanical failure, so you know what to expect from them.


     Andy B.
  • 0

    Sparkingchip:

    When I did the 16th edition course the lecturer said the authors of BS7671 like cartridge fuses as they are very predictable and there’s not any chance of mechanical failure, so you know what to expect from them.


     Andy B.




    Even B.S. 7671:2018 in Reg. 533.1.2.3 says that cartridge fuses are preferred to semi-enclosed fuses. I saw a promotional film once, made by a fuse manufacturer, extolling the virtues of cartridge fuses. The presenter said that the cartridge fuse is reliable, and when a new one is fitted it is totally unused and fully functional, unlike perhaps M.C.B.s where internal wear may occur with much use. Of course if the 8.5kW  (35.4 Amps) shower is rated at 240 Volts, its current will increase with a Voltage rise to say 250 Volts.


    Z.

  • 0

    Zoomup:




    Sparkingchip:

    When I did the 16th edition course the lecturer said the authors of BS7671 like cartridge fuses as they are very predictable and there’s not any chance of mechanical failure, so you know what to expect from them.


     Andy B.




    Even B.S. 7671:2018 in Reg. 533.1.2.3 says that cartridge fuses are preferred to semi-enclosed fuses. I saw a promotional film once, made by a fuse manufacturer, extolling the virtues of cartridge fuses. The presenter said that the cartridge fuse is reliable, and when a new one is fitted it is totally unused and fully functional, unlike perhaps M.C.B.s where internal wear may occur with much use. Of course if the 8.5kW  (35.4 Amps) shower is rated at 240 Volts, its current will increase with a Voltage rise to say 250 Volts.


    Z.


     




    Surely you mean reduce? 8.5kw @ 250V = 34 Amps

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