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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.
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  • 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.

Reply
  • 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.

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