I believe 'standard' fuses are designed to blow in around one second
at 100% overload.
At 50% overload, an hour or two - ish.

Back at school in the 1970's, "someone" rewired the 100A fuse
controlling the stage lighting box with 5A fusewire, and set up the
lights to draw 15A illuminating the morning assembly one day.

Worked to perfection - the headmaster and his deputy were plunged into
darkness midway through their spiel.  The large mass of ceramic
housing cooled the wire and extended the overload duration to 15
minutes at 200% overload.     The fuse and carrier were  VERY hot
afterwards, the brass contacts had even discoloured.  Good thing the
wire was surrounded by asbestos :-)

Also,  about that time, "someone" also wanted to see what a 30 A short
on the mains looked like.  Won't do that again intentionally!

Andy
www.g4jnt.com

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2009/12/4 John RABSON <[email protected]>:
>
> On 04/12/2009 at 20:22 Ken wrote:
>
>>The ceramic fuses in 13A plug tops usually have a 100% overload
>>capabiliy, this was also the case for some of the BT fuses.
>>
>>Ken
>>de M0KHW
>
> I believe they are supposed to be able to clear 7000A short circuit (in case 
> the installation is too close to the substation).
>
> John F5VLF
>
>
>
>
>