Fuses in plugs are made in standard ratings. The most common are 3 A, 5 A and 13 A. The fuse should be rated at a slightly higher current than the device needs. e.g. if the device works a 3 A use 5 A fuse. Fast and slow fuses are made. Fast acting fuses blow immediately once the current rating is exceeded. Slow blow fuses will let the current exceed the rating for a fraction of a second, to allow devices like motors to start up.