Some circuits have an initial surge of high current but then the current drops back down to a steady flow. You use a slow blow fuse so it does not blow with the initial high current. These will only blow if the high current such as a fault continues. If you use a fast blow where a slow blow was specified it will blow the fuse every time. If you use a slow blow where a fast blow was specified the fuse will not blow fast enough and could cause further damage. Only use the correct type of fuse as originally specified. I also come across alot of boards where 10A fuses are used for 1A fuses or pieces of wire have been soldered across fuse holders. Of course, rather the blow the fuse the circuit stays on and frys the boards or the coils.
