Let's not confuse amperage and voltage. Your fuse USUALLY won't blow because of overvoltage. Granted all fuses have a voltage rating, but your voltage regulator USUALLY won't produce enough voltage to burn up the fuse element. Notice I say "usually", because there are no certainties in electricity.
I've seen fuses run on voltages well over 1000% of their listed rating.
Fuses blow from an overcurrent condition. And Jim is probably right about the short-circuit. A short circuit draws incredibly high amperage. For instance, a short-circuit in a 120 volt receptacle will often pull 35,000 amps. Chances are your fuse is blowing from a short someplace. And yes, that short could be inside your alternator - possibly the windings are going to ground.
There are lots of other possibilities, so let us know what happens with your alternator test. Do you know how to determine if you have a voltage leak to ground with the ignition off?
