The purpose of fuses is to prevent whole systems from shorting out. Replacing an 120 amp fuse with one just 60 amp would mean the fuse blow earlier, not later, than normal thereby protecting the system. In order to short out the whole system, as Steve Sloan believes, you would have to replace the 120 amp fuse with a larger one, say 200 amps. This would result in the amp not blowing and shorting out the rest of the system.