Each S.P.S. has a built-in fuse for protection. Once the fuse is blown, it means that something is wrong inside the power supply. Normally you should return the unit to Wellforce for repair or replacement. But in some cases if you need to use the unit urgently, you can try to replace the internal fuse. If the power supply can not operate properly after replacing with a fuse of the same type and rating, please send back to the supplier for repair. If you want to add an external fuse, be sure that the rating of this fuse should be less than or equal to the internal fuse.