0451.500MRL 500mA FF Surface Mount Fuse, 125V ac/dc. Littelfuse 451 series of NANO² surface mount fuses are very small wire-in-air (WIA) square shape surface mount devices which are suitable for a variety of secondary side circuit over-current protection applications. Anti-surge ceramic cartridge fuses are designed to prevent damage to electrical equipment. These fuses allow an electric current to flow through until the current goes over the fuses rating then it will blow and stop the flow of electricity. This range of ceramic cartridge fuses are 5 x 20mm with a T speed rating