Bussmann Fuse: 40 A, 250V AC, 3 in L x 13/16 in dia Fuse Size, Cylindrical Body, FRN-R

These 250VAC UL Class RK5 fuses are time delay and nonindicating. They tolerate temporary inrush currents without opening, which prevents nuisance-blowing from motor and equipment startups. Also called slow-blow Class RK5 fuses, these 200kA IR fuses open to disconnect an electrical circuit when exposed to sustained loads and short circuits. They have no indicator for identifying a blown fuse, so they're sometimes used with an indicating fuse block or cover.

UL Class RK5 fuses provide moderate current limitation for general use applications where more current-limiting RK1 fuses aren't needed. These fuses have an interrupting rating (IR) of 200kA and are the same size as Class H and K fuses, but have a rejection feature that prevents an accidental downgrade from a current-limiting system. Their modified terminals ensure a match with the rejection clips in Class R fuse holders that prevent the installation of fuses with a lower interrupting capacity. They're typically used in newer equipment, but can also be used with a Class H fuse holder to upgrade older installations from Class H or K fuses for improved overcurrent protection. They are current-limiting to prevent electrical damage by disconnecting circuits before they're exposed to the full magnitude and duration of an overcurrent.