Bussmann Fuse, Time Delay, Class RK1, LPN-RK-SP Series, 100 A

Time-delay, nonindicating UL Class RK1 fuses tolerate temporary inrush currents without opening, which prevents nuisance-blowing from motor and equipment startups. Also called slow-blow Class RK1 fuses, they 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 RK1 fuses are more current-limiting than RK5 fuses to react more quickly to overcurrent. These fuses 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. These fuses are current-limiting to prevent electrical damage by disconnecting circuits before they're exposed to the full magnitude and duration of an overcurrent.