Fusetron dual element time-delay current limiting class RK5 fuse 250 volt maximum. Drop-in replacement upgrade for one-time and renewable fuses to aid in locating the open fuse when the power goes out. Time-delay - 10 seconds (minimum) at 500 percent rated current. Indicator on the side turns black when the fuse opens. Fits existing RK1, RK5, K5 H 30 and 60 amp fuse blocks and holders. UL Listed.