The Littelfuse 0451.630NRL is a 630 mA fast-blow fuse in the NANO²® 451 series, rated for 125 VAC and 125 VDC with a 50 A interrupting capacity at rated voltage. That 50 A breaking capacity means it safely clears a fault up to that current without rupturing the package — critical for secondary-side protection in a power supply where the available fault current can hit that level. Fast-blow response means it opens quickly on a short circuit but tolerates brief inrush pulses below the melting I²t of 0.143 — a typical fit for protecting control circuits, indicator lamps, or small DC motors where the start-up surge is short and the fault current needs to be cleared fast.
Package and board-fit — what the 2-SMD square end block demands
The 2-SMD square end block package, 6.10 mm long by 2.69 mm wide and tall, is a standard subminiature footprint for pick-and-place assembly. The 0.305 ohm cold resistance at 630 mA means a voltage drop of about 0.19 V at rated current — negligible for a 125 V rail but worth checking on a 5 V supply where every millivolt counts. Surface-mount mounting and Tape & Reel packaging mean it flows through standard reflow profiles.