630 mA fast-blow in a compact SMD footprint
The 0451.630MRL is a 630 mA fast-blow fuse from Littelfuse's NANO²® 451 series, rated for 125 VAC and 125 VDC. The fast-blow response means it opens quickly on overcurrents above the rating — designed for protecting sensitive downstream components that cannot tolerate sustained fault current. Breaking capacity is 50 A at the rated voltage — the fuse safely interrupts a fault current up to that level without arcing or rupturing the package. For a 630 mA rail, 50 A clearance covers most secondary-side faults in telecom and industrial control supplies.
Package, approvals, and temperature grade
Housed in a 2-SMD square end block package measuring 6.10 mm x 2.69 mm x 2.69 mm (0.240" x 0.106" x 0.106"). The end-block termination pattern mates with standard SMD pads — no through-hole keep-out zone, so the fuse sits alongside passives on the same side of the board.
Melting I²t and cold resistance — inrush sizing
Melting I²t is 0.143, and DC cold resistance is 0.305 Ohms. The I²t value tells you how much energy the fuse absorbs before opening — useful for checking whether the inrush of a downstream capacitor bank or motor start will hold. The 0.305 Ohm resistance means a voltage drop of roughly 0.19 V at rated current, negligible on a 125 V rail.