630 mA fast-blow SMD fuse for secondary-side protection
The Littelfuse 0451.630MRSN is a 630 mA, 125 VAC fast-blow surface-mount fuse from the NANO²® 451 series. It clears overcurrents quickly on secondary-side DC or AC rails where the load is sensitive to prolonged fault energy — think control boards, I/O modules, or sensor power inputs inside an enclosure.
Rated breaking capacity is 50 A at 125 VAC. That is the maximum fault current the fuse can safely interrupt without rupturing or arcing — if the available short-circuit current on the secondary rail exceeds 50 A, this fuse is not the right choice; step up to a higher-interrupting series. Melting I²t is 0.143. That figure governs how much energy passes through before the element opens — useful when sizing the fuse to ride through inrush from a capacitive input or a small motor start. A lower I²t means less let-through energy to the downstream components. DC cold resistance is 0.305 Ohms.
Package, mounting, and temperature range
Housed in a 2-SMD square end block package measuring 6.10 mm x 2.69 mm x 2.69 mm. The fuse element is sealed inside the ceramic body, so no outgassing or contamination risk in sealed enclosures. Supplied in Tape & Reel packaging, ready for automated pick-and-place assembly.
Approvals and compliance
Carries CSA and UR approval marks — accepted in North American and Canadian equipment listings.
Production status is Active per Littelfuse.