80 mA fast-blow for secondary-side protection
The Littelfuse 0451.080MRSN is an 80 mA fast-blow surface-mount fuse from the NANO²® 451 series, designed to clear overcurrent faults on densely populated boards where a through-hole pigtail fuse won't fit the layout. Rated 125 VAC with a 50 A interrupting capacity, it safely opens a fault up to that level without arcing across the element or damaging the surrounding traces — the 50 A ceiling is the maximum fault current the fuse can interrupt at its rated voltage.
Melting I²t and cold resistance — the coordination numbers
The melting I²t of 0.00033 A²s is the energy required to vaporise the fuse element — this is the number you compare against the I²t rating of the downstream semiconductor (diode, regulator, or IC input) to guarantee the fuse opens before the part fails short. Cold resistance of 4.05 Ohms means the fuse adds about 0.32 V drop at rated current — negligible on a 5 V rail but worth accounting for in a 1.8 V supply where every millivolt of IR drop eats into the load's headroom.
Industrial temperature range and approvals
Carries CSA and UR approval marks — accepted in North American and Canadian equipment without a separate component-level safety evaluation, which saves a qualification cycle on the bill of materials.