315 mA fast-blow — secondary protection for a 125 VAC rail
The 0451.315MRSN is a 315 mA, 125 VAC-rated fast-blow fuse from the Littelfuse NANO² 451 series — a board-mount surface-device fuse meant for secondary-side protection where a quick open on overcurrent is the goal, not a slow thermal trip. The 50 A breaking capacity at the rated voltage means it can safely interrupt a fault current up to that level without arcing across the package — enough for most low-power AC/DC converter outputs or control-transformer secondaries, where the prospective short-circuit current stays under that ceiling.
Melting I²t and cold resistance — the real fit numbers
The 0.0311 melting I²t tells you how much energy the element absorbs before opening — a low figure like this means it clears fast on a hard short, but it also means inrush from a capacitive input filter can nuisance-blow it if the charging surge exceeds the I²t budget. DC cold resistance is 0.3918 ohms — at 315 mA the voltage drop across the fuse is about 124 mV, which is negligible for a 125 V rail, but the I²R heating (roughly 39 mW) matters if the fuse is packed tight against other components in a high-ambient layout.
Surface-mount package and temperature range
Housed in a 2-SMD square end block package measuring 6.10 mm x 2.69 mm x 2.69 mm, this fuse reflows onto the board with standard solder profiles — no hand-soldering required, and the end-block terminations are visible for AOI inspection.
Approvals and compliance
Carries CSA and UR approval marks — CSA certifies compliance with Canadian safety standards for branch-circuit and supplementary protection; UR (UL Recognized) means it is tested as a component for use in UL-listed equipment, not as a stand-alone device.