315 mA fast-blow in a 2-SMD square end block
The Littelfuse 0451.315NR is a 315 mA fast-blow surface-mount fuse from the NANO² 451 series, housed in a 2-SMD square end block package measuring 6.10 mm x 2.69 mm x 2.69 mm. Rated for 125 V AC and 125 V DC, it provides a breaking capacity of 50 A at the rated voltage — enough to safely interrupt a hard short on a 125 V rail without the fuse casing rupturing.
What the ratings mean for your board
The 0.3918 ohm DC cold resistance means the fuse drops about 124 mV at rated current — negligible in a 125 V circuit, but worth checking if the load is a sensitive analog front-end that sees the voltage drop across the fuse during a transient. Melting I²t of 0.0311 A²s tells you how much energy the fuse absorbs before opening — useful for sizing the fuse to ride through inrush without nuisance trips. A downstream capacitor bank with a higher I²t surge will blow this fuse on power-up. No derating is required within this band, though the fuse's opening time shifts with temperature — it opens faster at 125°C than at 25°C.
CSA and cULus approvals are on file, so the part carries the same safety agency marks as when it was in production.