315 mA at 125 V — the protection envelope for this NANO² 451 fuse
The Littelfuse 0451.315MRL is a 315 mA fast-blow fuse in the NANO² 451 series, rated for 125 V both AC and DC. The 50 A breaking capacity at rated voltage means it can safely interrupt a short-circuit fault up to that current without rupturing — a critical spec for clearing faults in low-impedance DC rails where the prospective fault current can be high. The fast-blow response time opens quickly on overcurrent, making this fuse a natural fit for protecting semiconductor circuits — diodes, MOSFETs, or DC-DC converter inputs — where a slow-blow delay would let the fault energy destroy the device before the fuse clears.
Melting I²t and cold resistance — the inrush and voltage-drop numbers
A melting I²t of 0.0311 A²s tells you the energy needed to blow the fuse element. For a capacitive input circuit, the inrush current pulse must stay below this energy level or the fuse will nuisance-blow at power-up. The 0.3918 Ω cold resistance means at 315 mA the voltage drop across the fuse is about 124 mV — negligible in a 5 V rail but worth checking in a 1.8 V supply where every millivolt of headroom matters.
Surface-mount footprint and board integration
The 2-SMD square end block package, 6.10 mm x 2.69 mm x 2.69 mm, is a standard surface-mount fuse footprint that reflows on a standard lead-free profile. CSA and UL approvals are on file — the safety agency marks confirm the fuse is listed for overcurrent protection in equipment sold in North American and Canadian markets.