BOM-fit note for the 800 mA fast-blow fuse
The Littelfuse 0451.800MRSN is an 800 mA, 125 VAC fast-blow surface-mount fuse from the NANO²® 451 series, with a 50 A breaking capacity at rated voltage. This is the part to specify when the BOM calls for a compact, board-level overcurrent protector that clears quickly on a hard short — the fast-blow response time means it opens before downstream silicon sees the full fault energy.
Parametric details for circuit protection design
Melting I²t is 0.271 — this is the energy required to open the fuse element. For a downstream DC-DC converter with a 100 µF input capacitor, the inrush current I²t must stay below 0.271 to avoid nuisance opens at power-up. DC cold resistance is 0.212 ohms. Approved by CSA and UR — the UR mark means UL-recognized for North American equipment, so the fuse is accepted in UL 60950 and UL 62368 power supplies without additional agency testing.
Packaged in a 2-SMD square end block (6.10 mm x 2.69 mm x 2.69 mm) — the end-cap terminations are solderable to standard reflow profiles. The 0.106-inch height keeps it below low-profile components on the same side of the board. Supplied on Tape & Reel (TR) — compatible with automated pick-and-place assembly. The reel quantity is not specified here, but the TR suffix indicates it ships in a standard embossed carrier tape for 8 mm or 12 mm feeder width.