What the 800 mA fast-blow rating means on the bench
The 0451.800NR is a Littelfuse NANO² 451 series surface-mount fuse rated 800 mA with a fast-blow response characteristic — it clears quickly on overcurrents above the threshold, which matters when the downstream circuit can't tolerate sustained fault energy before the silicon lets go. Rated 125 V AC and 125 V DC, it covers common low-voltage rails in telecom, industrial control, and instrumentation — the same voltage ceiling on both sides simplifies BOM planning for mixed-signal boards. Breaking capacity is 50 A at rated voltage — adequate for most secondary-side protection zones but below the prospective short-circuit current on a high-capacity bus; upstream coordination with a higher-interrupting device is the call if the fault current exceeds that ceiling.
Melting I²t and inrush discrimination
Melting I²t is 0.2654 — this is the energy required to open the element, and it sets the fuse's tolerance to inrush pulses from capacitive or inductive loads. A downstream capacitor bank drawing 5 A for 2 ms delivers 0.05 A²s, well below the threshold; a motor start or hot-swap event that pushes 10 A for 10 ms hits 1.0 A²s and risks nuisance opening. DC cold resistance is 0.1355 Ohms — at 800 mA the I²R drop is about 87 mW, negligible for thermal budget but enough to shift the voltage seen by a sensitive load if the trace resistance is already marginal.
Obsolete — sourcing reality for this NANO² 451 fuse
The part carries CSA and cULus approvals, so compliance documentation for North American markets is available with the lot trace records. For a board repair or a last-time-buy fill, the 2-SMD square end block footprint (0.240" x 0.106" x 0.106") is shared across the NANO² 451 series — a parametric search for 800 mA fast-blow in that package yields the active siblings if a redesign is an option.