Obsolete — the BOM decision driver
Littelfuse has marked the 0451.400NR as Obsolete. For existing designs, procurement must source through independent or surplus distribution channels — the part is not available through the factory's standard ordering process. The 0451.400NR is a 400 mA fast-blow fuse from the NANO² 451 series, rated 125 V AC and 125 V DC with a 50 A breaking capacity at rated voltage. The 0.0484 melting I²t value is the energy let-through figure needed for downstream semiconductor protection coordination — a key parameter when the fuse sits ahead of a sensitive IC or regulator input.
Board-fit and thermal envelope
Housed in a 2-SMD square end block package measuring 6.10 mm x 2.69 mm x 2.69 mm, this surface-mount fuse occupies a compact footprint for dense PCB layouts. The tape-and-reel packaging supports automated pick-and-place assembly — no manual handling step in production. The 0.4233 ohm DC cold resistance sets the voltage drop at rated current — about 170 mV at 400 mA — which matters for low-voltage rails where every millivolt of headroom counts. CSA and cULus approval agency marks confirm acceptance for North American and Canadian equipment — a requirement for UL 508A or CSA C22.2 panel designs.