400 mA fast-blow SMD fuse — the selection parameters that decide the fit
The 0451.400MRSN is a 400 mA fast-blow fuse in Littelfuse's NANO² 451 series, housed in a 2-SMD square end block package measuring 6.10 mm x 2.69 mm x 2.69 mm. This is a surface-mount board-level fuse — not a cartridge style — designed to protect a secondary circuit or a single load on a PCB. The 50 A breaking capacity at 125 VAC means this fuse can safely interrupt a fault current up to 50 A without rupturing the package or sustaining an arc. For a 400 mA fuse, 50 A is a high interrupt rating — it clears a short-circuit fault with margin, protecting downstream traces and components from the fault energy. The DC cold resistance of 0.56 Ohms is the resistance of the fuse element at 25°C.
Thermal and environmental envelope
The 125°C upper limit means the fuse element does not derate significantly in a warm enclosure or near a power component — the melting point of the alloy is well above the ambient ceiling. CSA and UR approval agency marks confirm acceptance for North American and Canadian equipment. These are not optional — a UL-listed fuse is often a requirement for UL 508A industrial control panels or UL 60950-1 ITE equipment. The UR mark (UL Recognized Component) means the fuse is tested to UL 248-14 for supplemental fuses.
Lifecycle and supply posture — active, no end-of-life pressure
Sourced through independent distribution channels against an RFQ.