What this 4 A fast-blow fuse does on your board
The Littelfuse 0451004.NR is a 4 A surface-mount fuse from the NANO² 451 series, designed to clear quickly on overcurrent faults in low-voltage DC and AC circuits up to 125 V. Its fast-blow response means it opens within milliseconds on a high-magnitude fault — useful for protecting sensitive semiconductor loads where a slow-blow fuse would let through enough energy to damage downstream components. Rated 50 A breaking capacity at 125 V, it safely interrupts a fault current up to that level without rupturing the package.
Obsolete — sourcing through independent channels
No official successor order code is recorded from Littelfuse for this NANO² 451 series part. A board-level redesign using a current-production surface-mount fuse from the same family may be the practical migration path.
Package, approvals, and thermal fit
Housed in a 2-SMD square end block package measuring 6.10 mm x 2.69 mm x 2.69 mm, it reflows on a standard surface-mount line and mates with common fuse-holder pads for the NANO² footprint. Approved by CSA, PSE, and UL — the UL listing covers North American branch-circuit and supplementary protection applications; PSE acceptance allows use in equipment exported to Japan.
Melting I²t and coordination
With a melting I²t of 3.152, this fuse lets through a predictable energy pulse before opening — useful for coordinating with downstream components that have a known surge-withstand rating. The 0.016 Ohm cold DC resistance means the voltage drop at rated current is about 64 mV — negligible in a 5 V or 12 V rail, but worth checking in a 3.3 V low-dropout supply where every millivolt of headroom matters.