630 mA fast blow in a 2.69 mm profile
The Littelfuse 0451.630MR is a 630 mA fast-blow SMD fuse from the NANO² 451 series, rated for 125 V AC and DC with a breaking capacity of 50 A at rated voltage. The 0.143 melting I²t means it clears quickly on a hard short, but the energy let-through is low enough to protect downstream traces and components rated for that threshold. The 0.305 Ohm DC cold resistance at 630 mA produces about 120 mW self-heating at rated current — negligible in free air, but in a dense SMD layout the adjacent parts see that thermal rise.
Obsolete — sourcing the 0451.630MR today
Available through independent distribution and surplus channels; stock is lot-specific and traceable. No official successor order code is recorded from Littelfuse for this exact package and rating. A board-level review of the 451 series parametric range is the practical path to a replacement.
Package, approvals, and temperature range
Housed in a 2-SMD square end block package measuring 6.10 mm x 2.69 mm x 2.69 mm, surface-mount only — the end caps are the termination, no leads to bend or trim. CSA and UL approved — the UL recognition covers the 50 A interrupting rating at 125 V AC/DC, so it passes the branch-circuit protection requirements for most low-power DC rails and control transformers.