800 mA fast-blow SMD fuse — what the ratings mean for the BOM
The Littelfuse 0451.800MR is a NANO²® 451 series surface-mount fuse rated 800 mA at 125 VAC and 125 VDC. The 50 A breaking capacity at rated voltage means it can safely interrupt a short-circuit fault up to 50 A without rupturing or sustaining an arc — that is the number that matters for the secondary-side protection of a DC-DC converter or a low-voltage power rail. The fast-blow response time clears the overcurrent in under a few milliseconds at high fault multiples, which keeps downstream silicon safe during a hard short. The 0.271 melting I²t figure tells you the thermal energy the fuse absorbs before opening — useful for coordinating with the I²t rating of the PCB trace or the MOSFET it protects.
Obsolete — sourcing what is left
Littelfuse has marked the 0451.800MR as obsolete. That means the only supply channel is independent surplus and broker inventory — the part is not being made anymore, so whatever is in the distribution pipeline is what exists. For a BOM that already uses this fuse, the procurement move is to source it through an independent distributor that can verify authenticity and lot traceability. No pin-compatible drop-in replacement from Littelfuse is documented, so a redesign to a current-production NANO²® 451 variant or a different series may be needed for long-term supply.
Board-fit and approvals
The 2-SMD square end block package measures 6.10 mm x 2.69 mm x 2.69 mm — a compact footprint for dense PCB layouts. CSA and UL approvals mean the fuse is accepted for use in equipment sold into North American and Canadian markets — the UL listing covers the safety interrupt rating, and CSA is the Canadian equivalent.