800 mA fast-blow fuse for low-voltage DC and AC rails
The Littelfuse 0451.800MRL is an 800 mA fast-blow surface-mount fuse from the NANO² 451 series, rated for 125 V AC and 125 V DC circuits. Its fast-blow response means it opens quickly on overcurrent — useful for protecting sensitive electronics where a slow-blow part would let through damaging energy before clearing. With a breaking capacity of 50 A at the rated voltage, it can safely interrupt fault currents up to that level without rupturing or sustaining an arc. The 0.271 melting I²t figure is the energy required to melt the fuse element — a key number for coordination studies when sizing upstream protection or predicting let-through energy to downstream components.
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, this fuse mounts directly to the PCB surface — no through-hole leads. The DC cold resistance of 0.212 Ohms is the initial resistance before any current flows; it rises with temperature and after repeated surges, so the voltage drop at rated current stays under 200 mV in a cold circuit. CSA and UL approvals mean the fuse is accepted for use in equipment sold into North American markets without additional agency testing.