12 A fast-blow for tight overcurrent protection
The 0451012.NRL is a 12 A, fast-blow surface-mount fuse from Littelfuse's NANO²® 451 series, rated for 65 VAC and 65 VDC. The 50 A breaking capacity at rated voltage means it safely interrupts a fault up to that current without rupturing the package or sustaining an arc. In a 65 V bus feeding a 12 A load, a hard short can pull hundreds of amps for a few milliseconds; this fuse stops that event inside the 2-SMD square end block before the PCB trace vaporises.
Melting I²t and cold resistance for coordination
Melting I²t is 47.97 A²s — this is the energy needed to melt the fusible element. When sizing series protection (like a second fuse or a PTC), the upstream device must pass at least this much energy before opening, or the 0451012.NRL blows first and the coordination fails. For a 12 A rail, a 20 A slow-blow upstream fuse typically holds through the 47.97 A²s pulse. DC cold resistance is 0.0049 ohms. At 12 A the I²R loss is about 0.7 W — negligible for a 2-SMD package with good copper pours on the pads, but worth checking if the ambient inside the enclosure exceeds 85°C and the trace runs near the fuse's thermal limit.
Active production with CSA and UL marks
The 65 VAC/65 VDC rating is symmetric, so the same fuse works on both AC and DC buses — useful when a single BOM line covers a rectified supply.