Obsolete NANO² 451 SMD fuse — 3 A, 125 V, 50 A interrupting rating
The Littelfuse 0451003.NR is a 3 A fast-blow SMD fuse from the NANO²® 451 series, rated for 125 V AC and 125 V DC with a breaking capacity of 50 A at rated voltage. The 0.0227 Ω DC cold resistance and 1.65 melting I²t give a clear picture of the energy let-through before the element opens — useful for coordinating with downstream semiconductor protection.
Fast-blow response and interrupting duty
Rated 3 A continuous with a fast-blow characteristic — the fuse opens rapidly on overcurrents above the threshold, making it suitable for protecting circuits where the fault current must be interrupted before downstream silicon sees the energy. The 50 A interrupting rating at both 125 V AC and 125 V DC means it can safely clear a fault up to that level without rupturing the package. The melting I²t of 1.65 A²s defines the thermal energy required to open the element. For a given fault current, the time-to-blow can be estimated from this value — critical for coordinating with upstream breakers or downstream TVS diodes so the fuse opens before the protected device fails short. The cold DC resistance of 0.0227 Ω means the fuse contributes about 68 mW of self-heating at rated current — negligible for the 2-SMD package's thermal dissipation in still air.
Approvals and compliance basis
Carries CSA, PSE, and UL recognition — the three agency marks cover North American (UL/CSA) and Japanese (PSE) regulatory acceptance for branch circuit and supplementary protection.
Sourcing the 0451003.NR now
Because this fuse is obsolete, standard authorized-distributor channels no longer carry it. The remaining inventory is held by independent surplus and broker sources. No official successor order code has been published by Littelfuse for the 0451003.NR. A parametric replacement would need to match the 3 A rating, 125 V AC/DC voltage, fast-blow response, 50 A interrupting capacity, and the 2-SMD square end block footprint — any substitution should be validated against the original PCB layout and the protected circuit's fault current.