Current-limiting fuses reduce both the magnitude and duration of a fault current. A UL Listed, current-limiting fuse must clear a short circuit current in less than one half cycle. In its current-limiting range, the fuse will melt in the first quarter cycle and prevent the fault current from reaching the first peak of the asymmetrical waveform, tremendously limiting the total electrical energy delivered to the fault.

Incident energy chart of 400A Class J fuse.

This energy limitation enables class J, RK1, T and L fuses to reduce incident heat energy from an arc flash to very low levels. The incident energy performance curve of a 400 amp, Class J fuse shows that, for defined parameters, the predicted incident energy would be 0.3 calories/cm². Compare this to 9.2 cal/cm², the result for an overcurrent protective device clearing in six cycles. (Comparison made using IEEE 1584 fuse equations and standard equations for the cycle device).

IEEE 1584 offers standard equations to predict incident energy and arc flash boundary distances for Class RK1 and Class L fuses up to 2,000 ampres.

Extensive testing at our high-power lab, in accordance with IEEE 1584 test procedures, has shown that Ferraz Shawmut's Amp-Trap 2000® A6D (Class RK1) and A4BQ (Class L) fuses yield incident energy levels at or below those predicted by the IEEE 1584 fuse equations. Our Amp-Trap 2000® AJT (Class J) fuse exceeds the performance of these equations for the equivalent ampere rating.