Current Limitation” and why is it important? Article 240.2 of the National Electrical Code® (NEC®) defines a Current- Limiting Overcurrent Protective Device as: “A device that, when interrupting currents in its interrupting range, reduces the current flowing in the faulted circuit to a magnitude substantially less than that obtainable in the same circuit if the device were replaced with a solid conductor having comparable impedance.” What this really means is that a current-limiting device is one that opens and clears a fault before the first current zero after the fault occurs, and limits the peak fault current. In most cases the current-limiting device will clear a fault in less than one half cycle.National Electrical Code® and NEC® are registered trademarks of the National Fire Protection Association, Quincy, MA. of standard 60 Hz current (8.33 milliseconds). Figure 4 is a graphical representation of the effect of current limitation on a faulted circuit. As seen above, the total clearing time “t” occurs before the first zero. The I2t energy is the area under the curves. It is clear that I2t through the fuse is much less than would otherwise occur. Heating is a direct function of current squared x time (I2t). Reducing current in half reduces heat by 75%. Generally, the lower the peak instantaneous current is, the lower the I2t energy will be. The square of peak current determines the amount of magnetic stress. For a given circuit, cutting the peak current in half reduces magnetic stress by 75%.
