Let-Through Energy (I²t)

When a short circuit occurs, the protective device takes a finite time to detect and interrupt the fault. During this time, energy is delivered to the circuit in the form of I²t — the integral of the square of the instantaneous current over the total clearing time. This includes the pre-arcing time (before the device contacts begin to separate) and the arcing time (while the arc is being extinguished). Current-limiting circuit breakers and fuses can significantly reduce let-through energy by opening so quickly that the fault current never reaches its prospective peak value. Manufacturers publish I²t values for their devices at various prospective fault currents, both for pre-arcing (I²t let-through) and total clearing. The downstream cable must withstand this energy: the adiabatic equation k²S² ≥ I²t provides the check. If a cable's withstand capacity is exceeded, the engineer must either increase the cable size or select a device with lower let-through energy. Let-through energy coordination is also critical for back-up protection (cascade) arrangements, where an upstream device assists a downstream device that has insufficient breaking capacity alone.