Calculation [portable] - Fault Loop Impedance

The purpose of calculating ( Z_s ) is governed by a fundamental safety principle: Ohm’s Law. When a fault occurs, the fault current (( I_f )) is determined by the supply voltage (( U_0 )) divided by the loop impedance (( Z_s )). For a protective device (such as a circuit breaker or fuse) to clear the fault safely, it must trip within a prescribed time (typically 0.4 seconds for final circuits). This requires that the fault current be high enough to operate the device instantaneously. If ( Z_s ) is too high, the fault current will be too low, and the protection may not operate, leaving dangerous voltages present on exposed metal parts. The calculation of the fault loop impedance is deceptively simple in form but complex in its variables. The basic formula is:

[ Z_s = Z_{source} + (R_1 + R_2) ]

[ Z_s \times I_a \leq U_0 ]