Earth Fault Loop Impedance

When an earth fault occurs, current flows through a loop consisting of the supply transformer secondary winding, the phase conductor to the fault point, the fault itself (assumed to be zero impedance for worst-case analysis), the circuit protective conductor back to the distribution board, and the earthing system back to the transformer neutral. The total impedance of this loop — Zs — determines how much fault current flows and, consequently, how quickly the protective device operates. BS 7671 specifies maximum Zs values for each protective device rating to ensure disconnection within 0.4 seconds for final circuits and 5 seconds for distribution circuits. The loop impedance comprises the external impedance Ze (from the supply transformer to the installation origin) and the internal impedance R1 + R2 (phase and protective conductor impedances within the installation). Higher Zs values mean lower fault current, slower protective device operation, and potentially dangerous touch voltages persisting for longer periods.