Fault Loop Impedance Zs

Fault loop impedance is a critical parameter that links cable design to protection performance. Zs comprises two components: Ze, the external impedance from the supply transformer to the consumer's main earthing terminal (typically provided by the distribution network operator), and R1 + R2, the combined impedance of the phase conductor and circuit protective conductor from the distribution board to the furthest point of the circuit. Maximum permissible Zs values are tabulated for each protective device type and rating to ensure sufficient fault current flows to operate the device within the required disconnection time. For Type B MCBs, the instantaneous trip at 3–5 times rated current determines the maximum Zs, while for Type C MCBs (5–10 times), the maximum Zs is lower. Measurement is performed during commissioning using a loop impedance tester, and the measured value must be compared against the tabulated maximum after applying a correction factor (typically 0.8) to account for conductor temperature rise during a fault. For ring final circuits, both legs must be tested to verify continuity and correct interconnection.