To appreciate the calculation, one must first understand the loop itself. A "fault loop" is the closed path that an electric current takes during a fault condition—specifically, a phase-to-earth or phase-to-neutral short circuit. The journey begins at the source (the transformer), travels through the supply line (live conductor) to the fault point, and then returns via the protective earth conductor and any metallic bonding back to the source’s neutral point. The total impedance of this complete circuit is what engineers refer to as the Earth Fault Loop Impedance, denoted as ( Z_s ).
[ Z_s = Z_{source} + (R_1 + R_2) ]
In practice, engineers must account for temperature. Conductor resistance increases with temperature. A fault may occur when conductors are already hot under normal load. Therefore, calculations often apply a correction factor (typically 1.2 to 1.5 for copper conductors) to convert cold resistances to operating temperatures. Additionally, for circuits longer than 100 meters or with high current, the inductive reactance of the loop may become significant, requiring the use of impedance (( Z )) rather than pure resistance (( R )). fault loop impedance calculation
However, the single-phase earth fault loop impedance ($Z_s$) remains the most critical for shock protection. To appreciate the calculation, one must first understand
To ensure a device trips within required timeframes (e.g., 0.4s for portable equipment or 5s for fixed equipment), Zscap Z sub s must not exceed a maximum value derived from Ohm’s Law: The total impedance of this complete circuit is
): The path from the consumer unit's earth terminal back to the supply transformer, including the supplier's earthing system.
The relationship between these values is expressed as: