Prospective Fault Current

Determining prospective fault current is a fundamental requirement of every electrical installation design. The value represents the worst-case current that could flow if all insulation barriers failed simultaneously — a bolted fault with zero fault impedance. This value must be calculated or measured at every significant point in the installation: the main switchboard, each distribution board, and critical final circuits. The supply authority provides the prospective fault current at the point of supply (typically on the electricity bill or by request), and the engineer then calculates the reduction due to cable and equipment impedances at downstream points. Every protective device must have a breaking capacity equal to or greater than the prospective fault current at its installation point. Cables must withstand the fault energy (I²t) for the protective device operating time using the adiabatic equation. For TN systems, prospective earth fault currents are typically lower than phase-to-phase fault currents because the protective conductor impedance is higher. Measurement instruments can verify calculated values during commissioning using loop impedance testing.