What is prospective fault current?

• A. The maximum fault current that could flow from the supply to a fault
• B. The normal operating current of a circuit
• C. The current after protective devices have operated
• D. The maximum inrush current of a load

Answer: (A)

Prospective fault current is the maximum fault current that could flow from the power source to a fault, i.e., the short-circuit current available at the protection point if a fault occurred. It depends on the impedance of the source and the network up to the fault location, so it sets the upper limit of what protective devices must interrupt. This isn’t the normal operating current of the circuit, nor the current after protective devices have operated (that would be interrupted/zero), and it’s not the inrush current when a load energizes. Knowing the prospective fault current helps engineers select correctly rated fuses or circuit breakers and ensure cables and equipment can safely handle the fault before protection trips.