The available fault current is influenced by which factors?

• A. System impedance and source characteristics
• B. The color of conductors
• C. The height of the room
• D. The time of day

Answer: (A)

The amount of fault current is determined by the combination of the source strength and the impedance in the circuit. In an electrical system, the available fault current is approximately the system voltage divided by the total impedance seen from the fault point back to the source. So, a strong source with low internal impedance and a short, low-impedance path will deliver a high fault current. If the path includes more impedance—long feeders, transformers and cables with higher impedance, or other limiting elements—the fault current is reduced. This is why system impedance and source characteristics are the controlling factors for available fault current. The color of conductors, the height of the room, or the time of day do not affect the impedance or the instantaneous current that can flow during a fault.