What is earth fault loop impedance and why must it be low?

• A. The impedance of a cable; high is better for performance
• B. The loop impedance measures noise on the line
• C. The resistance of the earth electrode; low means better moisture
• D. The total impedance from the supply to earth in a fault; low impedance ensures sufficient fault current to trip protective devices rapidly

Answer: (D)

Earth fault loop impedance is the total impedance the fault current must travel from the supply, through the live and protective conductors, back via the earth path, and return to the source during a fault. It includes the source impedance, wiring, and the earth path. Keeping this impedance as low as practical is crucial because the fault current is determined by the fault voltage divided by this impedance (I = V / Z). A lower loop impedance means a larger fault current for the same fault voltage, so protective devices like fuses or circuit breakers will reach their trip current quickly and operate within their specified time. This rapid operation limits the duration of the fault, reducing the risk of electric shock, overheating, or fire. If the loop impedance is too high, the fault current may be too small or too slow to trip the protective device, leaving the fault present longer and increasing danger.