What is the correct expression for Zs in fault loop calculations?

• A. Zs = R1 + R2
• B. Zs = (R1 + R2) - Ze
• C. Zs = Ze
• D. Zs = (R1 + R2) + Ze

Answer: (D)

The fault loop impedance Zs is the total impedance the fault current must travel through in the loop from the source to the fault and back via the earth path. In a fault, current goes through the live conductor (R1), returns via the neutral conductor (R2), and finally returns through the earth path with impedance Ze. So all these impedances are in series and must be added together: Zs = (R1 + R2) + Ze.

Omitting Ze underestimates the loop impedance, while subtracting Ze or using only Ze ignores part of the path. This sum ensures the calculated fault current I = V / Zs reflects the actual loop so protective devices trip correctly within the required time.