Zero-sequence impedance (Z₀) is a value that represents the opposition a transmission line presents to the flow of zero-sequence current. In a balanced three-phase system, the sum of the currents is zero, so there is no zero-sequence current. However, during an unbalanced fault—such as a single-phase-to-ground fault—a zero-sequence current flows through the ground and back through the line. The zero-sequence impedance is crucial because it directly influences the magnitude of these fault currents.

Its importance lies in fault analysis and protective relaying. Accurate zero-sequence impedance values are essential for calculating fault currents and designing protective relay settings to ensure a ground fault is detected and isolated quickly and reliably.