Distance protection of Transmission line: Distance protection scheme uses impedance relay. The relay operation is based on the impedance (or distance) between the relay and point of fault. Figure shows arrangement for distance protection for typical transmission line. The voltage element of impedance relay receives supply from PT secondary and current element receives supply from CT secondary. It measures Impedance at relay location( Z = V / I ) The protection zone of line is between A and B under normal working conditions, the impedance of line is ZL. The impedance relay is so designed that, it operates only when line impedance becomes less than ZL. When fault occurs between points A & B, the impedance of line becomes less than ZL and impedance relay operates which operates the CB and line is protected.