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Summary form only given. Mutual coupling poses difficult problems witnessed during single phase to earth fault, in transmission line protection. In parallel lines on the same right of way, mutually coupled zero-sequence circuits cause error in the apparent impedance seen by the relay. This causes the distance relay at one end of the faulty line to over-reach while the one at the other end to under-reach. Incorrect sensing of the ground fault due to zero-sequence voltage inversion caused by mutually coupled zero-sequence network may lead to false trip of the neighbouring healthy line. The paper develops characteristic expressions for effective sequence impedances of twin circuit lines and presents a non-iterative microprocessor based algorithm for fault distance and under-reach estimation in real time, for under-reach correction in basic distance relaying scheme. The residual current measurement from the parallel circuit is not required for this purpose.