This paper aims to overcome the effect of transposed transmission lines with unsymmetrical impedances on protection systems. Transposition of phases can successfully overcome the problem of unsymmetrical impedances at the end of the line, but fails at the other parts of the line. The effect of this problem appears in the distance measured by the distance relay unit; the measured distance is usually not equal to the actual distance to the fault location. Moreover, this measured distance (measured impedance) has different values at the same fault location according to the fault loop for both the same and different fault types. Experiments have been carried out to study the effect of the unsymmetrical impedances on the operation of the system protecting against each fault type. The fault types under study are single line-toground, line-to-line and three-phase faults. The fault clearing time, the distance measured by the distance relay unit and the error in this measured distance were determined. A new setting rule is suggested in order to prevent failure of the distance relay caused by unsymmetrical impedances