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Jointed 12-phase-transmission lines are frequently used in power systems. However, it is subjected to a variety of technical problems on fault analysis and protection engineering. These problems are mainly due to the mutual coupling effects between adjacent circuits of the line. A method to decouple the mutual impedance completely was proposed in this paper. When fault occurs in the 12-phase-transmission lines, the faulted phase electrical measurements can be decomposed into twelve-sequence components which are completely independent from each other. The twelve-sequence components consist of positive sequence currents and inverted sequence components and the inverted sequence components will occurs under any internal faulted condition. When fault occurs in the jointed 12-phase-transmission lines, the inverted sequence component voltages calculated from two Bus sides are equal at the faulted point, which can be used to calculate the faulted location. As the twelve-sequence-component method has decoupled the mutual electromagnetic effect between the lines, the accuracy of the fault location is high. A large number of EMTP simulations indicate that the algorithm is completely suitable for the jointed 12-phase-transmission lines and the location accuracy is not affected by such factors as faulted resistance, generation mode and power angle, etc.