The paper presents a comparison between analytically rigorous and simplified methods in the calculation of a transient due to the initiation of a fault at any point on an overhead line connected to a crossbonded-cable line. In the simplified method, a supposed conductive medium surrounding cable insulation layer, the magnetic shielding effect of which is approximately equivalent to that of the actual cross-bonded-cable sheath circuit and earth, is introduced. By applying the simplified method, a 7-conductor inhomogeneous system of a single circuit of actual crossbonded-cable line including the earth has been reduced to a 4-conductor homogeneous system. A simple double-ended system has been modelled, in which transmission line is accurately represented. A 275kV line is considered, and the fault transient at various locations of protective relay along the line is evaluated for both rigorous and simplified analytical models. It has been proved that the simplified method takes the computing time down one place with practically sufficient accuracy