In this paper transmission line transposition is analyzed focusing electromagnetic transient phenomena in the range up to 10 kHz. Line transposition is implemented to decrease phase unbalances at fundamental frequency. The transposition section must be much lower than 1/4 of wave-length, which means, for 60 Hz, much lower than 1250 km. normally a 100-km section is used. For fundamental frequency it is then correct to consider the line as an ideally transposed line. In the present paper it will be analyzed the error of treating transmission line as ideally transposed for all frequency ranges, specifically when dealing with switching transients up to 10 kHz. A theoretical analysis is performed identifying the unbalances between the phases considering the transmission line ideally transposed and considering transmission line with actual transposition sections. The frequency dependence of transmission line parameters was properly represented. © 2006 IEEE. Dommel, H.W., EMTP theory book (1992) Microtran Power System Analysis Corporation, pp. 4-34. , 2nd. ed., Vancouver, British ColumbiaPortela, C., Tavares, M.C., Modelling simulation and optimization of transmission line. Applicability and limitations of some used procedures (2002) IEEE Transmission and Distribution Latin America 2002, p. 38. , SP, BrazilTavares, M.C., (1998) Modelo de Linha de Transmissao Polifasica Utilizando Quase-modos, , Dr. dissertation, Dept. Energy Control and Systems, State university of CampinasTavares, M.C., Pisolato, J., Portela, C.M., Mode domain multiphase transmission line model - Use in transient studies (1999) IEEE Trans. Power Delivery, 14, pp. 1533-1544. , OctWedepohl, L.M., Application of matrix methods to the solution of traveling-wave phenomena in polyphase systems (1963) Proc. IEE, 110, pp. 2200-2212. , DecWedepohl, L.M., Nguyen, H.V., Irwin, G.W., Frequency dependent transformation matrices for unstransposed transmission line using a Newton-Raphson method (1996) IEEE Transactions on Power Systems, 11, pp. 1538-1546. , Aug