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In order to protect the transmission lines against lightning strikes, it is important to investigate the tower grounding system. In this paper, a recent method called hybrid continuous circuit-trapezoidal integration method is used to calculate the impulse impedance of the grounding system. Moreover, conventional structures of the grounding systems have been simulated and the results show that, with the same wire length, the square with additional wire system has the least impulse impedance as compared to counterpoise, square and crow?s foot. Moreover, the effects of soil resistivity and lightning current rise-time on the impulse impedance of these grounding systems are investigated. It is concluded that the design of the grounding system depends on the geographic location of the site in terms of soil resistivity and isotropic characteristics of the area in terms of lightning current rise-time.