The tower's grounding system of the transmission line is essential for the dissipation of lightning current, decreasing overvoltage and flashover on the insulator. This paper presents the effects of the distance among counterpoises (the grounding wire placed deep underground) on the transient voltage of the tower grounding system and the insulators of the 220 kV overhead transmission line. Moreover, the influence of these distances on the outage rate of the transmission line (due to lightning phenomenon) is also determined and evaluated. The research results are based on the EMTP/ATP electromagnetic transient program and COMSOL software. These findings will assist power utilities in managing and operating transmission lines by providing a reference for designing an effective lightning protection grounding system.