The paper presents the theoretical background of the so-called of the Chowdhuri-Gross model for the analysis of overvoltage conditions on an overhead line (power line) that occur as a result of a close lightning strike. It is an analytical model that, among other things, takes into account the influence of the steepness of the lightning current front on the realized overvoltages on the water (in the case of a close lightning strike to the ground next to the transmission line). On the basis of the aforementioned theoretical background, a corresponding computer program was developed in the paper, which then analyzed the influence of close lightning strikes on the occurrence of overvoltage conditions in typical MV transmission lines. In the analysis, the input parameters of the mentioned model were varied, such as e.g. height of water above the ground, distance of lightning strike from water, height of clouds above the ground, speed of lightning return strike, steepness of the front of the lightning current, amplitude of the lightning current, etc. water.