The paper describes the issue of determining the expected number of shielding failure flashover rates (SFFOR) on HV overhead lines (transmission lines) using the so-called of the classical electrogeometric model. A detailed theoretical basis for the development of the mentioned model is presented. The influence of the use of electrogeometric models proposed by different authors and the application of logarithmic-normal distributions of lightning current amplitudes proposed by different working groups (CIGRE, IEEE, IEEE T&D Committee) on the amount of the expected number of SFFOR was then analyzed. This was done on the characteristic example of HV transmission lines 110 kV (fir-shaped pole heads) and 400 kV (portal-shaped pole heads).