This paper presents some practical modeling aspects related to incorporation of ageing effects into reliability model for power transmission network.Quantitative reliability estimation is being recognized as necessary in the planning of electric power systems. A comprehensive analysis to assess current system performance and prediction of further evaluation serves as a base for planning actions. But usuallyreliability models do not address the impact of ageing equipment on system reliability. In most cases assessment methods are based on constant failure rate assumption. In addition power system equipment can be considered as partially interdependent. Ageing of interdependent components because of various disturbances in the grid can differ significantly.