The aim of this work is to estimate the influence of random factors on the operational reliability of electricity supply networks. In order to achieve the proposed aim, the interruptions in operation, caused by the influencing factors, which occurred randomly in the process of operation of the examined power grids were recorded and processed. Specific random factors were selected as the object of the research, because at present they occur frequently and interrupt the electricity supply process unpredictably, essentially affecting consumers and causing them significant harm. The main results of the research consist in establishing the characteristics of outages and mathematical expressions that allow to estimate the influence of random factors on the level of reliability, depending on: The number of outages caused, their duration and the number of consumers affected. Moreover, the research has made it possible to highlight factors with a significant influence on reliability, which allows planning measures to prevent them and ensure the continuity of the electricity supply process. The significance of the results obtained lies in the solution to a number of problems currently faced by the electricity supply network operating services, in order to ensure operational reliability, providing the possibility of forecasting random influencing factors, which allows the justified implementation of prophylactic works and ensuring the level of reliability.