The reliability of electric supply is an important issue in power industry. Both the number of electricity consumers, and amount of energy consumed increases yearly. Wherewith losses from electric supply outages also increases for generation and transmission companies as well as for consumers. The reliability of electric supply largely depends on the reliability level of devices included in electrical power system from generators to consumers. The reliability of the particular device in its turn depends on various factors, focusing mainly on insulation evaluation in this article. Power transformer is one of the main elements of energy system determining largely the reliability of electric supply. Therefore such issues are analyzed in the presented paper as evolution of power transformers’ maintenance strategies from time based maintenance to condition based maintenance and technical condition assessment, and further to prognostic and predictive maintenance including unit’s work with acceptable risk level.