The contribution is focused on the diagnostics of transformers in power engineering practice. First, attention is paid to the brief description of the general expert systems. Furthermore, the classification of expert systems and the analysis of the diagnostic expert system without vagueness are presented. In this contribution, emphasis is laid on the description of the rule expert system which is applied in the diagnostics of transformers. The composition of this expert system and the specific example of the results obtained are analysed in detail.