The Cobriza underground mine, for the development of its operations, has an electric power distribution system, in which transformers submerged in 69 kV and 10 kV oil play a vital role in the reliability of the service; even more considering that a large part of them have been working for more than 38 years, it is necessary to know the status of their condition, to attend to their extensions, to optimize their life cycle and to make medium and long-term decisions to prioritize investment needs. a maintenance, relocation or replacement. For this purpose, we will use the concept called: Health Index, which consists of the combination of the relevant data obtained from the analysis of samples of the transformer insulation system and with the support of fuzzy logic using the software tool tool. Matlab, we will find a numerical value, which reflects the current health status of the transformer. The basic concepts of the fuzzy logic associated with the Health Index, the revision of the regulations in force, as well as the method of calculation are explained in a summarized form. For the application of the proposed method, we have considered real data from ten transformers that are currently working in the Cobriza mine, whose results are consistent with other similar works in this regard; In addition, an interface is designed to facilitate the application of the proposed method.