This research develops a theory applied to the reliability of electrical systems for determining the average time between failures (TPEF) in pole single-phase transformers using Weibull equations, which would determine the lifetime of this equipment. The results indicate a low level of reliability for transformers object of this study in Venezuela. For its part, the sample of teams in Colombia indicate that the probability of failure is influenced by environmental conditions. It was concluded that the investment required for this sector are postponed, therefore, recommended the implementation of maintenance programs in the short, medium and long term immediately.