This article presents a guide to designing a maintenance plan for any industrial system. As an example, it develops a maintenance plan for highly reliable equipment, such as a hydroelectric power plant, where instant availability and reliability are crucial in its operation. The development of the proposal serves as a basis for the transversal development of any industrial system that has the same operational objectives (manufacturing lines), transport (trains, aircraft) and also involves safety and environmental aspects in its proper functioning. Today’s society requires that there are more and more industrial processes in which the maximum availability of the systems must be guaranteed, and at the same time there must be a minimum number of incidents that prevent the unavailability of the process. The methodology used has consisted firstly of dividing the complex industrial system into systems to be analysed on the basis of the functions they have to perform, then on the basis of the fault history a list of potential faults to be analysed has been determined, taking into account the risk of the system itself. From here, the systems of the hydroelectric plant have been classified to determine the priorities of actions. The different maintenance techniques to be applied have been carefully considered, focusing on the need to analyse condition-based maintenance techniques, such as predictive techniques, which allow us to define the point of potential failure based on parameters, and thus be able to plan maintenance actions in a justified manner. In the specific case of a hydroelectric generation plant, the fundamental objective is based on the commitment to operate in the electricity market (high reliability and immediate availability), and the performance of maintenance actions imply in most cases the shutdown of the plant and therefore the loss of income from electricity production. Finally, a design of a justified maintenance plan for a hydroelectric power plant has been proposed based on the methodology explained.