Any industrial organization has to analyse the maintenance strategy that it is applying to ensure the maximum efficiency of the system. This includes to define the maintenance policy, a well-analysed maintenance plan, and correct maintenance management and operational procedures. The maintenance department in most industrial organizations has to be continuously justifying the costs associated with their activities to avoid the continuous cuts requested by management based on the economic business plan for asset management. This paper presents the bases of the reliability centered maintenance and describes a practical case referring to a critical electrical equipment of an electrical system, such as the power transformer of a hydroelectric power plant. The power transformer in a hydroelectric power plant constitutes a fundamental electrical equipment because a failure in its operation can generate a long lasting unavailability of the system in the electrical generation with the consequent losses associated to the business.