An electrical utility has for obligation to guarantee the continuity and the quality of the given service. In order to stimulate the quality of the service the Brazilian energy agency introduced penalties in contracts with the electrical utilities of transmission in case the transmission installations is off for accident, failure of equipment or scheduled maintenance. On the basis of practical Reliability Centered Maintenance, this thesis considers a determination of maintenance intervals to minimize the economical impacts due to equipment disconnection. These disconnections are represented through the probabilistic model of the protection of transmission systems. A method for determination of the steady states probabilities of the protection system and its transitions rates was developed using techniques of Frequency and Duration. The failure rate of the component was determined using data collected from protection of transmission systems throughout eleven years, where the maintenances had been carried through every three years. It was determined the protection failure rate considering others intervals of maintenance. For each interval of maintenance, one assumed that the failure rate meets the useful life period and is characterized by a constant hazard rate. The useful life period follows a good approximation to an exponential curve. For a utility it is preferable to take more risks of component failure than to have to pay penalties for maintenances, whereas for another one is recommendable to be cautious and pay more for accomplishment of maintenances taking little risks of payments for unavailability do to component failure. Techniques of the economic theory had been tried in order to determine reasonable levels of acceptable risks to the problem.