[en] In general, in calculations of reliability of systems with components in series, and particularly in fault trees and in method of parts count, the supposition is made that such components are independent. A realistic focus on the problem, however, shows that when determining the availability of a group of components in series, an interdependence exists among the components that should not be ignored: when one of the components fails, the group fails, and in practice it turns out that all the components are turned off until the one that failed is repaired. In this work this dependence is considered, generating a model closer to reality and less conservative. The method of solution of the resulting equations was shown to be accurate and fast, and could be included in the current applications with practically no performance reduction. The cases numerically investigated in this work are of two types. In the first, all components have constant failure rates. In the second case, the systems analyzed have components under aging. The results of the new method are compared with those obtained by a program which performs the discretization of the equations of the problem using the finite differences technique. For the first case, the differences among the new model and that which considers independent component are also analyzed. In both cases, systems with 2 components and with 100 components have been numerically evaluated. (author)