AbstractNowadays it is quite usual that systems are correctly maintained. This leads to a decreasing number of failures for the system. In this paper we discuss how to establish warning procedures that take into account features giving the chance of detecting eventual changes of trend in the systems and allowing taking measures before too much damage has actually happened. The proposed procedure is applied to the study of the reliability and maintenance of the railway electric supply system. In this kind of systems, complex and extended, testing the performance of the system and effectiveness of the maintenance tasks has special interest. Copyright © 2007 John Wiley & Sons, Ltd.