This article deals with the analysis of failures in the insulators used in electricity transmission lines. This analysis employs the methodology of Failure Modes and Effects Analysis - FMEA. Applied to the study of failures of one of the major equipment of the transmission lines, the isolator, FMEA helped to improve the reliability of a power transmission system. The construction of FMEA risk priority numbers by means of probabilistic composition made simpler and safer to use the tool, leading to satisfactory results on the prioritization of the problems causing failures in insulators.