Statistical Analysis of Reliability in a Real Urban Distribution System

Underground cables are generally used in urban Medium Voltage systems. In the last years, the cable lines appeared to be more fragile than expected, with an increase of the number of faults, especially in the warmest periods of the year with the occurrence of heat waves. This paper, starting from actual data gathered in urban areas, aims to verify the existence of a relation between the number of faults and the temperature increase, to understand the possible limitations in using the classical reliability hypotheses to study the probability distributions of the time between faults. The results indicate the presence of critical periods with remarkable discrepancy between the empirical probability distributions and the exponential distribution used in the classical reliability analysis.