Determination of failure rates of underground distribution system components from historical data

It is shown how failure rates of underground distribution system components can be determined from historical data. When these data are complete, the time dependence of the failure rates can be found. A complete set of data contains a record beginning with the first installation of the component type and including the number of components mailed (or removed) and the number which failed during each year of the record. The method is applied in the determination of failure rates for high molecular weight polyethylene (HMPE) 15 kV unjacketed cable, cross-linked polyethylene (XLPE) 15 kV unjacketed cable, single phase distribution transformers, and load break elbows. Data for these components were made available by San Diego Gas and Electric Company and the Northwest Electric Light and Power Association. A record spanning some 20 years of data was used