A procedure for estimating the flashover probability of aged nonceramic insulators in-service is presented. The study was carried out using two types of 138 kV EPDM line post insulators received from the Florida Power and Light system (USA). Presented are the laboratory characterization of the naturally contaminated insulators, the development of a linear regression model, and the estimation of the statistical parameters that define the relation between probability of failure and applied voltage for a given contamination level. The results obtained in the laboratory were in agreement with the current and past field experience. Conservative probabilities of flashover were estimated under the assumption of a cumulative normal distribution. The estimators of the parameters that define the normal distribution were assessed with the original up and down method. The results suggest that for one type of EPDM insulator, contamination flashover performance was distinctly inferior to the other.
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