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Flashover of contaminated insulators in polluted areas is one of the most important factors influencing the operation of transmission and distribution lines and substations. Hence predicting an approaching flashover is of a great importance. In this paper, a new method and criteria have been proposed to predict the flashover occurrence of SIR composite insulators and its probability by using leakage current harmonic component. The tests were performed on various profiles of artificially polluted SIR insulators under clean fog condition. It is proposed that an important index is the ratio of 5th harmonic component to the 3rd for which the operational and critical limits have been determined. The probability of flashover occurrence has also been calculated by geometric distribution. Only the measurements of a minimal set of parameters are required. The presented criteria can be used to optimally schedule overhead lines maintenance teams and to develop proper Condition-Based Monitoring Systems (CBMS). Experimental results illustrate high performance of this approach.