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Till today, flashover because of pollution of insulators of overhead transmission lines (OHTL) is a major source of trouble in the electrical power transmission system. The importance of this problem can be assessed from fault statistics of high-voltage transmission lines. In Egypt, for example, about 82% of the total faults of transmission and distribution OHTL are because of pollution. This study deals with laboratory tests, which have been carried out on polluted insulators of the OHTL that simulate desert environments. In this study, the effect of the variation of conductivity of the polluted layer by means of equivalent salt deposit density on flashover voltage (FOV) is investigated. The effect of variation of conductivity of the polluted, layer along the time required by the procedure needed to form dew, on FOV is also studied. This study is useful for indicating the worst period after the formation of dew, where the probability of flashover is increased.