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On March 10, 1986 the 500-kV transmission system in Southern Ontario sustained a total of 57 flashovers on outdoor insulators. Line suspension insulators accounted for 30 flashovers, station post insulators for the remaining 27. These flashovers have been attributed to a particular set of recurring atmospheric conditions which include a combination of freezing rain, followed by fog, and rising temperatures. A special testing technique has been developed to compare the performance of different insulator arrangements under simulated weather conditions. Tests on station post insulators have shown that a 20 % improvement in critical flashover voltage can be achieved by a change in shed shape and protected leakage distance. Improved insulator designs for both line and station insulators are being developed in co-operation with manufacturers.