A probability method is presented for the design of clearances and number of insulators for extra-high-voltage (EHV) lines. The method involves determination of a tolerable probability of flashover for critical switching operations and the corresponding switching-surge voltages. The probabilities of flashover for various insulation designs are then calculated from data on wind, conductor swing, climatic, and insulation-strength probabilities. From these, the adequacy and economics of the insulation design can be determined.