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This investigation aims to perform a probabilistic prediction of the lifetime of epoxy resin insulation of transformer windings. To support the prediction, several accelerated electrical aging tests were conducted at different voltage levels on epoxy resin insulated transformer winding specimens. The lifetimes achieved from the tests show a “flat z-shaped” V-t (voltage vs. lifetime) curve and indicate the well known threshold stress effect. Since the operation stress is in the convex part of the V-t curve, the life exponent n was not predetermined, but estimated from a data pool obtained at the lowest voltage levels. As a result, the operation life and its confidence intervals were predicted from the breakdown data at these low voltage levels.