Accelerated life tests using the brand-new electrical insulation have long been used to estimate the lifetime of the insulation. Although a variety of types of stresses are imposed on the insulation in the accelerated life tests, the insulation is often broken more quickly than the estimated lifetime due to unexpected deterioration factors. The step-up breakdown voltage tests using the working insulation are then performed to assess the actual remaining lifetime of the insulation. However, theoretical explanations of the relation between these two test methods have not been made rigorously. One way to combine these two may be the cumulative exposure model proposed by Nelson, or Sedyakin model which is usually applied to short time accelerated life testing. This paper shows that Sedyakin model is also useful to assess the actual remaining lifetime using the inspection breakdown voltage data, although the relation between the duration of use and the electrical strength in breakdown voltage data seems to differ from the relation between the imposed stress and the lifetime in the accelerated life test data.