Abstract A pragmatic extrapolation model is described which considers the entire measured breakdown distribution(s), so that distributions can be matched which do not exactly follow either a linear or an inverse dependence of breakdown time on the electric field. It is shown that this and other extrapolation models are restricted to a range below certain upper limits for the stress conditions for both temperature and electric field. The limits are caused by a transition of the degradation behavior at “high” fields, which is temperature-dependent. The physical mechanisms for the transition are reviewed. Furthermore, the activation energy for the temperature acceleration is shown to be dependent on electric field and the temperature range. The upper limits for relevant voltage and temperature stress conditions are derived from available data.