A phenomenological aging model for combined thermal and electrical stress [comment and reply]

For original paper see A.C. Gjaerde, IEEE DEIS, vol.4, no.6, pp.674-80, 1997. In the original paper, Gjaerde proposed a life model for insulating materials under combined stress, where the combined effect of temperature and PD is considered. The latter is taken into account by means of measurements of void gas pressure. The purpose of this comment is not to contest Gjaerde's model from the point of view of its inability to fit all experimental results, but to show that the model can be derived from the well-known model proposed by this author, but with several inadmissible simplifications and errors. A reply by Gjaerde to the comment is included.