Study of the synergistic effect of electrical and thermal stresses on insulation life

The synergism between stresses affects significantly insulating materials or systems subjected to multi-stress conditions, resulting in a reduction of life which can be particularly remarkable when stresses have comparable effects. A factor is defined here, able to quantify the synergism; in the case of two stresses, this factor has a maximum for large values of both stresses and tends to unity (the minimum value corresponding to absence of synergism) when one stress prevails. This behavior has been supported by calculations performed for polypropylene capacitors and EPR insulated cable models.