The difference in the long term accelerated ageing curves is essentially due to the difference in composition and manufacturing process of the tested ZnO products [70, 89]. Dissimilar shapes have been recorded for ZnO products showing identical initial V –I characteristic but having different compositions [89]. Another important factor, which could play a key role in the curve shape, is the nature of the collar material. It is known that the collar material can overshadow the characteristics of the ZnO material when it is of unsuitable nature [69, 70].