TABLE OF CONTENTS As described in the previous sections, the continuous applied voltage on ZnO surge arresters during their service duty causes a permanent flow of current which affects the arrester qualities in the long term. As a consequence of this degradation, the life span during which the arrester will provide efficient protection will be limited. In order to assess this life span, long term accelerated ageing tests have been performed [4, 65, 66, 69, 70, 75, 76, 84, 88, 89, 92, 96]. These tests are normally run for long periods (approx. 1500 hours) under a constant elevated voltage level (~ knee of conduction) and at a constant temperature which is considerably higher than that anticipated in actual use (70 150 C). The current I or power loss P is monitored versus time t until a given value is reached. This value is often referred to as the end of life criterion. Various criteria have been adopted:
when power loss reaches a critical value such as 1 W/cm of length [96]
when the leakage current reaches a fixed value, e.g. 100 mA [76]
when power loss reaches a value that is double the initial value measured at the beginning of the test [70, 84, 88].
During these long term tests, it has been observed that the P- t and I- t curves go through three stages: (i) a stable period of working life where the...
