Deducing metal oxide varistor life span from pulse rating curves for surges of different magnitudes

Metal Oxide Varistors (MOVs) are employed as surge protective devices due to their capability of dissipating reasonably large values of surges. The pulse (life span) rating curves of a MOV provide the estimated life span of the device when exposed to multiple surges of the same magnitude. However, in reality, a lightning event (flash) consists of multiple current surges (strokes), with the first stroke having a magnitude approximately three times that of the subsequent strokes. Furthermore, the sequence of occurrence of surge events (lightning and switching surges) is not always the same. In this paper two sequences are considered: sequence 1 - larger surge event, and then smaller; sequence 2 - smaller surge event, and then larger surge event. Several tests were conducted to deduce the life span of a MOV from pulse rating curves, and to determine the influence of sequence of occurrence of surges on the degradation of the MOV. A standard 20K201 MOV was tested under different levels of exposure of surges. The sequence of occurrence of surges was found to have a significant influence on the degradation of the MOV. The MOV showed a larger degree of degradation when it was first exposed to a surge of lower amplitude (2 kA) and then later to a surge of larger amplitude (4 kA), compared to when it was exposed to same surges but with opposite sequence. When the MOV was exposed to approximately 60% of rated surge current, it became more susceptible to surges and could easily fail when exposed to surges less than 32% of rated surge current.