In general, modern surge arresters utilizing ZnO blocks are very reliable apparatus with a low failure rate. A published number for distribution type arresters is 0.1% failures per year [1], [2], while for high-voltage arresters the estimated failure rate is even lower. However, since the primary duty of the arrester is to protect other equipment under all circumstances a slightly higher risk of failure compared to other apparatus is generally accepted. The philosophy has been that if something goes wrong the arresters should fail first, thereby saving other, more expensive, equipment from damage. Nevertheless, both among manufacturers and users, it is considered extremely important to ensure that the arresters fail in a "safe" way when overloaded. This is also reflected in the standardization work where large efforts have been made on test procedures for pressure relief i.e. short-circuit tests. When polymer-housed surge arresters were first introduced on the market, the manufacturer immediately claimed a better short-circuit performance compared with porcelain-housed arresters. This was due to the fact that the new type of arrester was lacking enclosed gas volumes as well as a housing comprising a brittle material like porcelain. It was thus considered that no explosions with a dangerous scattering of material were possible. Subsequently it was realized, both by manufacturers and users alike, that special precautions were also necessary for arresters with polymer housings to avoid explosive behaviour under short-circuit.
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