The relatively high protective levels provided by SiC arresters became more and more an economical disadvantage with the increase of maximum system voltages. In order to reduce the insulation levels of the apparatus it was, therefore, necessary to try to reduce the protection levels of the surge arresters. Furthermore, the increase of transmission line lengths resulted in an increase of the energy that the arrester had to absorb in case of a line discharge through it. The search for new materials to obtain superior non-linear V –I characteristics led to the discovery of zinc oxide (ZnO) varistors in the late 1960s. The impedance of ZnO varistors at voltages below the rated voltage is so high that the resulting current is in the milliampere range. The direct consequence of this lowcurrent consumptionwas the possibility of constructing surge arresters with no series gaps. The first power system gapless metal oxide surge arresters were completed in the mid 1970s [3, 4]. The absence of the gaps and the extreme non-linearity of the voltage–current characteristic of the material resulted in the following additional changes in the main features of arrester protection