Ultrasonic methods that are sensitive for detection of physical defects such as small voids or cracks, have been used to check the physical condition of overstressed ZnO elements [91]. There was no microstructure difference which could be revealed between new and degraded (aged) blocks when scanning electron micrography (SEM) was used [69]. However, when a high energy such as those of excessive direct or alternating voltage levels or a very long surge overvoltage (>100 μs) is absorbed, a puncture will occur in the ZnO element [99]. Such failure may be caused by current concentration in non-uniform elements. At 820 C, the main additive Bi melts. ZnO grains along the localised current path may then melt as a result of high temperature.