How to judge the quality of varistor?

Varistor it with the voltage applied on top of it increasing, its resistance value can be changed from MΩ (megohms) level to mΩ (milliohms) level. When low voltage TGE metal oxide varistor application works in leakage current region, showing great resistance, leakage current is very small; when the voltage rises into the non-linear zone, when the current is within a large range, voltage changes little , showed good pressure limiting characteristic; voltage rises again, the varistor into the saturated zone, showing a small linear resistance, due to the current large, over time it will make varistor overheating or even burst.
Set the multimeter measurement 10k file, table pen connected to both ends of the resistor, the varistor should exhibit marked resistance multimeter, if you exceed this value is large, then the TGE metal oxide varistor sizing has been damaged.