This part of IEC 61643 is a test specification for metal oxide varistors (MOV), which are used for applications up to 1 000 V a.c. or 1 500 V d.c. in power line, or telecommunication, or signalling circuits. They are designed to protect apparatus or personnel, or both, from high transient voltages. This specification applies to MOVs having two electrodes and does not deal with hybrid devices. This specification also does not apply to mountings and their effect on the MOV’s characteristics. Characteristics given apply solely to the MOV mounted only in the ways described for the tests.