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IEC 61643-331:2017E) is a test specification for metal oxide varistors (MOV), which are used for applications up to 1 000 V AC or 1 500 V DC 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 hybrid overvoltage protection components. 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. This second edition cancels and replaces the first edition published in 2003. This edition constitutes a technical revision. This edition includes the following significant technical changes with respect to the previous edition: a) Update of the nominal varistor voltage test method; b) Addition of thermally protected varistors - component symbol and test methods; c) Addition of nominal discharge current - test methods; d) Addition of voltage ratings for disc types (Table 1); e) Addition of test currents for clamping voltage of disc types (Table 2); f) Addition of typical voltage ratings of SMD types (Table 3); and g) Addition of Limited current and temporary overvoltage tests for thermally protected varistors. Keywords: metal oxide varistors (MOV), high transient voltages.