This part of IEC 60099 applies to non-linear metal-oxide resistor type surge arresters with spark gaps designed to limit voltage surges on a.c. power circuits. This standard basically applies to all metal-oxide surge arresters with gaps and housed in either porcelain or polymeric housings. This standard specifies requirements and tests for metal-oxide surge arresters with internal series gaps, with rated voltages 52 kV and below. The following arrester types and ratings are presently under consideration, but are not addressed in this standard. They will not be addressed until more information can be ascertained on the individual subjects: • series gapped arresters above 54 kV; • externally gapped arresters, all ratings; • shunt gapped arresters, all ratings; • line discharge class 2, 3, 4 and 5.