By definition, overvoltage category is a roman numeral defining a transient overvoltage condition. Overvoltage categories I, II, III and IV are used. The term “overvoltage category” is synonymous with “impulse withstand category” used in other standards. These categories are based in statistical probability rather than the idea of physical attenuation of the transient overvoltage downstream in an installation. Nevertheless, it is understood that as power is transformed through galvanic isolation or passes through a distribution center, there is a category reduction that takes place.
IV – Equipment of overvoltage category IV is for use at the origin of the installation. — Examples of such equipment are electricity meters and primary overcurrent protection equipment.
III – Equipment of overvoltage category III is equipment in fixed installations and for cases where the reliability and the availability of the equipment is subject to special requirements. — Examples of such equipment are switches in the fixed installation and equipment for industrial use with permanent connection to the fixed installation.
II – Equipment of overvoltage category II is energy-consuming equipment to be supplied from the fixed installation. — Examples of such equipment are appliances, portable tools and other household and similar loads.
I – Equipment of overvoltage category I is equipment for connection to circuits in which measures are taken to limit transient overvoltages to an appropriately low level. In general this means you have applied transient suppression or use regulation to limit transients to pre-defined levels.
* Source: IEC 60664-1 Ed. 2, section 220.127.116.11