Overvoltage Categories

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 predefined levels.   * Source: IEC 60664-1 section

3 Replies to “Overvoltage Categories”

  1. This is really helpful.
    What will be the category of appliance in case if it is energy consuming (not distributing) end device but installed in wall cavity instead of having a plug in connection?

    1. Hi, thanks for your question.

      As a general rule equipment of any kind, that is downstream of a distribution panel or other protective device, does not decrease its OV category with distance. When evaluating OV categories for individual devices, it is the general practice to use the same category as the equipment feeding the circuit. In your example, a device installed in a wall or even a power outlet installed in a wall assumes the same category seen at the output connection of the distribution panel immediately upstream. If that happens to be a OV Cat II circuit then the device hanging on that circuit is also operating at OV Cat II. In the other direction, going further upstream from the distribution panel toward the mains source, you may be close to the facility mains which is usually OV Cat III. It is almost never the case that OV Cat IV exists within a building since the outdoor transformer reduces the OV Category by one level from OV Cat IV.

      Just to clarify, back to the end device case, it is usually not the case that you can reduce OV Cat by simply adding longer length circuits. Reducing from OV Cat II to OV Cat I requires active transient suppression to a predefined impulse voltage level. For guidance about impulse voltage levels, you can refer to IEC 60664-1, which is the source document for most world-wide safety standards in use today.

      All the best, Doug

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