Roman numeral is used to define a transient overvoltage condition. That is, a short time overvoltage lasting not more than few milliseconds, oscillatory or not, usually strongly damped.

• Category I (OVC I) applies to equipment connected to a circuit where measures have been taken to reduce transient overvoltage to a low level. Example, inside the electrical circuit.
• Category II (OVC II) applies between the circuits not directly supplied by mains or the environment. Examples, appliances, portable tools and other plug-connected equipment.
• Category III (OVC III) applies to the circuits directly supplied by mains or the environment. Examples, downstream of and including the main distribution board like switchgear and other equipment in an industrial installation.
• Category IV (OVC IV) applies to equipment permanently connected at the origin of an installation. Examples, upstream of the main distribution board like electricity meters, primary overcurrent protection equipment and other equipment connected directly to outdoor open lines.

  Figure 1-4 shows an example of overvoltage category reduction in industrial drive system with electrical circuit connected directly to the supply mains.

Figure 1-4 OVC Level With Electrical Circuit Connected Directly to the Supply Mains