As far as I know, IEC has not mentioned medium voltage in any of its standards except in IEC 61000-3-6.
The following table provides the different voltage levels as per IEC 61000-3-6.
|Voltage Level||Range (line-to-line)|
|LV (Low voltage)||Un ≤ 1kV|
|MV (Medium voltage)||1kV < Un ≤ 35kV|
|HV (High voltage)||35kV < Un ≤ 230kV|
|EHV (Extra high voltage)||Un > 230kV|
NOTE The boundaries between the various voltage levels may be different for different countries
It is good to note that  is the only IEC standard that mentions about Medium voltage. All other IEC standards consider voltages above 1kV to be high voltage.
- IEC/TR 61000-3-6
Electromagnetic compatibility (EMC) – Part 3-6: Limits – Assessment of emission limits for the connection of distorting installations to MV, HV and EHV power systems