is the primary international standard for electrical supply voltages, providing a universal framework to harmonize power systems and equipment design globally. The current edition (2009, with 2021 amendments) defines standard nominal voltages for AC and DC systems from low to ultra-high voltage. Core Voltage Classifications
A very likely candidate is the continuation of a major table listing Um values for systems above 1 kV. For example: iec 60038 standard voltages pdf 103
When you download the IEC 60038 PDF, you will often need these companion standards: is the primary international standard for electrical supply