What abbreviation does they use for nominal voltage (Un) in Russian?
Personally I'd go for "Uн" / "Vн", or "Uном" / "Vном" if you can afford longer labels. Note that equipment I have seen would cite the nominal voltage itself (i.e. "220/230 В", "380/400 В", "10кВ" etc.) without calling it nominal.

Russian Wikipedia has an article about nominal voltage, but it doesn't mention any official abbreviations.