I have a military grade filament transformer made by GE. Primary is multitap 115-132V with 535V.WVM. Secondary is 6.6V with 1.07 KV. WVM. This is from a an early 1950's radar set. So some transformers used in high voltage equipment are designed to handle that kind of voltage in terms of insulation, even when operating at much lower voltage. Wayne WB5WSV