It is described a single-phase or three-phase distribution electric transformer, of solid insulation, for use in aerial installation, in poles, in platforms or pedestals, or installation in underground distribution network for operation in air environment, semi-submerged or submerged. Such transformer comprises at least one high voltage winding (3) and at least one low voltage winding (2) concentrically assembled around a core column (1.2,1.3), the low voltage and high voltage windings (2,3) being electrically isolated from a solid material, a core window (20) being defined as a space between two core columns (1.2,1.3), the transformer comprising at least one electrical insulation sheet (4) assembled on at least a core window (20) of said transformer, the assembly of the electrical insulation sheet (4) being defined in the longitudinal direction (300) of the transformer. Such sheet (4) avoids the formation of spiral around the core by immersion water or conductive dust. The use of this dry transformer in the underground distribution networks in the cities offers more safety to the population, because the dry transformer does not explode, apart from eliminating the risk of environmental contamination through the leakage of oil from transformers in oil. Once the dry transformer of the invention does not explode, the underground installation chambers can be simpler and more economic once they do not need to be resistant to explosion.