This document covers transformers from 50 kVA to 2 500 kVA intended for operation in three-phase distribution networks, for indoor or outdoor continuous service, 50 Hz, immersed in mineral oil, natural cooling, with two windings: - a primary (high-voltage) winding with a highest voltage for equipment from 3,6 kV to 36 kV - a secondary (low-voltage) winding with a highest voltage for equipment not exceeding 1,1 kV. NOTE 1 This document may be applied, either as a whole or in part, to transformers immersed in a synthetic insulating liquid. NOTE 2 This document may be applied, either as whole or in part, to transformers having windings with more than one rated voltage. In this case the rated power for each rated voltage shall be specified by the purchaser. The object of this document is to lay down requirements related to electrical characteristics and design of three phases distribution transformers immersed in mineral oil.