Defines transformers from 50 kVA to 2500 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.