A transformer for mounting on a mast of an air-insulated energy distribution network has a core which is protected against corrosion and environmental influences, while at the same time stable retention is provided. The transformer contains a winding block having a solid insulating body, in which at least one upper voltage winding and at least one lower voltage winding are arranged. The winding block delimits a completely peripherally closed central retention opening. Furthermore, a core unit is provided which has at least one magnetizable core and is inductively coupled to each lower voltage winding by each upper voltage winding. The core unit extends through the retention opening via at least one core leg and annularly extends around the winding block on the outside by a ring portion. Retention devices are provided for fastening the ring portion to the mast.