A transformer (3, 3') includes a housing (6), a magnetic core (10, 10') and transformer coils (12, 12') wound around the housing. The housing (6) defines a magnetic core receiving cavity (20, 20') in which the magnetic core (10, 10') is mounted. The housing further comprises positioning portions (8) projecting from walls of the housing in the magnetic core receiving cavity, configured to position the magnetic core in the magnetic core receiving cavity to form a dielectric air space (15, 15') between the magnetic core and the transformer coil.