A transformer having a plastics coil former on whose hollow winding tube (7) between flanges (3, 5) windings (11, 13) of different potentials are wound with one winding wound on top of the other. The flanges (3, 5) are provided with spacers (17, 19, 23) to keep the windings (11a, 13a) axially spaced from the flanges (3, 5). An insulating layer (21), which extends from flange to flange, is interposed between the windings (11, 13). The winding (11) nearer the coil-former axis is spaced from the flanges (3, 5) by fixed spacers or stepped portions (17, 19) provided on the winding tube (7). The winding (13) arranged on the insulating layer (21) is spaced from the flanges (3, 5) by spacing blocks (23) which are connected to the flange areas (24) of the coil former by integral hinges (25). The spacing blocks are operable for swinging down onto the insulating layer (21).