An electrical unit such as a lighting dimmer pack incorporating one or more semi-conducting switching devices such as triacs or thyristors, wherein the usual insulated mounting base (18) of the triac (30) is in turn mounted on the unit casing via an electrically insulating heat conductor, and the said mounting base is connected to the phase or neutral conductor (52), preferably the latter. In a preferred arrangement, a plurality of triacs are clamped by a printed circuit board which provides the connection from the triac bases to the neutral conductor.