To direct cooling air to components which, in operation, require different amounts of cooling due to different heat dissipation, a cover cap is provided which prevents accidental contact with current-carrying elements and which is formed with slits or openings therein located with respect to the components requiring cooling to direct cooling air, upon induced axial airflow, to the respective components in relationship to their cooling requirements. Some of these openings may be formed with in-turned collars surrounding, for example, diode rectifiers, to direct air on and around the diode rectifiers, while preventing contact with their terminals.