Sealing internally screened cables against moisture penetration - by intermediate sheath ribs, desiccant and a helical rubber seal

摘要

Electric cables for submerged or underground use and incorporating metal foil as a concentric intermediate screen are sealed against longitudinal penetration of moisture by wrapping the foil over an intermediate sheath having springy longitudinal or helical ribs and coated with a powdered material (I) which swells when wetted. Pref. a rubber band of about 1.2 mm square cross section is wound helically in the sheat/foil interface with a wide pitch and in a direction which intersects the rib pattern and isolates the powder into a series of interfacial troughs of limits length. The elastic ribs compensates for the tendency for thermal cycling to cause delamination between the metallic sheath and the underlying intermediate sheath. Specifically the intermediate sheath is of semiconductive carbon filled rubber surrounding e.g. crosslinked polyethylene insulation, and the foil screen is enclosed by an extruded PVC sheath. Pref. (I) is a cellulose ester.