Tight buffered fibres are surrounded with a helical steel hose and a layer of aramid yarns as the strength member, and then a LSZH sheath is extruded to form an optical sub-unit. Optical sub-units and copper wires are stranded around a non-metallic central strength member to form a cable core. The core is wrapped with water blocking tape. Finally, a LSZH outer sheath is extruded. Other sheath materials are available on request.