Introduction his paper will overview the evolution of moisture barrier cables and the data that support their use. T There has been extensive development of moisture barrier medium and high voltage cables as a countermeasure to water treeing. The moisture barrier can take traditional forms, including extruded or welded metallic tubes. The latest developments in moisture barrier cable design use laminate sheaths. This paper will discuss the traditional approaches and present an evolution of the laminate sheath. An overview of the various designs, under-jacket with foil or sheet, or oncore, will be given. Design considerations also will be discussed. Information on deployment in the field and standardization activities will complete the report.As the reader will discover, an immense amount of data and information on moisture barrier cable has emerged. In spite of this body of knowledge on the effectiveness of moisture barriers against treeing and the commercialization of moisture barrier cable, the treeing phenomena is still being extensively researched and studied. Well over a thousand papers have been presented on treeing related research. Great energy is still being expended to understand the mechanism and the material constraints. Yet the causes of treeing or its acceleration are well known. They are the presence of water and the presence of ions in an electric field concentration. The evidence is indisputable that if moisture and ions are kept out of the insulation, cable service life is assured. To this end, radial and longitudinal moisture barriers have been developed to the degree that practical, cost-efficient cable designs are available. In point of fact, there has been a proliferation of moisture barrier or water impervious cable designs in recent years. Field data on performance is starting to emerge and it is excellent.To set the stage, some background data on the use of jackets to increase moisture resistance will be presented, followed by general construction details with data on There is overwhelming evidence that the use of moisture barriers on medium and high voltage power cables will solve the problem of water treeing. the performance of moisture barrier sheaths. Details on the types of sheaths in use for both medium voltage and high voltage cable applications comprise the body of the report.

In recent years, environmental issues have had a profound influence on cable designs and materials used in cable construction. This paper will address the issues concerned with the effects of cable on the environment, as well as, the effects of environmental conditions on cable performance and reliability. Effects on the environment include release of toxic materials and contamination of ground water. Environmental effects on cable are related to the presence of moisture, chemicals, ions, oils or solvents. All such issues must be considered in the evolution of environmentally safe cables. This also extends to the manufacturing processes for producing cable insulation, jacketing, and sheathing materials as well as the cable manufacturing process itself.

Mechanical strength in the cable sheath is particularly valuable for both fiber optical and copper pair cable. By bonding a PVC jacket to a coated metal tape, a mechanically strong sheath construction is obtained. Changes in PVC jacket technologies have created the need for PVC compatible coated metals with greater tolerance for variations in PVC jacketing materials. This need has been met by the development of a variety of coated metals having thermoplastic coatings which adhere to PVC. As a consequence of this development, new cable sheath designs for use in a variety of applications, such as riser cable and direct buried cable destined for local area networks, are possible. These cables may utilize both copper conductors and/or optical fibers for signal transmission. Data will be provided in the paper to show the effects of extrusion process conditions on adhesion for a variety of PVC resins. The properties of a variety of coated metals—aluminum, copper, and steel—will be discussed. Data on environmental tests of adhesion will be presented. Relationships between adhesion, metal characteristics, jacket properties, and mechanical performance of the sheath will be discussed. Cable applications for the PVC compatible metals will be discussed. Data on the performance of the bonded sheath in riser cable and buried local area network cable will be presented. New cable applications where coated metals in the sheath can provide lightweight armoring will also be discussed.

Materials have been developed to allow rapid commercialization of under-jacket moisture barrier sheathed medium voltage power cables. Plastic coated aluminum and copper tapes meet the needs for a radial metallic moisture barrier. These tapes have outstanding initial adhesion to polyethylene, chlorinated polyethylene or polyvinyl chloride jackets and excellent bond stability during thermal loading, temperature cycling, water aging, and exposure to elevated temperatures. Semi-conductive water-swellable tapes have been developed which function as cushion (thermal expansion) layers and longitudinal water blocks. These tapes are commercially available from several sources. Commercially available hot melt adhesives are effective in sealing the overlap of the metallic moisture barrier and meet the criteria for adhesion and bond retention. These materials have been combined to produce a viable water impervious medium voltage power cable. Underjacket moisture barrier cables have been commercially manufactured and have passed several key industry tests. The propemes and installation characteristics of the cable are detailed in this paper.

Lightning can cause damage to electrical systems from direct and induced effects. A study was conducted to devise means to prevent or control the effects of lightning on instrument and control systems. The results indicate that an acceptable approach is to use instrument cables with a relatively thick overall metallic shield grounded at both ends. The overall shield functions as a magnetic shield, not because of the mere presence of metal, but because it generates a counter, or opposition, c m n t that nearly cancels out the interfering current on the signal conductors. The data show that a twisted pair by itself is not adequate to protect against lightning. The overall shield is used in conjuiction with a single point ground system to provide this protection.