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The paper describes the considerations underlying the development of 400 kV internally oil-cooled cable systems, on both theoretical and practical grounds, with particular reference to coolant selection and duct diameters, cooled-section length and critical areas for development. Various conductor designs are described and their merits, leading to the adoption of two cable designs using copper and aluminium conductors, are discussed. Accessory design is also considered; particularly the design of stop joints suitable for operation at large continuous oil-flow rates. The requirements for ancillary plant and oil pumps are stated, and the results of investigation of long-term circulation on the dielectric properties of cable oil are reported. The paper also considers possible practical cooling schemes. Future applications of this technique for cooling very-heavy-duty cable circuits are indicated, and the advantages of extension of operating limits (such as temperature and pressure) for the cable are discussed. The basis for calculation of coolant flow and pressure drop is stated.