Electric cable

March 7, 1939.

2,149,772

P. V. HUNTER ET AL.

ELECTRIC CABLE

Filed April 28, 1937

Sheets-Sheet 1

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ATTORNEYS

March 7, 1939.

P. V HUNTER ET AL

ELECTRIC CABLE

2,149,772

Filed April 28, 1937

Sheets-Sheet 2

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ATTORNEYS

Patented Mar. 7, 1939

2,149,772

UNITED STATES PATENT OFFICE

2,149,772

ELECTRIC CABLE

Philip Vassar Hunter, London, and Harry Hill, Belvedere, England, assignors . to Callendar’s Cable and Construction Company Limited, Lon- , don, England, a. British company

Application April 28, 1937, Serial No. 139,519 In Great Britain May 9, 1936

4- Claims.

This invention relates to electric cables of the kind having two or more cores which are so shaped that when laid up together they form by themselves an approximation to a complete ,5 cylindrical body. The cores in such cases are in the form of sectors with more or less rounded corners or in the form of ovals approximating to such sector shapes. The invention is concerned with the production of a cable of im10 proved and relatively cheap construction which results in minimum overall dimensions and minimum quantities of material used in the construction. The basic feature of the improved construction is the use for each conductor of 15 a solid rod of non-circular cross-section with a surface of a particular kind, which rod has been accurately formed before the insulating material is applied to it so that the core when laid up with the others occupies the appropriate po20 sition without tendency to spring therefrom. Such conductors will hereinafter for the sake of brevity be termed pre-spiralled, solid conductors. The conductor is covered with insulating material (usually paper) in the form of strips 25 lapped on and impregnated. Each core may, as in other constructions, have a conductive covering of metallised paper or foil. The assembled cores are sheathed and the sheath may be protected in any appropriate manner.

If the surface of such a conductor is made perfectly smooth it has both advantages and disadvantages as compared with the surface of a conductor made of stranded wire. The advantage of the smooth surface is that it reduces the 35 value of the maximum intensity of electric stress in the dielectric at the surface of the conductor. The disadvantage is found in the difficulty of obtaining complete impregnation of the insulating material which is in contact with the con40 ductor. This is, of course, the place where it is most important that impregnation should be complete. By utilising the particular form of surface, to be described, the advantages of smoothness are obtained, while the disadvan45 tages as regards impregnation are avoided. According to the present invention the surface is provided in certain parts with shallow- grooves or depressions running substantially longitudinally. These parts are so chosen that the smooth 50 surfaces at the place of minimum radius of transverse curvature are not broken into; that is to say, the minimum radius of curvature to be found on the surface of the conductor is not decreased by the provision of the longitudinal grooves or 55 depressions. They are provided where the radius (Cl. 174—26) of transverse curvature approaches a maximum so that the provision of the depressions or grooves does not decrease this radius to an undesirable extent. It can, for instance, be kept, to a value not less than .that which occurs on the smooth 5. parts of the conductor owing to the general transverse shape thereof.

Examples of conductors constructed in accordance, with the invention are shown in the accompanying drawings, wherein: 10

Figure 1 is a perspective. view showing one form of pre-spiralled sector shaped conductor constructed in accordance with the intention,

Figure 2 is a cross-section through a 3-core cable comprising three conductors of the form 15 shown in Figure. 1,

Figure 3 is a perspective view of a second form of sector shaped conductor, constructed in accordance with the invention,

Figure 4. is a cross-section through a 4-core 20 cable comprising four conductors of the form shown in Figure 3, . . ;· .

Figure 5 is a perspective view of a pre-spiralled oval conductor constructed in accordance with the invention, and . . 25

Figure 6 is a-cross-section of a cable comprising three oval conductors of the form shown in Figure 5.

Referring now to the drawings, it will be seen that each of the flat sides 2 of the conductors 30 shown in Figures 1 and 2, has a centrally disposed groove 3 and the curved side 4 Is provided with a pair of parallel grooves 5, all extending substantially longitudinally of the conductor. It will be seen from Figure 2, that the three con- 35 ductors when insulated with a layer of insulation 6 and assembled together, form an approximation to a cylindrical body and that very little packing 7 is required to fill the small spaces between the assembled cores and the surround- 40 ing lead sheath 8. In the case of the conductors shown in Figures 3 and 4, each flat side 2 of each conductor is provided with a groove 3 as described with reference to Figures 1 and 2 but the curved, face 4 of the conductor is furnished 45 with only one groove 5. In the case of the conductors shown in Figures 5 and 6, a pair of grooves 9 are formed on each of the two flatter sides 10 of the conductor, one on each side of the minor axis of the conductor, i. e., in the 50 region where the radius of transverse curvature approaches a maximum. It will be observed that in all cases the grooves are kept away from the corner parts or other surfaces where the radius of curvature approaches a minimum and 55

2,149,772 that the sides of the grooves are so rounded off that the radius of curvature of the solid conductor in the neighbourhood of the grooves Is not decreased to an undesirable extent by the 5 provision of the grooves. It will be appreciated s of course that the conductors shown in the drawings are described merely by way of example and that the number, location and cross-secj tional form of the grooves may be varied, for instance to suit the shape of the conductor and the type of cable in which it is to be employed. For example they may be made deeper where the viscosity of the impregnant at the temperature of impregnation is high and instead of being IS continuous, as shown, they may be of comparatively short length and provided at frequent intervals along the length of the conductor.

By the. use of this improved form of conductor the advantage of complete impregnation com20 bined with' minimum dielectric stress and minimum size of cable for a given area of the conductor cross-section is obtained. The smooth form of the solid conductor also has the advantage of favouring the smooth and regular ap25 plication of the insulating material. Although the rigidity of the solid conductor may in large sizes place a limit on its use, there is a large and important range of sizes within which the construction may be employed with advantage.

³⁰ The improved conductor is preferably made by a rolling process, in which the shape in crosssection and the helical form are produced in one operation, this is not, however, essential.

The solid conductor has the further advantage S5 over the stranded conductor that it can be annealed, after forming and thereby be made better ; adapted for handling in the subsequent stages of the manufacture of the cable.