US1805513A - Shielding means for high tension cable - Google Patents

Description

y 1931. E. L. CRANDALL SHIELDING MEANS FOR HIGH TENSION CABLES Filed Nov. 20, 1929 @%Q ..d n rm. r L? .0 n m. W A I e .e n H @y ub E 20 insulation on the conductors.

Patented May 19,1931

UNITED STATES PATENT OFFICE EUGENE L. CRANDALL, F SGE EN'ECTADY, NEW YORK, ASSIGNOB TO GENERAII ELEG- TBIG COMPANY, A CORPORATION OF NEW YORK SHIELDDTG MEANS FOR HIGH TENSION CABLE Application filed November 20, '1929. Serial Io. 408,589.

In the manufacture of cable for the transmission of high tension electric current for power purposes, and specifically of cable'comprising a plurality of insulated conductors located within the same sheath for carrying multi phase alternating current, it is common practice to provide each conductor covering, such as paper, with an electro-static shield for the purpose of preventing ionization within the cable both between the conductor coverings themselves and between said coverings and the metallic enclosing sheath. Shields for this purpose have been made of metal foil and also of metal ribbon which latter although thin has in reality a substantial thickness and moreover has a certain stillness which renders it diflicult in some cases to apply the same smoothly and evenly over all parts of the surface of the paper The metal ribbon is made by a cutting or shearing operation. Upon examination of the edges under a microscope numerous minute projections will be observed and at these projections there will be a concentration of electrical stresses, especially if a'relatively wide space is left between the turns or wraps around the conductor insulation. In the cases mentioned the shields .are spirally wrapped around the conductors which are subsequently cabled with a relatively long lay or pitch to facilitate bending, as in reeling and installing, and are then fed through a press and a lead sheath ap lied thereby. p X shield made of foil has the advantage that it is very thin'andhence well adapted to make good electrical and mechanical contact with the paper insulation on the conductors. It has also the advantage of low 40 cost. On the other hand, because the'foil is very'thin and delicate it'is difiicult to apply the same to the conductor coverings in the first instance, and when properly ap lied it is liable to be broken in the subsequent 'a-ndling of the conductors both before and while passing throu h the. ress which applies the lead sheath. veil a er the sheet is applied it may be broken when the cable is bent as the bending causes relative movements of the conductors and their coverings one with respect to another and with respect to the sheath itself. In this connection it may be stated that the continuity of any shield is essential to the successful operation of the cable. Furthermore the shield must be in electrical communication with the sheath at frequent intervals so that any current induced therein will be conveyed to ground through the sheath. After the sheath is applied to the cable there is no means whereby it can be determined whether the shield is intact or otherwise. The first intimation of a defective shield is usually when a burnout occurs and this commonly requires the substitution of a new seer tion for the injured one.

I have invented or discovered a means or arrangement of parts whereby the mechanical weakness of a shield made of foil is overcome, and whereby the currents induced therein are "freely conveyed to the sheath.

In 'carryin out my invention the foil is combined wit a carrier of paper or equivalent material which is of suiiicient strength to permit of the same being tightly wound on the conductor coverings and to be subsequently handled without injury. The carrier is made narrower than the foil and the edges of the latter are folded over the edges of the paper carrier. The folding over of said edges looks or fastens the two together, and also presents a slightly rounded edge which is free of minute projections which cause concentration of electrical stresses. I may depend upon the interlocking of the edges alone or may use an adhesive between all parts of the-'ioil and the carrier as an additional. uniting means. "When an adhesive is used it should be of such character as not to have an injurious effect on the'oil or other insulation with which the paper insulation is impregnated. For this purpose a phenolic condensation product or an asphalt mixture will be found satisfactory. The foil may be of any suitable metal, as tin or aluminum, for example. Such foils are of the order of one mill in thickness, from which an idea may be had of their delicate structure, and their liability to injury, especially during manu-.

facturing operations. The paper of which the carrier is formed may be of the kind commonly used in cable manufacture and is many times thicker than the foil and may be of the order'of five mills in thickness.

In order that the impregnating fluid may freely enter the paper insulation on the conductors and also to facilitate the removal of air and other gases from the cable both the carrier and the foil are perforated. To attain the best results the perforating die should pass first through the foil and then through the paper carrier. In this manner the minute pro ections or ragged edges about the perforations will be firmly embedded in the paper. After the foil and carrier are united as above described it is advantageous to pass the same through a pair of rollers to press the same firmly together. By folding the foil back over the paper at the edges and perforating and rolling the foil and its carrier, the needle point effect of the tape edges is reduced to a minimum.

In applying the tape it should be done with the metal foil next to the core or conductor insulation and with the carrier on the outside. With this arrangement only the narrow edges of the foil are exposed at the peripheral surface of the conductor coverings and these are in electrical communication with the sheath when the latter is applied. Assuming the case of a three core or conductor cable, each core is covered with the foiland its turnedover edges will appear as spirals on the outer surface. As the cores are cabled together the edges of the foil on one core will cross similar edges on an adjacent core, and being also in electrical communication with the lead sheath,

the currents induced in the foils will be suitably grounded and at frequent intervals. The foils will also serve to convey heat from the interior of the cable to the sheath where it is dissipated. The edges of the "foil being turned over the edges of the paper carrier are in this manner well supported and protected against external injury and the main body of the foil is protected by the carrier which overlies it. It will also be seen that the main body of the foilis protected from injury during the factory operations as well as after the cable is completed when the bending thereof causes the conductors to move slightly one with respect to another.

In the accompanying drawings which is illustrative of my invention, Fig. 1 is a perspective view of a piece of foil and its carrier; Fig. 2 is a cross-section of the foil and carrier on a greatly exaggerated scale; and Fig. 3 is a cross-section of a three-core cable, each core being covered with foil.

3 indicates the paper carrier, 4 the main body of the foil and 5 the edges which are turned or folded back over the top surface of the carrier. 6 indicates the perforations which are relatively numerous and so spaced as not to unduly weaken the carrier.

In Fig. 3, 7 indicates the cores or conductors and 8 the paper insulation which is tightly wound thereon. The heavy black line 9 about each insulated covering indicates the foil and its carrier. Enclosing the cores is animpervious lead sheath 10 of the ordinary kind.

The spaces 11 between cores may be filled with .to prevent them from spreading, said band being located inside the lead sheath.

By passing the carrier and foil between pressure exerting rollers the foil is largely embedded in the paper carrier at its folded back edges or sides and thus presents a smooth peripheral surface. In Fig. 2 the rough or ragged edges 13 of the foil about the perforations will be seen embedded in the paper carrier due to the fact that the punches which form them first pass through the foil and then through the paper.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electrostatic shield for an electric cable comprising a strip of insulating material and a strip of metal foil wider than said material having its edges folded back over the edges and part of the top surface of said material and embedded therein.

2. An electrostatic shield for an electric cable comprising a relatively thick and narrow strip of paper which forms a carrier, and a thin strip of metal foil wider than and supported by the carrier and having its edges folded back over the edges and part of the top surface of the paper and embedded therein.

3. An electrostatic shield for an electric cable comprising a relatively narrow strip of paper which is perforated along itslength and forms a carrier and a strip of thin metal foil which contains perforations registering with those in the paper and has its edges folded back over the edges of the paper whereby the carrier and foil are united.

4. An electrostatic shield for an electric cable comprising a relatively thick strip of paper and a thin strip of foil, the edges of the foil being folded back over the paper to unite the parts and also to form smooth edges, and perforations which extend through both paper and foil, the raw edges of the foil around each perforation projecting into'the paper.

5. An electric cable comprising more than one insulated conductor and an enclosing sheath, electrostatic shields for the'conductors, each comprising a strip of paper and a thin strip of foil, the. edges of which are folded back over the paper, the body portion of the foil lying flat-wise on the conductor insulation and the folded. over edge portions;

being in electrical communication with the sheath.

6. An electrostatic shield for electric cable comprising a relatively thick and narrow flat paper tape forming a carrier of substantial stren h and a metallic foil much thinner and 0 less strength than the paper which completely covers one flat surface of said paper and both side ed and also a portion of the other flat su ace.

In witness whereof, I have hereunto set my hand this 19th da of November, 1929.

EUGE L; CRANDALL.

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