US2446387A - Shielded cable - Google Patents

Description

ug. 3, 1948. T. F, PETERSON 2,446,387

SHIELDED CABLE Filed May 19, 1945 .www /6 m Mp5 /3 Kwam MP5 /Z /fvsaMr/a/v 607500470 INVENToR. Thomas Pe/efsof? AT TORNE Y atented ug. 3, 1948 SHIELDED CABLE Thomas F. Peterson, Shaker Heights, Ohio Application May 19, 1943, Serial No. 487,577

Claims. (Cl. 174-105) This invention relates to the shielding of electric cables for confining electrical stresses or eliminating interference by the cable with external circuits or interference from sources external to the cable. In a copending application Serial No. 373,181 filed June 24, 1929, that has since become Patent No. 2,322,702, .Tune 22, 1943, there is described a cable structure embodying a semi-conducting layer in intimate contact with a surface of the conductor insulation for dissipating or conducting away charges that may otherwise build up a potential difference at the surface under the applied electrical stress, which potential dierence may cause ionization with consequent breakdown of the insulation. In general the layer is described therein as one given semi-conducting properties by conductive carbon or metal particles embedded in, sprayed on, or included in a coating medium, such as a paint or lacquer, directly on the conductor insulation itself or on the surface of a base material that is normally insulating such as rubber, paper, varnished cambric or fabric tapes and in intimate contact with the conductor insulation. The present application is a, continuation-impart directed to the coating or the paint and lacquer type of semi-conducting layer.

Coating media that form adherent continuous lms o-r layers upon deposition and drying cover a wide range of known materials; they are applied as relatively thin solutions for spraying or brushing or as pastes which can be frictioned or rubbed on. Solutions, dispersions, or pastes, made with spirit (alcohol) or other volatilizable solvents or vehicles to leave solid continuous films upon solvent removal after application, are known as and herein intended by the term lacquers; liquids or solutions that in themselves solidify after application into continuous films, either through oxidation or polymerization or both, are kno-wn as and herein intended by the term paints. The lacquer type depends upon film-forming salutes or phases (either the continuous or the disperse phases) and these comprise natural and synthetic resins, cellulose esters, polymerized and copolymerized compounds, natural and synthetic rubber, etc. Paints generally depend for film-forming upon oils which are drying, i. e. air-oxidizing, or heatpolymerizlng, such as tung, oiticica, linseed, soya bean, etc., and the oils are normally extended with compatible natural or synthetic resins, or the like, to give body and durability. Both lacquers and paints are insulating electrically, form tough elastic lms and can be utilized as media or carriers for conductive particles, such as conductive carbon (of which acetylene black is an example), graphite, metallic powders, etc.; but for most installations requiring resistance to oxidation or other properties, lacquers are preferred.

While cellulose ester (cellulose nitrate and acetate) lacquers have found extensive use in the cable art so as to be regarded as synonymous with lacquers and are useful as media for conductive particles to form semi-conducting coatings or layers, they are open to such objections as insumcient resistance to heat and oils. It has been found that improvements in these respects are obtained when elastomers, i. e. inherently flexible or elastic compounds or compounds made so by plasticizers, are utilized as the film-forming constituents in lacquers; known elastomers that are suitable for the purpose include organic polymers, natural and synthetic rubbers and plasticlzed or oil-modiiied synthetic resins, such as Thiokol-organic polysulde Neoprene-chloroprene polymer Vinylite-plasticized vinyl chloride-acetate copolymers Koroseal-vinyl chloride polymer Vistanex-lsobutylene polymer (butyl rubber) Buna S--butadiene-styrene copolymer Buna N-butadiene-acrylonitrile copolymer Alkyd resinsfatty acid-modiied polyhydric alcohol-dibasic acid esters Phenolic resins--oil-modied phenolic aldehyde condensation products.

Among these, neoprene and the alkyd resins have been found particularly e'ective when the cable insulation is wrapped paper filled with viscous petroleum compounds.

The lacquers need not be true solutions; they can be colloidal dispersions or suspensions so long as the non-volatile phase becomes a continuous solid and adherent film upon deposition and removal of the volatile component. A distinguishing characteristic of the compositions is that the lacquers or paints forming the media for the conductive particles are in themselves essential constituents of the coatings or films and have their modifying effect on the electrical properties, such as conductivity.

The preparation of the semi-conductive compositions requires no particular procedure. Normally, however, conductive particles are premixed with a lacquer or paint to form a. solution or paste. For lacquers including elastomers and some resinous media, a kneading or rolling action may be required to secure a uniform distribution o f the conductive particles in the lacquers.

Without any intent of limiting the physical formation or the electrical behavior of the semiconducting films or layers to any particular theory or explanation, it appears that the conductive particles are present in the electrically-insulating matrices in varying degrees of contact; even in the case of "high resistivity films, there are undoubtedly paths of continuous contact between the particles. Such contacts between individual particles, however, are evidently at points forming but small portions of the surface areas of the average particles and through the carrier separating the particles to impart semi-conductivity, for the particles are in and are coated by the insulating matrix; the conductivity so manifested is what is known as non-metallic conductivity. since it is of the nature of an insulating material that allows but slight current leakage through or along its surface. By an increase of the particle contact areas, as by an increase of the number of particles in the matrix or increase of the average relative contact area to particle surface area. the film resistivity (which, as hereinafter stated, can be varied from a few ohms to many megohms) is found to be correspondingly reduced; for instance, burnishing of the films reduces the resistivity. The surface resistivity may be millions of times that of metal foil or inlay or sprayed or otherwise deposited metal lm that in continuous metal-to-metal contact has resistivities in the order of .001 ohm/cm. square; the resistivity of such a metal lm, however, can become infinite upon a crack or break developing in the film. Since typical non-metallic lms containing conductive carbon particles are produced with particles having an average size approximating 12 microns, it is practically impossible to determine the relative spacings and nature of the conducting paths or leakages which contribute to the'film conductivity. The non-metallic semiconducting film, moreover, has a. negative temperature coefficient. i. e., the resistance decreases with increase in temperature, whereas theresistance of a continuous metal foil or film increases with increase in temperature.

Insulation on electrical conductors can be wrappings of paper or varnished cambric, preformed synthetic resin lms, etc.; strips of rubber, etc., applied longitudinally of a conductor; or a homogeneous wall structure such as is obtained by the extrusion of rubber, thermoplastic resins, and the like about the conductor; and any of these insulation structures can have overall "filled-fabric" or other tapes, such as rubber-filled cotton fabric tapes. A semi-conducting coating can be formed directly on a surface of such insulation, as by the conventional dipping, spraying or frictioning methods on an outside surface or by a transfer of a lm to an inside surface accomplished for instance by applying a coating to the copper conductor and leaving the coating in a condition, when the insulation is placed about the conductor, to unite or adhere intimately to the insulation. When the semi-conducting film-forming compositions are coated on or made into base materials as on tapes, fabrics, etc., or preformed into films before application to the insulation, the compositions can have adhesive backings of rubber or the like, and in this form they can be wound either about the insulation or directly on the conductor to contact the inner surface of the cable insulation. For coating tapes or other base materials, calendering or knife-coating machines can be used for pastes or viscous solutions; thin liquid solutions are conveniently incorporated by passing the base materials through a bath of the solution and then drying to permit further handling. Preformed films of coating materials can be made by pouring a solution on a plate, drying and then stripping; the stripped films can be sliced into tapes, made adhesive on the backs with solvent or with other suitable medium and wrapped intimately about the insulation, since they are flexible and elastic, to give the equivalent of a coating applied in liquid form and dried or polymerized in place on the insulation.

After the application of the semi-conductive composition, either to the cable insulation or to a separate tape or other base for winding about or in place on or under the insulation, it is customary to heat in order to hasten the drying or volatilizatlon of solvent. Heating is required when a high boiling solvent is used or when the nlmforming constituent is one that sets or is cured to an insoluble or a higher-melting form by heat; tape coatings are usually heated before their application to the cable insulation. Since heating, however, is necessary to vulcanize rubber insulation, that has been extruded on the cable conductor in its unvulcanized form and customarily has a cable tape wound about it to retain it in place and keep it from swelling, the cable assemblage including the tape can be submitted to heat to simultaneously vulcanize the rubber insulation and to set the semi-conducting compound on the surface or embedded in the interstices of the tape.

When the paints and lacquers as herein described are coated on cable insulation, they function as shielding layers in the manner explained in the parent application; they need have but little material thickness, since it is the surface resistivity that is of moment, and they usually vary from .0001 up to .010 inch. the latter including a base material or tape. For this purpose the relative proportion of conductive particles can be varied to give any degree of surface resistance along the cable from a few ohms to many megohms; megohms per centimeter per centimeter (or per inch per inch) of surface is found highly satisfactory for high tension (e. g. 3,000 volts or more) cables having frequent splices and terminals, since the conductivity longitudinally of the cable is sulcient to dissipate destructive charges and the splicing and terminal constructions are greatly simplified. Resistance grading of shielding at terminals can be accomplished by coating the insulation with progressively higher resistivity lacquers as the bared end of the cable is approached, or by successively fewer coats of a given lacquer applied along insulation from the edge of shielding to the end of cable.

Lacquers and paints deposited on the insulation as coatings, and particularly as coatings on or in tapes, have a most practical advantage over layers made integral with the conductor insulation of being removable. where desired as at splices and terminals, as by the stripping of the tapes and applying solvents to clean the insulation surface; since the tapes are separately applied, their adhesion to the insulation in general is not such as to prevent their removal. Lacquers and paints are also distinguished by the ease of application to cable insulations without interfering with or requiring modification in the manufacture of the cable. In addition they combine stability with any desired range of conductivity while the bond is intimate and amply sumcient to accomplish the electrical eects.

By choice of types and relative loading with carbon, graphite or metal, varying resistivities and physical properties are attainable to meet specific requirements.

Some specific forms of semi-conductive compositions are hereinafter described as illustrations of the invention.

Example 1.-A cable tape for wrapping about the rubber insulation. consisting of a cotton cloth fabric containing unvulcanized rubber in its interstices and being adhesive. had brushed on one surface a lacquer made from conductive carbon particles dispersed in a thin solution of unvulcanized rubber in benzene; about to 20 per cent of carbon based on the weight of rubber was used. The tape resistivity per inch per inch along Y the coated surface was reduced from 15,000 megohms to less than 100,000 ohms, along the opposite uncoated surface from 14,300 megohms to 104 megohms (showing penetration into the tape interstices), and perpendicularly` through the tape from 27,100 megohms to Il megohms.

Example 2.-A lacquer of a flexible oil-extended phenol-aldehyde resin in volatile solvent had metal powder dispersed in it in amount to give, when coated on a fabric, a surface conductivity per inch per inch of 10,000 ohms to -1 meghom.

Example 3.-Paints having a linseed oil base were made semi-conducting by incorporating litharge and Aquadag which is a colloidal water-dispersion of conductive carbon. When coated on a cotton cloth tape, these showed re- Example 4.--The following synthetic resin composition, in an acetone solvent for painting or spraying, yielded when coated on paper, Cellophane, or the like, illms having surface resistivities between 2,000 to 100,000 ohms per inch per inch:

Parts Vinyl chloride (85-88%)acetate copolymer Example 5.--For paper cable, i. e. cable in which the insulation is made of layers of paper and the spaces filled withfvaseline or a viscous petroleum fraction, a semi-conducting tape'was made by coatixm fabric or conventional paper insulating tape with a composition of Parts Neoprene G `(chloropr'eijiepolymer) 4 Magnesium oxide/--- v4 Acetylene black l .32 Cottonseed oil- 10 and minor amounts of stearic acid, barium sulfate and pine tar. The'ltape so coated was wrapped about the paper insulation and also on the conductor to contact the inner surface of insulation. Surface resistivities between k5,000 and 100,000 ohms perv inch per inch were maintained even after long periods ofi'mmersion in and contact with hot insulating oils.

Example 6.--Thiokol (an organic lconductor of thel cable for this purpose. `semi-conducting layer can be interposed between Grams Colloidal graphite dispersion 20 Butyl acetate 5 Acetone 5 Lacquer 20 When this was further thinned with a conventional thinner, such as acetone, and sprayed on a base fabric or sheet material, films after drying were obtained with resistivities of approximately 50,000 ohms to one megohm per inch per inch. Example 8.-A solution in toluol of an element'- convertible alkyd ester resin, prepared by the reaction`of an aliphatic dibaslc acid, castor oil and glycerol and having dispersed thereincolloidal graphite, was sprayed on paper and Cellophane strips, dried and cured at 350 F. for one hour, to develop resistivities from 10,000 ohms to 50,000 ohms per inch per inch. The lms have been found very durable, and they retain their conductivity after prolonged immersion in hot oil.

In the usual cable structure there is armor, rubber, braid, or other protective sheath over the insulation. Such a sheath when of metal provides a conductor for return currents at ground potential and thus connes the electrical stress to the insulation. A metal sheath also serves to drain charging current from the semi-conducting layer on the exterior surface of the insulation by contact at intervals along the length of the cable with the semi-conducting layer on the outer surface of 4the insulation; calculations show that in a cable operating at 13,000 volts with a semi-conducting layer having a surface resistance as high as megohms per centimeter per centimeter, such contact intervals can be about one-third oi an inch apart without the building up of harmful charges, which is a distance much greater than exists or is likely to occur in a metallicsheathed cable during use. When the sheath or covering is of rubber or other insulating material, a longitudinally extending external wire or netal tape is placed about or in longitudinal contact with the semi-conducting layer; for example, in a three-conductor cable a wire can be placed in the center or a valley of the cable to contact each of the layers on the insulation of the conductors, or a wire or metal tape can be helically wrapped about the semi-conducting layer on each The the conductor of the cable and the insulation about it to serve in this position as a preventative for corona discharges, and the conductor itself then' serves to drain charging currents from the layer and to prevent development of destructive potential diierences at the layer; in this construction the coating is applied to the conductor, the insulation, if rubber. is extruded about it into intimate contact, and the coating upon vulcanlzation intimately adheres to the insulation. In the case of wrapped insulation, the rst tape (next to conductor) whether rubber, paper, cloth, etc., can be coated with the semi-conducting film-forming paint or lacquer.

In the accompanying drawing there are illustrated some typical cable structures in which Fig. -1 is an elevation of a single conductor cable;

Fig. 2 is a radial cross-section of F18. 1;

Fig. 3 is a radial cross-section of a three-conductor cable;

Fig. 4 is a radial cross-section of another form of three-conductor cable; and

Fig. 5 is a radial cross-section of a multiple conductor cable designed for X-ray voltages and supplying filament current.

In the single conductor cable, illustrated in Figs. l and 2, there is provided a central conductor I; this can be either solid or stranded. About the conductor is shown a semi-conducting coating II of either lacquer or paint carrying conductive carbon or metal particles; for manyY types of cable structures the coating II is not necessary and can be omitted. Over the coating is the cable insulation I'Z` of rubber, paper, varnished cambric or the like; and about the insulation there can be a wrapping of rubber-filled or other cable tape I3. Applied to the insulation I2 or to the wrapping I3 is a continuous semi-conducting coating 'I4 (shown as a tape in Fig. 1) similar to the coating- II. In contact with the coating I4 is a conductor I5, shown as a metal tape, but the conductor can be wire braid, helically wrapped wire, etc., and it serves as a ground rconnection to drain charges which may build up on the coating I 4 and to carry any return currents. A protective covering I6, such as a saturated braid, an abrasive-resistant rubber sheath, a lead sheath or a metal armor, encloses the cable parts; when the sheath is metallic or conducting and grounded, the tape I5 'is not required.

A cable, such as is shown in Figs. 1 and 2, can be made, for example, by running the conductor through a solution of lacquer containing conducting particles.' The coated conductor is then passed vthrough a wiping die to form a thin uniform film on the surface, and then through a drying oven to remove solvent and to leave a tacky coating on the conductor.- Unvulcanized rubber insulation can then be extruded on, following the known practice, and tape can be wrapped about the rubber. The tape is coated by passing through a solution of the lacquer, and the assembly passed through an oven to vulcanize the insulation andftfo set the semi-conducting coatings in intimate and adhering contact with the insulation. The; metallic tape or Vother conductor and outside covering are4 then applied to the cable.` A

The multiple conductor cable,\shown in crosssection in Fig. 3 comprises conductors I0, each provided with insulation I2. The conductors are helically, twisted. about each other, and the valleys thusformed are filled out with jute I1. A tape I8 is Wrapped about the insulated conductors and jute filling, and the tape is provided with a semi-conducting coating on its outer surface. Overthe tape is a conductive metal tape er braid or wire I9 and an insulating covering or sheath Ifprotects the metal tape.

A modified threeeconductor cable is shown in cross-section in Fig. 4. In thisform each of the conductor insulations I2b has an exterior semiconductive coating I3, In the valleys formed by the insulated conductors extend wires ZI in longitudinal contact with the coatings |31'. The valleys are filled out with jute I'Ib, and a sheath II'b encloses'the cable parts.

The X-ray cable of Fig. 5 .has acore 30. As

- shown the core includesconductors 3I, 32, 33 for carrying filament-heating currents (the voltage drop for this purpose is only about 6 volts), and

. 8 the conductors haveinsulations 34, 35, 38 respectively; (in the figure, these insulatlons are shown disproportionately large for clearness) Twisted with the insulated conductors, and extending the length of the conductors, arewires 31, 38, 39 to serve as drain wires for any charging currents, and wrapped or formed about the conductors and wires is ajacket 40 which can be of tape or other base material having a semi-conducting coating on its surface in contact with the drain wires. The jacketed assembly constitutes the core.

For X-ray use the core is maintained at a high potential of from 50,000 to 100,000 volts; a heavy body of insulation 4I about the core is therefore required. As shown, the insulation 4I is rubber extruded about the core and therefore intimately contacting thesemi-conductive jacket 40. 0n the exterior of the insulation is provided a semiconductive tape orcoating 42, a groundedwire Ybraid 43 over the tape or coating, and a woven cotton-braid covering orsheath 44.

With this construction thesemi-conducting jacket 40 is external to theconductor insulations 34, 35, 36 but internal to the main body of insulation 4I. Conductors 3|, 32, 33 are at potentials approximately equal (within volts) to the high potential ofjacket 40 and the potential leads 31, 38, 39. The latter are carried to the source of highpotential and with thejacket 40 maintain the inner surface of insulation 4I and the entire core at a potential which prevents disturbances or excessive stresses on the insulation III or spaces within the insulation 4I. Charging currents to insulation 4I, carried from the source bywires 31, 38, 33 to thejacket 40 into the insulation 4|, are picked up by thesemi-conductive coating 42 and returned cr drained to the ground by thelbraid 43.

What is claimed is:

l. In an electric cable including insulation for a conductor of electric current, a semi-conducting layer in intimately-contacting continuous form at a surface of the insulation comprising a carrier selected from the group consisting of flexible insulating paints and lacquers, said carrier having conductive particles therein in amount to provide a surface resistivity with a lower limit of an order of ohms per square of area under normal operating conditions, and said layer serving to carry charging current to a conductive element having electrical contact with the layer at intervals suiciently close along the length of the cable for maintaining the electrical potential of any portion of said semi-conducting layer at a value precluding harmful electrical disturbances.

2. Inan electric cable including insulation for a conductor of electric current, a semi-conducting layer in intimately-contacting continuous form at a surface of the insulation comprising a carrier selected from the group consisting oi' Iflexible insulating paints and lacquers, said carrier having conductive particles therein in amount to provide a surface resistivity of the order of a few ohms to 100 megohms per square of areaunder normal operating conditions, and said layer serving to carry charging current to a conductive element having electrical contact with the layer at intervals sufciently close along the length of the cable for maintaining the electrical potential of any portion of said semi-conducting layer at a value precluding harmful Aelectrical disturbances.

3. In an electric cable including insulation for a conductor of electric current, a semi-conduct.-

arrasar lng layer in intimately-contacting continuous form at a surface of the insulation comprising a normally insulating lacquer having an elastomer as the nlm-forming constituent and conductive particles in the lacquer in amount to provide a. surface resistivity with a lower limit of an order of ohms per centimeter squared under normal operating conditions, and said layer serving to carry charging current to a conductive element having electrical contact with the layer at intervals sumciently close along the length of the cable or maintaining the electrical potential of any portion of said semi-conducting layer at a value precluding harmful electrical disturbances.

i. In an electric cable according to claim 3 in which the elastomer comprises a chloroprene polymer.

5. In an electric cable according to claim 3 in which the elastomer comprises a plasticized vinyl chloride type polymer.

6. In an electric cable according to claim 3 in which the elastomer comprises an element-convertible alkyd resin.

7. In an electric cable according to claim 3 in which the particles are conductive car-bon.

8. In an electric cable including insulation for a conductor of electric current, a semi-conducting layer in intimately-contacting continuous form at a surface of the insulation comprising a c ing layer at a value precluding harmful electrical disturbances.

9. I n an electric cable including insulation for a conductor of electric current. a semi-conducting layer in intimately-contacting continuous form at a surface of the insulation comprising a flexible base material having associated therewith a carrier selected from the group consisting of insulating paints and lacquers, said carrier having conductive particles therein in amount to provide a surface resistivity with a lower limit oi an order of ohms per square of area under normal operating conditions, and said layers serving to carry charging current to a conductive element having electrical contact with the layer at intervals sufilciently close along the length of the cable for maintaining the electrical potential of any portion of said semi-conducting layer at a value precluding harmful electrical disturbances.

10. Cable suitable for X-ray voltages comprising in combination a core including an insulated conductor, a jacket comprising a carrier of insulating character selected from the group consisting of paints and lacquers and conductive particles in the carrier to impart semi-conductivity, and a non-insulated wire enclosed within and contacting the jacket along its length; insulation enclosing the core and intimately contacting the jacket; a semi-conductive film on the outer surface of the insulation; a conductor in contact with the iilm along the length of the cable; and an enclosing sheath.

THOMAS F. PETERSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 90,089 Foucant May 18, 1869 1,199,789 Hochstadter Oct. 3, 1916 1,235,373 Peek July 3l., 1917 1,290,673 Sonneborn Jan. 7, 1919 1,348,248 Steinberger Aug. 3, 1920 1,585,125 Simons May 18, 1926 2,142,625 Zoethout Jan. 3, 1939 2,165,738 Van Hoffen July 11, 1939 2,211,584 Ruben Aug. 13, 1940 2,282,832 Spooner May 12, 1942 2,322,702 Peterson June 22, 1943 FOREIGN PATENTS Number Country Date 756 Great Britain 1880 19,882 Great Britain 1914 404,519 Great Britain Jan. 18, 1934 450,805 Great Britain July 24, 1936 526,895 Great Britain Sept. 27, 1940 OTHER REFERENCES Rubber Chemistry 8x Technology. vol. 15, 1942, pages 146 to 157; article therein entitled Electrically Conducting Neoprene and Rubber," by B. J. Habgood et al.

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