US2454625A - Insulated electrical conductor and method of fabricating the same - Google Patents

Description

Nov. 23, 1948. L. A. BoNDoN INSULATED ELECTRICAL CONDUCTOR AND KETHOD 0F FABRICATING THE SAME Filed April 9. 1947 INVENTOR. Lewis A. Bondon WM LM ATTORNEYS Patented Nov. 23, 1948 INSULATED ELECTRICAL CONDUCTOR AND METHOD OF FABRICATING THE SAME Lewis A. Bondon, Arlington, N. J.

Application April 9, 1947, Serial No. 740,310

1 claims. l

.This invention relates to insulated electrical conductors and cables and a method of fabricating them and has more particular reference to insulated conductorsand cables wherein the insulating covering is formed of laminated layers of insulating material.

Conventional forms of insulating covering for electrical conductors and cables usually comprise a solid, homogeneous layer of insulating material such as synthetic resin or rubber, or physically reinforced dielectric tapes of various types of insulating material such as asbestos and varnished cambric, which also may be saturated with various dielectric impregnants. The latter form of insulating coverings are usually fabricated by helically or spirally winding several layers of varnished cambric tape about the conductor, after which an outer textile cover is superimposed on the cambric by a braiding or similar operation. In certain types of taped coverings, a layer of felted asbestos is applied directly to the conductor before being wrapped with the varnished cambric tape; in other types, additional layers of asbestos are used before the braided covering is applied.

Another form of taped insulating covering cornprises a layer of felted asbestos applied directly to the conductor; a layer of rubber hydrochloride formed by helically winding a thin tape of rubber hydrochloride around the asbestos; a second layer of elted asbestos superimposed on the rubber hydrochloride; and an outer covering of asbestos braid.

All of the foregoing forms of insulating coverings are subject to one or more of the following objectionable features:

( a) Poor thermal stability.

(b) Poor dielectric performance.

(c) Inability to perform dielectrically in continuous high' temperature service without becoming brittle and fracturing upon slight impact or shock.

(e) Lack of flexibility.

(f) Progressive deterioration due to age.

(g) Inflammability of material.

(h) Limited in application to certain types of Wires and cables.

(i) Restricted to low voltage applications.

(7') Unable to perform in humid atmosphere without introducing large voltage losses.

One object of the present invention is to provide an insulated electrical conductor or cable having an insulating covering in which all of the above mentioned objectionable features are overcome.

Another object of the present invention is to provide an insulated electrical conductor or cable having an insulating covering which is physically strong and flexible, possesses high electrical and thermal stability values, and is impervious to moisture.

Another object of the present invention is to provide an insulated electrical conductor or cable having an insulating covering generally in the form of a thin unreinforced dielectric lm or tape of insulating material helically or spirally wound around the conductor or cable, with each convolution of the tape overlapping the preceding convolution a predetermined amount, and with a semi-liquid insulating material forming a dielectric seal between the overlapped surfaces of the tape and filling the open spaces formed at each overlapped edge of the tape, thereby preventing any air from being trapped in the covering.

Another object of the present invention is to provide an insulated electrical conductor or cable, as characterized above, wherein the tape is made of polytetrafluoroethylene and the semi-Huid insulating sealant is of the organo-silicone group.

Another object of the present invention is to provide an improved method of fabricating an insulated electrical conductor or cable, as characterized above.

Another object of the present invention is to provide an insulated electrical conductor or cable, as above characterized, wherein the laminar insulation is enclosed in a braided outer jacket and given a coating of waterproof, Iire resistant material,

A further object of the present invention is to provide a braided outer covering for an insulated conductor or cable which is formed of interwoven glass and cotton thread with the cotton thread having a larger diameter than the glass thread, thereby providing improved physical confinement and better abrasive resistance.

Other objects and advantages of the invention will become apparent from the speccation when considered with the accompanying drawings, wherein:

Fig. 1 illustrates a multiple strand cable embodying the invention;

Fig. 2 is a vertical cross-sectional View taken on the line 2--2 of Fig. 1; and

Fig. 3 is a partial longitudinal sectional view, on a slightly enlarged scale, taken on line 3-3 of Fig. 1, with the braid fibres being shown on an exaggerated scale.

In manufacturing insulated conductors or cables of the invention, the conductor element is first coated with a semi-liquid insulating material, preferably one of the organo-silicone group. Then a thin, fiexible, unrelnforced and initially unstressed film `or tape of insulating material, preferably poiytetrafluoroethylene, coated or wetted with the semi-liquid insulating material, is spirally or helically wrapped, under high tensile stress, around the conductor element, with each convolution of the tape overlapping the preceding convolution by a predetermined amount, to form an insulating layer of a predetermined thickness.

The tape is wrapped around the conductor element in a suflicient number of layers to provide the necessary dielectric strength for the perfoi-mance required. As the tape is tightly wrapped around the conductor element, the semiliquid coating thereon is squeezed out to iill the open spaces formed at each overlapped edge of the tape. This prevents the entrapment of any air between the overlapped layers and provides a thin dielectric film of the semi-liquid or grease between the overlapped surfaces of the tape, which acts as a lubricant to facilitate the movement of the overlapped surfaces when the insulated conductor or cable is bent or flexed. After the conductor element has been Wrapped to produce the required thickness of insulation, a reinforcing, physically connng jacket or outer covering is formed thereon, preferably by braiding. The jacket may be made of woven cotton or glass fibres; preferably, however, the jacket is made of an interwoven combination of glass and cotton fibres. After the braid has been applied it is impregnated with a suitable flame-proof saturant, preferably silicone varnish, or encased in a silicone rubber jacket and subsequently exposed to a curing and stress relieving heat exposure. The particular heat exposure necessary to stress relieve the fabricated dielectric core must be varied because of the temperature limitations of the reinforcing braid.

Glass braided cores can be relieved in 1.5 to 2.0 hours at 250 C. Cotton-glass combination braided cores can be relieved in 6 to 8 hours at 150 C.

If the braid is to be finished with well known lacquer formulations, it is necessary that such finishes be applied after the heat treating process, due to the low heat deformation temperature of such synthetic resin finishes.

Polytetrafiuoroethylene possesses an inherent ory. This characteristic will cause initially mechanically unstressed polytetrauoroethylene forms to return or tend to return to their original form after having been mechanically stressed in process and then subsequently subjected to a stress relieving heat exposure. Full advantage is taken of this physical characteristic in the fabrication of the dielectric core. The initially unstressed polytetrauoroethylene tape is maintained under sufcient tension during the wrapping process to cause it to be extended under such stress. In addition, the tape, while under such tensile stress will be subjected to varying degrees of elongation across the tape Width, due to the various diameters that it lays upon and that are produced by any predetermined amount of overlapping.

The resultant fabricated dielectric material will thus be under various degrees of mechanical stress within a single tape Width and will, upon a subsequent subjection to a stress relieving heat exposure, tend to assume the original unstressed form and in so doing will cause the entire crosssection to shrink radially, thus binding the layers together as a solid homogeneous mass. In this process, the organo-silicone semi-fluid acts as a lubricant and prevents the seizing of the lapped surfaces, thus permitting uniformly distributed radial confining pressure throughout the entire dielectric cross-section.

Referring now to the drawing, there is shown, in Fig. i, an insulated electrical cable embodying the invention and comprising a conductor element i0, consisting of a plurality of metallic conductors il an insulating covering I2 formed on the conductor element; and an outer braided reinforcing cover member I3, encasing the conductor element and the insulating covering.

The insulating covering I2 is formed on the conductor element I I by spirally or helically wrapping, under high tensile stress, a thin, ilexible, unreinforced and initially unstressed film or tape lli of insulating material around the conductor element, with each convolution of the tape overlapping the preceding convolution a predetermined amount. The tape I4 may be of any desired thickness or width, depending upon the requirements of the particular cable being constructed. In the particular embodiment shown, the tape is made of polytetrafiuoroethylene and is .005" thick and l in width. It possesses high tensile strength and is resilient and flexible. The tape is spirally wound upon the conductor element so that each convolution overlaps the preceding convolution by three-fourths of its width. When wrapped in this manner, an insulating layer of 300% overlap is formed on the conductor element; in other words, at all points along the conductor element, the insulating layer is formed of four tapes or an equivalent solid dielectric thickness of three times the tape thickness. As many layers of insulating tape as desired may be applied. As shown in Fig. 1, the tape is wound around the conductor to form two layers indicated generally at I5 and I5, thereby providing a wall insulation having a thickness of .040", or .020 wall thickness per layer.

In order toseal any pores which may exist in the polytetrafluoroethylene tape and to exclude all air from within the insulated cable, the outer surface of the conductor is filled with, and the conductor and the tape are coated with a semiliquid insulating material Il, preferably one of the organo-silicone group, prior to the wrapping of the tape around the conductor. This results in the formation of a film of the semi-liquid organo-silicone material between the surfaces of the conductor and the tape and between the overlapping surfaces of the tape. In addition, as the tape is tightly wrapped around the conductors, the semi-liquid material is distributed throughout the overlapping surfaces and fills the openings formed at each overlapped edge, thereby preventing any air from being entrapped within the insulating cover. The semi-liquid or grease performs an additional-function in acting as a lubricant to facilitate the sliding movement and minimize mechanical stressing of the dielectric cross-section when the cable is bent or flexed.

After the conductor element has been wrapped with the number of layers necessary to provide the required dielectric performance which, in the modification shown in Fig. 1, is two layers, a reinforcing, physical conning jacket or outer covering i2 is formed thereon, preferably by braiding. The jacket I2 may be made of Woven cotton or glass fibres; preferably, however, and as shown in Fig. 1, the jacket is made of an interwoven combination of cotton fibres I8 and glass fibres `nated with a suitable flame-proof saturant, prefg erably an organo-silicone varnish. Then the cable is subjected to a heat of 150 C. for a period of from 6 to 8 hours for curing purposes vand toy 'stress relieve the dielectric core for the purpose and in the manner hereinbefore described. Organo-silicone varnish on cotton-glass fibre interwoven surface can be rated conservatively at 150 C., while the same varnishon the glass fibre will permit operation up to250 C.

While a braid made lsolely of cotton fibres or of glass iibres'may be-used, as above stated, a cotton-glass fibre interwoven braid is preferable for the following reasons:

1. The glass fibre has high tensile strength and does not yield or stretch, thus maintaining uniform radialconflnement of the dielectric core, which requires such physical support.

2. 'Ihe proper size cotton fibre, when interwoven with the glass fibre, forms a larger diameter and thus provides abrasion or scu protection to the interwoven glass libres. `The glass fibres alone are unsatisfactory physically, because they will on continuous flexing, fracture at the points of crossing in the braid, where they are subjected to their own sawing action. 3. Upon exposure to flame or intense heat, cotton fibres will disintegrate and, if it were not for the remaining confining glass fibres, the dielechaving been mechanically stressed in process and then subsequently subjected to a stress relieving heat exposure.

v 3. The organo-silicone iluid or grease has excellent wetting properties and will uniformlxn wet the surfaces ofthe polytetrafluoroethylene tapes.

C. Electrically 1. Polytetrafiuoroethylene dielectrically is a superior insulating material and has .electrical properties which enable it to provide improved given conductor size produces a smaller diameter with improved dielectric strength. This feature is advantageous for all wires and cables and especially for wires and cables used in aircraft applitric core would be unsupported and fail electrically. With the combination cotton and glass fibre braid, the wire is able to supply service after flame exposures which would render conventional wire useless.

Cables and conductors provided with an insulating core employing polytetrafiuoroethylene and an organo-silicone grease or semi-liquid as dielectrics and fabricated as above described, will have superior thermal, physical, electrical and chemical qualities and will be free from all of the objectionable features as above pointed out which are inherent ina more or less degree in all conventional types of cables and conductors. This is due to the particular construction of the insulating core and the particular characteristics of the dielectrics employed, which are as follows:

A. Thermally l. Polytetraluoroethylene as a solid is unaffected by heat from -70 C. to 300 C.

2. Organo-silicone compound is heat stable from 70 C. to 250 C. 3. Neither of these materials will carbonize upon flame exposure.

B. Physically i. Polytetrafluoroethylene in conventional solid extruded form on a wire is a very rigid member cation where light weight is essential.

(b) Power factor or loss angle is extremely low and is stable over a wide frequency range, thereby permitting the use of polytetrafluoroethylene in all types of wires and cables for instruments, heater cords, low-tension, high tension, ignition` transformer leads, coaxial, heater elements, high tension aircraft ignition terminal disconnect seals for very high and low temperature application, etc.

(c) Dielectric constant-Has extremely lowv specific inductive capacity value and can be used where other types of insulation would be objectionable, due to their higher capacities.

(d) Insulation resistance-Extremely high.

2. Organo-silicone compound:

(a) Has dielectric properties which improve the performance of polytetrailuoroethylene in this laminated construction by excluding air occlusion from the conductor surface and in-between the lapped surfaces.

(b) Wets polytetrafluoroethylene surfaces and reduces the danger of porosity effect of the polytetrafluoroethylene which, alone, would cause the cable to fail, due to premature local ionization and flashover. This porosity accompanies all polytetrafiuoroethylene tapes and solid extruded forms to a certain degree and will result in unstable dielectric performance. Therefore, with this dielectric combination there is improved dielectric stability and a resultant increase in corona initiation voltage level.

(c) High insulation resistance which does not vary to any great degree with prolonged heat or humidity exposure.

D. Chemically l. Polytetrailuoroethylene and organo-silicone compounds are unaffected by chemical or metallic reactions and arenot affected by corona which may form about the cable in operation. Both dielectrics in combination, therefore, act together to form a barrier tc prevent moisture or conductive gases from entering into the dielectric core. which normally would cause reduced insulation resistance and also subsequent ilashover along the moist surfaces. l

2. This dielectric combination will not carbon ize as do conventional organic insulations which renders a cable inoperative due to the resultant carbon track formed.

From the foregoing, it will be seen that there has been provided an improved insulated construction for wires and cables which has the following advantages over the present conventional type insulated wires and cables, in addition to those hereinbefore mentioned:

A. Higher current rating The dielectric combination of the present invention is rated'from 70 C. to 25 C. Therefore, for a given conductor size, increased ratings beyond the present conventional vtypes are apparent. l

B. Flame proof Both poiytetrailuorcethylenc and Iorgano-silicone compounds are flame prooi.

C. Reduced diameters for given size conductors The high dielectric strength of the dielectric combination, which is unailected by prolonged aging, will permit reduced diameters for given size conductors.

D. High insulation resistance Because of the nature and construction of this dielectric combination, insulation resistance values are well above those of conventional dielectrics in wire applications and will remain intact over long periods of high humidity exposures as well as continuous submersion inwater.

The dielectrics in combination are both nonhygroscopic and will not take up moisture within its own mass. However, if used alone, the polytetrailuoroethylene tapes with conventional liabrication with the short overlapped surface will pick up moisture after afshort application to ilexing and bending stresses which would extend and compress the tape at the stressed areas and develop laminar porosity. While the degree of this porosity is minute, it is detrimental to the insulation resistance as well as other electrical properties. However, when insulating walls are applied in combination as disclosed, it eliminates this possibility as the organo-silicone lubricant wets and seals the tightly laminated surfaces.

E. High dielectric strength When compared with currently known tapes and their lubricating agents (usually e, petroleum derivative) this combination exceeds their performance dlelectrically, thermally and physically, and it is only necessary to apply this Vdisclosed combination in thin wall insldation to achieve superior performance.

Because of the high tensile strength of polytetraluoroethylene tape, as Well as its ability to yield to the laminated profile in process, it is pos.. sible to overlap itself many times to provide a laminar dielectric wall of a number of times the tape thickness in one taping operation, which is not possible with conventional physically reinforced dielectric tapes. Flexibility is provided by the organo-silicone compound which wipes each. lap and excludes minute air particles to provide a solid mass in the finished combination dielectric cross-section.

The iinished cross-section is electrically proper from the standpoint of dielectric placement which requires thepositioning of high specic inductive capacity materials to be adj acent to the conductor surfaces to avoid distress to the other dielectrics in the cross-section in order that they may be used most eiectively. The semi-huid organosilicone compound has a specific inductive capacity value of 2.4i, while polytetrafluoroethylene has a specific inductive capacity value of 2.0,

F. Higherv corona forming voltages Due to the exclusion of air from the cross-sec- 'tion of this construction mainly along the conprovided than that which exists inthe solid in sulated polytetrauoroethylene wires. This is wires cannot be processed to guarantee uniform dielectric strength. They cannot be extruded in small diameter or to provide thin wall insulation and they have non-uniform wall thickness. The improved construction of the present invention provides the solution to all the above diilculties without sacrificing any of the good properties of the polytetrafluoroethylene but rather improving its performance physically and electrically. The resultant flexibility reduces fatiguing efects which tend to reorient the originally stress-relieved dielectric molecular pattern.

All the materials proposed in this construction, as well as the finished reinforcing braid are not sensitive to the action of corona and will, therelore, perform eiliclently under prolonged corona exposure.

G. High heat stability Because of the extreme temperature limits of all the materials used in this construction, it is possible to provide a wire which will perform beyond temperatures presently permitted by the National Board of Fire Underwriters.

The highest current rating presently assigned to approved asbestos types is specified for dry location only and is considerably below that possible ln the proposed construction.

Conventional varnish cambric types are limited in temperature performance and will, after prolonged exposure .at allowable temperature, become brittle and fracture on slight impact or shock. Such cables, when exposed to various sealing compounds in which they are immersed, will rea-ct chemically and often fail dielectrically after curing process.

Conventional synthetic resin or rubber insulated types are -aiected by soldering operationA Imperviousness to humidity and liquid Tests have shown that this dielectric combination, which is totally inert chemically and nonhygroscopic, is not affected dielectrically by prolonged immersion in liquid or exposure to highly humid or reactive atmospheres.

With a lacquered iinished interwoven cottonglass libre jacket, which is sensitive to mild alkalis and acid solutions, the dielectric performance of the core combination remains intact even though the surface finish and the cotton nbre may disintegrate.

The insulating covering of the present invention may be used for all types of wire service and, by applying the number of proper gauge tapes in suiicient layers, with proper degree of overlap, will satisfy any insulating wall dimension that may be specied and will provide accurate .diametric concentricity.

While in the particular modiilcation of the invention illustrated, the conductor element has been shown as a seven strand cable, obviously, the conductor element may be of any type such as single conductor, multiple strand conductor or concentric conductors.

Having thus described the invention, what is claimed is:

1. An insulated electrical conductor comprising a metallicconductor element insulated by a surrounding layer of laminated polytetrafiuoroethylene having a film of dielectric lubricant interposed between the contacting surfaces of the 1aminated polytetrafluoroethylene.

2. An insulated electrical conductor comprising a metallic conductor element insulated by an insulating layer of high dielectric strength and composed of a thin tape of polytetrauoroethylene wound helically about the conductor element, with each convolution of the tape overlapping the preceding convolution by a. predetermined amount, and with a thin illxn of d-ielectric lubricant coating the surfaces of the conductor element and the tape.

3. An insulated electrical conductor comprising a metallic conductor element insulated by a surrounding layer of laminated polytetrailuoroethylene having a th'n film of organo-silicone compound interposed between the contacting surfaces of the laminated polytetrauoroethylene.

4. An insulated electrical conductor as set forth in claim 1, including a protecting confining covering fabricated from cotton and glass nbres with the cotton bres being of a larger diameter than the glass fibres to provide an outer circumferential surface of cotton fibres.

5. An insulated electrical conductor comprising a metallic conductor element insulated by an insulating layer of high dielectric strength and composed oi' a thin tape of polytetrailuoroethylene wound hellcally about the conductor element, with each convolution of the tape overlapping the preceding convolution by a predetermined amount, and with a thin nlm of an organo-silicone compound coating the surface or the conductor element and the tape.

6. An insulated electrical conductor as set forth in claim 5, including a protecting conning covering of braided cotton and glass' fibres impregnated with an organo-silicone compound.

7. A process of fabricating insulated electrical conductors or the like, characterized by helically wrapping. a thin, flexible, unreinforced and initially unstressed tape in the form of a 111m of polytetrafiuoroethylene about a. conductor element under suiiicient tension to cause each convolution of the tape to tightly conform to the surface'about which it is wrapped and with each con# volution of the tape overlapping Ithe preceding convolution by at least three-fourths of its width to form a laminated ldielectric core; coating the outerv surface of the conductor element and said tape? with an organo-silicone semi-liquid prior to wrapping said tape thereon to prevent any air from being entrapped within the dielectric core and -to lubricate the overlapping convolutions; braiding a reinforcing, physically confining outer jacket on said core; and subjecting the complete assembly -to suicient heat to stress relieve the laminated core.

LEWIS A. BONDON.

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

v UNITED STATES PATENTS Number Name Date 232,122 Hammesfahr Sept. 14, 1880 1,749,740 Frederickson Mar. 4, 1930 2,004,004 Knoderer June 4, 1935 2,164,904 Cook July 4, 1939 2,258,218 Rochow Oct. 7, 1941 2,335,088 Shoemaker Nov. 23, 1943 2,392,388 Joyce Jan. 8, 1946

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