Nonflammable Electrical Cable

Abstract

Nonflammable electrical cable resistant to combustion under current overload conditions. The cable conductor is constituted by one or more composite metal strands. Each strand has an aluminum base core clad with copper and has an outer layer of silver, nickel or tin. The conductor is wrapped with flexible fire-resistant insulating material and the facing areas of the wrapping are sealed with an adhesive which is kept out of contact with the conductor. When subjected to a current overload in an oxygen atmosphere the strand fuses, thereby interrupting the current, before either the insulating material or the adhesive can ignite.

Description

BACKGROUND OF THE INVENTION

In the design of high performance aircraft and space vehicles, one of the more difficult problems is the provision of electrical systems which present minimum hazard when the vehicle is in operation. Electrical systems for both power and signal purposes are essential to each of the various vital functions of the vehicle, including propulsion, directional control and guidance. As a consequence, electrical energy must be transmitted from the electrical power sources to a multitude of points located throughout the vehicle.

To insure the safety of the vehicle during operation, the electrical transmission equipment must be secure against short-circuiting or other misdirection of electrical energy and against fire hazards under both normal and current overload conditions. Thus the electrical conductor of the transmission equipment must be insulated in a manner which prevents current leakage to the surroundings and exposure of the surroundings to excess temperatures, even when the conductor is seriously overloaded. Moreover, the insulating material itself must be resistant to combustion even when exposed to the temperatures to which the conductor rises when overloaded.

Electrical cable which has been available heretofore has not proved to be flame resistant under all the conditions to which it is exposed in modern aircraft or spacecraft. The atmosphere in a space vehicle normally consists of 95 percent or more by volume oxygen. The combustibility of almost all materials is much greater in such an atmosphere than it is in air, which contains only about 20 percent oxygen. Thus, even the most flame-resistant cable insulation materials known will burn when exposed to such an atmosphere at the temperatures to which conventional electrical cable rises during an overload.

Among the best flexible cable insulation materials presently available are fluorinated ethylene propylene resin (sold under the trade designation "FEP Teflon" by E. I. DuPont de Nemours and Company) and a polyimide resin (sold under the trade designation "Kapton" by E. I DuPont de Nemours and Company). A high performance cable which has been commercially available and which has been used in the electrical systems of spacecraft consists of a copper conductor surrounded by an insulating sheath of "FEP Teflon," the latter in turn being surrounded by a dip coating of polyimide film. Though quite satisfactory under normal service conditions, this cable has not proved to be flame resistant when exposed to an oxygen-rich atmosphere under electrical current overload conditions. The inability of this cable to withstand such conditions has been demonstrated by a standard test developed by the National Aeronautics and Space Administration at the George C. Marshall Spaceflight Center (Specification 101A, Jan. 12, 1970). In this test, a sample of insulated wire or cable is placed in a chamber whose atmosphere contains 95 percent by volume oxygen at the operating pressure of a spacecraft, i.e., about 6.5 psia. After the cable sample has been allowed to "soak" in the oxygen atmosphere for a period of 10 minutes, a current is applied to the sample by means of an external d.c. electrical power supply. The initial test current is 5-20 amps. below the nominal fusion current of the cable conductor, depending on the size of the wire tested. If ignition is not obtained within one minute of the application of current, the current is raised in 5-amp. steps at one minute intervals until the conductor fails or ignition occurs. When subjected to this test, the aforementioned cable has burned, as evidenced by the emission of fumes and smoke, before the fusion temperature of the conductor has been reached, despite the presence of the external sheathing of polyimide material.

A substantial amount of research has been conducted by numerous workers in the art in an effort to provide a nonflammable electrical cable for use in aircraft and spacecraft which can survive the NASA tests. Prior to the present invention, however, it is believed that none of these efforts have been successful. Numerous insulating materials and combinations thereof have been tried with uniformly unsatisfactory results. The various insulating materials which have been unsuccessfully tested on copper conductors include all types of fluorocarbon tapes and extrusions, including fluorinated ethylene propylene resin (sold under the trade designation "FEP Teflon" by E. I. DuPont de Nemours and Company), tetrafluoroethylene resin (sold under the trade designation "TFE Teflon" by E. I. DuPont de Nemours and Company) and poly vinylidene fluoride (sold under the trade designation "Kynar" by the Pennwalt Corporation); tapes and braids of glass wool fibers and "Kapton"; and various combinations of these insulators including combinations of "Kapton" and glass fibers.

To insure against the possibility of fire generated by a current overload on any of the cables which have been commercially available heretofore, elaborate precautions are necessary if the cable is used in an oxygen atmosphere. One accepted approach is to encase the cable in ablative material and house the resulting structure in aluminum channels partitioned at frequent intervals to isolate various sections of the cable from one another. These two measures serve to isolate a portion of cable whose insulation has started burning and to impede access of oxygen to a site of combustion. Though reasonably effective, these measures are not only expensive in themselves but, more seriously, occupy space and add a substantial amount of weight to a spacecraft or aircraft, thus severely reducing its payload. A critical need has existed in the art, therefore, for an electrical cable which is nonflammable under overload conditions in an oxygen atmosphere and which, consequently, does not require the use of elaborate space-wasting and weight-wasting measures to avoid the serious hazards which flammable-type cables otherwise present.

SUMMARY OF THE INVENTION

It is an object of the present invention, therefore, to provide a high performance electrical cable which is nonflammable under overload conditions even in an oxygen environment. It is a particular object of this invention to provide such a cable which can be subjected to the sever NASA test conditions without burning, fuming or smoking. A further object of the invention is to provide such a cable which possesses advantageous mechanical and electrical properties. Other objects will be in part apparent and in part pointed out hereinafter.

In substance, the present invention is directed to nonflammable electrical cable, resistant to combustion under current overload conditions in an oxygen atmosphere, comprising a composite metal strand and an outer wrapping constituted by a flexible fire-resistant insulating material. The composite strand comprises an aluminum base conductor core, an annular cladding of copper metallurgically bonded to the surface of the aluminum base core, and an annular coating of a metal selected from the group consisting of silver, nickel and tin overlying the outside surface of the copper cladding. The wrapping of film insulation has facing areas with adhesive therebetween for sealing purposes, but the adhesive is entirely out of contact with the composite strand. The cable is resistant to combustion when subjected to a current overload in an oxygen atmosphere with the composite strand being fused and the current being interrupted before ignition of the insulating material or the adhesive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged transverse cross-sectional view of the cable of the invention;

FIG. 2 is a plan view illustrating a preferred form of the film insulation wrapping; and

FIG. 3 is a plan view showing another alternative form of the wrapping.

Corresponding reference characters indicate corresponding parts throughout the several views of the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Unlike the electrical cables which have hitherto been employed in the electrical systems of high performance aircraft and spacecraft, the cable of the present invention is resistant to combustion even under current overload conditions in an oxygen atmosphere. More particularly, the cable of this invention does not support combustion when tested to failure in accordance with NASA's George C. Marshall Spaceflight Center test (Specification 101A, dated Jan. 12, 1970). Thus, the cable may be used in oxygen atmospheres without the elaborate and expensive precautions and resultant reduction in payload which are required when prior art cables are used. In its preferred embodiment, the cable of the invention possesses advantageous mechanical properties which satisfy the requirements for utilization of the cable in high performance aircraft and spacecraft. These properties are preserved during sealing of the insulation by careful control of temperature conditions in the sealing oven. The cable also possesses outstanding electrical properties and may be utilized in essentially any power or signal service without short circuiting, current leakage or excessive power consumption. Among particular applications in which this cable has been found highly useful is shielding against radio frequency interference and electromagnetic interference.

The novel and unique combination of four essential design principles provides the flame resistance of the cable of this invention in an oxygen atmosphere. First, an insulating material such as polyimide or an amide-modified polyimide is used which is resistant to combustion at very high temperatures. Second, the aluminum core conductor, which is used instead of conventional copper fuses at a temperature on the order of 660.degree. C., far below the 1,083.degree. C. fusion temperature of copper. The resultant interruption of current, which takes place very quickly once the fusion current is reached or exceeded, prevents the insulation from reaching a temperature at which it can ignite. The thermal insulating properties of the insulation contribute to rapid fuse action by impeding dissipation of the heat generated on overload. Third, the adhesive material, which normally has an ignition point well below that of the insulation, is kept out of contact with the conductor strand, being separated therefrom by at least one layer of the insulation. In conjection with the quick fuse action of the core, this prevents combustion from arising with the adhesive. By contrast, conventional cables heretofore available have commonly included a sheathing of polytetrafluoroethylene, or similar materials having softening points and ignition temperatures in the range of the adhesives, in direct contact with the conductor. Fourth, the use of a wrapping of insulation instead of an extruded sheathing provides a more uniform thickness of insulation, free from holidays or other defects which occasionally allow moisture leakage or current leadage in conventional cable. Wrapped insulation is also generally more flexible than extruded sheathing and is thus more resistant to cracking under conditions of flexure.

Referring to FIG. 1 of the drawings, the novel cable of this invention has an aluminum base conductor core 1 which is clad with an annular layer of copper 3. The copper cladding is in turn coated with an annular layer 5 of either silver, nickel or tin. The copper cladding provides desirable termination properties not otherwise possessed by aluminum conductors and the outer coating of silver, nickel or tin further enhances the termination properties and provides corrosion resistance. As FIG. 1 shows, the cable preferably includes a plurality of composite metal strands bundled together inside the insulation. The insulation is constituted by a wrapping 7 of an insulation material such as polyimide or amide-modified polyimide tape, with an adhesive 9 lying between facing areas of the insulation wrapping for purposes of sealing the insulation.

Different arrangements of the insulation are shown in FIGS. 2 and 3. FIG. 2 shows a preferred embodiment of the invention in which two or more individual layers of insulation tape are helically wrapped around the conductor with a layer of adhesive lying between the two layers of insulation. Such an arrangement is also indicated in FIG. 1. Alternatively, of course, a single layer of tape may be helically wrapped around the conductor, with the trailing edge of each wrap lapping the leading edge of the preceding wrap and a layer of adhesive material lying between the facing areas thus presented to form a helical seam. FIG. 3 shows another embodiment in which the longitudinal center line of the tape is oriented parallel to the longitudinal center line of the conductor with one edge of the tape lapping the opposite edge in a single longitudinal seam which is also parallel to the center line of the conductor. The seam of FIG. 3 is formed by a layer of adhesive lying between facing areas of the tape. In each embodiment, the adhesive material is kept out of contact with the conductor, being separated therefrom by at least one layer of the film insulation tape.

The composition of the aluminum base core is not critical insofar as the nonflammability characteristics of the cable are concerned. Thus, essentially any aluminum base alloy whose melting point is on the order of that of aluminum will provide the fuse action which protects against combustion of insulation materials such as polyimide or amide-modified polyimide. However, because of the mechanical and electrical properties which are desirable in an electrical cable adapted for use in high performance aircraft or spacecraft, it is preferable that the aluminum base core be constituted by an alloy containing between about 0.07 percent and about 0.65 percent by weight iron, up to about 0.12 percent by weight silicon, up to about 0.03 percent by weight magnesium, between about 0.01 percent and about 0.03 percent by weight manganese, between about 0.02 percent and 0.04 percent by weight copper, and between about 0.006 percent and about 0.011 percent by weight boron with no more than 0.001 percent by weight each of titanium, vanadium, nickel or chromium. The presence of the indicated proportions of iron is especially important in increasing the tensile strength of the aluminum alloy. Among the aluminum alloys whose compositions fall within the above indicated ranges may be mentioned the alloys sold under the trade designations "EC Aluminum No. 1," "EC Aluminum No. 2," "CK-76" and "EC Aluminum No. 3" by the Aluminum Company of America, and the alloy sold under the trade designation "Triple E" by the Southwire Company. The compositions of these alloys and their associated physical properties are shown in Table I. "CK-76" and "Triple E" are particularly preferred alloys for the conductor core.

TABLE

I Chemical Composition - Percent Physical Properties Ti, elon- V, ga- Conduct- Ni, Uts ti- ivity % Fe Si Mg Mn Cu B Cr KSI on I.A.C.S. __________________________________________________________________________ "EC .09 .054 .002 .001 .003 .007 less 14.2 12% 62.6 Al- than no. 1" "EC .10 .052 .002 .001 .003 .006 .00170 15.7 9% 62.2 Al- no. 2" "EC .14 .053 .002 .001 .002 .011 each 10.0 37% 63.6 Al no. 3" "Trip- .60 .050 .002 .002.004 .006 17.0 18.002 .002 60.5 le E" "CK- .10 .12 .003 .003 .003 .003 17.0 16% 61.0 76" __________________________________________________________________________

the copper cladding constitutes between about 12 percent and about 20 percent by volume of the composite metal conductor strand and is metallurgically bonded to the aluminum base core. A number of conventional methods may be employed to provide a metallurgical bond of cladding to the aluminum conductor core. Among such methods may be noted hot-dipping, flame-spraying, electroplating, or solid-phase bonding (as described in U.S. Pat. Nos. 2,691,815 and 2,753,623).

Copper-clad aluminum alloy rod stock, from which the conductor may be produced by conventional wire-drawing techniques, is commercially available. Such stock having a diameter of approximately five-sixteenths inch, for example, is available from Texas Instruments Incorporated. By a series of conventional drawing and annealing steps the rod stock may be drawn to any convenient gauge. Preferably the stock is drawn to a diameter corresponding to between 8 and 40 A.W.G. Strands of this size may be conveniently woven into a cable bundle having a relatively high degree of flexibility. Where there are no narrow radius bends in the cable as installed or where no significant flexure in use is anticipated, larger diameter strands may be used. In the latter case a bundle of strands may be unnecessary, and the conductor may be constituted by a single composite metal strand.

As indicated above, size reduction of copper-clad rod stock may be accomplished by conventional techniques to produce a finished strand having the desired mechanical and electrical properties. For use in high performance aircraft and spacecraft, the finished strand should have a tensile strength of at least about 9,000 psi, an elongation of not less than about 8 percent and a conductivity of not less than about 60 percent I.A.C.S. (conductivity relative to conductivity of copper conductor of same cross section). As indicated in Table I, the preferred aluminum base conductor core materials have properties which are substantially superior to these minimums. During size reduction of the copper-clad rod stock, the ratio of copper volume to total volume remains substantially constant. Thus the volume of copper produced on the finished conductor strand can be predetermined by cladding the rod stock with that proportionate volume of copper.

The annular layer of nickel, silver or tin is preferably applied to the copper-clad aluminum base stock prior to the drawing operation, though silver or tin may be applied after drawing if desired. A silver or tin layer may be provided by hot-dipping, while a layer of nickel must be applied by extrusion. Copper-clad aluminum rod having an annular outer layer of nickel is commercially available from Texas Instruments Incorporated and is sold by it under the trade designation "DFE3."

Regardless of which metal is used for it, the outer layer should have a thickness of at least about 40 microinches after drawing. As with the copper cladding, the annular cross-sectional area of the silver or nickel coating is reduced proportionately to the reduction of the core area during drawing. The thickness of the outer coating may then be similarly predetermined.

The wrapping of insulation is preferably constituted by a film of polyimide resin (sold under the trade designation "Kapton" by E. I. DuPont de Nemours and Company) or amide-modified polyimide resin (sold under the trade designation "AI" by Westinghouse Electric Corporation). These resins are described in U.S. Pat. Nos. 3,179,634 and 3,179,635, respectively. As will be apparent to anyone skilled in the art, however, other flexible fire-resistant insulating materials which will survive the failure of the conductor under overload conditions in an oxygen atmosphere, as demonstrated by the NASA test conditions, can also be utilized. Very few such materials are known in the present state of the art. None has been found which survives failure of a copper conductor. By use of a copper-clad aluminum conductor and by keeping the adhesive out of contact with the conductor, polyimide and amide-modified polyimide film insulations survive the fusion of the conductor on overload. Thus, any flexible insulation material which resists combustion at the temperatures to which it is exposed under such circumstances would serve equally well.

The thickness of the insulation wrapping should be at least about 0.5 mil. Desirably, the wrapping is constituted by two or more layers of 1-2 mils thick polyimide film having a backing of 0.1-0.5 mil "FEP." Thus the insulation as a whole has a total thickness of 3-10 mils, depending in part on the extent of lapping. Greater thicknesses can be utilized but are not normally necessary.

The use of an adhesive material is necessary to seal the insulation wrapping in order to impede electrical current leakage or the access of either moisture or oxygen to the conductor. The adhesive material should be flame resistant under normal conditions and should have a softening point of between about 300.degree. F. and about 600.degree. F. If the softening point of the adhesive is substantially less than 300.degree. F., it may melt prematurely on overload and seep past the film insulation into contact with the conductor, thus being exposed to high temperatures and raising the hazard of fire. If the adhesive has a softening point substantially greater than 600.degree. F., on the other hand, the mechanical and electrical properties of the conductor may be adversely affected by the excessive temperatures required in the process of sealing the insulation with the adhesive. Among the adhesive materials which possess the characteristics necessary for use in the cable of this invention may be noted the epoxy adhesive films sold under the trade designations "CMC 15" and "CMC 16" by the Circuit Materials Co., the polyester adhesive sold under the trade designation "46950 Polyester Adhesive" by E. I. DuPont de Nemours and Company, the polyamide-imide sold under the trade designation "TR 150-25" by Thermo-Resist, Inc., the silicones sold under the trade designations "SR-585" by the General Electric Company and "DC-280" by Dow Corning Corporation, the epoxy sold under the trade designation "D.E.N. 438" by the Dow Chemical Company, the nitrile rubber phenolic sold under the trade designation "Plastilock 605" by the B. F. Goodrich Company, and the fluorinated ethylene propylene resin sold under the trade designation "FEP Teflon" by E. I. DuPont de Nemours and Company. "FEP Teflon" is a preferred adhesive since its softening point is at the upper end of the 300.degree. - 600.degree. F. range. It thus has a high degree of stability during the subjection of the cable to a current overload, without creating insuperable problems in the process of applying the insulation to the conductor.

After the tape insulation has been wrapped around the conductor strands the adhesive lying between facing areas of the insulation is fused to seal said facing layers together. The adhesive is fused by raising it to a temperature at or above its softening point for a period sufficient for it to form a strong bond to each of the facing areas of the tape between which it lies. This operation is conveniently performed in an oven. Where an adhesive having a relatively high softening point such as "FEP Teflon" is employed, residence time in the oven is preferably held to a minimum to avoid adverse effects on the mechanical properties of the conductor strands. Exposure to sealing temperatures for excess periods of time can also result in seepage of the adhesive past the tape insulation and into contact with the conductor or can adversely affect the silver or nickel plating.

A method has been developed for applying the film insulation wrapping and sealing it with the adhesive which assures a high integrity seal while protecting the other essential properties of the cable. In this method, an insulation tape is used which has a backing of adhesive material. The tape is wrapped around the conductor with the backing facing outwardly. The wrapped cable is then moved continuously through an oven having a temperature profile which is a function of the nature of the adhesive. Thus, the inlet temperature of the furnace should not be higher than about 600.degree. F. while the outlet temperature should be between the softening point temperature of the adhesive and about 850.degree. F.

To avoid overheating of the wire with consequent loss of mechanical properties and also to avoid seepage of the adhesive or deterioration of the outer layer of the composite conductor, the cable is moved through the oven at a rate sufficient to raise the adhesive temperature up to its softening point but not substantially above it. Because the conductor strands not only act as a heat sink but through axial heat transfer cause the loss of heat from the furnace, the residence time required to bring the adhesive to the desired temperature varies radically with the cross-sectional area of the conductor strands contained in the cable. Thus for "FEP Teflon" adhesive, where the inlet oven temperature is 600.degree. .+-. 10.degree. F. and the exit oven temperature is 850.degree. .+-. 10.degree. F., it has been determined that the following residence times must be maintained within .+-. 5 percent to insure a high integrity seal without adversely affecting the properties of the cable.

TABLE II

Residence Cable Size Stranding Time (Min.) __________________________________________________________________________ 30 1 .times. 30 solid 0.33 26 19 .times. 38 ga. 0.40 24 19 .times. 36 ga. 0.45 22 19 .times. 34 ga. 0.50 20 19 .times. 32 ga. 0.55 16 19 .times. 29 ga. 0.71 12 19 .times. 27 ga. 0.91 128 .times. 38 1.8 162 .times. 40 2.0 __________________________________________________________________________

The following examples illustrate the invention.

EXAMPLE 1

A cable was prepared in accordance with the invention having 19 29-gauge woven composite metal strands. The copper cladding constituted 15 percent by volume of each strand and was coated with a 50 microinch thick layer of silver. Two individual layers of "Kapton" film insulation tape were helically wound around the woven strands. The inner layer was 2 mils thick and had a 0.5 mil thick layer of "FEP" on its outer side. The outer layer was 1 mil thick, and had a 0.1 mil thick layer of "FEP" on each side. The insulation was sealed by passing the cable through an oven with an inlet temperature of 600.degree. F. and an outlet temperature of 850.degree. F. with the residence time in the oven being about 43 seconds.

From the cable thus prepared, a number of lengths of cable samples were cut. From these cable samples, a test bundle was prepared consisting of seven lengths of cable, six of which were 12 inches in length, and one of which was 13 inches in length. The bundle was bound together in three places 4 inches apart using lacing tape. The 13 -inch length of cable was positioned on the exterior of the bundle and had a 1/2 -inch length of insulation stripped from each of its ends. The lengths of cable which formed the bundle were located parallel to each other and one end of the bundle was twisted 180.degree. relative to the other end.

The exposed ends of the 13 -inch length of cable were connected to two horizontally mounted electrical terminals in a test chamber having a volume of 98 l. The chamber included a center support for the cable sample spaced approximately half way between the two electrical connections. The chamber was also provided with a pressure gauge capable of measuring pressure to an accuracy of .+-. 0.1 psia, an oxygen supply and a window for observing and photographing test results.

The electrical terminals inside the chamber were externally connected to a d.c. electrical power supply capable of supplying a 250 amp. steady current.

The test chamber was evacuated to a total pressure of less than 5 torr. Oxygen gas containing less than 5 percent by volume of nitrogen and other inert gases was then introduced into the chamber until the pressure of the chamber reached 6.5 psia. The cable sample bundle was allowed to "soak" in the oxygen atmosphere for 10 minutes, and then a current of 100 amp., 20 amp. below the nominal fusion current, was applied to the 13 -inch cable by means of the external d.c. power source. Twelve seconds following the application of current, the cable failed. No evidence of combustion, either smoke, fumes or darkening, was evident either before or after cable failure. The sample bundle was removed from the test chamber and examined. By cutting away a portion of the insulation of the 13 -inch cable, it was determined that fusion of the composite metal conductor strands had taken place and that the conductor strands had consequently ruptured, causing an interruption in current.

EXAMPLES 2-12

Additional sample lengths were cut from cable prepared in accordance with the method described in Example 1. Eleven bundles of cable samples, each containing six 12 -inch lengths and one 13 -inch length, were prepared and tested according to the method described in Example 1. Each of these samples failed at 100 amp. within the time set forth in Table III.

TABLE

III Example Current Time to Failure __________________________________________________________________________ 2 100 amp. 13 seconds 3 100 amp. 13 seconds 4 100 amp. 14 seconds 5 100 amp. 15 seconds 6 100 amp. 21 seconds 7 100 amp. 17 seconds 8 100 amp. 3 seconds 9 100 amp. 9 seconds 10 100 amp. 10 seconds 11 100 amp. 12 seconds 12 100 amp. 7 seconds __________________________________________________________________________

No evidence of combustion was evident, either before or after cable failure. Examination of the cable showed that fusion and rupture of the composite metal conductor strands had taken place as in Example 1.

No other flexible electrical cable, either commercial or experimental, is known to have passed this test.

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As various changes could be made in the above products without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Claims

What is claimed is:

1. Nonflammable electrical cable, resistant to combustion under current overload conditions in an oxygen atmosphere, comprising a composite metal strand comprising an aluminum base conductor core, an annular cladding of copper metallurgically bonded to the surface of the aluminum base core, an annular coating of a metal selected from the group consisting of silver, nickel and tin overlying the outside surface of the copper cladding, and an outer wrapping of flexible fire-resistant insulating material on said composite strand, said wrapping having facing areas with adhesive therebetween for sealing purposes, said adhesive being entirely out of contact with said composite strand, said cable being resistant to combustion when subjected to a current overload in an oxygen atmosphere with said strand being fused and the current being interrupted before ignition of said insulating material or adhesive.

2. A cable as set forth in claim 1 wherein the insulating material is selected from the group consisting of polyimide and amide-modified polyimide film.

3. A cable as set forth in claim 1 wherein said adhesive has a softening point of between about 300.degree. F. and about 600.degree. F.

4. A cable as set forth in claim 1 having a plurality of said strands within said wrapping.

5. A cable as set forth in claim 4 wherein the aluminum base core is an alloy having an ultimate tensile strength of not less than about 9,000 psi, an elongation of not less than about 8 percent, and a conductivity of not less than about 60 percent I.A.C.S.

6. A cable as set forth in claim 5 wherein the aluminum base core is constituted by an alloy containing between about 0.07 percent and about 0.65 percent by weight iron, up to about 0.12 percent by weight silicon, up to about 0.03 percent by weight magnesium, between about 0.01 percent and about 0.03 percent by weight manganese, between about 0.02 percent and about 0.04 percent by weight copper, and between about 0.006 percent and about 0.011 percent by weight boron, the balance essentially aluminum with no more than 0.001 percent by weight each of titanium, vanadium nickel or chromium.

7. A cable as set forth in claim 6 wherein said adhesive material is fluorinated ethylene propylene resin.

8. A cable as set forth in claim 4 wherein said wrapping has a substantially uniform thickness of at least about 2 mils.

9. A cable as set forth in claim 4 wherein said wrapping includes at least two individual layers of helically wrapped polyimide tape.

10. A cable as set forth in claim 4 wherein the strands are woven.