US2019297A - Electric cable - Google Patents

Description

1935- I l. 'r. FAUCETT 2,019,297

' ELECTRIC CABLE Filed June 28, 1932 +1 INVENTOR ATTORN EYJT Patented 29, 1935 UNITED STATES PATENT ow-101a:

a General Cable corporation, New YorkJl. 1., a corporation of New Jersey This invention relates to insulated electric cables, and more particularly to shielded cables. It is an object of .the invention to provide improved cable constructions. Other objects and i advantages of the invention will appear hereinafter An illustrative embodiment of the invention selected merely for descriptive purposes is shown in the accomp nying drawing, in which:

Fig. l is a longitudinal elevation of a single conductor cable with portions of the sheath, shielding layers and insulation removed to. disclose the invention more clearly:

Pig. 2 is an end elevation of the cable shown in Fig. l;

Fig. 3 is a longitudinal elevation of a three conductor cable with portions of the sheath, binding tape, shielding layers and insulation removed to disclose the invention more clearly; and

Fig. 4 is an end elevation of the cable shown in Fig. 8. L

It has been proposed heretofore to provide one or both surfaces of the wall of insulation surrounding .a cable conductor with a thin electrostatic shield for purposes of distributing electrical stress within the insulation, and limiting the electrical stress to the wall of insulation, thereby preventing or reducing ionization at the surface of the insulation. l

The electrostatic shield ordinarily takes the form of a metal ribbon or metallized paper tape helically wound about the conductor before the insulation is applied, or over the conductor insul'ation, with the adjacent turns of theshielding ribbon or tape lying side by side. In practice, adjacent turns of the shielding strip are spaced apart slightLv. It is practically impossible to wind the shielding strip in place with adjacent edges exactly abutting, and with-a metal ribbon it is desirable to have a slight spacing between adjacent turns to permit bending of the cable with a minimum danger of buckling of the shielding strip and damage to the cable insulation.

The shielding layer desirably is very flexible so as to conform closely to the surface of the insulation at all times. Otherwise spaces would be formed betweenthe shielding layer and the cable insulation when the cable is handled, and such spaces would be under electrical stress during operation of the cable. Ionization in these spaces would injure the cable insulation and eventually might result in breakdown of the cable.

In the known constructions the shielding is not perfect because small ionizable spaces are present between adjacent turns of the shielding strip June 38, 1m, Serial No. 619,80 1 Claim. (01. 173-266) where the turns lie side by side, and at the inner edgeoftheshieldingstripiftheadjacentturns are overlapped. These spaces ordinarily .are very small, but it has been found that the dielectric strength of an insulated cable is subs stantialiy increased if these unshielded spaces are eliminated or reduced in size.

According to this invention insulated cable-constructions are provided in which one or both surfaces of the -isulating wall surrounding the cable conductor may be completely shielded, and in which ionizable spaces at the surfaces of the conductor insulation are reduced to a minimum.

Referring first to Figs. 1 and 2, there is shown by way of illustration a single conductor cable in which both the inner and outer surfaces of the insulating wall are shielded. The conductor II is a stranded conductor, and it will be seen from Fig. 2 that small valleys I! exist on the outer surface of the conductor between adjacent conductan ing strands. If the insulating wall were applied immediately over the stranded conductor, these small spaces l2 would be under electrical stress when the cable is in operation.

In accordance with this invention the spaces i2 are shielded by wrapping helically about the conductor a conducting ribbon ll whose adjacent turns lie side by side, and simultaneously therewith an overlapping conducting tape Ii. This ribbon it should have a fairly high tensile 80 strength, and desirably is-a thin metal tape, conveniently copper. Desirably the metal ribbon is perforated as at it to reduce electrical losses due to currents induced therein. It will be apparent that the helically wound ribbon ll shields to a large extent the spaces l2, but that if it were not for the overlapping conducting tape I! there would be unshielded spaces between I adjacent turns of the ribbon.

The ribbon II is substantially incompressible, and is fairly still, so that if adjacent turns were overlapped to eliminate the spaces between turns the flexibility of the cable would be lessened, and there would be a small unshielded helically extending space adjacent the overlapping edge of the ribbon, which space wouldnot be completely filled by insulation applied over the shielding layer. Furthermore, in such a construction bending of the cable might result in buckling of the shielding strip, which would injure the cable insulation or enlarge the existing unshielded spaces.

As the metal ribbon It and conducting tape II are helically wrapped on the conductor, the tape i5 overlaps and extends beyond one edge of the thisinsulationordinarilytakestheiormoi mam plurality of overlying layers of helically paper tape. the adjacent each layer lying side by side.

ordinarilytakestheiormotaleadtube I'igs.3and4 disclose the/inventionasapplied to three conductor cable. In the illustrative showingtheconductorinsulationisshieldedonLvs onitsoutersuriace. Itwillbeobvious thatthe insulatimeouldalsobeshieldedonitsinner-suriaceinthesamemannerasisshownforsingle conductorcable.

Threes'imilarconductorsfl areeachenclosed inawallfloiinsulation. Overeacholthe insulatingwallsilisappliedashieldinglayer the overlapping helically wrapped metal ribbon 23 and conducting tape 24. The

three insulated and shielded conductorsare cabled together, and the valleys between the-conductors conveniently are rounded out with filler 'material ll. Denrably the insulated conductors andtheiillersareheldinassembledpositionby meansoiahelicallywrappedbindingtapefl.

'll'hisbindingtapeconvenientlyisathinsteelor bronze tape, and is applied simultaneously with anoverlapping tape ll whichmaybe aninsulating tape. or which may be a fibrous tape treated with conducting material. It the tape 21 is aconducting tape, the electrical resistance betweenthe shielding layers overlying each in-v mlatedconductorandthecablesheathllwill bereduced. Overthebindingstrlpllisthe cablesheathll,whichordinarilywouldheoi leadoraleadalloyincableoithetypeshown.

gltwillbeapparent'thatthisinveniion provides shieldedcabl'einwhich ionimblespaces adiacentthesuriaceot-thecableinsiflationaresubstsntially eliminated. rests have shownthst across? cable constructed according to this invention hli greatly improved dielectric strength as compare to cables heretofore known. In addition to thi improvement in dielectric strength, such cabin wouldbeexpectedtohaveamuchlongerliie.

The foregoing description of certain speciii' ts oi the invention is illustrative mere ly.andisnotintendedasdeiiningthelimitse the invention.

I claim: 1.Anelectr1ccablecomprising,incombina tion, conductor. a wall of insulation surround ing conductor. an electrostatic shield over lying the insulation said shield comprising helically wound metallic ribbon whose turns an positioned side by side and a relatively more flexi ble iibrous tape treated with conducting mate rial wound on simultaneously with said metallt ribbmsothataturnoitheflbroustapeunder liesoneturnottheribbonandoverliesanad iacentturnoitherlbbomandasheathenclosim v the insulated'conductor.

2.Anelectrlccable,incombina tion, a conductor. a wall 0! insulation surround ing the conductor, an electrostatic shield inter posed between the conductor and the insulation saidshieldcomprisingahelicallywoundmetalli ribbon whose turns are positioned side by aid and a relatively more fleaible nbrous tape treate1 with conducting material wound on simultane ously with said metallic ribbon so that a turi oi the fibrous tape underlies one turn of the rib bonanddverliesanadiacentturnottheribbon and a sheath enclosing the insulated conductor .3. An electric cable comprising", in combina tion, a conductor. a wall of insulation surround ing the conductor, an electrostatic shield imme diatelyaddacentonesuriaceotthewalloiinsu lation. said shield comprising a metallic ribboi q and a relatively more flexible fibrous tape treat ed with conducting material helically wound to gether with the tape interlocked with the rib bon in-lapping relation, and a sheath enclosim the insulated conductor;

4.Anelectric cablecomprising,incombina tion.aconductor. awallotinsuiationsurround ing the conductor, a sheath enclosing the insu lated conductor. and an electrostatic shield im mediately adiacent one surface oi the insulatim wall, said shield comprising a helically wouni metallic ribbon, the turns of which are positione side by side, and a helically wound. metallize fibrous tape, said metallised fibrous vtape beim more flexible than the metallic ribbon and eacl turn of the tape overlying one turn of the ribboi and underlying an adjacent turn oi! the ribbon 5. An electric cable comprising,- in combina tion, a conductor, a wall of insulation surround ing the conductor, a sheath enclosing the insu lated conductor, and an electrostatic shield im mediately adjacent one surface of the insulatim wall, said shield comprising a helically woum metallic ribbon, the turns of which are positionei side by side, and a helically wound, metallizel fibrous tape more flexible than the metallic rib hon wound on simultaneously with the ribbon said tape being interlocked with the ribbon am extending transversely beyond one longitudina edge of the ribbon to overlap an adiaceht tun oi the ribbon. K I

6. An electric cable comprising, in combination, a conductor, a wall of insulation surrounding the conductor. a sheath enclosing the insulated conductor. and an electrostatic shield im: medistelyadiacentonesuriaceoitheinsulatim wail, said shield comprising a metal ribbon and a relatively more flexible metallized fibrous tape aelically wrapped about the conductor with 2. :um 01 the metallized fibrous tape underlying me turn of the metal ribbon and overlying an ad- Iacent turn of the metal ribbon.

7. An electric cable comprising, in combinaiioni ahcbnductor, a wall of insulation surroundng the conductor, a sheath enclosing the insuated conductor, and an electrostatic shield immediately adjacent one surface of the insulating wall, said shield comprising two conducting strips, one 01' which is relatively stiiI and the other of which is relativeLv more flexible, simultaneously helically wound about the conductor with the turns oi. the relatively stifl' strip side by side and with the turns of the more flexible strip extending between and underlying and overlying adjacent turns 01' the relatively stifl. strip.

IRVING T. FAUCE'IT.

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