US1818127A - Submarine cable insulation - Google Patents

Description

Aug. ll, 1931. J. J. GILBERT 1,818,127

-SUBMARINE CABLE INSULATION Filed April 29. 192s 2 sheets-sneer 1 my\ y Fresa/rr lbs. per s; /z

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' I SUBMARINE CABLE INSULATION Filed April 29, 192e 2 sheets-sneer 2 7/ I /N vE/v ran J2 JOHN J @/mr 2 low temperature presentat great depths.

Patented Aug. 1.1,l 1931 UNITED STATES-PATENT OFFICE *Y JOHN JOSEPH GILBERT, OF PORT WASHINGTON, NEW "YORK, ASSIGNOB TO WESTERN ELECTRIC COMPANY, INCORPORATED, 0F NEW YORK, N. Y., A CORPORATION OF NEW YORK SUBM'ARINE CABLE INSULATION Application led April 29,4

This invention relates to submarine cables for telephony and telegraphy and particularly to cables for deep sea servicewhere the conductor is subjected to high pressure and 5 low temperature. l

One of the most serious problems in signaling by submarine cable is the attenuation of the signals. -While this is detrimental in low frequency telegraphy, the effect is much o greater in high frequency telegraphy and the still higher frequencies of telephony for which present day cables are designed. More especially is this so when cables are subjected to the extremely high pressure and An object of the present invention is to reduce the attenuation of signals transmitted over submarine cables.

A more specific object is the reduction of the leakance losses in such cables. j One method of attaining these objects becomes evident from a study of the factorsj upon which the attenuation constant of the 2K5 -cable depends. This constant a may be calculated from; the formula weee ,l

in which R is. the effective resistance, L the 5 inductance, C thecapacity and G. the leakancenper nautical mile of cabl'e. The! eX- 4 pression is considered as asingle quantity throughout this. specification because in the'testing of dielectric-materials it has become .the general practice to consider thi? expression rather-than thelleakance Gr alone. This is because the shape of the sample under test will affect both the capacity and the leakance but in such a way that the quantity .dielectric material. Considerable mathematical calculation isthereby avoided.

'1926. Serial No. 105,390.

It isobvious that forja cable having a given resistance, inductance. and capacity,

the attenuation constant vvill have the smallest value when an insulating material is employed which has a value of as small as possible. It is also seen that the Aeffect'of lealrance upon the attenuation con loaded cable is largely dependent 'upon the leakancev of the sections of non-loaded conductor between the loading coils and may The leakance of a coil therefore be reduced by thesame means;

'which ivill reduce the leakance of al nonloaded cable.

In the formula for the attenuation con#A stant the quantity e is to b multipliaiby the inductan L. yI

'ris' might therefore appear that for va so-called 1 non-loaded cable the leakance .would have no effect. It must be remembered however,

that even a non-loaded? conductor has an appreciable inductance amountin henry or more per nautical mile ue to the.

`magnetic fields set up in the sea water surrounding the cable. Y

Submarine cables when laid in deep water are ordinarily `subjected to very great pressure due to the weight of the water above the cable, the depth at which the cable is laid determining the amount of -this pressure.

Trans-oceanie cables are subjected to pressures of over v2500 pounds per square inch for the greater part of their length and a considerable portion of they cable is'usuall'y subjected to pressures ranging from 5000 to 10,000 pounds per square inch.' v

The temperature of the water at the ocean ybottom is usually in the nei hborhood of 4 C. and-the temperature of t e cable and in- I sulation'is therefore the same.` l v- Although these extreme conditions ofpresexist, but little attention has been given to the effect upon the insulation material other than to insure that the insulation was impervious to water under these conditions.

' The insulation used at the present time for submarine cables is usually gutta percha ap- Aplied in a layer about 120 mils or more thick,

the thickness depending upon such factors as the operating potential and the insurance of mechanical integrity.

In the manufacture of submarine cables it is usual to apply a flux material, such as Chattertons compound to the conductor in order to insure adhesion between the conductor and insulation. This flux is applied in a very thin layer, about 1 mil'in thickness, before the insulation is applied. In a copending application for A. R. Kemp, issued as Patent No. 1,700,766 on February 5, 1929 is described a continuously loaded conductor in which bitumen was applied in a thick layer filling the interstices between the loading material and the conductor so as to act asa cushion substance to relieve strains set up in the loading material during the manufacture and the laying of the cable. It also served the additional purpose of a flux for the gutta percha insulation. Up to the present time, however, there has been no use of bitumen as a flux between the conductor and the insulation of non-loaded or coil loaded cables.

The insulation materials applied to conductors heretofore have seldom been chosen with reference to their characteristics under the actual operating conditions of high pressure and low` temperature. It has, however, been known for some time, among cable manufacturers, that the capacity and the leakance of insulation varies with changes in pressure and temperature and specifically that the leakance of gutta percha increases lwith an increase in pressure and with a decrease in temperature.

It has also been known that the leakance is, to a limited extent, proportional to the frequency of the impulses transmitted over the cable. F or'this reason the leakance of a Icable for high frequency telegraph or telephone operation is considerablygreater than that of the same cable for directcurrent .telegraph operation. Furthermore the attenuation constant is mostly affected by the characteristics of the layers of insulation nearest the conductor, that is within a thickness of from 20 to 30 mils from the surface of the conductor.

From experiments conducted in connection with the present invention it is now found that the quantity i for various substances depends upon the frequency, temperature and pressure at which it is measured.

In accordance with this invention a submarine cable is contemplated for the A4transmission of signals, the frequencies of which are those met with in telephone work or high speed telegraphy, under conditions of high pressure and low temperature present at great depths. Such a cable has a material, combined with the usual insulation, which decreases the leakance of the cable, under conditions of high pressure. This material is applied t0 the conductor and acts as a binder between the conductor and main insulation. Certain of the bitumens, and especially asphalt, have been found very satisfactory for this purpose. The invention further contemplates a cable having one insulation, usually gutta percha, at moderate depths and a combination of gutta percha and a material such as asphalt for the eXtreme depths.

In the accompanying drawings Fig. 1 indicates the variation of for several materials when the pressure is varied and the frequency of the measuring current 1s 100 cycles per second;y Fig. 2 is a 'set of curves for the same materials subjected to varying pressures when the frequency of the measuring current is 1000 cycles per second; Fig. 3 shows a cross section of a cable using asphalt as the flux or binder between the conductor and gutta percha; and Figfi'shows a cable with different sections` constructed to meet the requirevments -at the respective ocean depths at phalt flux. For protection etc., a layer of jute 13 is usually applied, then a layer of armor 14 and finally a second layer of jute 15.

The thickness of the layer of flux or binder is dependent upon the material employed but for asphalt this layer should be about l0 to 20 mils. thick. Since the greater part of the leakance lossesoccur's within a thickness of about 30 mils. of insulation, it would seem that a thicker flux would be of advantage but an increase in the thickness of asphalt beyond about 2O mils presents considerable difficulty in the manufacture of the cable and is therefore not feasible.

lIt must be remembered that the capacity of an insulated conductor is largely determined by the dielectric constant of the malac terial closest to the conductor and it is necessa n to insure vthat the arrangement de- Y -scrbed for reducing the value of does not increase the lcapacity of the conductor and hence increase the attenuation constant. U der sea bottom conditions the dielectric co stant of asphalt is lessv thanI that of`gutta percha so that a decrease in capacity of the cable accompanies the decrease in obtained by employing a layer of asphalt between the conductor and the gutta percha. In Figs. 1 and Qcurves A represent the variations in l due to varying ressures, at a temperature 25A of4 (land at requencies of 100 and 1000A cycles per second, respectively, for a cable having a heavy layer; of asphalt flux surrounded by gutta percha as shown in Fig. 3.

for a cable with a heavyv layer of asphalt decreases with increasing pressure, whereas the value of for a cable with the gutta percha insulation close to the conductor in general increases with the pressure. l.

' From the curves of Fig. 1 taken at a transmitting frequency of 100 cycles per second it is apparent that with respect to ,the value and its effect upon the attenuation constant v f the cable having a heavy asphalt layer has an advantage over the cable having the insulation of ordinary gutta percha close to the conductor when used on depths where the pressure is above 4500 pounds per square inch. At pressures around 10,000 pounds per square'inch it has been found that 'the cable with a layer of asphalt in this respect is equal to that having the insulation of improved utta percha close to the conductor, the va ue of then' being 14 in both cases.

The advantage of the asphalt flux is more pronounced at higher frequencies. From the 'curves of Fig. 2 it is evident that the cable having a layer of asphalt is far superior at all pressures to that having the ordinary 'gutta percha insulation close to the conductor, and excels at pressures above 2500 pounds per s uare inch'the cable having the improve gutta percha insulation close to the conductor.

The use of a heavy layer of ux, such as mentioned above, therefore, has the effect, not only of serving` as an effective binder, but also of reducing the leakance of a cable at the higher Apressures to which submarine cables generally aresubjected.

From a consideration of Figs. 1 and 2, it will be seen that some advantages may be obtained` in both lowv frequency and high frequency cables by employing an insulation havinglow leakance under high pressure for the deep sea portions of the cable and guttapercha or other insulation material having low'leakance at low pressures for the terminal or shallow water portions o f the cable. This could be accomplished by employing a very thin layer of flux consisting of as halt or similar material, 1 mil or so in thic (ness for shallow water portions and a thicker layr of flux for the deep sea portions of the ca e.

, Although bitumens and particularly asc phalt have been mentioned as suitable for this purpose, it is contemplated that any material which has low leakanceat high pressures and low temperatures may be employed to advantage.

The manner in which the insulation is applied to the conductor and its chemical composition is of little importance provided it has the desirable properties described above and` is not in any way injurious to the remainder of the cable. 1

Such an arrangement is shown in Fig. 4, in which the cable 20 is shown as having a section 21 placed at a comparatively great ocean depth and a section 22 ata smaller depth. The figure particularly shows detail cross sections 23 and 24 of the respective sections, the cross sections showing a cable core similar to that shown in' Fig. 3 without the protecting layers of jute and armoring. In the cross section 23 corresponding to the greater depth the layer -31 of a low leakance insulating material, placed between the conductor 30 and the outer insulation 32, is comparatively heavy. In the cross section 24 corresponding to the smaller depths the layer 31 is comparatively thin and may merely serve as a binder. The selection of the insulating materials and the dimensions of the insulating layers would of course depend upon the transmission frequency `and the depth at which the cable is to be located.

It is to be understood that this invention is applicable to the sections of coil loaded telegraph cable comprising a conductor and an insulating material closely surrounding said conductor, said insulating material having a decreasing leakance with decreasing temperature.

3. A non-loaded submarine telephone or telegraph cable comprising a conductor and an insulating material closely surrounding said conductor, said insulating material having a decreasing leakance with increasing pressure and decreasing temperature.

4. A non-loaded section ,of a submarine tele-)hone or` telegraph cable comprising a conductor, insulation for said conductor, and a second insulating means between said conductor and insulation and engaging said conductor, said second insulating means having a lower leakance than improved gutta percha for frequencies of the order of 1000 cycles when subjected tohigh pressure and low temperature. Y

5. A non-loaded section of a submarine telephone'or telegraph'cable for transmitting frequencies of the order of 1000 cycles per second, comprising a. conductor, insulation `for for said conductor, and an insulating flux betweenv said insulation and conductor and engaging said conductor, said iux comprising a material having a leakance lower than that of improved gutta percha at pressures in eX- cess of 2500 pounds per square inch, and at temperatures in the neighborhood of 4 de? grees centigrade.

G. A non-loaded submarine telephone or telegraph cable comprising a conductor7 insulation for said conductor, and a layer of bitumen between said conductor and insulation and engaging said conductor.

7. In a non-loaded submarine telephone or telegraph cable a conductor, insulation for said conductor comprising gutta percha, and means for reducing the leakance of said cable comprising a layer of bitumen between said conductor and said gutta percha insulation.

8. A submarine telephone and telegraph cable comprising a conductor and composite insulation therefor of bitumen and other insulating material, the amount of bitumen applied per unit length of the conductor being smaller on portions of the cable. near the terminals than on portions more remote from the terminals to compensate for the effect of the difference in water pressures acting on said portions.

9. A submarine telephone or telegraph cable comprising a conductor, a main body of insulating material for said conductor and a layer of an insulating material different from that in said main body between said conductor` and insulation, the thickness of said layer being greater for certain portions of the cable than for other portions of the cable to compensate for the effect of the difference in waterpressures acting on said porf tions.

10. In a submarine telephone or telegraph cable, a conductor, an insulation therefor,y

said insulation comprising an insulating material having a decreasing leakance characteristic with respect to pressure and another insulating material having an increasing leakance characteristic with respect to pressure, the weight ratio between said materials being cable. Y

11. In a submarine telephone or telegraph cable, a conductor, an insulation therefor, said insulation comprising an insulating material having an increasing leakance characteristic with respect to temperature and another insulating material having a decreasing leakance characteristic with respect to temperature, the"weight ratio between said materials being different at different p0rtions of said cable.

12. In a submarine telephone or telegraph cable, a conductor and insulation therefor, said insulation comprising an insulating material having a decreasing leakance characteristic with respect to pressure and an increasing leakance characteristic with respect 1|() to temperature, and a second insulating material having an increasing leakance characteristic with respect to pressure and a decreasing leakance characteristic with respect to temperature, the weight ratio between said materials being different at different portions of said cable..

13. A submarine cable for signaling fre-y quencies of 1,000 cycles per secondhaving a non-loaded deep sea section and a non-loaded a layer of bitumen contiguous to said conductor.

In witness whereof, I hereunto subscribe my name this 27th day of April, A. D., 1926.

JOHN J. GILBERT.

different at different portions of saidv

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