US873216A - Electric cable. - Google Patents

Description

N0. 873,216. PATBNTBD 1126.16, 1907.

I c. w. DAVIS.

ELECTRIC CABLE.

A'rrmonlox run we. 29. 1906.

(EL- 2.. w. 1%., Q. Qgekd STATES PATENT OFFICE.

OHARLES W. DAVIS, OF EDGEWORTH, PENNSYLVANIA, ASSIGNOB TO STANDARD UNDER- GROUND CABLE COMPANY, OF lffITTSBURG, PENNSYLVANIA, A CORPORATION OF PENN- SYLVANIA.

. ELECTRIC CABLE.

Patented Dec. 10, 1907.

To all whom it Be it known that 1, CHARLES W. DAVIS,

residing at Edgeworth in the. county of Alleghenyand State of Pennsylvania, a citizen of the United States, have invented or stranded cables' that is, cables with con-.

ductors of relatively large size which to meet 7 the requirements of flexibilityarecomposed of an aggregation of individual. wires, usually of copper, laid together without intervemng insulation, so that each strand carries a part" of the current, the whole mass of wires being covered by an envelop of insulating material, protected or not protected by an inclosing metallic sheath, as the nature of the insulating material requires.

The object of my improvement is to in:- crease the current ca g capacity of such stranded cables accor g to the principle and in the manner hereinafter stated'and described. A

When a current of electricity passes through any conductingmedium a certain portion of the currentis transformed into heat. This heat tends to raise the temperatureof the conducting medium, the temperature finally attained being dependent among other things upon the resistance of the conductors, the superficial area of the conductors, the amount of the current, the temperature of the surrounding medium and the ability of both conductor and surrounding medium to dissipate by radiation or conduction the heat generated in the conductors.

If the insulating medium around the conductors, and the various mediums which in turn surround the insulation are of high thermal conductivity, the energy transformed into heat within the conductor itself, will easily be dissipated from the conductor with a relatively small increase in temperature in both the conductors and the surrounding medium;

if, on the other hand, the various mediums surroundin the conductors'are of relatively low therm conductivity there will of necessity be a relativel high temperature attained by the cond which lies in close proximity thereto, the temperature at any point outside of the conuctors and the material the conductors. I

As the insulating materials commonly made use of, such as rubber, paper, jute, varnished cloth, etc., are deleteri'ousl affected by high temperature-the actual e ectbeing' dGPGDdG IILHPOD the degree of heat as well as /4:heleiigthof time it is applied, it becomes vnecessary from a practical standpoint to prevent the insulation from becoming e osed to tem eratures of such value as to ring about t deterioration. It will readily be ductor, made up as I have indicated above, surrounded by the usual insulating wall of some one of the usual insulatin materials, "and placed in an environment 0 any given character, there would be a limiting value to the current carried, in order that the temperature finally attained by the conductor with shall not "be above the temperature selected as be the maximum to which that kind of insulation should beexposed.

. It is a well known fact, that as insulated conductors solid as distinguished from an'nu lar in cross section are increased in cross sectional area, their carrying ca acities, although actually greater, are re atively less. That is to say, under like environment, the lar er the cable conductor becomes the lower be' its current carrying capacity per unit of cross sectional area for any given limit of maximum temperature. And this. result is due, among other things, to the fact that the superficial area of the conductor in contact with the insulating material, or in other words the area through which the heat is transmitted from the conductor to and through the surrounding insulating walls, is relatively greater in the small sized cable than in the large sized cable, this being eral area of a cylinder varies as the diameter while the circular cross section varies as the square of the diameter. That is, given two cables, the conductor having in one case twice the cross-sectional area of the otherthe larger is not ca able of carrying a current (other conditions eing the same) twice as lar e-as thesmaller.

t is to increase the carrying capacity of the conductors, for any limiting maximum temperatures, by taking advantage of my ductors being lower as it is more remotefromf seen, therefore, that for any given size of con and theinsulating material in contact thereclearly explained by a consideration of the well known geometrical fact that the periphknowledge of the fact that increased super ficial area ofthe conductor increases the radiating .power and consequently reduces In t e accompanyi wlng's forming a part of this specification I have shown in transverse section three forms or embodiments of my improvement;

My-invention in broad terms consists in' r of anfrom 011'- forming the body of the cable ro nularfcross section as distinguishe cular cross sectionthat is, in f0 through the center of such a standard conductor a spaceor a hole. Such a construction is indlcated in Figure 1, and, assuming that the crosssectiona area of the'conductor shown in Fig. 1 is the mile cross sectional area of the solid conductor, I find that the maximum current which may be .passed through the cable of Fig. 1 without in'ury to the insulating envelop very materially exseeds in strength the maximum current which the cable having the solid conductor may carry. It is manifest from the form of the cable that (assuming their cross sectional areas equal) the outer surface of the body of the conductor of Fig. 1 is of-greater extent than the outer surface of the body of a solid conductor of the same current carrying capacity. Accordingly, the heat radiated is in the former case spread over a larger. surface and into a larger body of the insulating envelop; and for any given current flow the I temperature attained after a stable condition -is reached, is less than with cable having the solid conductor. I have found that in such a construction the increase in carrying capacity isat a greater rate than the increase in cost, within practical limits, and from this the practical benefit of the invention becomes apparent. For example, I have found in a particular instance that whereas the cost surrounded by an envelop of insulati invention in its broadest terms. 'In Fig. 1 I

show the strands of the cable laid up on a lead pipe, in Fig. 2, a coiled spring forms the hollow core, and in Fig. 3 the core is composed of hemp or jute. Practical conditions mand a cable' suflic iently flexible for insertion through surface manholes into underground conduits, and the various forms shown possess such flexibilit otherwise, there is no need that the core e flexible.

With a form such as shown in Figs. 1 and 2 a hollow core is present; and in such case a cooling stream of air, water or other fluid may be made to circulate therethrough, to carry off a portion of the heat generated in the conductor, and in so doi still greater degree its maximum 0 capacity. This additional cooli wilfinsuitable conditions of service e found of ractical value.

t is characteristic of my invention that the strands forming the conductor areso arrangedthat the conductor shall have such cross-sectionalandperiplieralmto be cap-able of carrying a given current and of ra 'atingheat at such a rate that the heat generated by such current will not raise the temperature of the cable to a value sufficient to injuriously affect the cable.

I claim as my invention:

A stranded cable for electrical purposes com osed of a sheaf or bundle of strands in con uctive contact with one another and material of such character or nature as to e injured when heated beyond a certain critical temperature, the said sheaf or the strands composing said sheaf being disposed in annu lar form of such peripheral extent that the rate of radiation from the conductor when carrying a current of desired strength will prevent a rise in. the temperature of the in sulation beyond the critical point thereof, substantially as' described.

In testimony whereof, I have hereunto set my hand.

CHARLES W.-DAVIS.

Witnesses CHARLES BARNETT, WILLIAM H. WILSON.

to increaseto means

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