US1982539A - Insulated electrical conductor - Google Patents

Description

Nov. 27, 1934. B. H. REEVES INSULATED ELECTRICAL CONDUCTOR Filed July 11, 1931 gwuentm flia/fu-vd H. Ms

Patented Nov. 27, 1934 UNITED STATES PATENT OFFICE to Rockbestos Products Corporation,

Haven, Conn.

New

Application July 11, 1931, Serial No. 550,063

4 Claims.

This invention relates to electrical conductors and it has more particular reference to electric cables for power lines carrying low voltages, al-

6 though it has application to other conductors as well.

In order to save the cost of transformers and for other reasons, electric power is distributed in large cities and elsewhere at low voltages. The low voltage current traverses relatively long distances and many cables are used in such networks and frequentlya large number of cables are grouped together in a relatively small space under the surface of the ground. When a fault occurs, as, for example, when a laborer in doing excavating work inadvertently penetrates one of these cables with his pick, serious consequences are apt to follow. The amperage of the current is so high that under such circumstances or under similar circumstances a large arc is apt to be established. This frequently leads not only to failure of important power lines and to buming and destruction of cables, but to disastrous underground explosions. In such cases the metallic conductors and certain gases generated by the heat feed the fire, and in many cases the insulation on the cables is of such a nature that when it is burned or highly heated it evolves gases which are explosive in themselves or combine with others to form explosive gases. In the past it has been practically impossible to prevent failure of cables resulting from the accidental or undesirable drawing of arcs or grounding, and where a fault has occurred giving rise to the establishment of an arc, the latter has usually been maintained for such length of time as to cause considerable damage.

One of the objects of my present invention is to avoid or to a large extent overcome the abovementioned difliculties incidental to the use of cables such as now found upon the market.

Another object of my invention is to provide an electric cable or other conductor of such character that upon the formation of an arc in proximity to the cable, agencies will automatically be set in operation for bringing about the quenching of the arc and for preventing it from becoming reestablished.

Another object of my invention is to provide an electric cable particularly applicable to low voltage distributing networks, having about the metallic element or conductor proper an enclosing wall which, when subjected to temperatures such as occasioned by the more or less close proximity of an electric arc, evolves a volume of inert gas which is effective to quench the arc.

Another object of the invention is to provide an electric cable or like conductor having an insulating medium whose dielectric qualities are very satisfactory and which can be conveniently embodied in a cable or other conductor structure at relatively low cost, and which, in addition to its dielectric properties, has the property of throwing off, when heated, vapors which are unfavorable to the persistence of an arc from or involving the metallic core.

Another object is to provide an electric cable which can be used for distributing power at low voltages more safely and economically than has been possible heretofore.

To these and other ends the invention consists in the novel features to be hereinafter described and claimed.

In the accompanying drawing, the single figure is a perspective view of one form of electric conductor made in accordance with my invention.

In the preferred practice of my invention, I build up a cable having a stranded copper or like core which is shown in the drawing at A. Around this core is placed insulation which preferably, for reasons to be hereinafter mentioned, is quite absorptive in character. This insulation may be in one or more layers. In the form shown in the drawing the outer layer B is a braided layer enclosing an inner insulating body C. I prefer to use asbestos as the insulating material, but cotton or other absorbent textile material could be used, and I could use a construction in which the body C shown in the drawing was made, for example, partly of rubber and partly of linen or cotton. In the form which I prefer to employ the body C is made substantially entirely of felted asbestos fibres; the layer B is a braided layer of asbestos yarn; and around layer B there may be an outer layer D which may be constituted by the ordinary lead sheath or its equivalent.

In providing a cable having insulation which when heated has the property of throwing off gases for the extinguishment of the arc, I preferably surround the metallic core with one or more layers of absorbent material, and then saturate or impregnate this material with a substance which volatilizes upon heating or which provides with sufficient promptness the necessary volume of vapor or gas of the proper kind for displacing the air adjacent the arc and causing the latter 1 to be extinguished. Preferably, the substance which I employ is urea, the commercial grade of which is comparatively inexpensive and quite well suited to the purposes which I have in view. This substance is readily meltable and when melted forms a. thin liquid so as readily to penetrate the spaces or interstices of the absorbent carrier, and. it is quite stable and serves as a part of an insulating cover which, when heated, will evolve very satisfactory arc-extinguishing vapors or gases. This material also has a relatively high dielectric strength. I prefer to use it upon a carrier of asbestos fibres which in themselves have good dielectric properties and are fireproof, but in the broader aspects of my invention the gas-evolving substance need not be employed as an impregnant of asbestos fibres.

In producing such a cable as shown in the drawing, I prefer to form the insulating body C of asbestos fibres laid about the core in long helical windings of sliver, each winding having a fine cotton carrier thread therein, said windings being thereafter compacted and compressed so as to form a single felted fibre sleeve or layer. One or more of such layers may be used as desired. After the body C has been applied, the conductor is moved through a pot or receptacle containing the impregnation as thus described, it will be evident that a relatively large amount of the impregnant is taken up and held in the absorbent layers of the cover. These layers dry quite promptly upon the solidification of the urea incident to its removal from the source of heat. After impregnation of the outer layer B the con-' ductor can be encased in a lead or like sheath D if this is desired.

As the melting point of urea is approximately 132 centigrade, there is no difficulty in practice in impregnating the insulation with urea in machines having gas-heated dope pots, and after the insulation has been saturated with the melted urea it solidifies very promptly so that when the wire reaches the take-up reel of the machine in question it is not unduly sticky. Even if left subject to the action of the atmosphere, the

substance in the insulation remains therein without passing into the atmosphere in any appreciable quantity, and the urea, while soluble in water, does not readily absorb water from the atmosphere or from any other source of moisture, being relatively non-hygroscopic.

By saturating a relatively open porous material such as constituted by asbestos or like fibres with urea, the covering of the conductor is given a very satisfactory dielectric value.

When a fault occurs in a cable such as above described, creating an are from the metallic element to some other conductor located in proximity to the cable in question, the heat which is generated causes the impregnant to volatilize and create an atmosphere unfavorable to the persistence of the arc. Upon the creation of an electric arc with its acompanying high temperature, the urea carried by the insulation or in the cable structure is subjected to sufiicient heat to cause the evolution of a volume of gas having a tendency to displace air in the neighborhood of the arc and cause its extinguishment by providing an inert atmosphere of gas which will not ionize and through which the arc will not pass. As urea volatilizes with decomposition at a temperature considerably below that of the electric arc, the insulation is affected usually over a considerable length of cable, thus setting free a large volume of C02 and NH: which, especially in a. confined space, such as a conduit or manhole, is quickly brought in quantity into proximity to the arc. When the cable is treated in the manner above described, the inert arc-quenching atmosphere is developed with great promptness upon the drawing of an are or occurrence of other fault, such as the melting down of the metallic conductor through an overload of current, so that the inert gas acts quickly to quench the arc. By acting promptly the gases prevent the melting down of copper, lead, or other metal to such a degree or extent as to produce favorable conditions for the reestablishment of an are or the establishment of new arcs, such as occur when faults arise in ordinary cables. Cables of my invention, therefore, are much safer than ordinary cables. Promptly acting agencies for the extinguishment of the are are set in operation upon the insulation reaching a temperature of, say, 200 C., which is a temperature easily attained in the near proximity of an electric are occurring in connection with a cable or like .conductor. Where the metallic core is heated up by an overload of current the inert gas is produced in immediate proximity thereto and is very effective to arrest arcing regardless of whether or not the cable is in a confined space.

Not only is damage due to cable failure reduced to a minimum by my invention, but likewise explosions such as now commonly occur in conduits and manholes when cables are highly heated or catch fire. cables the insulating material or some of the insulating material is of such a character that heat or combustion causes the generation of vapors or gases that are quite explosive. Explosion may also be caused by using certain impregnating materials in connection with the insulation. With my cable, however, no such explosive vapors or gases can be evolved. Where asbestos is used as a carrier for the arc-quenching substance, there is no difficulty due to the inherent nature of such asbestos because of the fact that it is fireproof and upon heating evolves no vapor or gas. If all of the insulation be of asbestos impregnated with urea the cable structure will be incombustible and there will be no problem of combustion caused by an arc, but it will be desirable to eliminate continued arcing and melting of the conductor and other bad eifects, and this the urea will do by immediately providing a nonionizing gas where the arc is located. The cable shown herein by way of example is practically incombustible as the cotton carrier threads pre viously referred to are of such minute bulk as to be negligible as a combustible factor, but in certain constructions they may be omitted entirely. If the carrier for the arc-smothering substance be of some textile material such ascotton, the fireproof feature will be absent, and the gases caused by the heating of the textile material may be explosive, but the inert gases will be produced in large volume upon the application of a relatively low heat to the insulating cover of the conductor so as to create an atmosphere unfavorable to combustion. or explosion. Any dangerous flame, spark or are is usually quenched In some of the ordinary promptly as a consequence of the effect of heat upon the urea.

Upon the heating of the urea by the are a volume of gas is created in which ionization will not take place and which is electrically nonconducting. This is a feature of the greatest importance. The body of gas produced is also non-combustible, non-explosive and non-toxic. It is understood, of course, that the carbon dioxide and ammonia gas given off from the urea are unfavorable to combustion, non-explosive and non-toxic. The manner of action of the body of gas upon the arc is in some particulars uncertain, but it has been amply demonstrated that the gas produced is non-conducting and will promptly quench the arc. The ammonia gas gives off an unpleasant smell, but this is a useful feature inasmuch as the smell of this gas is a signal indicating that the cable is being subjected to an undue amount 01' heat. After the insulation has been highly heated and cooled it will be found that the residue has considerable dielectric and mechanical strength. There remains a stable body having satisfactory electrical and stituted urea is meant urea in which the hydrogen atoms linked to nitrogen have been replaced by I hydrocarbon radicals, either alkyl or aryl.

Ammonium carbonate. is another ammonia compound possessing some of the advantageous properties Yer-rein mentioned. When heated likewise evolves CO: and NI'IE, but its er to the conductor so as to remain perz ently in the insulation is impractical or extremely difllcult, and in a number 01 other particulars also I regard it as distinctly interior to the preferred impregnant mentioned above.

What I claim is:

1. An electrical conductor having an insulating covering containing urea in a condition to produce ammonia gas upon heating.

covering for an electrical conductor compri......g a body of asbestos saturated with urea in a condition to produce ammonia gas upon heating.

3. An e ectrical conductor comprising a covering impregnated with urea, said covering being initially devoid of substances producing inflammable or explosive gases under heat and having the urea chemically uncombined therewith and in a condition to produce carbon dioxide and ammonia gas upon heating to decomposition.

4. A power cable for low voltage networks comprising a metallic core adapted to carry a current of low voltage and high amperage, a lead jacket, and a body of insulation filling the space between said core and said Jacket, said body of insulation being incapable of evolving an inflammable or explosive gas when subjected to high temperatures and comprising interentangled asbestos fibres impregnated with urea in a condition to produce ammonia gas upon heating.

BEAUFORD H. REEVES.

Images