US 2105060 A
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Description (OCR text may contain errors)
Jan. 11, 1938. s o ET AL 2,105,060
. NETWORK CABLE Original Filed March 10, 1931 fig]. Hg? 2'.
Comp/"armed Zo/v'c 170/211? @m orenred Ear/t Fold WITNESSES: p INVENTORS r v A fiber) 5 an; Cfih/bnLDLMz/I X/ 2 I and lean MiCu/kvch. 9 3 ATTQRNEv Patented Jan. 11, 1938 .NETWORK CABLE Albert P. Strom, Wilkinsburg, Clinton L, Dammit, Forest Hills, and LeonMcCullocll, Pittsburgh, Pa, assignors to Westinghouse Electric and Manufacturing Company,
Pennsylvania a corporation of Original application March 10, 1931, Serial No.
521,500. Divided and this application February 14, 1934, Serial No. 711,150
an insulating material in the form of com pressed hloclrs, slabs, plates or other shapes which is capable of evolving nou inilauunahle cases when exposed to the heat of an electric are.
A further object of our invention is to provide a cable comprising an inner conductor, an outer conducting sheath, anti an insulating? material interposed between the conductor and sheath, which is capable of evolving nou iuilammahle cases when the insulation is exposed to the heat developed by an electric are.
In constructing cables, comprising an inner insulated conductor and an outer sheath, especially those utilized in electric fighting service or in the transmission of power, it has heretofore been the practice to utilize an organic insulating material, such as oil impregnated paper, rubber, etc, as an insulator between the inner conductor and outer sheath. Inorganic material, such as porcelain, glass or earthenware has also been ernployecl for this purpose.
When .faults occur in cables of such types either through defective material or construction or when the cable is accidentally punctured hy workmen, an arc is often produced between the conductor and the outer sheath, or between the broken or punctured conductor, which, if not promptly, extinguished, is liable to cause explosions in the event that the surrounding atmosphere of the duct contains inflammable or combustible gases. When organic insulating material is utilized, the danger from explosion is accentuated because in such cases when an arc definitely ascertained that where gases are gen Another object oi our invention is to provide erated by the decomposition of material by the heat of the are, the characteristics are remark ably changed. For example, in cables of, the type mentioned, in which the insulated material interposed between the inner conductor and the outer sheath is capable of generating gases in the presence of an electric are, when a fault occurs in the outer sheath or in the inner conductor, the arc occurs in the insulation in the same way as the oil circuit breaker are is in the oil. in both cases, the arc is in a gas pocket or bubble. This gas and the arc in it, however, are not in a quiescent state but are being" violently stirred lay the turbulent mixing cl 2. large into the gas containing the are which comes from the decomposing insulation, This turhu lent iooouring of fresh gas causes the extinction of the arc in circuits of higher voltage than would otherwise he possihle. The gases coming from the thermal decomposition oi organic in sulatiou in cables are generally regarded as a nuisance, and since they are inflammable, they may even constitute-an explosiohhaaard. Nev ertheless, our researches show that, because of the effectiveness of gases in extinguishing arcs, they are extremely useful, if not essential in the proper functioning of cables utilized in low volt-= age, alternating current networks.
We have made the discovery that, ii an in sulating' material that is capable of generating a hon-inflammable gas is utilized between the conductors in which arcs are liable to occur, such as cables, the are which usually occurs will he promptly extinguished. By utilizing such a material, we are, therefore, not only ahle to secure the benefit resulting from the generation of gases but since the gases generated are hon-inflate Enable, the danger from explosion is avoided.
Our invention will he hotter understood by reference totheaccompanying drawing, in which Figure l is a perspective view of a compressed cylindrical block composed of our improved in. sulating material.
Fig. 2 is a similar view in which the insulating material is compressed in a solici -cylindrical form.
his. 3 is a cross sectional view cl 2:. cable, showing" a central conductor, an outer sheath, and insulating material interposed between the two conductor and sheath.
Fig. 4 is a similar view showing another modification, and s Fig. 5 is a sectional view showing a plurality of volume of fresh, relativelyccol, unionized gas conductors surrounded by a sheath containing an insulating material.
Referring to Figs. 1 and 2 of the drawing, the numeral l designates a block of our improved insulating material which has been consolidated into cylindrical form by means of pressure and the numeral 2 designates a block which has been compressed into semi-cylindrical shape by similar means.
Figs. 3, 4 and 5 illustrate how the insulating material may be applied to cables such as those utilized in low-voltage, alternating current networks, which are employed for the transmission of. electricity for light or power. Such cables usually consist of an inner conductor 3, which may be either a single conductor or a plurality of strands, as shown in the drawing, and an outer sheath 4 which is preferably made of lead. The inner conductor and the outer sheath are insulated from each other by means of our improved insulation which may be formed entirely of compressed material, as indicated at 5. In order that the cable may be slightly flexible, however, the insulation is preferably formed of alternatelayers of a pulverulent insulation interposed between pieces of the compressed insulation. The compressed pieces may be ofthe form disclosed at 5 in Fig. 3 or of the form indicated at T in Fig. a. If flexibility is of paramount importance, however, pulverulent material may be utilized alone between the inner conductor and the outer sheath.
In Fig. 4, the compressed pieces are of substantially elliptical shape, as shown at l, and the interstices between the outer edges of the elliptical pieces and the additional space between the inner conductor and the outer sheath is filled with pulverulent material, as shown at 8. It will be understood, however, that the shape of the compressed pieces and the form which the intervening pulverulent material assumes, as shown in the drawing, is merely illustrative as other shapes will be apparent to those skilled in the art without'departing from the spirit of the invention. It is highly desirable, however, to utilize, as much as possible, of the compressed material because of its greater mechanical strength, but when some flexibility is essential, a portion of the insulation should be in the pulverulent form, as illustrated in Figs. 3 and 4 of the drawing.
Another method of utilizing our improved material in cables is shown in Fig. 5 of the drawing. In this example, the insulating material is molded to the desired shape or placed in a pulverulent form in a sheath of insulating material l0, such as asbestos or spun glass and then wrapped around the conductor or strands or the asbestos sheath may be first impregnated with the pulverulent material and a binder, or treated with a material, such as sodium or potassium silicate before being wrapped around the cable. In the latter examples, the sheath may be utilized by itself or may contain the compressed or pulverulent material. Either one or a plurality of conductors 9 may be utilized, depending upon the particular lighting or power system employed. In case a plurality of conductors are utilized, as in the three-phasethree-conductor lighting or transmission system, an extra additional water proof insulating material H such as impregnated cloth or mica tape may be employed for the purpose of binding the conductors compactly together and to fill the space between the conductors and the outer sheath. While it is-true that in and preferably less than 5%.
this modification some organic material is utilized, the amount will be relatively small.
The particular insulating material utilized must have a comparatively high insulating value and must be capable of generating a gas in the presence of the are, which is not inflammable. We have found that boric acid, gypsum, plaster of Paris, borax, magnesium carbonate, basic magnesium carbonate, ammonium carbonate, aluminum ammonium sulphate, or the carbonates or bicarbonates of the alkali or alkaline earth metals, or a mixture of two or more of such compounds, are suitable. As a rule such compounds decompose and give off vapor at temperatures below 500". The insulating material may be utilized in the pulverulent state, or it may be compressed into the form of blocks, as indicated in the drawing. Asbestos fibers, porcelain, glass or other inert material may be substituted in amounts ranging from one part of asbestos, or other inert material, up to 30 parts of the insulating material, although higher amounts, even up to 80%, of the inert material may be utilized. When asbestos is utilized as a filler, however, the amount should be kept low, say less than 30%, If desired, the insulating material may be provided, especially for some purposes, with a supporting or backing plate or with suitable heat conducting fins.
We prefer to utilize boric acid or gypsum. These materials will extinguish short circuit arcs between conductors spaced one-fourth inch-apart at voltages up to 750 r. m. s. Boric acid is only one-tenth as conductingas talc, and one-thirtieth as conducting as magnesium carbonate. It is not only an excellent insulator, but it also has very favorable arc extinguishing properties, when decomposed in the presence of the arc. Its excellent arc extinguishing properties are undoubtedly caused by the fact that gases are given oil in the form of water vapor, when the material is decomposed in the arc. Boric acid may also be readily compressed into cakes, cylinders or other shapes suitable for use in cables. Pressure in amounts up to 25 tons per square inch has been utilized although the cakes may be formed with much less pressure. For example, cakes suitable for insulating purposes have been formed in laboratory experiments by means oi. simple pressure clamps.
Improved cables, in which our insulating material is utilized in compressed form, have excellent physical properties, and the danger from explosions caused by the decomposition of inflammable gases is completely avoided.
Although we have described our invention in considerable detail, it will be understood that various modifications may be made therein without departing from the spirit of our invention. For example, the insulating material, either with or without fillers, supporting plates or cooling fins, may be utilized for various purposes, such as fuses, or inserts in the arc chamber walls of circuit breakers. The material may also be subject to heat treatment either during or subsequent to the molding operation. The temperature of such heat treatment however, should not exceed the decomposition temperature of the insu-"- lation.
aioaose We claim as our invention:
1. An electrical conductor including an inner conductor and a body of insulating material around said inner conductor, said body of insulating material being capable of extinguishing arcs when subjected to the heat thereof and including at least 20% of horic acid in the free state.
2. A cable comprising an inner conductor, an outer sheath and a quantity of boric acid in the free state between said inner conductor and said outer sheath.
3. A cable comprising an inner conductor, an outer sheath and a quantity of horic acid between said inner conductor and said outer sheath, said boric acid being at least partly in the form of compressed blocks.
4. A cable comprising an outer sheath of Weather-proof material, an inner conductor, and a non-inflammable insulating material between said conductor and said sheath including at least 20% of boric acid in the free state.
5. A cable comprising an outer sheath of weather-proof material, a plurality of inner conductors, said inner conductors being separated from each other and from the outer sheath by a non-inflammable insulating material including at least 20% of boric acid in the free state.
6. A. cable comprising an outer sheath of Weather-proof material, an inner conductor and a non-inflammable insulating material between said conductor and said sheath including boric acid partly in powdered form and partly in solid I block form.
'1. A cable comprising an outer sheath of weather-proof material, a plurality of inner conductors and means of insulating material separating said conductors from each other and from the outer sheath, said means comprising a quantity of boric acid in the free state.