US2157815A - Electrical protective device - Google Patents

Description

May 9, 1939. v. BOYER V ELECTRICAL PROTECTIVE DEVICE Filed Jan. 27, 1938 Fifl.

:L V/ vY//// /////////////////////////////////////////////A n y fim w w u w n A w s h m i aw/H R Patented May 9, 1939 UNITED STATES PATENT OFFICE Ralph V. Boyer, Pittefleld, Masa., assignor to General Electric Company, a corporation o! New York Application January 27, 1938, Serial No. 1s7,2o9

12 claims.

This invention relates to electrical protective devices. The invention is more particularly concerned with electrical devices having terminals between which an arc is formed and a protective insulating structure disposed adjacent and in confining relation to said arc. Ex'amples of such devices are electric circuit interrupters, for instance, circuit interrupters of the air-break expulsion type, space discharge devices for draining excess voltages from transmission lines (so-called "lightning arresters") arc chute barriers and the li e.

In a well-known form of circuit interrupter of the air-break expulsion type, the arc is drawn into a. narrow channel or tube composed of, or

suriaced with, material which, under the infiuence of the heat of the arc, emits gases or vapors which at least in part aid in extinguishing the arc. Inorganic materials such as, !or example, boric acid, ammonium alum, etc., have been proposed as the gas-emitting material.

These vapor-producing inorganic materials, when used alone, in general lack desirable mechanical characteristics such as strength and rigidity,

2 which are necessary in connection with the explosive pressures developed during interruptionof the circuit.

Most other inorganic and Organic materials while they may be ormed readily in tubular and 30 other shapes and have inherently high mechanical strength introduce certain disadvantages in the use of circuit interrupter tubes. Inorganic materials such as quartz, alundum, steatite, etc., fuse and become conducting when 35 subjected to the high are temperatures. Organic materials such as phenolic resin compounds, etc., are decomposed in the heat of the arc resulting in a partial reduction to carbon. The conducting surfaces thus formed with *both types of mam terial result inutter failure to extinguish the arc and interrupt the circuit, particularly in the case of high voltages. Vulcanized flbre, the only herebefore known material which is not so affected by the heat of the arc and which is other- 45 wise suitable for interrupter tube construction, absorbs water to such an extent that unless a tube formed of such material be protectively covered with a moisture-resisting composition, the tube swens, warps and soon becomes useless.

50 Also, the high degree of water absorption crorganic flbers tends to cause formation of low resistance "creepage surface, which decrease the resistance to flash-over and permit the formation oi carbonized paths ("tracking"), particu- 55 larly in the case of high voltages. Even in indoor installations a fiber tube may absorb a harmful amount of moisture. Another disadvantage in the use oi tubes formed of fibrous material alone is the fact that frequent heating of the tube resulting from the PR. losses in the cur- 5 rent-carrying parts, under normal or unusual service conditions, tends to drive off the chemical water, thus partially reducing the material to carbon. This produces brittleness and inability to withstand the arc gas pressures produced dur- 10 ing current interruption, especially with the currents of high magnitude at the high voltages.

It is a principal object of the present invention to provide electrical protective devices comprising relatively inexpensive arc-extinguishing or ex- 15 pulsion tubes of long service life which are substantially non-hygroscopic and non-tracking, of good mechanical strength and have desirable electrical and arc-extingu shing characteristics, that is, are capable of producing gas under an arc while maintaining a non-conducting surface throughout the process of arc extinction.

The stated objects are attained, in accordance with the present invention', by the use of expulsion tubes formed of, or surfaced with a polymerized acrylic compound such as a polymerized alkyl acrylate, and preferably with polymeric alpha methyl methacrylate (methyl alpha methacrylate). The scope of the invention also includes tubes formed of such material in combination with carrier materials, for example, Organic fibrous materials such as linen, cotton, paper, wool and the like.

My invention will be more fully set forth in the following description with reference to the accompanying drawing, and the features of novelty which characterize my invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.

Referring to the drawing: Fig. 1 is an elevational View partly in section of an electric circuit breaker of the expulsion type to which the present invention is applicable; Fig. 2 is a sectional View of an expulsion fuse unit consisting of a tuse holder of conventional design and a renew- 4 able fuse link, and Fig. 3 is an enlarged sectional view of the fuse link shown in Fig. 2.

There is shown merely by way of example in Fig. 1 a circuit breaker having means, such as the statonary contact l and the movable rod contact 2, for opening the circuit so as to form an are, and an insulating structure 3 forming an arc chamber for closely confining the arc between the Contacts. The insulating structure 3 consists of a tubular member closed at the one end and open at the other end for receiving the rod contact. Upon opening of the circuit, separation of the contacts :I and 2 causes formation of an arc which tends to decompose some of the material of the arc chamber walls. The air already in the arc chamber and the gas emitted from its walls is under considerable pressure at the heat of the arc by reason of the comparatively close fit of the rod contact 2 in the tube 3, 'with the result that when the rod leaves the tube a blast of gas is released, as indicated, and the arc is interrupted.

'In accordance with one embodiment of the present invention switch tube 3 is composed of, or surfaced with, a p'olymerized acrylic compound such, for instance, as polymeric alpha methyl methacrylate. Polymerized methyl methacrylate is a glass-like, substantially water-impermeable material, having a softening point above about' 100 C. and characterized by its toughness, hardness and thermoplasticity. It is formed by polymerizing liquid monom'eric alpha methyl methacrylate produced as described, for example, in Hill Patent No. l,980,483, with the aid of heat or light, or heat and light, with or without a Catalyst such as oxygen, ozone, an organic or inorganic peroxide, an ozonide, or other agent which gives cit oxygen.

In accordance withianother embodiment of the invention switch tube 3 comprises Organic fibrous material in combination with polymeric alpha methyl methacrylate. The fibrous material may be impregnated either in a compressed or in a non-compressed (loosened) state with a suitable amount of 'said polymer. If solid compressed fibrous material be impregnated with the mono meric reshi, polymerization of the same con= veniently may be efiected in situ. Either monomeric or polymeric alpha methyl methacrylate may be employed in impregnating the loosened fibers. When using the monomer in this manner, it may be partly polym'erized simultaneously with the iormation of tubes from the mass and polymerized further in situ. On the other hand, ii desired, the polymeric resn in finely divided state may be thoroughly mixed with the flbrous mate- Trial and the mass then molded into tubes of the desired size and shape. The inner surface of tube that is the surface in close proximity of the arc fot-med when thecircuit is interrupted, preferaloly comprises solid polymerized methyl methacrylate, alone or mixed with a suitable inert ller.

The polymerized methyl methacrylate used in 'the manner hereinbefore set iorth may contain, as desired or as may be required, non-conducting filling material such, for example, as quartz, magnesia, alumina, i'lint or other materials of high thermal conductivity, wcod flour, china clay, ashestos, spun glass in woven, felted or other form, mica, etc. ;it may also contain coloring materials, as well as plasticizing or soitening agents-such, i'or instance, as phthalates, waxes, Vegetable or animal oils, particularly no-n-drying or semidrying oils, chlorihated diphenyls, esters 'of poly- -hydric alcohols, polymerized or co-pclymerized esters of acrylic or methacrylic acid, -o' other addition or modifying agents.

To increase the gasemitting properties of the tubes or otherwise to improve its useful properties, urea, urea-formaldehyde resins, urea' phosphates, or other compounds of urea, ammonium carbonate or ammonium bicarbonate or other organic or inorganic substances which in the circuit-interrupting process emit gases (nitrogen, carbon dioxide, water vapor, etc.) that extinguish or aid in extinguishing the arc, may be added to the methylmethacrylate, or other acrylic compound employed, prior to forming a tube comprising the same. Or such materials may be incorporated with a fibrous or other carrier material prior to being impregnated or otherwise brought into intimate association with the methyl methacrylate. In a similar manner substances that are carriers of oxygen, which is released in the heat of the arc as such or in the form of an oxidizng gas, may be incorporated in or with the tube during or 'after its fabrication, so that carbon produced in the tube under the influence of the arc will not be deposited as soot on the surfaces of the tube but instead will be converted to carbon monoxide and carbon dioxide and thus will be more readily expelled. Common examples of such oxidizing agents and illustrative of those which may be utilzed in carrying this embodiment of my invention into effect are oxides, chlorates, perchlorates, perborates, nitrates and ni trites that decompose in the heat of the arc.

For certain applications a tube of porous refractory ceramic material impregnated with polymerized methyl methacrylate, or other suitable polymerized acrylic compound, may be used advantageously, for example, as a liner of vulcanized fiber fuse tubes of the expulsion type.

Fig. 2 is illustrative of a iuse holder having within a main tube portion an inner or axlliary expulsion tube surrounding a renewable fu'se link. The casing or fuse holder of the expulsion fuse unit shown in Fig. 2 consists of a fuse tube' d comprising, or having an insulating base material surfaced with a polymerized acrylic compound; as, for instance, polymeric alpha methyl methacrylaw alone or modified as described with reference to the tube 3 (Fig. 1). The Contacts 5 and ti are mounted at either end of the fuse tube 6 for connecting the fuse unit in the circuit by mounting in a suitable iuse support or otherwise. The cap M, which closes one end of the tube l, is threaded onto the contact providing a clamped electrical contact with the button head cap &2 of the fuse link 9. Other suitable clamping means i and ti are provided for similarly clamping the lower terminal [lt of the i'use link ti to the contact 5. The explosion chamber of the fuse-holder consists of the central bore of the use tube and the chamber formed by the contact 6 and its extension iii. The walls of the iuse tube i are formed of a substance comprising or having an insulating base material surfaced with polymerbefore described with referenceto the tube 3 (Fig. l) and the tube 'i (Fig. 2). The fusible element id may be of tin or other usible metals or alloys either in wire or strip form, soshaped that blowing occurs near the upper end of tube M. The tube iii is closed at one end by a stopper iii which is cemented in position in. the end of the tube to' seal the end. A stopper i@ is placed in the bottom of the tube i l with a snug fit to permit gas pressure to build up within the tube when the fuse link is blown by a light current.

However, this stopper is not indispensable and may be omitted. The fusible element ll is connected by means of a hard metal wire l'l which passes through stopper |5 to the button !2. The other end of fusible element |3 is connected to theterminal wire !8 which passes through the stopper IG. The tube ll is provided to obtain an auxiliary explosion chamber surrounding the fusible element l3, which chamber is small enough to produce the necessary gas pressure for the operation of the fuse on light currents. When a heavy current fault occurs the extremely high gas pressure, generated in a manner such as hereinbefore described with particular reference to Fig. 1, bursts this tube and permits the gas to expand within the larger explosion chamber, thus reduclng the gas pressure to a safe value and furnishing the normal arc-extinguishing action of the larger chamber. 'I'hus the combination of the small tube ll of the fuse link 9 with the larger tube 4 produces an expulsion fuse which has improved Operating 'characteristics on small curhents and yet operates with equally satisfactory characteristics on high short-circuit current values;

In a fuse structure such as has been described with particular reference to Flgs. 2 and 3, either or both of the main and auxiliary tubes may be surfaced on either their inner or outer, or on both their inner and outer, body portions with a substance comprising a, polymerized acrylic compound such as methyl methacrylate resln in a solid state of polymerization. Preferably, however, at least the inner surfaces of both of such tubes comprise such resin.

It is to be understood that this invention is not limited to the specific structures described and illustrated. 4 The invention may be utilized with any type of electric' circuit interrupter or fuse structure having as an essential element an insulating member disposed adjacent and in conning relation to an arc formed when the circuit isopened. For example, the arc-confining structures shown in such United States patents as, for example, 1,828,659, Hinds; 1,962,654 and 2,'081,813, Hermann; 2,081,826, Lincks; 1,398,981, Tritle; and 1,742,565, Traver, may comprise polymerized methyl methacrylate or similar acrylate as hereinbeiore more fully set forth. The Tritle and Traver patents describe and illustrate structures having arc-chute barriers which, in accordance with the present invention, may comprise acrylic compounds. Such barriers confine an arc in a way that is well understood in the art. The term "arc-confining structure" and similar terminology, as used herein and in the appended claims, includes within its meaning any structure disposed adjacent an arc, whether such structure is in the form of a tube or spacedapart plates or walls. 4

Arc-confining structures formed of, or surfaced with polymerized acrylic compound such as polymerized methyl acrylate, polymerized methyl methacrylate, polymerized mixtures thereof alone or in combination with other polymerized acrylic compounds as, for instance, polymerized ethyl esters of acrylic' and methacrylic acid, or such resins in combination with flbrous or other carrying or filling materials have good insulating and arc-extinguishing characteristics. Tubes formed of these materials do not erode in service use as much as conventional tubes, for example those' formed of vulcanized 'or hornrlber, and

hence have longer service life than tubes heretofore in` common use. Further, metallic particles do not adhere during service use to tubes surfaced with polymerized methyl methacrylate, thus eliminating the possibility of the dielectric breakdown of the surfaces should small particles of metal bumed oif from the contacts during arcing be deposited on the tube surfaces.

Tubes formed wholly of polymerized methyl methacrylate are transparent and this transparency is not appreciably reduced by the deposition of arc products. This facilitates the examination of the interior of fuses, arc chutes, protective gaps and other parts of electrical devices enclosed by walls formed of a transparent material adapted to evolve an arc-interrupting gas in the heat of the arc. For example, when a transparent acrylic compound such, for instance, as polymerized methyl methacrylate is used in constructing arc chutes or arc quenchers of aircircuit breakers, the contacts enclosed therein are always readily visible, thus permitting inspection without physically removing the chutes or quenchers from the breaker.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electric device having terminals between which an arc is formed and an insulating structure disposed adjacent and in conning relation to said arc, said insulating structure being formed of a hard, solid, transparent Organic polymer adapted to evolve 'an arc-interrupting gas in the 'heat of the arc.

2. An electric device having terminals between which an arc is formed and an insulating structure disposed adjacent and in confining relation to said arc, said insulating structure having a surface comprising anacrylic compound polymerized to solid state.

3. An electric device having terminals between which an arc is formed and an insulating 'structure disposed adjacent and in conflning relation to said arc, said insulating structure comprising an alkyl acrylate in a solid state of polymerization.

4. An electric device having terminals between which an arc is formed and an insulating structure disposed adjacent and in conflning relation to said arc, said insulating structure comprising an intimate association of mechanically strong organic material and a substance comprising solid, polymerized methyl methacrylate.

5. An arc-confining structure comprising fibrous Organic material surfaced with polymerized methyl methacrylate in solid state.

6. An electric device having terminals between which an arc is formed and an insulating structure disposed adjacent and in confining relation to said arc, said insulating structure having a surface comprising solid, polymerized methyl methacrylate and a modifying agent which upon decomposition by said arc yields an arcinterrupting gas. 4

7. An electric circuit interrupter comprising means for opening the circuit so as to form an arc, and an insulating structure disposed adjacent and in conflning relation to said arc, said insulating structure having a surface comprising methyl methacrylate in a. solid state of polymerarc-confining structure formed or a porous refractory ceramic material impregnated with methyl methacryate in a. solid state of polymerization.

10. An expulsion fuse including an are-confining structure formed of a substance comprising solid, polymerized methyl methacrylate and an inert filling material.

-11. An expulson fuse including an are-con- 10 fining structure formed of fibrous Organic material coated and impregnated with polymerized methyl methacrylate in solid state.

12. In an expulsion fuse, walls which define a primary fuse chamber and walls which define an ization.

RALPH V. BOYER.

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