CIRCUIT BREAKER WITH ARC CHAMBER VENTS

Abstract: The invention relates to a molded-case circuit breaker with arc chamber vents formed in a housing end wall, and with line terminal members extending into the flow paths of arc gases issuing from the respective vents. For each line terminal member (92), the front cover (14) of the circuit breaker housing is provided with a tubular portion (86) which extends telescopically over the associated terminal member to shield it from contact with arc gases being vented. Preferably, the distal end of the tubular portion (86) is bevelled (102) to provide a syphoning effect causing ambient air to be drawn (arrow 100) through the tubular portion and admixed with the arc gas.

Description: This invention relates generally to electric circuit breakers and, more particularly, to a molded-case circuit breaker with line terminal shields. It has always been somewhat of a problem to control the venting of arc gases generated in a molded-case circuit breaker during contact separation effected under load. This is particularly true of molded-case circuit breakers of relatively small physical size but high current-interrupting ratings and which have wiring terminals disposed in close proximity to the arc gas vents of the breaker so that ionized arc gases issuing from the vents can cause voltage breakdown between the terminals of the circuit breaker and the metallic enclosure in which the breaker ordinarly is mounted when in use. A breakdown of this kind can develop into a ground fault and, if severe enough, can create a phase-to-phase fault outside the circuit breaker. US-A-2 225 791 discloses a circuit beaker having an inclined insulating plate for shielding a terminal screw from contact with ionized arc gases and for assisting to direct gases blast upwardly through an aperture. In this circuit breaker, fastening screws mounting a protective cover part and extending in the vent area are insulated from the arc gases by a tubular sleeve of insulating material. However, this cannot solve the problems mentioned above. US-A-2 727 965 discloses a circuit breaker embodying a protective cover for the breaker terminals for preventing accidental contact with a line terminal. The protective cover has a tubular member which surrounds the terminal. However, the function and the shape of this tubular member is quite different and cannot solve the problems mentioned above. It is the object of the invention to provide an improved protection against the danger of establishing arcing between adjacent terminals due to ionised arc gases bridging them. The invention accordingly resides in a circuit breaker as defined in claim 1. The tubular member, extending from an interior surface of the housing toward the line terminal and telescopically over said portion thereof, in effect diverts the vented arc gas around the line terminal portion, namely, through flow passages defined between surface portions of the housing end wall and the tubular member on opposite sides thereof. Thus, the ionized arc gas being vented will flow around the line terminal portion without touching it, and it will be discharged into the ambient in a direction parallel to the line conductors and without being blasted into the enclosure which houses the circuit breaker. Preferably, the tubular member communicates directly with an opening formed through the housing wall and has its distal end bevelled or tapered and facing downstream of the arc flow path so that vented arc gas flowing past the bevelled end of the tubular member will create a low-pressure region causing cool air to be syphoned from the ambient and to be mixed with the ionized arc gas, thereby cooling it and hence reducing its conductivity. The tubular member preferably is formed as an integral part of the cover of the insulating housing. Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:- Figure 1 is a vertical sectional view of a circuit breaker; Fig. 2 is a horizontal sectional view taken along the line II-II of Fig. 1; Fig. 3 is a fragmentary sectional view of a conventional venting arrangement; Fig. 4 is a fragmentary sectional view of the venting arrangement Figs. 5 and 6 are fragmentary sectional and elevational views, respectively, of the venting arrangement embodying the invention; and Fig. 7 is a vertical sectional view taken along line VII-VII of Fig. 6. The molded-case circuit breaker illustrated in Fig. 1 and generally designated with numeral 10 includes a molded, electrically insulating housing comprising a base 12 and a cover 14 abutting the base along line 16 and secured thereto by means of suitable fasteners, such as screws (not shown). The circuit breaker as illustrated is a three-pole breaker having three pole units each of which includes a line terminal 18, a load terminal 20, a pair of cooperating contacts 30 and 32 one of which is connected to the line terminal 18 through a conductor 19 and the other of which is disposed on a movable contact arm 28, an arc extinguishing device or arc chute 72 for extinguishing electric arcs drawn, as indicated at 70, between the contacts 30,32 upon separation thereof under load, and a trip device 24 for tripping the circuit breaker open in response to predetermined overcurrents occurring in the associated pole unit. The contact arm 28 in each pole unit is connected through a flexible conductor 36, a bimetallic element 38 (forming part of the trip device 24 of the associated pole unit), and a conductor 40 to the load terminal 20, and it is pivotally supported, as at 42, on a contact-arm carrying portion 44 of an insulating crossbar 26 which in turn is pivotally supported in the base 12 for rotational movement of the crossbar about its longitudinal axis. The circuit breaker 10 includes further an operating mechanism 22 which is common to all pole units and located in the center pole unit, being supported therein by and between two support plates 45 (only one is shown) secured to the base 12. The operating mechanism 22 comprises a releasable member or cradle 56 pivotally supported on the plates 45 by means of a pin 58, a pair of toggle links 52 and 54 pivotally connected together by means of a knee pin 60 to form a toggle linkage which is pivotally connected to the cradle 56 at 62 and is pivotally connected at 42 to the crossbar portion 44 associated with the center pole unit. The operating mechanism 22 includes further an operating lever 46 having generally the form of an inverted U with two legs having their ends pivotally supported in generally U-shaped notches 48 formed in the respective support plates 45, and with a bight to which is secured a manually operable insulating handle 50 extending through an opening in the cover 14. Overcenter operating springs 64 are connected under tension between the bight of the operating lever 46 and the knee pin 60 of the toggle linkage 52,54. The cradle 56 has associated therewith a latch 66 which in Fig. 1 is shown in an ineffective position but is engageable, in a known manner, with a latch surface 68 on the cradle 56 so as to releasably latch the latter in a "reset" condition enabling the contacts 30,32 to be closed through movement of the handle 50 causing the toggle linkage 52,54 to be straightened and consequently effecting movement of the contact arm 28 from the contact open position thereof to its contact closed position indicated in Fig. 1 in phantom at 28a. Upon the occurrence of an overload or fault current in any of the pole units, the associated trip device 24 will respond and, acting through a trip bar 47 common to all pole units, will cause the latch lever 66 to release the cradle 56 and thereby enable the toggle linkage 52,54 to collapse under the action of the overcenter springs 64, thereby effecting a contact opening movement of the crossbar 26, all generally as known in the art (e.g. see U.S. patent specifications Nos. 4503408 and 4220935). From Fig. 1, it will also be noted that each contact arm 28 is supported and arranged on the crossbar 44 in a manner adapting it to be electrodynamically driven, upon the occurrence of a fault or short-circuit current above a predetermined level, to the contact open position independently of the crossbar and at current-limiting speed, generally as described in Applicant's EPC patent publication No. 0145990. As mentioned hereinbefore and as generally well known in the art, when the contacts 30,32 are opened under load either manually or automatically, an electric arc 70 is established between them and is driven into the arc chute 72, there to be stretched further, broken up and extinguished. The formation of such an arc is accompanied by the generation of arcing products in the form of ionized gases which must be appropriately vented in order to prevent structural damage to or rupture of the circuit breaker housing due to the resultant pressure increase. For this purpose, the arc chute 72 has vent holes 76 formed in the end or rear wall 74 thereof which is located adjacent the circuit-breaker housing end wall separating the interior of the housing from the end cavity containing the line terminal 18, and said housing end wall is provided in each pole unit with a vent opening 78 formed in an end wall portion which is part of the housing cover. Referring in this context to Fig. 3 which illustrates a conventional arc-gas venting arrangement, it will be seen therefrom that arc gases are vented, as indicated by arrows 82, through a cavity 94 defined in the cover 15 of the conventional circuit breaker housing. Into this cavity 94 there extends the line terminal screw 92 of the associated pole unit, which terminal screw is accessible to a screw driver (not shown) or the like adapted to be inserted through an opening formed in a front wall portion of the cover 15. With this conventional arrangement, the terminal screw 92 lies exposed to the ionised gases being vented, and some of the ionized gas is blown through the screw insertion opening of the cover 15 into the metallic receptacle (not shown) enclosing the circuit breaker. As a result, voltage breakdown may occur between the line terminal including the terminal screw 92 and the line terminals of adjacent poles and/or nearby metal parts of the receptacle. This problem is alleviated by, broadly speaking, providing means for shielding and isolating each terminal screw from direct contact with the ionized arc gases being vented. More specifically, and with particular reference to Figs. 1, 2 and 4, the cover 14 has, for each pole unit, a tubular insulating member 86 extending therefrom toward the associated terminal screw 92 and telescopically over at least that portion thereof which is located in the flow path of arc gases issuing from the respective vent 78 in the end wall portion 80 of the cover 14. As seen best from Fig. 2, each tubular member 86, in conjunction with adjacent surface portions of the cover end wall 90, defines a pair of flow passages 84 extending from the vent 78 round opposite sides of the tubular member 86, and jointly terminating in an outlet 88 provided in the cover end wall and communicating with the atmosphere. Thus, arc gases blown through the respective vent 78 will be vented, again as indicated by arrows 82, through the passages 84 and the outlet 88 without coming into contact with the terminal screw 92, the latter being sheathed in and protected by the tubular member 86 associated therewith. Moreover, since in the preferred embodiment illustrated each tubular member 86 is formed, i.e. molded, integral with the cover 14 and in effect forms an axial extension of a screw insertion opening 87 therein, it affords access for a screw driver to the terminal screw 92 but provides essentially no path for ionized gas to be blown into the metallic receptacle housing circuit breaker when in use. Referring now to Figs. 5, 6 and 7 each tubular member 86 comprises a bevelled distal (i.e. free) end so as to provide the tubular member with a biased or tapered end face 98 (Fig. 5) or 102 (Figs. 6 and 7) facing downstream of the gas flow path, i.e. away from the associated vent 78. Thus, the tubular member 86 still shields the terminal screw 92 from contact with ionized arc gas issuing from the vent 78 but, at the same time, also enables the gas flowing past its bevelled end to create a reduced-pressure region causing cool ambient air to be syphoned through the tubular member 86, as indicated by arrows 100 in Figs. 5 and 7, and to be mixed with the issuing ionized arc gas, whereby the latter is cooled and, hence, rendered less conductive.

Claims: A circuit breaker (10) comprising an insulating housing and, supported therein, at least one pole unit comprising a line terminal (18), a load terminal (20), and a pair of cooperating contacts (30,32) electrically connected in series between said line and load terminals and disposed in an arc chamber (72), and an operating mechanism (22) for opening and closing said contacts, said housing having an end wall including a wall portion (80) which separates said arc chamber from said line terminal and has an arc-gas vent (78) formed therethrough proximate to a portion (92) of said line terminal, characterized in that said line terminal (18) has associated therewith a tubular member (86) which surrounds said portion (92) thereof so as to shield it from contact with arc gases issuing from said vent (78) and that said tubular member (86) communicates with an opening (87) formed through the wall said housing and extends into the flow path of arc gas issuing from said vent (78), said tubular member having a bevelled end (98 or 102) which faces downstream of the flow path so as, upon flow of arc gas therein, to cause ambient air to be syphoned through said opening (87) and to be mixed with the arc gas. The circuit breaker according to claim 1, characterized in that said tubular member (86) extends from an interior surface of said housing toward said line terminal (18) and telescopically over said portion (92) thereof. The circuit breaker according to claim 1 or 2, characterized in that said portion (92) of said line terminal (18) is a terminal screw (92) said opening (87) and said tubular member (86) being axially aligned with the terminal screw so as to render it accessible to a tool adapted to be inserted through said opening (87) and the tubular member (86). The circuit breaker according to any of claims 1,2 or 3 characterized in that said tubular member (86) is formed integral with said insulating housing. The circuit breaker, according to any of claims 1 to 4, characterized in that said insulating housing includes a cover (14) having thereon said interior surface from which the tubular member (86) extends, said tubular member (86) being formed integral with said cover (14). The circuit breaker according to any one of the preceding claims, characterized in that said tubular member (86), together with adjacent surface portions of said end wall (90), defines two gas flow passages (84) which extend from said vent (78) round said tubular member on opposite sides thereof, and to an outlet (88) formed in said end wall and communicating with the ambient.

Inventors: GRUNERT, KURT ALBERT, LEONE, DAVID ANTHONY, PATON, CHARLES RICHARD, WALKER, ROGER EUGENE
Applicants: WESTINGHOUSE ELECTRIC CORPORATION
Application year: 1986
Main classification: H01H73
IPC codes: H01H1, H01H9, H01H73