ARC RESTRICTING DEVICE FOR CIRCUIT BREAKER

Abstract: The present invention provides a circuit breaker of improved performance by providing arc shields (6, 7) and arc runways (601, etc.) together with the contacts (202, 302) on the rigid conductors (201, 301) of the contactors (2, 3) of a circuit breaker, and disposing said arc runways (601, etc.) on the stationary contactor side with a predetermined rela­ tionship to arc extinguishing plates (501) such that a portion of a cut-out (503) in at least one of said arc extinguishing plates (501) substantially overlaps said arc runway (601, etc.).

Description: This invention relates to a circuit breaker and particularly to an improved circuit breaker constructed so as to be able to efficiently cool an arc drawn between the contacts when breaking a fault current. Prior circuit breakers show the drawback that the foot of the arc drawn between the contacts spreads to the rigid conductor of the contactor, on which the contact is mounted, such that it is not possible to cool the arc efficiently. Further, in prior circuit breakers there is inadequate contact between the arc and the arc extinguishing plates such that the arc cooling and extinguishing effects of the arc extinguishing plates are not satisfactory. According to document EP-A-0 054 833 (which falls within the terms of Art.54(3) EPC) a circuit breaker comprises a pair of contactors including rigid conductors with contacts fastened thereto, wherein the contactors are functioning to open and close an electric circuit. With this proposed construction an arc shield is disposed on at least one of the contactors, said arc shield having a resistivity higher than that of the conductors and surrounding the contact. There are also provided arc driving means to drive in a predetermined direction an electric arc struck across the gap between the pair of contactors and an arc runway adjoining the contact provided with said arc shield and extending in said predetermined direction. The arc runway has a resistivity lower than that of the arc shield and is narrower than the side of the contact to which it adjoins. However, with this proposed construction there is also no possibility to cool the arc efficiently. Further relevant prior art is disclosed in US-A-2 244 061. Therefore an object of this invention is to improve the construction of a circuit breaker so that the efficiency of the extinguishing and in particular of the cooling of the arc is raised. For solving this object the invention provides a circuit breaker as specified in claim 1. With such construction of the circuit breaker the foot of the arc is prevented from expanding by the arc shield, and the transfer of the arc by means of the arc runway as well as cooling and extinguishing of the arc by the arc extinguishing plate assembly are effectively carried out. Preferred ways of carrying out the invention are described below with reference to the drawings, in which: Figure 1 (a) is a sectional plan view of a general circuit breaker to which the present invention can be applied; Figure 1(b) is a side sectional view along line b-b of figure 1 (a); Figure 2(a) is a side view of the main parts of the stationary contactor provided in the circuit breaker of figure 1 (a); Figure 2(b) is a plan view of the main parts of the stationary contactor of figure 2(a); Figure 2(c) is a front view of the stationary contactor of figure 2(a); Figure 3 is a model diagram showing the behaviour of the arc drawn across the contacts of the circuit breaker of figure 1(a); Figure 4(a) is a sectional plan view of a circuit breaker according to this invention; Figure 4(b) is a side sectional view along line b-b of figure 4(a); Figure 5(a) is a plan view of the main parts of a stationary contactor provided in the circuit breaker of figure 4(a); Figure 5(b) is a side view of the main parts of the stationary contactor of figure 5(a); Figure 6(a) is a perspective view of the stationary contactor, the movable contactor and the arc extinguishing plate assembly of the circuit breaker of figure 4(a); Figure 6(b) is a perspective view of the movable contactor shown in figure 6(a) seen from the opposite side to figure 6(a); Figure 7 is a model diagram showing the effects of the arc shield provided in the circuit breaker of figure 4(a); Figure 8 is a model diagram showing the behaviour of the arc drawn between the contacts of the circuit breaker of figure 4(a); Figure 9(a) is a sectional plan view showing another embodiment of a circuit breaker in accordance with this invention; and Figure 9(b) is a side sectional view along line b-b of figure 9(a). In the figures, like reference numerals denote identical or corresponding parts. Description will now be made of a conventional circuit breaker to which this invention is applicable, with reference to Figures 1(a) and (b). An enclosure 1 is made of insulating material and forms the housing of a circuit breaker which comprises a pair of electrical contactors a contact arms 2 and 3 which are a stationary contactor and a movable contactor, respectively. An electrical contacting surface of a stationary rigid conductor 201 which forms the main part of the stationary contactor 2, has affixed thereto a stationary-side contact 202, shown in figures 2(a) to (c), while an electrical contacting surface of a movable rigid conductor 301 which forms the main part of the movable contactor 3, has affixed thereto a movable-side contact 302, shown in figure 3. An operating mechanism 4 operates to open or close the circuit breaker by moving the movable contactor 3 in or out of contact with the stationary contactor 2. An arc-extinguishing plate assembly 5 comprising a plurality of arc extinguishing plates 501 supported by frame plates 502 cools an electric arc A struck across the stationary-side contact 202 and the movable side contact 302. The operating mechanism 4 and the arc-extinguhing plate assembly 5 are well known in the art and described, for example, in U.S. Patent 3,599,130. As appears from this U.S. patent, the operating mechanism includes a reset mechanism. An exhaust port 101 is formed in the enclosure 1. In figures 1(a) and (b), when the movable-side contact 302 and the stationary-side contact 202 are in contact, current flows from a power supply side to a load side along a path from the stationary rigid conductor 201 to the stationary-side contact 202, to the movable-side contact 302 and to the movable rigid contactor 301. When, in this state, an over-current such as a short-circuit current flows through the circuit, the operating mechanism 4 operates to separate the movable-side contact 302 from the stationary-side contact 202. At this time, an arc A appears across the gap between the stationary-side contact 202 and the movable-side contact 302, and an arc voltage develops thereacross. The arc voltage rises as the distance of separation of the movable-side contact 302 from the stationary-side contact 202 increases. In addition, at the same time, the arc A is drawn by the magnetic force of attraction in the direction of the arc-extinguishing plate assembly 5, and the arc-extinguishing plates cause the arc to be stretched, thus further raising the arc voltage. In this way, the arc current reaches the current zero point, the arc A is extinguished, and the interruption is completed. During the interrupting operation thus far described, large quantities of energy are generated by the arc A across the gap between the movable-side contact 302 and the stationary-side contact 202 in a short period of time of the order of several milliseconds. In consequence, the temperature of the gas within the enclosure 1 rises abruptly, as does the pressure thereof, and the high temperature and pressure gas is emitted into the atmosphere through the exhaust port 101. The circuit breaker performs the interrupting operation as described above to interrupt overcurrents. The performance required of a circuit breaker that operates in this way is that the arc voltage be high, whereby the arc current flowing during interruption is suppressed, reducing the magnitude of the current that flows through the circuit breaker. Accordingly, a circuit breaker which generates a high arc voltage, exhibits excellent protection performance with regard to all serially connected electrical equipment, including the wiring. Heretofore, in circuit breakers of this type, in order to establish a high arc voltage, the movable conductor 301 was caused to separate at high speed or the arc A was caused to stretch by using magnetic force, but in these cases there was a certain limit to the rise in arc voltage, and satisfactory results were not obtained. Explanation is now given with regard to the behaviour of the arc voltage, etc., across the statonary-side and the movable-side contacts 202 and 302 of the circuit breaker shown in figures 1(a) and (b). In general, the arc resistance R(Ω) is given by the following expression: R=ρ I S where ρ: arc resistivity (Ω.cm) I: arc length (cm) S: arc sectional area (cm²) In general, in a short arc A with a high current of at least several kA and an arc length I of at most 50 mm, the arc space is occupied by contact particles. The emission of metal particles from the rigid conductors occurs orthogonally to the contact surfaces, and at the time of the emission, the emitted particles have a temperature close to the boiling point of the contact metal. Further, whether or not they are injected into the arc space, they are injected with electrical energy, rising in temperature and pressure, and taking on conductivity, and they flow (are blown) out of the arc space at high speed in a direction away from the conductors while expanding in a direction according to the pressure distributon in the arc space. Thus, the arc resistivity ρ and the arc sectional area S in the arc space are determined by the quantity of contact particles produced and the direction of emission thereof. Accordingly, the arc voltage is also determined by the behaviour of such contact particles. The behaviour of such electrode or contact particles is explained in conjunction with figure 3. In figure 3, a stationary contactor 2 and a movable contactor 3 form a mutually confronting pair of contactors in which the movable contactor 3 moves in or out of contact with the stationary contactor 2 to make or break an electric circuit. The surfaces X of the respective contactors 2 and 3 are opposing surfaces which become contact surfaces when the contactors 2 and 3 make contact, and the surfaces Y of the respective contactors 2 and 3 indicate the electrically contacting surfaces of the contactors other than the respective opposing contact surfaces X. A contour Z indicated by a dot-and-dash line in figure 3 indicates the envelope of the arc A struck across the contactors 2 and 3. Further, the contact particles emitted from the contactors 2 and 3 are shown in model form by "a", "b" and "c", wherein "a" represents the contact particles emitted from the vicinity of the center of the opposing surfaces X, "b" represents particles emitted from the other surfaces Y of the contacts and the rigid conductors 201 and 301, and "c" represents the contact particles emitted from the peripheral region of the opposing surfaces X, at a position lying between the areas of origin of the particles "a" and "b". The paths of the particles "a", "b" and "c" after emission respectively follow the flow lines shown by arrows "m", "n" and "o". Such contact particles "a", "b" and "c" emitted from the contactors 2 and 3 have their temperature raised from approximately 3,000°C, the boiling point of the metal of the contactors, to a temperature at which the metal particles take on conductivity, i.e. at least 8,000°C, or to the even higher temperature of approximately 20,000°C, and so energy is taken out of the arc space and the temperature of the arc space lowers, the result of which being to produce arc resistance. The quantity of energy taken from the arc space by the particles "a', "b" and "c" increases with the rise in the temperature, and the degree of rise in temperature is determined by the positions and emission paths in the arc space of the electrode particles "a", "b" and "c" emitted from the contactors 2 and 3. However, in figure 3, the particles "a" emitted from the vicinity of the center of the opposing surfaces X take a large quantity of energy from the arc space, whereas the particles "b" emitted from the surfaces Y of the contacts and rigid conductors, compared to the particles "a", take little energy from the arc space, and further the particles "c" emitted from the peripheral portion of the opposing surfaces X take out only an intermediate amount of energy approximately midway between the amounts of energy taken by the particles "a" and "b". That is to say, within the range in which the particles "a" flow (are projected), it is possible to take out or absorb large quantities of energy and to lower the temperature of the arc space, and hence to increase the arc resistivity ρ, but within the range in which the particles "b" and "c" flow, large quantities of energy are not taken out, and so the lowering of the temperature in the arc space, too, is small, and so an increase in the arc resistivity ρ cannot be achieved. Moreover, since the arc is produced from the opposing surfaces X and the contactor surfaces Y, the arc's cross-sectional area increases, and the arc resistance is consequently lowered. This energy outflow from the arc space due to the contact particles is proportional to the electrically injected energy, and so if the quantity of particles "a" produced between the contacts 202 and 302 and injected into the arc space were increased, the temperature in the arc space would, of course, be greatly lowered, with the result that the arc resistivity could be increased and the arc voltage greatly raised. This invention extends the limits with regard to the increase in arc voltage in conventional circuit breakers as described above, and by means of arc shields provided on the electrical contactors it is able to increase the quantity of contact particles produced between the contacts and which are injected into the arc space, and to greatly raise the arc voltage, to improve the current limiting effect of the circuit breaker, and particularly as is to be described below, by specifying the positional relationship between an arc runway provided adjacent to the contact on the stationary contactor and a cut-out slit in the arc extinguishing plates directly thereabove, it is possible to cause the arc to travel rapidly from the contacts to the arc extinguishing plates, to extinguish the arc effectively. This travelling and extinguishing of the arc permit both to raise the interruption performance of the circuit breaker and to prevent wear of the contacts. Figures 4(a) and (b) are a sectional plan view and a side sectional view, respectively, of a circuit breaker according to an embodiment of the present invention. Figures 5(a) and (b) are a pian view and a side view, respectively, of the contact portion of a stationary contactor. Figure 6(a) is a perspective view showing the relationship between the stationary contactor, the movable contactor, and the arc extinguishing plates, and figure 6(b) is a perspective view of the movable contact portion. In these figures, the arc shields 6 and 7 are respectively affixed to the stationary rigid conductor 201 and the movable rigid conductor 301 so as to cover and conceal therebehind the portions of the rigid conductors 201 and 301 other than the contacting surfaces of the respective contacts 202 and 302. Additionally, slits are provided in the arc shields 6 and 7 to extend in the lengthwise direction of the rigid conductors 201 and 301 from the contacts 202 and 302, in the direction of the arc extinguishing plate assembly 5, to form arc runways 601 and 701. In this embodiment, the end portion of the arc runway 601 on the stationary contactor side is positioned such that it lies directly under at least one of the arc extinguishing plates 501, when viewed in plan view as in figure 4(a). That is to say, the arc runway 601 extends in the direction from the contact 202 towards the arc extinguishing plate assembly 5 farther than the cut-out slit 503 in at least one of the arc extinguishing plates 501, traversing the bottom edge of the cut-out slit 503. In practice, it is best to think of the arc extinguishing plate or plates 501 as overlapping the arc runway, or being transected by an imaginary line extending vertically upwards from the periphery of the arc runway 601 as defined by the cut-out portion of the arc shield 6. The material of the arc shields 6 and 7 has a resistivity higher than the resistivity of the material forming the respective rigid conductors 201 and 301. For example, this material may be an organic or inorganic insulator or a high resistivity metal, such as copper-nickel, copper-manganin, manganin, iron-carbon, iron-nickel, or iron-chromium, etc. In this embodiment, the arc shields are formed with a plate shape, but in instances where they are formed to cover the rigid conductors, particularly on the movable contactor 3 side, the weight can be made low, making the moments of inertia low, for a high opening speed. The effects of employing plate-shaped arc shields in this embodiment are as stated hereinbelow. In the instance of a circuit breaker rated at 100A, the contacts 202 and 302 according to this embodiment would suitably measure substantially 4.5 mm by 4.5 mm, and the arc runways 601 and 701 would suitably be approximately 2 mm in width. The arc extinguishing plates 501 may be constructed of a magnetic or a non-magnetic material, but if they are constructed of a non-magnetic material, it is possible to eliminate the problem of a temperature rise in the arc extinguishing plates due to eddy currents that occur with magnetic materials. Next the operation of the above construction is explained. The operation is substantially the same as that of the circuit breaker shown in figures 1(a) and (b), so that the explanation may be brief, but the behaviour of the contact particles between the two contacts differs from that of the device shown in figures 1(a) and (b), and so this is explained hereinbelow in conjunction with figure 7. In Figures 7 and 8, the arc shields 6 and 7 are provided together with the contacts 202 and 302 on the rigid conductors 201 and 301. The arc shields 6 and 7 are formed with a plate shape, and are disposed to surround the contacts 202 and 302 so as to cover and conceal the rigid conductors therebehind. In Figure 7, X, "a", "c" and "m" denote the same items as in figure 3, and the dot-and-dash line Z₁ indicates the envelope of the space of arc A contracted by the circuit breaker of this invention, while the dot-and-dash line Z₂ (figure 8) indicates the envelope of the arc A when the arc spot is shifted in the arc runways 601 and 701. The arrow o₁ indicates the flow lines of the contact particles "c" which in the circuit breaker of this invention flow (are projected) in a path different from that in the prior device, and the intersecting oblique (hatching) lines Q indicate the space in which the pressure generated by the arc A is reflected by the arc shields 6 and 7, raising the pressure which was lowered in the prior device without the arc shields 6 and 7. The electrode particles between the contacts in the circuit breaker of this invention behave as follows. The pressure values in the space Q cannot exceed the pressure value of the space of the arc A itself, but much higher values are exhibited, at least in comparison with the values attained when the arc shields 6 and 7 are not provided. Accordingly, the relatively high pressure in the space Q produced by the arc shields 6 and 7 acts as a force to suppress the spread of the space of the arc A, and the arc A is confined to a small area. In other words, the flow lines of the contact particles "a" and "c" emitted from the opposing surfaces X are narrowed and confined to the arc space. Thus, the contact particles "a" and "c" emitted from the opposing surfaces X are effectively injected into the arc space, with the result that a large quantity of effectively injected contact particles "a" and "c" take a quantity of energy out of the arc space of a magnitude that greatly exceeds that taken out in the prior device, thus markedly cooling the arc space and hence causing a marked increase in the arc resistivity ρ, i.e. the arc resistance R, substantially raising the arc voltage. Further, the pressure between the contacts 202 and 302 is raised as above described, and so a strong gas flow towards the exhaust port 101 is produced, and the arc spot on the contacts 202 and 302 runs on the arc runway 601 and 701 in the arc shields 6 and 7, as shown by Z₂ in figure 8. For this reason, wear of the contacts 202 and 302 is radically reduced, and the length of the arc is increased, enabling a remarkable current limiting effect to be achieved. Furthermore, since the arc column positively contacts the bottom edge of the cut-out slit 503 in the arc extinguishing plates 501 and is thus cooled, the heat exchangeability of the arc heat is improved, and so less accordingly, is the interruption performance. Figures 9(a) and (b) are respectively a sectional plan view and a side sectional view of a circuit breaker according to another embodiment of the present invention. In the arc extinguishing plates 501 in the circuit breaker of this embodiment, an additional slit 504 is provided at the bottom of the cut-out portion 503. Said slit 504 extends parallel to the arc runway 601 on the side of the stationary contactor 2, and its width t₁ is the same as the width t₂ of the arc runway 601, or is smaller than the same, i.e. the relationship t₁≦t₂ is formed. When the circuit breaker operates, the arc A drawn across the stationary contact 202 and the movable contact 302 runs along the arc runways 601 and 701 in the direction towards the arc extinguishing plate assembly 5, as shown in figure 8. In this instance, the width of the arc positive column comes to be substantially the same as the width t₂ of arc runways 601 and 701, because of the arc shields 6 and 7, and so the arc A running in the abovementioned runways 601 and 701 contacts the arc extinguishing plates 501 with the slits 504 of width t₁, which is the same as or smaller than the abovementioned width t₂, during the run in the runways 601 and 701, and the arc A is thus cooled and extinguished. If the width t₁ of the slits 504 at the bottom of the abovementioned cut-outs 503 were larger than the width t₂ of the arc runway 601, the arc A would contact the arc extinguishing plates 501 at the end of the slits 504, thus extinguishing the arc, but by making the slits 504 narrower than the width t₂ of the arc runway 601, as in this embodiment, cooling is effected while the arc A is still running, thus aiding the arcextinguishing effect of the arc extinguishing plates 501.

Claims: 1. A circuit breaker comprising a pair of rigid contact arms (2,3) with contacts (202,302) directly affixed thereto, said contact arms (2,3) functioning to open or close an electric circuit and including an arc shield (6,7) disposed on each of said contact arms (2,3) so as to cover and conceal therebehind the portions of the particular contact arm (2,3) other than the conducting surfaces of the respective contacts (202,203) and having a resistivity higher than that of said contact arms (2,3); an arc runway (601,701) formed on at least one of said contact arms (2,3) by a slit within the particular arc shield (6,7) so that said runway (601,701) is obtained by the surface of the contact arm (2,3) underneath the slit, one end of the slit lying adjacent to said contact (202,302); and an arc extinguishing plate assembly (5) comprising a plurality of arc extinguishing plates (501) for extinguishing an arc drawn across the contacts (202,302) during the operation of opening the contacts (202,302), wherein each of said arc extinguishing plates (501) has a cut-out portion (503) to be contacted by said arc arranged in such a way that the bottom of the periphery of said cut-out portions (503), when viewed in plan view, substantially overlaps the end portion of the other end of the arc runway (601) formed on said stationary contact arm (2). 2. Circuit breaker as claimed in claim 1, wherein in the aerea of the bottom of the cut-out portions (503) the arc extinguishing plates (501) are provided with slits (504) the width of the same being substantially identical or smaller than the width of said arc runways (601, 701) whereby the lateral edges of those slits (504) are disposed in such a way that they overlap substantially the side portions of said arc runways (601) when viewed in the plan view.

Inventors: HISATSUNE, FUMIYUKI, TERACHI, JUNICHI, YAMAGATA, SHINJI, YOSHIYASU, HAJIMU
Applicants: MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1982
Main classification: H01H9
IPC codes: H01H9