Switch and arc extinguishing material for use therein

Abstract: A switch comprising a switch case, contacts adapted to be opened and closed, an arc extinguishing chamber disposed in the vicinity of the contacts, and an arc extinguishing material capable of reducing the amount of metal particles and free carbons to be scattered from components disposed within the switch by an arc generated when the contacts are opened or closed or capable of insulating the metal particles and the free carbons to convert into an insulator, thereby inhibiting a decrease in arc resistance expected to occur upon the generation and extinction of the arc and a decrease in insulation resistance expected to occur within and around the arc extinguishing chamber and at inner wall surfaces of the switch case upon and after the extinction of the arc. The switch according to the present invention is applicable to a switch expected to generate an arc upon interruption of electric current flowing therethrough such as a circuit breaker, current-limiting device or electromagnetic contactor and is capable of immediately extinguishing the arc and inhibiting the decrease in insulation resistance within and around the arc extinguishing chamber and at inner wall surfaces of the switch case.

Description: The present invention relates to an arc extinguishing material according to the precharacterizing part of claim 1. The invention further relates to a switch, such as a circuit breaker, current-limiting device or electromagnetic contactor, which is expected to generate an arc when the current passed therethrough is interrupted, said switch comprising said arc extinguishing material capable of immediately extinguishing the arc and inhibiting a decrease in insulation resistance within and around an arc extinguishing chamber of the switch and at inner wall surfaces of the switch box. In a switch kept applied with an overcurrent or rated current, when the contact of a moving contact element is opened from the contact of a fixed contact element, an arc is generated between the two contacts. To extinguish this arc, there is used an arc extinguishing device 8 as shown in Fig. 1-14 having insulator-(1) 1 and insulator-(2) 2 provided around a region where arc 9 is expected to generate between the moving contact 4 (not shown) of moving contact element 3 fixed movably by axis 7 and the fixed contact 5 of fixed contact element 6. The term "contact portion" on "contact section" as used herein means a portion where the contact point 4 or 5 is located and which includes the contact point and its peripheral portion in the contact element. The insulator (1) 1 and insulator (2) 2 of the arc extinguishing device 8 generate a thermal decomposition gas owing to the arc 9, and the thermal decomposition gas cools and extinguishes the arc 9. Examples of such arc extinguishing devices include one employing an insulator comprising polymethylpentene, polybutylene or polymethyl methacrylate and 5 to 35 wt% of glass fiber included therein, one employing an insulator comprising an acrylic acid ester copolymer, aliphatic hydrocarbon resin, polyvinyl alcohol, polybutadiene, polyvinyl acetate, polyvinyl acetal, isoprene resin, ethylene-propylene rubber, ethylene-vinyl acetate copolymer or polyamide resin, and 5 to 30 wt% of glass fiber included therein, and one employing an insulator comprising a melamine resin containing at least two of ε-caprolactam, aluminum hydroxide and an epoxy resin. If the width W of the insulator (2) 2 is reduced as compared to a typical one for the purpose of scaling down the arc extinguishing device, the distance between the insulator (2) 2 and the plane including the locus of an opening or closing movement of the moving contact element is shortened, with the result that the pressure of the thermal decomposition gas generated from the insulator (2) 2 by the arc rises as compared to the case of the typical insulator. Further, if the decrease in distance between the aforesaid plane and the insulator (2) 2 causes the insulation resistance of the inner wall surfaces of the insulator (2) 2 extending along that plane to decrease, an arc current is more likely to flow in the inner wall surfaces than in the typical switch. During generation of an arc in a switch, metal particles are scattered from the contact elements, contacts and other metal components existing adjacent the contacts in an arc extinguishing chamber and are deposited onto wall surfaces within and around the arc extinguishing chamber. A conventional switch does not take a measure for the problem of such scattered metal particles. When the arc extinguishing device is scaled down, however, the density of the scattered metal particles adhering to the wall surfaces within the arc extinguishing chamber is increased, so that the insulation resistance of such wall surfaces is considerably lowered. Further, if the distance between the insulator (2) 2 and the aforesaid plane is shortened, the pressure of thermal decomposition gas to be generated from the insulator (2) 2 by an arc is increased to scatter the metal particles farther than in the conventional switch, so that the insulation resistance of wall surfaces existing outside the arc extinguishing chamber is also considerably lowered. Such scattered metal particles may reach and adhere to the inner wall of the switch box. To realize a switch having the arc extinguishing device 8 miniaturized and exhibiting an improved current limiting or interrupting property, the provision of the insulator (1) covering a contact portion from which an arc will be generated or the insulator (2) disposed on opposite sides of the aforesaid plane or around the contact portion is effective. In this case, the arc extinguishing property of the insulators (1) and (2) is required to be enhanced. Where the moving contact element or fixed contact element is reduced in cross-sectional area as compared to the conventional one for the purpose of miniaturizing the arc extinguishing device 8, the electrical resistance thereof is increased and, hence, the temperatures of the contact portion and the periphery thereof at the time when current is being applied to the switch are raised to higher temperatures than in the conventional switch. For this reason, the insulators (1) and (2) are required to have a higher heat resistance than the conventional ones. As described above, where the width W of the insulator (2) is reduced as compared to that of the conventional one in order to miniaturize the arc extinguishing device 8, the distance between the insulator (2) and the plane including the locus of the opening or closing movement of the contact element is shortened, resulting in increase of the pressure of thermal decomposition gas to be generated from the insulator (2) by arc. Therefore, the insulators (1) and (2) are required to have a higher pressure withstand strength than the conventional ones. Further, if the distance between the aforesaid plane and the insulator (2) is shortened, the insulator (2) will be much more consumed by arc. Hence, the insulator (2) is required to have improved consumption-by-arc resistance, specifically to such a degree that a hole is not formed therein. As described above, with the miniaturization of the arc extinguising device 8, the metal scattered and deposited on wall surfaces within and around the arc extinguishing chamber causes the insulation resistance of the wall surfaces to be considerably decreased. Accordingly, it is required to insulate the metal particles to be scattered from metal components existing within the arc extinguishing chamber at the time of arc generation to prevent the decrease in the insulation resistance of the wall surfaces attributable to a metal layer formed of such deposited metal particles. According to document EP-A-346 824 , there is disclosed a molding composition suitable for producing molded articles of polyamide having improved dimensional accuracy, dimensional stability, deformation, resistance and mechanical properties. This composition comprises a polyamide or a blend of a polyamide and a vinyl polymer, and an inorganic filler consisting of a glass filler composed of glass fibres and glass beads and calcium carbonate. In Römpps Chemie-Lexikon, 7th ed., Stuttgart 1972 (pages 1486 and 1584 ), glass fibers made of so called E-glass containing less than 1% K 2 O+Na 2 O are described. In Elektrie, 37, 1983, p. 656ff, D. Amft et al. report an arc-extinguishing material which comprises a polyamide resin reinforced by a glass fiber having a low content of alkaline metals. EP-A-0 571 241 refers to a flame retardant material comprising glass fibers as an inorganic filler, polyamide as a matrix resin and magnesium hydroxide. The object of the present invention is to provide an improved arc extinguishing material having excellent properties such as arc extinguishing property, heat-resistance, pressure resistance and resistance to consumption by arc. A further object of the invention is to provide a switch comprising such an arc extinguishing material. The object of the invention is achieved by means of the combination of the features defined in claims 1 and 6, respectively. Preferable embodiments of the arc extinguishing material are set forth in the subclaims. According to the present invention, there is provided an arc-extinguishing material comprising an arc-extinguishing insulator molded product having a double-layered structure, said product comprising: an arc-receiving layer made of a reinforced arc-extinguishing insulator composition comprising 5 to 20 % by weight of at least one filler selected from the group consisting of a glass fiber containing not more than 1 % by weight of compounds of group 1A metals of the periodic table in total, an inorganic mineral containing not more than 1 % by weight of compounds of group 1A metals of the periodic table in total and a ceramic fiber containing not more than 1 % by weight of compounds of group 1A metals of the periodic table in total, and a matrix resin containing as a principal component at least one member selected from the group consisting of a polyolefin, an olefin copolymer, a polyamide, a polyamide polymer blend, a polyacetal and a polyacetal polymer blend; and a base layer underlying said arc-receiving layer and made of an arc-extinguishing insulator composition comprising 20 to 65% by weight of at least one filler selected from the group consisting of a glass fiber, an inorganic mineral and a ceramic fiber, and a matrix resin containing, as a principal component thereof, a thermoplastic resin or a thermosetting resin. Further, the present invention provides a switch comprising a contact section including contacts (4, 5) from which an arc is generated, and an arc extinguishing device (8) comprising an insulator (2) disposed on both sides with respect to a plane including the locus of an opening or closing movement of the contacts or around the contact section, wherein the insulator (2) is formed of an arc extinguishing material according to the present invention defined in claims 1 to 3. In the following the invention is further illustrated by embodiments with reference to the accompanying drawings in which: Fig. 1-1 is a schematic side view showing the closed state of an arc extinguishing device (III) according to the present invention; Fig. 1-2 is a schematic side view showing the opened state of the arc extinguishing device (III) according to the present invention; Fig. 1-3 is a schematic plan view showing the opened state of a general arc extinguishing device ; Fig. 1-4 is a schematic plan view showing the closed state of an arc extinguishing device (III) of which insulator (2) is of double-layered structure according to the present invention; Fig. 1-5 is a perspective view illustrating an insulator (1) molded from an arc extinguishing material composition according to the present invention; Fig. 1-6 is a perspective view illustrating a reference embodiment of an insulator (2) of single layer structure molded from an arc extinguishing material composition; Fig. 1-7 is a perspective view illustrating another reference embodiment of an insulator (2) of single layer structure molded from an arc extinguishing material composition; Fig. 1-8 is a perspective view illustrating an embodiment of an insulator (2) of double-layered structure molded from an arc extinguishing material composition according to the present invention; Fig. 1-9 is a perspective view illustrating another embodiment of an insulator (2) of double-layered structure molded from an arc extinguishing material composition according to the present invention; Fig. 1-10 is a perspective view illustrating Yet another embodiment of an insulator (2) of double-layered structure molded from an arc extinguishing material composition according to the present invention; Fig. 1-11 is a schematic side view showing the opened state of an arc extinguishing device (I) having an insulator (1); Fig. 1-12 is a perspective view showing the opened state of an arc extinguishing device (II) having an insulator (2) according to the present invention; Fig. 1-13 is a schematic side view showing the opened state of the arc extinguishing device (II) having the insulator (2) according to the present invention; Fig. 1-14 is a perspective view of a conventional arc extinguishing device for illustrating an arc generation state; Fig. 1-15 is a schematic plan view of the closed state of the conventional arc extinguishing device; The present invention concerns arc extinguishing insulative material compositions, molded products of those arc extinguishing insulative material compositions and arc extinguishing devices using the compositions and the molded products. More specifically, the present invention relates to arc extinguishing devices for use in circuit breakers, current limiting devices, electromagnetic contactors and the like, each of which generate an arc in the casing thereof when the current passing therethrough is interrupted, and to arc extinguishing insulative material compositions and arc extinguishing insulative molded products for use in such arc extinguishing devices. In circuit breakers, current limiting devices, electromagnetic contactors and the like, when the contact of a moving contact element is opened from the contact of a fixed contact element with an overcurrent or rated current being passed through those contacts, an arc is generated between the two contacts. To extiguish such an arc, there is used an arc extinguishing device comprising insulator (1) 1 and insulator (2) 2 which are disposed around an arc 9 which will be generated between the moving contact of moving contact element 3 and the fixed contact 5 of fixed contact element 6, as shown in Fig. 1-14. Numeral 7 denotes pivoting center of the moving contact element 3. The insulator (1) 1 and insulator (2) 2 of the arc extinguishing device 8 generate a thermal decompostion gas due to the arc 9, and the thermal decomposition gas cools down the arc 9, thereby extinguishing it. Such arc extinguishing devices and arc extinguishing insulator materials for use therein are disclosed in, for example, Japanese Unexamined Patent Publications Nos. 126136/1988 , 310534/1988 , 77811/1989 , 144811/1990 and 256110/1990 . For instance, Japanese Unexamined Patent Publication No. 126136/1988 discloses an arc extinguishing device employing an insulative material comprising polymethylpentene, polybutylene or polymethyl methacrylate and 5 to 35 % of glass fiber filled therein. Polymethylpentene, polybutylene or polymethyl methacrylate generates a large amount of hydrogen gas, which has a good heat conductivity and hence exhibits a rapid cooling effect. Japanese Unexamined Patent Publication No. 310534/1988 discloses an insulative material comprising an acrylic acid ester copolymer, aliphatic hydrocarbon resin, poly(vinyl alcohol), polybutadiene, poly(vinyl acetate), poly(vinyl acetal), isoprene resin, ethylene-propylene rubber, ethylene-vinyl acetate copolymer or polyamide resin and 5 to 35 % of glass fiber filled therein. Japanese Unexamined Patent Publication No. 77811/1989 discloses insulative materials such as polymethylpentene and melamine resin which generate hydrogen in an amount of 2.5 x 10 -2 ml/mg or greater when heated at 764°C for one second in a nitrogen gas atmosphere. Further, Japanese Unexamined Patent Publication No. 144811/1990 discloses insulative materials such as a melamine resin containing ε-caprolactam and aluminum hydroxide and a melamine resin containing an amine-terminated imide compound. Still further, Japanese Unexamined Patent Publication No. 256110/1990 discloses insulative materials such as a melamine resin containing glass fiber or epoxy resin and a melamine resin containing at least two of ε-caprolactam, aluminum hydroxide, glass fiber and epoxy resin, as well as a melamine resin containing ε-caprolactam and aluminum hydroxide. To miniaturize the arc extinguishing device 8 and to improve the current limiting or interrupting property thereof, it is effective to use an insulator (1) 1 covering a contact section in which an arc is generated or an insulator (2) 2 disposed on opposite sides of a plane including the locus of an opening or closing movement of the contacts or around the contact section. In this case the insulator (1) 1 and insulator (2) 2 are required to be improved in arc extinguishing property. Where the sectional area of the moving contact element or fixed contact element is reduced as compared to conventional one for the purpose of miniaturizing the arc extinguishing device, the electrical resistance of the moving contact element or fixed contact element is increased and, hence, when electric current is passed through the contacts, the temperature of the contact portion and its surroundings is elevated to degrees higher than with the conventional one. Accordingly, the insulator (1) 1 and insulator (2) 2 are required to have a higher heat resistance than the conventional ones. Alternatively, where the width W of the insulator (2) 2 is reduced than that of the conventional one for the purpose of miniaturizing the arc extinguishing device 8, the distance between the insulator (2) and the plane including the locus of an opening or closing movement of the contacts is shortened and, hence, the pressure of thermal decomposition gas to be generated from the insulator (2) by arc becomes higher than in the conventional ones. Accordingly, the insulator (1) 1 and insulator (2) 2 are required to have a higher strength against pressure than the conventional ones. In addition, since the distance between the insulator (2) 2 and the plane including the locus of an opening or closing movement of the contacts is shortened, the insulator (2) 2 is much consumed by arc. Accordingly, the insulator (2) 2 is required to have an improved consumption-by-arc resistance, specifically to such a degree that a hole is not formed therein. Where there is used the aforementioned conventional insulator containing a melamine resin or modified melamine resin as a matrix material thereof or a conventional melamine-phenol type insulator, a problem arises that the insulators (1) and (2) which have an insufficient strength against pressure are likely to be broken to pieces by an increased pressure in the periphery of the contacts due to a thermal decomposition gas generated from the insulators when exposed to an elevated temperature of an arc that is generated upon the opening movement of the moving contact. Further, when the distance between the insulator (2) and the contacts is shortened for the miniaturization of the arc extinguishing device, the amount of a filler to be used needs to be increased so as to improve the consumption-by-arc resistance of the insulator (2). However, the use of C glass containing about 8 % of sodium oxide and about 1 % of potassium oxide or A glass containing about 15 % of sodium oxide as a filler causes a problem of degraded arc extinguishing property. Still further, the use of a heat-resistive thermoplastic resin containing a large amount of aromatic ring in the arc receiving portions of the insulators (1) and (2) brings about a problem that an insulation failure arises because the surfaces of the insulators (1) and (2) are carbonized by arc 9 and free carbon will be scattered around, though the heat resistance of the insulators (1) and (2) is improved. It is, therefore, an object of the present invention to provide an arc extinguishing insulative material composition, an arc extinguishing insulative molded product and an arc extinguishing device using those composition and molded product, which are free from the problems essential to the prior art and are excellent in arc extinguishing property, heat resistance, strength against pressure, consumption-by-arc resistance and the like. According to the embodiment 1-1 of the present invention, there is provided an arc-extinguishing material comprising an arc-extinguishing insulator molded product having a double-layered structure, said product comprising: an arc-receiving layer made of a reinforced arc-extinguishing insulator composition comprising 5 to 20 % by weight of at least one filler selected from the group consisting of a glass fiber containing not more than 1 % by weight of compounds of group 1A metals of the periodic table in total, an inorganic mineral containing not more than 1 % by weight of compounds of group 1A metals of the periodic table in total and a ceramic fiber containing not more than 1 % by weight of compounds of group 1A metals of the periodic table in total, and a matrix resin containing as a principal component at least one member selected from the group consisting of a polyolefin, an olefin copolymer, a polyamide, a polyamide polymer blend, a polyacetal and a polyacetal polymer blend; and a base layer underlying said arc-receiving layer and made of an arc-extinguishing insulator composition comprising 20 to 65 % by weight of at least one filler selected from the group consisting of a glass fiber, an inorganic mineral and a ceramic fiber, and a matrix resin containing, as a principal component thereof, a thermoplastic resin or a thermosetting resin. According to the embodiment 1-2 of the present invention, the inorganic mineral of the arc-receiving layer according to the embodiment 1-1 is a member selected from the group consisting of calcium carbonate, wollastonite and magnesium silicate hydrate. According to the embodiment 1-3 of the present invention, the ceramic fiber material of the arc-receiving layer according to the embodiment 1-1 is a member selected from the group consisting of an aluminum silicate fiber material, an aluminum borate whisker and an alumina whisker. According to the embodiment 1-4 of the present invention, the polyolefin of the arc-receiving layer according to any one of the embodiments 1-1 to 1-3 is polypropylene or polymethylpentene. According to the embodiment 1-5 of the present invention, the olefin copolymer of the arc-receiving layer according to any one of the embodiments 1-1 to 1-3 is an ethylene-vinyl alcohol copolymer. According to the embodiment 1-6 of the present invention, the polyamide polymer blend of the arc-receiving layer according to any one of the embodiments 1-1 to 1-3 is a member selected from the group consisting of a combination of a polyamide and a polyolefin, a combination of a polyamide and a thermoplastic elastomer and a combination of a polyamide and a rubber. According to the embodiment 1-7 of the present invention, the polyamide of the arc-receiving layer according to any one of the embodiments 1-1 to 1-3 and 1-6 is a member selected from the group consisting of nylon 6T, nylon 46 and nylon 66. According to embodiment 1-8 of the present invention, the arc-receiving layer according to any one of the embodiments 1-1 to 1-3 and 1-6 contains nylon 6T as the polyamide. According to embodiment 1-9 of the present invention, the arc-receiving layer according to any one of the embodiments 1-1 to 1-3 and 1-6 contains nylon 46 or nylon 66 as the polyamide. According to embodiment 1-10 of the present invention, the polyacetal polymer blend of the arc-receiving layer according to any one of the embodiments 1-1 to 1-3 comprises a polyacetal and a thermoplastic resin which is incompatible with the polyacetal and has a melting point not less than that of the polyacetal. According to embodiment 1-11 of the present invention, the polyacetal polymer blend of the arc-receiving layer according to any one of the embodiments 1-1 to 1-3 comprises a combination of a polyacetal and nylon 6. According to embodiment 1-12 of the present invention, there is provided an arc extinguishing insulative molded product which is comprised by the inventive arc-extinguishing material and comprises itself : an arc receiving layer made of an arc extinguishing insulative material composition comprising not more than 20 % of at least one filler selected from the group consisting of a glass fiber containing not more than 1 % of compounds of group 1A metals of the periodic table in total, an inorganic mineral containing not more than 1 % of compounds of group 1A metals of the periodic table in total and a ceramic fiber containing not more than 1 % of compounds of group 1A metals in total, and a matrix resin containing as a main component at least one member selected from the group consisting of a polyolefin, an olefin copolymer, a polyamide, a polyamide polymer blend, a polyacetal and a polyacetal polymer blend, or made of a non-reinforced, arc extinguishing insulative material composition comprising as a main component at least one member selected from the group consisting of a polyolefin, an olefin copolymer, a polyamide, a polyamide polymer blend, a polyacetal and a polyacetal polymer blend; and a base layer underlying the arc receiving layer and made of an arc extinguishing insulator composition comprising 20 to 65 % of at least one filler selected from the group consisting of a glass fiber material, an inorganic mineral and a ceramic fiber material, and a matrix resin containing as a main component thereof a thermoplastic resin or a thermosetting resin. According to embodiment 1-13 of the present invention, the thermoplastic resin or thermosetting resin contained in the arc extinguishing insulator molded product according to the embodiment 1-12 is at least one member selected from the group consisting of nylon 6T, nylon MXD 6, polyethylene terephthalate and polybutylene terephthalate. According to embodiment 1-14 of the present invention, the polyamide for use in the arc receiving layer and/or the base layer of the arc extinguishing insulative molded product according to the embodiment 1-12 is nylon 46 or nylon 66. According to embodiment 1-15 of the present invention, the inorganic mineral for use in the arc receiving layer and/or the base layer of the arc extinguishing insulative molded product according to any one of the embodiments 1-12 to 1-14 is at least one member selected from the group consisting of calcium carbonate. wollastonite and magnesium silicate hydrate. According to embodiment 1-16 of the present invention, the ceramic fiber for use in the arc receiving layer and/or the base layer of the arc extinguishing insulative molded product according to any one of the embodiments 1-12 to 1-14 is at least one member selected from the group consisting of an aluminum silicate fiber, an aluminum borate whisker and an alumina whisker. According to embodiment 1-17 of the present invention, the glass fiber material for use in the base layer of the arc extinguishing insulative molded product according to any one of the embodiments 1-12 to 1-14 contains not more than 1 % of compounds of group 1A metals of the periodic table in total. According to embodiment 1-18 of the present invention, the arc receiving layer of the arc extinguishing insulative molded product according to any one of the embodiments 1-12 to 1-17 further contains a substance capable of generating H 2 O, O 2 and O (atomic oxygen) by thermal decomposition. According to embodiment 1-19 of the present invention, the substance capable of generating H 2 O, O 2 and O (atomic oxygen) by thermal decomposition which is contained in the arc receiving layer of the arc extinguishing insulative molded product according to the embodiment 1-18 is at least one member selected from the group consisting of aluminum hydroxide, magnesium hydroxide, antimony tetroxide and antimony pentoxide. According to embodiment 1-20 of the present invention, there is provided an arc extinguishing device comprising an arc extinguishing insulative material composition or an arc extinguishing insulative molded product according to any one of the embodiments 1-1 to 1-19. According to embodiment 1-21 of the present invention, there is provided an arc extinguishing device comprising an insulator (2) disposed on both sides with respect to a plane including the locus of an opening or closing movement of contacts of a switch or around a contact section of the switch, the insulator (2) being formed of an arc extinguishing insulative material composition or an arc extinguishing insulative molded product according to any one of the embodiments 1-1 to 1-19. According to embodiment 1-22 of the present invention, there is provided an arc extinguishing device comprising an insulator (1) covering a contact section of a switch excepting contact surfaces of contacts of the switch, and an insulator (2) disposed on both sides with respect to a plane including the locus of an opening or closing movement of the contacts or around the contact section, the insulator (1) being formed of an arc extinguishing insulative material composition according to any one of the embodiments 1-1 to 1-11, the insulator (2) being formed of an arc extinguishing insulative material composition or an arc extinguishing insulative molded product according to any one of the embodiments 1-1 to 1-19. In each of the embodiments 1-1 to 1-11 of the present invention, the arc-receiving layer comprises at least one filler selected from the group consisting of a glass fiber containing not more than 1 % of compounds of group 1A metals of the periodic table in total, an inorganic mineral containing not more than 1 % of compounds of group 1A metals of the periodic table in total and a ceramic fiber containing not more than 1 % of compounds of group 1A metals of the periodic table in total, and a matrix resin containing as a main component at least one resin selected from the group consisting of a polyolefin, an olefin copolymer, a polyamide, a polyamide polymer blend, a polyacetal and a polyacetal polymer blend. The arc-receiving layer of such constitution has improved arc extinguishing property, strength against pressure and consumption-by-arc resistance. Further, since the matrix resin of the arc extinguishing insulative material composition contains a thermoplastic resin as a main component thereof, the time period required for the molding of the arc extinguishing insulative material composition is shortened relative to that required for the case of a thermosetting resin which requires a setting time in molding. In each of the embodiments 1-2 and 1-3 of the present invention, the arc-receiving layer contains, as the inorganic mineral, calcium carbonate, wollastonite or magnesium silicate hydrate, or, as the ceramic fiber, an aluminum silicate fiber, an aluminum borate whisker or an alumina whisker. The arc-receiving layer of such constitution exhibits an improved arc extinguishing property. In the embodiment 1-4 of the present invention, the arc-receiving layer contains polypropylene or polymethylpentene as the polyolefin. Since polypropylene or polymethylpentene is of a small specific gravity, the insulative material is of a relatively small weight. Polymethylpentene, in particular, is a crystalline resin having a melting point of 240°C and hence imparts the arc-receiving layer with a high heat resistance. In the embodiment 1-5 of the present invention, the arc-receiving layer contains an ethylene-vinyl alcohol copolymer having a high strength as the olefin copolymer. Hence, the arc-receiving layer enjoys a further improved strength against pressure. In the embodiment 1-6 of the present invention, the polyamide polymer blend for use in the arc-receiving layer comprises a combination of a polyamide and a polyolefin, a combination of a polyamide and a thermoplastic elastomer, or a combination of a polyamide and a rubber. The arc-receiving layer of such constitution has an improved impact resistance and hence exhibits a further improved strength against pressure. In the embodiment 1-7 of the present invention, the polyamide for use in the arc-receiving layer is at least one member selected from the group consisting of nylon 6T, nylon 46 and nylon 66 which are crystalline polyamides having high melting points. Accordingly, the arc-receiving layer has a high heat distortion temperature and hence enjoys a further improved heat resistance. In embodiment 1-9 of the present invention, the polyamide for use in the arc-receiving layer is nylon 6T which is a crystalline polyamide having a high melting point. Accordingly, the insulator composition has a high heat distortion temperature and hence enjoys a further improved heat resistance. In embodiment 1-9 of the present invention, the polyamide for use in the arc-receiving layer is either one of nylon 46 and nylon 66 which are crystalline polyamides having high melting points. Accordingly, the arc-receiving layer has a higher heat distortion temperature and hence enjoys a further improved heat resistance. Still further, since nylon 46 and nylon 66 are each free of any aromatic ring in the chemical formula thereof, the arc-receiving layer is likely to be less carbonized at its surface by arc and hence enjoys a further enhanced arc extinguishing property. In the embodiment 1-10 of the present invention, the main component of the matrix resin contained in the arc-receiving layer comprises, as the polyacetal polymer blend, a combination of a polyacetal and a thermoplastic resin which is incompatible with the polyacetal and has a higher melting point than the polyacetal. Where an insulator has an arc receiving surface formed of, for example, a polyacetal rich layer, the insulative material exhibits an enhanced arc extinguishing property by virtue of the gas to be generated from the polyacetal by an arc. Further, the arc-receiving layer can have a higher heat resistance than the polyacetal depending on the material combined with the polyacetal in the polymer blend. The arc-receiving layer further contains at least one filler selected from the group consisting of a glass fiber containing not more than 1 % of compounds of group 1A metals of the periodic table in total, an inorganic mineral containing not more than 1 % of compounds of group 1A metals of the periodic table in total and a ceramic fiber containing not more than 1 % of compounds of group 1A metals of the periodic table in total. This allows the arc-receiving layer to exhibit improved consumption-by-arc resistance and strength against pressure. In the embodiment 1-11 of the present invention, the main component of the matrix resin contained in the arc-receiving layer comprises a combination of a polyacetal and nylon 6 as the polyacetal polymer blend. Since nylon 6 is free of any aromatic ring in its chemical formula, the arc-receiving layer is likely to be less carbonized by arc and hence offers a further improved arc extinguishing property together with the features of the embodiment 1-10. In each of the embodiments 1-12 to 1-19, the arc extinguishing insulative molded product is of a double-layered structure and hence is possible to have a layer of an excellent arc extinguishing property and a layer of excellent strength against pressure, consumption-by-arc resistance and heat resistance. In each of the embodiments 1-12 to 1-13 of the present invention, the arc receiving layer of the arc extinguishing insulative molded product is made of an arc extinguishing insulative material composition comprising not more than 20 % (but at least 5%) of at least one filler selected from the group consisting of a glass fiber containing not more than 1 % of compounds of group 1A metals of the periodic table in total, an inorganic mineral containing not more than 1 % of compounds of group 1A metals of the periodic table in total and a ceramic fiber containing not more than 1 % of compounds of group 1A metals of the periodic table in total, and a matrix resin containing as a main component at least one resin selected from the group consisting of a polyolefin, an olefin copolymer, a polyamide, a polyamide polymer blend, a polyacetal and a polyacetal polymer blend, or is made of a non-reinforced, arc extinguishing insulative material composition comprising as a main component at least one resin selected from the group consisting of a polyolefin, an olefin copolymer, a polyamide, a polyamide polymer blend, a polyacetal and a polyacetal polymer blend. The arc extinguishing insulative molded product of this constitution offers an improved arc extinguishing property. In each of the embodiments 1-12 and 1-13 of the present invention, the arc extinguishing insulative molded product comprises an arc receiving layer, and a base layer underlying the arc receiving layer and made of 20 to 65 % of at least one filler selected from the group consisting of a glass fiber, an inorganic mineral and a ceramic fiber, and a matrix resin containing as a main component thereof a thermoplastic or thermosetting resin selected from the group consisting of e.g. nylon 6T, nylon MXD6, polyethylene terephthalate and polybutylene terephthalate. The arc extinguishing insulative molded product of such constitution offers improved strength against pressure and and consumption-by-arc resistance. Nylon 6T, in particular, has a higher melting point than nylon 46 and nylon 66 and hence will contribute to a further improvement in the heat resistance of the molded product. In the embodiment 1-14 of the present invention, the polyamide for use in the arc extinguishing insulative molded product is either one of nylon 46 and nylon 66, each of which is free of any aromatic ring in its chemical formula. The molded product is likely to be less carbonized at its surface by arc and thereby offers a further enhanced arc extinguishing property. In each of the embodiments 1-15 to 1-17 of the present invention, the inorganic mineral is calcium carbonate, wollastonite or magnesium silicate hydrate, the ceramic fiber is an aluminum silicate fiber, an aluminum borate whisker or an alumina whisker, and the glass fiber contained in the base layer is a glass fiber containing not more than 1 % of compounds of group 1A metals of the periodic table in total. The molded product of this constitution enjoys an enhanced arc extinguishing property. In the embodiment 1-18 of the present invention, the arc extinguishing insulative molded product according to any one of the embodiments 1-12 to 1-17 includes the arc receiving layer containing a substance capable of generating H 2 O, O 2 and O (atomic oxygen) by thermal decomposition. These gases which will be generated by thermal decompostion act to inhibit the generation of free carbons and, hence, the molded product enjoys a further enhanced arc extinguishing property. In the embodiment 1-19 of the present invention, the substance capable of generating H 2 , O 2 and O by thermal decomposition is at least one member selected from the group consisting of aluminum hydroxide, magnesium hydroxide, antimony tetroxide and antimony pentoxide. Those substances act to inhibit the generation of free carbon more efficiently and thereby impart the molded product with a further enhanced arc extinguishing property. In the embodiment 1-20 of the present invention, the arc extinguishing device comprises an arc extinguishing insulative material composition or arc extinguishing insulative molded product according to any one of the embodiments 1-1 to 1-27. Such arc extinguishing device is possible to be miniaturized and to exibit an enhanced current limiting or interrupting performance. In the embodiment 1-21 of the present invention, the arc extinguishing device comprises insulator (2) disposed on both sides with respect to a plane including the locus of an opening or closing movement of the contacts or around the contact section, the insulator (2) being formed of an arc extinguishing insulative material composition or arc extinguishing insulative molded product according to any one of the embodiments 1-1 to 1-19. Such arc extinguishing device is possible to be miniaturized and to exibit an enhanced current limiting or interrupting performance. In the embodiment 1-22 of the present invention, the arc extinguishing device comprises insulator (1) covering the contact section excepting the contact surfaces, and insulator (2) disposed on both sides with respect to a plane including the locus of an opening or closing movement of the contacts or around the contact section, the insulator (1) being formed of an extinguishing insulative material composition according to any one of the embodiments 1-1 to 1-11, the insulator (2) being formed of arc extinguishing insulative material composition or arc extinguishing insulative molded product according to any one of the embodiments 1-1 to 1-19. Such arc extinguishing device is possible to be miniaturized and to exibit an enhanced current limiting or interrupting performance. The arc extinguishing material mainly comprises the matrix resin specified above which contains the specified filler. The filler used therein is at least one member selected from the group consisting of a glass fiber containing not more than 1 % of compounds of group 1A metals of the periodic table in total, an inorganic mineral containing not more than 1 % of compounds of 1A group metals of the periodic table in total, and a ceramic fiber containing not more than 1 % of compounds of group 1A metals of the periodic table. The above filler is used to improve the consumption-by-arc resistance, strength against pressure and arc extinguishing property of the insulative material composition. The compounds of group 1A metals (Li, Na, K, Rb, Cs, Fr) of the periodic table herein are in the form of metal oxide M 2 O (Na 2 O, K 2 O, Li 2 O, and the like). The total amount of these compounds allowable in the filler is not more than 1 %. If it exceeds 1 %, the insulative material composition exhibits a degraded arc extinguishing property. The total amount of such compounds is preferably not more than 0.6 %, more preferably not more than 0.15 % in view of arc extinguishing property. It is noted that the total amount of the compounds is measured by X-ray diffraction. The glass fiber material is used to improve the strength against pressure and consumption-by-arc resistance of the insulative material composition by virtue of its reinforcing effect. The glass fiber herein is a fibrous material of glass, and any particular limitations are not imposed on such fibrous material as far as it contains not more than 1 % of comounds of group 1A metals of the periodic table in total. Examples of the specific glass materials usable for the glass fiber include E glass, S glass, D glass, T glass and silica glass. Preferable are S glass, D glass, T glass and silica glass since they are free of any of compounds of group 1A metals. Examples of the specific glass fiber products usable for the glass fiber material include a long fiber product, a short fiber product and glass wool. Preferable is the short fiber product from the viewpoint of use as a filler for a thermoplastic resin. The glass fiber preferably has a fiber diameter of 6 to 13 µm and a fiber aspect ratio of 10 or more for imparting the insulative material composition with an improved strength against pressure. Further, the glass fiber may be processed with a treating agent such as a silane coupling agent for imparting the insulative material composition with a further improved strength against pressure. The inorganic mineral is used to enhance the arc extinguishing property, consumption-by-arc resistance and strength against pressure of the insulative material composition. Any particular limitations are not imposed on the inorganic mineral as far as it contains not more than 1 % of compounds of group 1A metals of the periodic table in total. Preferable examples of such minerals are calcium carbonate, wollastonite, and magnesium silicate hydrate such as talc, Aston, chrysotile or sepiolite. These minerals act to improve the consumption-by-arc resistance of the insulative material composition. Calcium carbonate is preferably treated with a surface modifier such as stearic acid in order to improve the dispersibility in a resin from the viewpoint of the strength against pressure of the insulative material composition. Wollastonite is preferably in a fibrous form having a high aspect ratio in view of the strength against pressure of the insulative material composition. Magnesium silicate hydrate is preferably a fibrous one such as Aston in view of the strength against pressure of the insulative material composition. The ceramic fiber is used to improve the consumption-by-arc resistance and strength against pressure of the insulative material composition, as well as the arc extinguishing property thereof. The ceramic fiber herein is a fibrous material of a ceramic. Any particular limitations are not imposed on the ceramic fiber as far as the total amount of compounds of group 1A metals contained therein meets the requirement. Preferable examples of such ceramic fiber include an aluminum silicate fiber, an aluminum borate whisker and an alumina whisker. Those ceramic fiber advantageously improves the arc extinguishing property and strength against pressure of the insulative material composition. The ceramic fiber preferably has a fiber diameter of 1 to 10 µm and a fiber aspect ratio of 10 or higher in view of the strength against pressure. One or more kinds of the fillers are used. Where two or more kinds of such materials are used, preferable combinations are: the glass fiber and the inorganic mineral; the glass fiber and the ceramic fiber; the inorganic mineral and the ceramic fiber; two or more of the glass fibers; two or more of the inorganic minerals; two or more of the ceramic fibers; and the glass fiber, the inorganic mineral and the ceramic fiber. These combinations advantageously contribute to improvement in the arc extinguishing property of the insulative material composition. The weight ratios of such combinations are: preferably 5/50 to 50/5, more preferably 10/30 to 30/10 in the case of glass fiber inorganic/mineral combination, glass fiber/ceramic fiber combination, and inorganic mineral/ceramic fiber combination, and preferably 1 : 1 1 to 1 : 1 : 10 in the case of glass fiber/inorganic mineral/ceramic fiber combination. The matrix resin is selected from the group consisting of a polyolefin, an olefin copolymer, a polyamide, a polyamide polymer blend, a polyacetal and a polyacetal polymer blend. The matrix resin is used to enhance the arc extinguishing property, strength against pressure and consumption-by-arc resistance of the insulative material composition and further to shorten the time required for molding the insulative material composition. The polyolefin is free of any aromatic ring and is excellent in impact resistance, and is therefore used to impart the insulative material composition with satisfactory arc extinguishing property and strength against pressure. Examples of the polyolefins are polypropylene, polyethylene and polymethylpentene. Among these, polypropylene and polymethylpentene which have a small specific gravity are preferred for providing the insulative material composition of a lighter weight. Polymethylpentene is particularly preferable, since it is a crystalline resin having a melting point of 240°C and hence imparts the insulative material composition with a high heat resistance. The olefin copolymer is free of any aromatic ring and hence is used to impart the insulative material composition with a satisfactory arc extinguishing property. Examples of the olefin copolymers are ethylene-vinyl alcohol copolymer and ethylene-vinyl acetate copolymer. A resin of a high strength such as the ethylene-vinyl alcohol copolymer is preferred for improving the strength against pressure of the insulative material composition. To realize the insulative material composition of improved strength against pressure, the copolymerization ratio of the ethylene-vinyl alcohol copolymer is preferably within the range of 30/70 to 45/55 by weight, more preferably 30/70 to 35/65 by weight. The polyamide herein is a high molecular compound having an amido bond and includes a polyamide copolymer in the present invention. The polyamide is a high strength resin and hence is used to impart the insulative material composition with a satisfactory strength against pressure. Examples of the polyamides include nylon 6T, nylon 66, nylon 46, nylon MXD6, nylon 610, nylon 6, nylon 11, nylon 12 and copolymer of nylon 6 and nylon 66. It is noted that nylon in general means a linear synthetic polyamide among polyamides. Nylon mn results from polycondensation of a diamine having m number of carbon atoms (NH 2 (CH 2 ) m NH 2 ) and a dibasic acid having n number of carbon atoms (HOOC(CH 2 ) n-2 COOH). Nylon n is a polymer of an ω-amino acid (H 2 N(CH 2 ) n-1 COOH) having n number of carbon atoms or of a lactam having n number of carbon atoms. Among the above polyamides, there are preferred crystalline polyamides having high melting points such as nylon 6T (melting point: 320°C), nylon 46 (melting point: 290°C) and nylon 66 (melting point: 260°C), since they can impart the insulative material composition with a high distortion temperature and a further improved heat resistance. Chemical formulae of the representative polyamides are as follows. The polyamide polymer blend herein is a blend of a polyamide polymer and another polymer. The polyamide polymer blend is used to impart the insulative material composition with an improved impact resistance. Examples of such polyamide polymer blends include a polyamide-polyolefin blend, a polyamide-thermoplastic elastomer blend, and a polyamide-rubber blend. Any of the aforementioned polyamides can be used as the polyamide in the polyamide polymer blend. Among such polyamides, there are preferably used nylon 46, nylon 66 and the like which are free of any aromatic ring and have high melting points, since they provide the insulative material composition with improved heat resistance and arc extinguishing property. Any of the aforementioned polyolefins can be used as the polyolefin usable in the polyamide polymer blend. Among these, polypropylene is preferred, since it provides the insulative material composition with an improved strength against pressure. Examples of thermoplastic elastomers usable in the polyamide polymer blend include a polyolefin elastomer, a polyamide elastomer and a polyester elastomer. Among those, the polyolefin elastomer is preferably used, since it imparts the insulative material composition with an improved strength against pressure. Examples of rubbers usable in the polyamide polymer blend include a butadiene rubber, an ethylene; propylene rubber and an acrylic acid rubber. Among these, ethylene-propylene rubber is preferably used, since it imparts the insulating composition with an improved strength against pressure. In the polyamide polymer blend, the blending ratio of the polyamide to any one of the polyolefin, thermoplastic elastomer or rubber is preferably 100 : 1 to 100 : 15 by weight, more preferably 100 : 5 to 100 : 10 by weight, taking account of the heat resistance and strength against pressure of the insulating composition. The polyacetal is used to enhance the arc extinguishing property of the insulative material composition, since a gas to be generated from the polyacetal by arc acts to extinguish the arc. Examples of the polyacetals are homopolymer and copolymer of polyoxymethylene. The polyacetal polymer blend is used to enhance the arc extinguishing property of the insulative material composition, since a gas to be generated from the polyacetal component thereof acts to extinguish the arc as described above, and to impart the insulative material composition with a higher heat resistance than the polyacetal alone by virtue of the thermoplastic resin other than the polyacetal in the blend. In the polyacetal polymer blend, the polyacetal component thereof is the same as described above, and the other polymer thereof is a thermoplastic resin which is incompatible with the polyacetal and has a melting point not less than that of the polyacetal, but preferably not more than 230°C. The incompatibility of the thermoplastic resin with the polyacetal herein is a characteristic that the two show a marked change in modulus of elasticity and a peak of loss tangent at the respective glass transition temperatures. It is to be noted that the polyacetal has a melting point of 178°C in the case of the homopolymer thereof, and a melting of 167°C in the case of the copolymer thereof. Examples of the thermoplastic resins for use in the polyacetal polymer blend include nylon 6 and polybutylene terephthalate. Among those, nylon 6 is preferred, since it is free of any aromatic ring in the chemical formula thereof and hence will be less carbonized at its surface by arc thereby further improving the arc extinguishing property of the insulative material composition. In the polyacetal polymer blend, the blending ratio of the polyacetal component to the other component is preferably 100 : 100 to 100 : 400 by weight, more preferably 100 : 200 to 100 : 300 by weight, taking account of the heat resistance of the insulative material composition. The matrix resin contains any one of the foregoing resins and, optionally, accessary constituents, such as a flame retardant, other than the filler. Preferable as such flame retardant are a phosphoric flame retardant free of any aromatic ring and an inorganic flame retardant. Preferably, the arc extinguishing material further contains a substance capable of generating H 2 , O 2 and O (atomic oxygen) by thermal decomposition for inhibiting the generation of free carbon thereby enhancing the arc extinguishing property of the insulator composition. Such a substance will hereinafter be referred to as "free carbon inhibitor". To verify whether or not a substance is capable of generating H 2 O, O 2 or O (atomic oxygen), it is possible to employ, for example, a method in which the substance is subjected to thermal decomposition in a nitrogen gas atmosphere, and the gas generated from the substance by thermal decomposition is allowed to pass through a gas detector tube to measure the concentration of H 2 O, O 2 or O therein. Examples of the free carbon inhibitors include aluminum hydroxide, magnesium hydroxide, antimony tetroxide and antimony pentoxide. These compounds are preferred in view of their free carbon generation inhibiting effect. Aluminum hydroxide or magnesium hydroxide generates H 2 O by thermal decomposition, on the other hand antimony tetroxide or antimony pentoxide generates O 2 or O by thermal decomposition. H 2 , O 2 or O thus generated reacts with particles of metals generated from an electrode material or the like, or with free carbon generated from the arc extinguishing material to give metal oxide, carbon monoxide or carbon dioxide, thereby inhibiting the occurrence of insulation failure. The proportion of the free carbon inhibitor in the arc extinguishing material is preferably not more than 20 %. The use of free carbon inhibitor in an amount of more than 20 % tends to degrade the strength against pressure of the insulative material composition particularly comprising a combination of nylon and magnesium hydroxide. The constitution of the arc extinguishing insulative material composition (I) to which the free carbon inhibitor is to be added is not particularly varied. The arc extinguishing insulative material composition (I) can be prepared by any method which is capable of mixing the filler and accessory constituents with the matrix resin, but is usually by an extrusion mixing method, roll mixing method or the like into a pellet form, sheet form or another form. A representative example of generally preferred arc extinguishing material is as follows: An arc extinguishing insulative material composition comprising the constituents of either one of the above generally preferred compositions, and further 5 to 20 % of magnesium hydroxide. This insulative material composition is preferred, since it exhibits a further enhanced effect of inhibiting the generation of free carbons and hence of inhibiting the occurrence of insulation failure. Reference is made to a further embodiment of the arc extinguishing material of the present invention. The arc extinguishing material of this embodiment comprises, as a principal component thereof, a polyacetal polymer blend composed of a polyacetal and a thermoplastic resin which is incompatible with the polyacetal and has a higher melting point than the polyacetal. In the insulative material composition (II), the polyacetal component of the polyacetal polymer blend serves to enhance the arc extinguishing property of the insulative material composition by virtue of the gas generated therefrom, and the thermoplastic resin component other than the polyacetal imparts the insulative material composition with a higher heat resistance than that of the polyacetal. With respect to the polyacetal, the thermoplastic resin which is incompatible with the polyacetal and has a higher melting point than the polyacetal, blending ratio therebetween, kinds of accessary constituents, blending amounts thereof, shape of the insulator composition, preparation method therefor and the like, those are the same as described above and, therefore, the description thereon is herein omitted. The material of this embodiment of the present invention also may further contain the free carbon inhibitor. In this case the insulative material composition exhibits a further improved arc extinguishing property by virtue of the effect of inhibiting the generation of free carbon. With respect to examples of the free carbon inhibitor, preferred examples thereof, content thereof in the insulative material composition and other particulars, those are the same as described above and, therefore, the description thereon is herein omitted. Generally preferred examples of the arc extinguishing material of this embodiment include one comprising, as a principal component thereof, a polyacetal polymer blend comprising 100 parts (parts by weight, hereinafter the same) of nylon 6 and 100 to 25 parts of a polyacetal, in view of the arc extinguishing property and heat resistance thereof, and one further comprising 5 to 20 % of magnesium hydroxide or aluminum hydroxide, in view of effect of inhibiting the generation of free carbon, hence, of inhibiting the occurrence of insulation failure. Reference is then made to another embodiment of the arc extinguishing material of the present invention. The arc extinguishing material of this embodiment comprises a substance capable of generating H 2 , O 2 and O (atomic oxygen) by thermal decomposition, and a matrix resin containing as a principal component at least one member selected from the group consisting of nylon 6T, nylon 46 and nylon 66. The insulative material composition (III) exhibits an enhanced arc extinguishing property since it is capable of generating H 2 O, O 2 and O (atomic oxygen) which serve to inhibit the generation of free carbon. With respect to the free carbon inhibitor, nylon 6T, nylon 46, nylon 66 and the like for use in the material of this embodiment, those are the same as described above and, therefore, the description thereon is herein omitted. Preferable as the free carbon inhibitor are magnesium hydroxide, antimony tetroxide and antimony pentoxide, because they can easily be incorporated into the resin. The content of the free carbon inhibitor in the arc extinguishing insulative material composition (III) is preferably within the range of 5 to 20 %. If the content is less than 5 %, the material is likely to exhibit an insufficient free carbon generation inhibiting effect, while if it exceeds 20 %, the insulative material composition is likely to demonstrate a degraded strength against pressure. With respect to the preparation method for the arc extinguishing material of this embodiment, the shape of the foregoing embodiment and the like, those are the same as described above and, therefore, the description thereon is herein omitted. The arc extinguishing material can be molded into specific forms. Such molded products can be used in, for example, an arc extinguishing device comprising insulator (2) disposed on both sides with respect to a plane including the locus of the opening or closing movement of contacts or around the contact section and optionally comprising insulator (1) covering a contact section to generate an arc excepting the contact surfaces thereof in a switch. Although the shape, structure and size of the molded product vary depending on the current interrupting mechanism of the switch, exemplary molded products are as shown in Figs. 1-5 to 1-7. The molded product can be prepared by, for example, an injection molding method or hot press method. The injection molding method is preferably employed in view of its mass productivity. Next, reference is made to a reference arc extinguishing insulative molded product (I). The arc extinguishing insulative molded product (I) comprises: an arc receiving layer made of an arc extinguishing insulative material composition comprising not more than 20 % of at least one filler selected from the group consisting of a glass fiber containing not more than 1 % of compounds of group 1A metals of the periodic table in total, an inorganic mineral containing not more than 1 % of compounds of group 1A metals of the periodic table in total and a ceramic fiber containing not more than 1 % of compounds of group 1A metals of the periodic table in total, and a matrix resin containing as a main component at least one member selected from the group consisting of a polyolefin, an olefin copolymer, a polyamide, a polyamide polymer blend, a polyacetal and a polyacetal polymer blend, or made of a non-reinforced, arc extinguishing insulative material composition comprising as a main component at least one member selected from the group consisting of a polyolefin, an olefin copolymer, a polyamide, a polyamide polymer blend, a polyacetal and a polyacetal polymer blend; and a base layer underlying the arc receiving layer and made of an arc extinguishing insulative material composition comprising 20 to 65 % of at least one filler selected from the group consisting of a glass fiber, an inorganic mineral and a ceramic fiber, and a matrix resin containing as a main component at least one member selected from the group consisting of a polyolefin, an olefin copolymer, a polyamide, a polyamide polymer blend, a polyacetal and a polyacetal polymer blend. The molded product is of double-layered structure of arc extinguishing insulative materials, and hence advantageously includes the arc receiving layer exhibiting a further enhanced arc extinguishing property as compared to the case of forming insulator (2) into a single layer of the above described arc extinguishing material, and a layer laminated on the arc receiving layer (hereinafter sometimes referred to as "base layer") exhibiting excellent strength against pressure, consumption-by-arc resistance and heat resistance. The arc receiving layer provides for an enhanced arc extinguishing property. The same description as with the foregoing arc extinguishing material of the first embodiment is adapted to the purposes of the fillers for use in the arc receiving layer containing the filler (hereinafter sometimes referred to as "arc receiving layer A"), particulars and content of compounds of group 1A metals of the periodic table, and purposes, particulars and preferable examples of the glass fiber material, inorganic mineral and ceramic fiber material, and is therefore omitted herein. Further, the same description as with the arc extinguishing material of the first embodiment is incorporated into the purpose of the matrix resin, the purpose, particulars, examples and preferable examples, together with reasons therefor, of each polymer, and the particulars and contents of the accessary constituents of the matrix resin, and is therefore omitted herein. It is to be noted that where the matrix resin comprises nylon 46 or nylon 66, the molded product is less carbonized at its surface, since each of these thermoplastic resins is free of any aromatic ring in the chemical formula thereof and hence imparts the molded product with a further enhanced arc extinguishing property. The arc receiving layer A contains not more than 20 % of the foregoing specified filler in the matrix resin. The content of the filler not more than 20 % provides an arc extinguishing device with a satisfactory arc extinguishing property for a switch of high current. The content of the filler is preferably within the range of 5 to 20 % for assuring the consumption-by-arc resistance and arc extinguishing property of the molded product. Another embodiment of the arc receiving layer in the arc extinguishing insulative molded product (I) is an arc receiving layer B which is non-reinforced and comprises not any filler but a matrix resin. The same description as with the arc receiving layer A is incorporated into the purpose of the matrix resin forming the arc receiving layer B, the purpose, particulars, examples and preferable examples with reasons therefor of each thermoplastic resin, the particulars and contents of the accessary constituents of the matrix resin, and the like, and is therefore omitted herein. As the current to be interrupted by the arc extinguishing device grows higher, the arc receiving layer B becomes more preferable than the arc receiving layer A in view of its arc extinguishing property. Reference is then made to the base layer. The base layer plays the role of improving the consumption-by-arc resistance and strength against pressure of the molded product. The glass fiber, inorganic mineral or ceramic fiber contained in the base layer serves to improve the consumption-by-arc resistance and strength against pressure of the molded product. The total amount of compounds of group 1A metals of the periodic table contained in the filler is not particularly limited. This is because the base layer is so positioned as not to be exposed to arc and hence is not particularly required to be enhanced in arc extinguishing property. Nevertheless, the total amount of compounds of group 1A metals of the periodic table contained in such a filler as glass fiber is preferably not more than 1 % in view of the safety of the arc extinguishing device. The same descrition as with the arc extinguishing material of the first embodiment is incorporated into other descriptions on the glass fiber, inorganic mineral or ceramic fiber contained in the base layer, i.e., the purpose, particulars and preferable examples of each filler, the purpose of the matrix resin, the purpose, particulars, examples and preferable examples with reasons therefor of each polymer, and the particulars and contents of the accessary constituents of the matrix resin, and is therefore omitted herein. It should be noted that the base layer can also be suitably used, which contains a filler containing more than 1 % of compounds of group 1A metals of the periodic table such as clay, kaolin or mica. The matrix resin of the base layer preferably comprises nylon 46 or nylon 66 in view of the safety of the arc extinguishing device. Further, the base layer preferably comprises a resin of the same type as used in the arc receiving layer for assuring good adhesion therebetween, since the arc receiving layer overlies the base layer. The base layer contains 20 to 60 % of the foregoing filler. If the content of the filler is less than 20 %, insufficient consumption-by-arc resistance and strength against pressure are likely to result, while if it is more than 65 %, the moldability of the base layer is likely to degrade. The content of the filler is preferably within the range of 35 to 50 % in view of the consumption-by-arc resistance, strength against pressure and moldability of the base layer. The arc extinguishing insulative molded product (I) is a laminate of the arc receiving layer and the base layer. The shape, structure and size of the molded product vary depending on the current interrupting mechanism of a switch including the arc extinguishing device. Nevertheless, exemplary molded products (I) are as shown in Figs. 1-8 to 1-10. The molded product (I) is preferably prepared by an injection molding method, especially a two color injection molding method. Description of the arc extinguishing insulative molded product (II) according to the present invention follows. The arc extinguishing insulative molded product (II) comprises: an arc receiving layer made of an arc extinguishing insulative material composition comprising 5 to 20 % of at least one filler selected from the group consisting of a glass fiber containing not greater than 1 % of compounds of group 1A metals of the periodic table in total, an inorganic mineral containing not more than 1 % of compounds of group 1A metals of the periodic table in total and a ceramic fiber containing not greater than 1 % of compounds of group 1A metals of the periodic table in total, and a matrix resin containing as a main component at least one member selected from the group consisting of a polyolefin, an olefin copolymer, a polyamide, a polyamide polymer blend, a polyacetal and a polyacetal polymer blend; and a base layer underlying the arc receiving layer and made of an arc extinguishing insulative material composition comprising 20 to 65 % of at least one filler selected from the group consisting of a glass fiber, an inorganic mineral and a ceramic fiber, and a matrix resin containing, as a principal component thereof, a thermoplastic resin or a thermosetting resin. The arc extinguishing insulative molded product (II) is different from the molded product (I) in that the base layer thereof comprises the arc extinguishing insulative material composition containing the matrix resin of which the principal component is a thermoplastic resin or a thermosetting resin. Therefore, the molded product (II) is further improved in consumption-by-arc resistance and strength against pressure than the molded product (I). The thermoplastic resin or thermosetting resin is used to improve the consumption-by-arc resistance and strength against pressure of the molded product (II). Examples of the thermolastic or thermosetting resins include nylon 6T, nylon MXD, polyethylene terephthalate, polybutylene terephthalate, modified polyphenylene oxide, polyphenylene sulfide, polysulfone, polyether sulfone, polyether ketone. These resins may be used either alone or in combination. Preferable among those are nylon 6T, nylon MXD, polyethylene terephthalate and polybutylene terephthalate in view of their moldability and economical feature. The same descrition as with the arc extinguishing insulative molded product (I) is incorporated into the particulars of the molded product (II) such as the arc receiving layer A containing filler or the arc receiving layer B free of filler, the materials, shape and structure of the base layer thereof, and the shape of and preparation method for the molded product (II), and is therefore omitted herein the filler content is, however, necessarily 5 - 20 wt.%. Preferably the arc extinguishing insulative molded product (II) further comprises the aforementioned free carbon inhibitor, since the inhibitor inhibits the generation of free carbon and thereby enhances the arc extinguishing property of the molded product. Examples and preferable examples of the free carbon inhibitor are the same as in the arc extinguishing material of the first embodiment and, therefore, description thereon is herein omitted. The free carbon inhibitor can be contained in the arc receiving layer, since free carbon is generated when the arc receiving layer is exposed to arc. Examples of such free carbon inhibitors include aluminum hydroxide, magnesium hydroxide, antimony tetroxide and antimony pentoxide. Among those, magnesium hydroxide is preferred, since it can easily be incorporated into the arc receiving layer. The content of the free carbon inhibitor in each of the arc receiving layers A and B is preferably not more than 20 %. If the content exceeds 20 %, the arc receiving layer particularly including a combination of a nylon and magnesium hydroxide is likely to show a degraded strength against pressure. The following are generally preferable examples of the arc extinguishing insulative molded product (II) of the present invention. An arc extinguishing insulative molded product comprising: an arc receiving layer made of a matrix resin containing, as a principal component thereof, nylon 46 or nylon 66, which contains 5 to 10 % of an aluminum borate whisker or aluminum silicate fiber containing not more than 1 % of compounds of group 1A metals of the periodic table in total, and a base layer made of a matrix resin containing, as a principal component thereof, nylon 46 or nylon 66, which contains 35 to 50 % of an aluminum borate whisker or an aluminum silicate fiber. Such an insulative molded product is preferable in view of its heat resistance, arc extinguishing property and strength against pressure. An arc extinguishing insulative molded product comprising: an arc receiving layer made of a matrix resin containing, as a principal component thereof, nylon 46 or nylon 66, which contains 5 to 10 % of an aluminum borate whisker or aluminum silicate fiber containing not more than 1 % of compounds of group 1A metals of the periodic table in total, and a base layer made of a matrix resin containing, as a principal component thereof; nylon 46 or nylon 66, which contains 35 to 50 % of a glass fiber material of E glass containing not greater than 1 % of compounds of group 1A metals of the periodic table in total. Such an insulative molded product is preferable in view of its heat resistance, arc extinguishing property and strength against pressure. An arc extinguishing insulative molded product comprising: an arc receiving layer made of a matrix resin containing, as a principal component thereof, nylon 46 or nylon 66, which contains 5 to 10 % of an aluminum borate whisker or aluminum silicate fiber containing not more than 1 % of compounds of group 1A metals of the periodic table in total, and a base layer made of a matrix resin containing, as a principal component thereof, nylon MDX, nylon 6T, polyethylene terephthalate or polybutylene terephthalate, which contains 35 to 50 % of a glass fiber of E glass containing not greater than 1 % of compounds of group 1A metals of the periodic table in total. Such an insulative molded product is preferable in view of its arc extinguishing property, consumption-by-arc resistance and strength against pressure. The synthetically preferable arc extinguishing insulative molded product (II) preferably further contains 5 to 20 % of magnesium hydroxide in the arc receiving layer thereof from the viewpoint of an improved effect of inhibiting the generation of free carbon, hence of inhibiting occurrence of insulation failure. Next, reference is made to the arc extinguishing device according to the present invention. The arc extinguishing device of the present invention is characterized by using the aforementioned arc extinguishing insulative molded product. Examples of the arc extinguishing devices include the arc extinguishing devices (I) to (III) (device (I) is for reference purposes, only). The arc extinguishing device (I) comprises the aforementioned insulator (1) provided to cover a contact section excepting the contact surfaces thereof, the insulator (I) comprising any one of the arc extinguishing insulative material compositions according to the embodiments 1-1 to 1-10. The arc extinguishing device (II) comprises the insulator (2) disposed on both sides with respect to the plane including the locus of an opening or closing movement of contacts or around a contact section, the insulator (2) comprising any one of the arc extinguishing insulative material compositions and the arc extinguishing insulative molded products according to the embodiments 1-1 to 1-19. The arc extinguishing device (III) comprises the insulator (1) provided to cover a contact section excepting the contact surfaces thereof, and the insulator (2) disposed on both sides with respect to the plane including the locus of an opening or closing movement of contacts or around the contact section, the insulator (1) comprising any one of the arc extinguishing insulative material compositions according to the embodiments 1-1 to 1-10, the insulator (2) comprising any one of the arc extinguishing insulative material compositions and the arc extinguishing insulative molded products according to the embodiments 1-1 to 1-19. In the above arc extiguishing devices, the insulator (2) of the arc extinguishing devices (II) and (III) is preferably disposed in a U-shaped fashion as surrounding the plane including the locus of an opening or closing movement of the contacts on both sides thereof and as closing in the arching direction of arc, as shown in, for example, Figs. 1-3, 1-4 and 1-6 to 1-10. The arc extinguishing devices (II) and (III) each comprising such insulator (2) are suitable, since they advantageously provide the effects of the present invention. Hereinafter, the present invention in use mode will be described in detail with reference to the drawings. Fig. 1-1 is an explanatory side view of one example of a switch in opened state including the arc extinguishing device (III) comprising the arc extinguishing insulative material composition according to the present invention. Fig. 1-2 is an explanatory side view of the switch in closed state including the arc extinguishing device (III). Fig. 1-3 is an explanatory plan view of the switch in closed state including the arc extinguishing device (III). In Figs. 1-1 to 1-3, the switch comprises a moving contact element 3 adapted to pivot about a pivoting center 7, a moving contact 4 disposed on the side opposite to the pivoting center 7, a fixed contact element 6 having a fixed contact 5 in one end portion thereof at a position corresponding to the moving contact 4, an insulator (1) 1 having a thickness T1 and disposed as covering the periphery of each of the moving contact 4 and fixed contact 5, and an insulator (2) 2 having a thickness T2 and a width W and disposed as encompassing the moving contact 4 and fixed contact 5. The dimensions of the moving contact element 3 are, for example, 3 mm wide x 5 mm thick x 25 mm long, and those of the moving contact 4 are, for example, 3 mm square x 2 mm thick. The insulator (1) has, for example, a thickness T1 of 0.8 to 1.0 mm, a face including the corresponding contact and having an area of 5 mm square (including 3 mm square contact area), and a length perpendicular to the 5 mm square face of 5.8 to 6.0 mm. The dimensions of the fixed contact element 6 are, for example, 3 mm wide x 5 mm thick x 25 mm long, and those of the fixed contact 5 are, for example, 3 mm square x 2 mm thick. The dimensions of the insulator (2) are 0.8 to 1.2 mm in T2, 8 to 12 mm in W, and 10 to 15 mm in height, preferably 0.8 to 1.0 mm in T2 and 8 to 10 mm in W. Where the insulator (2) is of double-layered structure, T2 is 1.5 to 2.0 mm, the thickness of the arc receiving layer is 0.5 to 1.0 mm, and the height is 10 to 15 mm. The distance N1 between the end edge of the fixed contact and the insulator (2) is 2 to 8 mm, preferably 3 to 5 mm, and the distance N2 between the lateral side of the fixed contact and the insulator (2) is 2 to 5 mm, preferably 3 to 4 mm. Fig. 1-4 is an explanatory plan view of a switch in closed state provided with the arc extinguishing device (III) including insulator (2) of double-layered structure. Fig. 1-15 is an explanatory plan view of a switch in closed state including a conventional arc extinguishing device. As is apparent from Figs. 1-3, 1-4 and 1-15, the distance N1 between the end edge of the fixed contact and the insulator (2) and the distance N2 between the lateral side of the fixed contact and the insulator (2) in the arc extinguishing device of the present invention are both smaller that those in the conventional arc extinguishing device. The arc extinguishing device of the invention is thus miniaturized because the arc extinguishing insulative material composition or arc extinguishing insulative molded product used in the insulators (1) and (2) is significantly improved in the above-mentioned performances. In the arc extinguishing device (III), the insulator (1) comprises the arc extinguishing insulative material composition according to any one of the embodiments 1-1 to 1-10, which are described earlier, and hence the description on which is herein omitted. Of such insulative material compositions for the insulator (1) of the arc extinguishing device (III), those according to the embodiment 1-8 are preferable in view of the heat resistance, consumption-by-arc resistance, strength against strength against pressure and arc extinguishing property thereof. Such preferable compositions each comprise the constitution according to any one of the embodiments 1-1, 1-2, 1-3 and 1-6. In the arc extinguishing device (III), the insulator (2) comprises the arc extinguishing insulative material composition or arc extinguishing insulative molded product according to any one of the embodiments 1-1 to 1-19, which are described earlier, and hence the description on which is herein omitted. Of such insulative material compositions for the insulator (2) of the arc extinguishing device (III), those according to the embodiment 1-8 are preferable in view of the heat resistance, consumption-by-arc resistance, strength against pressure and arc extinguishing property. Such preferable compositions each comprise the constitution according to any one of the embodiments 1-1, 1-2, 1-3 and 1-6. Of the arc extinguishing insulative molded products for the insulator (2) of the arc extinguishing device (III), those according to the embodiments 1-14 to 1-16 are preferable in view of the arc extinguishing property, strength against pressure and consumption-by-arc resistance thereof. Such preferable molded products each comprise an arc receiving layer made of an arc extinguishing insulative material composition comprising 5 to 20 % of at least one filler selected from the group consisting of a glass fiber containing not greater than 1 % of compounds of group 1A metals of the periodic table in total, calcium carbonate, wollastonite or magnesium silicate hydrate containing not more than 1 % of compounds of group 1A metals of the periodic table in total, and an aluminum silicate fiber, aluminum borate whisker or alumina whisker containing not more than 1 % of compounds of group 1A metals of the periodic table in total, and a matrix resin containing, as a principal component thereof, a polyamide such as nylon 46 or nylon 66, or made of a non-reinforced, arc extinguishing insulative material composition comprising, as a principal component thereof, a polyamide such as nylon 46 or nylon 66; and a base layer underlying the arc receiving layer and made of an arc extinguishing insulative material composition comprising 20 to 65 % of at least one filler selected from the group consisting of a glass fiber containing not more than 1 % of compounds of group 1A metals of the periodic table in total, calcium carbonate, wollastonite or magnesium silicate hydrate containing not more than 1 % of compounds of group 1A metals of the periodic table in total and an aluminum silicate fiber, aluminum borate whisker or alumina whisker containing not more than 1 % of compounds of group 1A metals of the periodic table in total, and a matrix resin containing as a main component at least one member selected from the group consisting of a polyolefin, an olefin copolymer, a polyamide such as nylon 46 or nylon 66, a polyamide polymer blend, a polyacetal, a polyacetal polymer blend, and a thermoplastic or thermosetting resin such as nylon 6T, nylon MXD6, polyethylene terephthalate or polybutylene terephthalate. Other embodiments of the arc extinguishing device include the arc extinguishing device (I) comprising only insulator (1) as shown in Fig. 1-11, and the arc extinguishing device (II) comprising only insulator (2) as shown in Figs. 1-12 and 1-13. It has been conventionally considered that the insulation failure of a switch occurring upon the generation of an arc is caused by a decrease in the electric resistance due to carbons resulting from the decomposition of an organic substance and adhering to wall surfaces of an arc extinguishing device of the switch or to the contact section of the switch. The present inventors made detailed analysis on the deposit adhering to wall surfaces and contact section within the arc extinguishing chamber of a switch. As a result, there was found the fact that a metal layer was formed from metals that were scattered from electrodes, contacts and other metal components in the vicinity thereof upon an open-close operation of the electrodes of the switch, and such a metal layer greatly influenced the decrease in electric resistance. Accordingly, the conventional method of inhibiting only the deposition of carbon was found to be incapable of satisfactory preventing the decrease in electric resistance. The composition of the present invention may contain a gas generating source compound which is capable of scatteredly generating an insulation imparting gas combinable with metal particles scattered from the electrodes, contacts and other metal components of a switch by an arc generated when the contacts are operated to be opened or closed, thereby insulating the scattered metal particles. The gas generating source compounds for use in the present invention include those compounds which are each adapted to generate a gas that is reactive mainly with metals and those compounds which are each adapted to generate a gas that is, per se, electrically insulative. Preferable compounds of the former type include, for instance, a metal peroxide, a metal hydroxide, a metal hydrate, a metal alkoxide hydrolysate, a metal carbonate, a metal sulfate, a metal sulfide, a metal fluoride and a fluorine-containing silicate. These compounds offer a great insulation imparting effect. Representative examples of the metal peroxides are calcium peroxide (CaO 2 ), barium peroxide (BaO 2 ) and magnesium peroxide (MgO 2 ). Representative examples of the metal hydroxides are zinc hydroxide (Zn(OH) 2 ), aluminum hydroxide (Al(OH) 3 ), calcium hydroxide (Ca(OH) 2 ), barium hydroxide (Ba(OH) 2 ) and magnesium hydroxide (Mg(OH) 2 ). Aluminum hydroxide and magnesium hydroxide are preferred in view of the quantity of the gas generated by thermal decomposition. Of these, magnesium hydroxide is more preferable in view of its effect in insulating metal particles. Representative examples of the metal hydrates are barium octohydrate (Ba(OH) 2 ·8H 2 O), magnesium phosphate octohydrate (Mg(PO 4 ) 2 ·8H 2 O), alumina hydrate (Al 2 O 3 ·3H 2 O), zinc borate (2ZnO·3B 2 O 3 ·3.5H 2 O) and ammonium borate ((NH 4 ) 2 O·5B 2 O 3 ·8H 2 O). Among these, alumina hydrate is preferred in view of its metal insulating effect. Representative examples of the metal alkoxide hydrolysates are silicon ethoxide hydrolysate (Si(OC 2 H 5 ) 4-x (OH) x , where x is an integer of 1 to 3), silicon methoxide hydrolysate (Si(OCH 3 ) 4-x (OH) x , where x is the same as above), barium ethoxide hydrolysate (Ba(OC 2 H 5 )(OH)), aluminum ethoxide hydrolysate (Al(OC 2 H 5 ) 3-y (OH) y , where y is 1 or 2), aluminum butoxide hydrolysate (Al(OC 4 H 9 ) 3-y (OH) y , where y is the same as above), zirconium methoxide hydrolysate (Zr(OCH 3 ) 4-x (OH) x , where x is the same as above) and titanium methoxide hydrolysate (Ti(OCH 3 ) 4-x (OH) x , where x is the same as above). Among these, silicon ethoxide is preferred in view of its metal insulating effect. Representative examples of the metal carbonates are calcium carbonate (CaCO 3 ), barium carbonate (BaCO 3 ), magnesium carbonate (MgCO 3 ) and dolomite (CaMg(CO 3 ) 2 ). Among these, calcium carbonate and magnesium carbonate are preferred in view of their metal insulating effect. Representative examples of the metal sulfates are aluminum sulfate (Al 2 (SO 4 ) 3 ), calcium sulfate dihydrate (CaSO 4 ·2H 2 O) and magnesium sulfate (MgSO 4 ·7H 2 O). Representative examples of the metal sulfides are barium sulfide (BaS) and magnesium sulfide (MgS). Of these, barium sulfide is preferred in view of its metal insulating effect. Representative examples of the metal fluorides are zinc fluoride (ZnF 2 ), iron fluoride (FeF 2 ), barium fluoride (BaF 2 ) and magnesium fluoride (MgF 2 ). Among these, zinc fluoride and magnesium fluoride are preferred in view of their metal insulating effect. Representative examples of the fluorine-containing silicates are fluorophlogopite (KMg 3 (Si 3 Al)O 10 F 2 ), fluorine-containing tetrasilicate mica (KMg 2.5 Si 4 O 10 F 2 ) and litium taeniolite (KLiMg z Si 4 O 10 F 2 ). Among these, fluorine-containing phlogopite is preferred in view of its metal insulating effect. The foregoing gas generating compounds which are each adapted to generate a gas that is reactive mainly with metals can be used either alone or as mixtures thereof. Among these, particularly preferable are magnesium hydroxide, calcium carbonate and magnesium carbonate because these compounds each generate a gas exhibiting a great insulating effect and are less expensive. Preferable gas generating compounds of the type which mainly generate an electrically insulative gas include, for instance, a metal oxide, a compound oxide and a silicate hydrate. These compounds exhibits a great insulation imparting effect. Representative examples of the metal oxides are aluminum oxide (Al 2 O 3 ), zirconium oxide (ZrO 2 ), magnesium oxide (MgO), silicon dioxide (SiO 2 ), antimony pentoxide (Sb 2 O 5 ), ammonium octamolybdate ((NH 4 ) 4 Mo 8 O 26 ). Representative examples of the compound oxides are zircon (ZrO 2 ·SiO 2 ), cordierite (2MgO·2Al 2 O 3 ·5SiO 2 ), mullite (3Al 2 O 3 ·2SiO 2 ) and wollastonite (CaO·SiO 2 ). Representative examples of the silicate hydrates are muscovite (KAl 2 (Si 3 Al)O 10 (OH) 2 ), kaoline (Al 2 (Si 2 O 5 )(OH) 4 ), talc (Mg 3 (Si 4 O 10 )(OH) 2 ) and ASTON (5MgO·3SiO 2 ·3H 2 O). Among these, ASTON is preferred in view of its metal insulating effect and mechanical strength. These compounds of the type which generates a gas that is, per se, electrically insulative can be used either alone or as mixtures thereof. Hydroxides, hydrates, oxides and the like have a good effect of converting the metallic substances into insulative substances. In particular, magnesium hydroxide is very easy to generate H 2 O, O 2 , atomic oxygen, oxygen ion and oxygen plasma by dehydration reaction owing to arc and is easy to cause a reaction to insulate metals and, hence, magnesium hydroxide is advantageous in reducing the amount of electroconductive substances. To obtain a molded product from the gas generating source compound and an organic binder, it is possible that 25 to 300 parts, preferably 40 to 100 parts of the binder and 100 parts of the gas generating source compound are homogeneously mixed using a roll kneader or extrusion kneader, and then the resulting mixture is molded using an injection molding machine or press molding machine. If the proportion of the binder is less than 25 parts, the kneadability and moldability of the mixture tend to degrade, whereas if it exceeds 300 parts, the metal insulating effect of the molded product tends to become poor. The present invention will be more fully described by specific examples thereof. In those examples were conducted the following interrupting test, short circuit test and durability test. Interrupting test A circuit breaker including an arc extinguishing device of the aforementioned arrangement in closed state is applied with a current six times as high as a rated current (for example, a circuit breaker rated at 100 A being applied with a current of 600 A) and a moving contact 4 is separated away from a fixed contact 5 by a contact gap distance L (distance between moving contact 4 and fixed contact 5) of 15 to 25 mm to generate an arc current. If the circuit breaker successfully interrupts the arc current predetermined times, the circuit breaker is regarded as passed the test. Short circuit test A circuit breaker as above in closed state is applied with an overcurrent of 10 to 100 kA and a moving contact element is separated away from a fixed contact to generate an arc current. If the circuit breaker successfully interrupts the arc current with no damage, the circuit breaker is regarded as passed the test. Durability test A circuit breaker as above in closed state is applied with a normal current (for example, a circuit breaker rated at 100 A being applied with a current of 100 A) and a moving contact element is mechanically separated away from a fixed contact to generate an arc current. If the circuit breaker successfully interrupts the arc current predetemined times and the arc extinguishing insulative material used therein exhibits a consumption-by-arc resistance, specifically to such a degree that a hole is not formed in the insulative material by the arc, the breaker is regarded as passed the test. EXAMPLES 1-1 to 1-7 Arc extinguishing devices each having only insulator (2) were fabricated by using the insulative materials shown in Table 1. The insulator (2) had a thickness T2 of 1.5 mm and a width W of 10 mm and was of a double-layered structure comprising an arc receiving layer (1 mm thick) and an outer base layer (0.5 mm thick) covering the arc receiving layer. The arc receiving layer comprised nylon 46 or 66 reinforced with 20 % of a filler or non-reinforced nylon 46 or 66, while the outer base layer comprised nylon 46, nylon MXD6, PET or nylon 6T which was reinforced with GF. The arc extinguishing devices thus fabricated were subjected to the tests under the following conditions: Interrupting test: one-phase 420 V/1500 A, open contact distance L = 25 mm Durability test: three-phase 550 V/225 A, open contact distance L = 25 mm Short circuit test: one-phase 265 V/25 kA, open contact distance L = 25 mm. The results of the tests were as shown in Table 1. Particulars of the matrix resins and fillers were as follows: PA6T: nylon 6T, ARLEN (trade mark) produced by MITSUI PETROCHEMICAL INDUSTRIES, LTD.; PA66: nylon 66, NOVAMID (trade mark) produced by MITSUBISHI KASEI CORPORATION; PA46: nylon 46, UNITIKA NYLON 46 (trade mark) produced by UNITIKA Ltd.; PA·MXD6: nylon MXD6, Reny (trade mark) produced by Mitsubishi Gas Chemical Company, Inc.; PET: polyethylene terephthalate, NOVAPET (trade mark) produced by MITSUBISHI KASEI CORPORATION; GF : glass fiber formed of E glass Table 1 Ex. No. Arc extinguishing insulative material Test result Insulator (1) Insulator (2) Short circuit test Interrupting test (Number of times of success) Durability test (Formation of hole) Arc receiving layer Base layer (Damage to insulator) 1-1 not present PA66/GF (10%) PA46/GF (50%) no 20 not formed after 4000 interruptions 1-2* not present PA66 PA46/GF (50%) no 20 not formed after 4000 interruptions 1-3* not present PA66/Mg(OH) 2 (10%) PA46/GF (50%) no 20 not formed after 4000 interruptions 1-4* not present PA66 PA6T/GF (50%) no 20 not formed after 4000 interruptions 1-5* not present PA66 PAMXD6/GF (50%) no 20 not formed after 4000 interruptions 1-6* not present PA66 PET/GF (45%) no 20 not formed after 4000 interruptions 1-7 not present PA46/GF (20%) PA46/GF (40%) no 20 not formed after 4000 interruptions *Reference Example As apparent from Table 1, the arc extinguishing devices of these Examples were not damaged at insulator (2) in the short circuit test, succeeded in interrupting an arc 20 times in the interrupting test, and did not suffer the formation of hole in the durability test. Therefore, the devices were regarded as passed. Like those nylon 46, nylon MXD6, PET and nylon 6T, satisfactory test results were obtained when the base layer was formed of any one of modified polyphenylene oxide, polycarbonate, polyphenylene sulfide, polysulfone, polyether sulfone and polyether ketone which were each reinforced with GF. The fillers used in those Examples did not allow the respective insulation resistances thereof to lower even when exposed to the heat of arc. Accordingly, there were obtained arc extinguishing materials of high insulation resistance. It should be noted that although the insulative materials in Examples 1-1 to 1-7 exhibited an excellent effect when used in the insulator (2), they exhibited a satisifactory effect when used in the insulator (1).

Claims: An arc-extinguishing material comprising an arc-extinguishing insulator molded product having a double-layered structure, said product comprising: an arc-receiving layer made of a reinforced arc-extinguishing insulator composition comprising 5 to 20 % by weight of at least one filler selected from the group consisting of a glass fiber containing not more than 1 % by weight of compounds of group 1A metals of the periodic table in total, an inorganic mineral containing not more than 1 % by weight of compounds of group 1A metals of the periodic table in total and a ceramic fiber containing not more than 1 % by weight of compounds of group 1A metals of the periodic table in total, and a matrix resin containing as a principal component at least one member selected from the group consisting of a polyolefin, an olefin copolymer, a polyamide, a polyamide polymer blend, a polyacetal and a polyacetal polymer blend; and a base layer underlying said arc-receiving layer and made of an arc-extinguishing insulator composition comprising 20 to 65 % by weight of at least one filler selected from the group consisting of a glass fiber, an inorganic mineral and a ceramic fiber, and a matrix resin containing, as a principal component thereof, a thermoplastic resin or a thermosetting resin. The arc-extinguishing material according to claim 1, wherein said matrix resin in said base layer is at least one member selected from the group consisting of a polyolefin, an olefin copolymer, a polyamide, a polyamide polymer blend, a polyacetal and a polyacetal polymer blend. The arc-extinguishing material according to claim 1, wherein said matrix resin in said base layer is at least one member selected from the group consisting of nylon 6T, nylon MXD, polyethylene terephthalate, polybutylene terephthalate, modified polyphenylene oxide, polyphenylene sulfide, polysulfone, polyether sulfone and polyether ketone. The arc-extinguishing material according to claim 1, wherein at least one of said arc-receiving layer and said base layer contains at least one polyamide selected from the group consisting of nylon 46 and nylon 66 as the principal component of the matrix resin. The arc-extinguishing material according to claim 1, wherein at least one of said arc-receiving layer and said base layer contains at least one filler selected from the group consisting of calcium carbonate, wollastonite, magnesium hydrate, aluminum silicate fiber, aluminum borate whisker and alumina whisker. A switch comprising a contact section including contacts (4, 5) from which an arc is generated, and an arc-extinguishing device (8) comprising an insulator (2) disposed on both sides with respect to a plane including the locus of an opening or closing movement of the contacts or around the contact section, said insulator (2) being formed of an arc-extinguishing material according to one of claims 1 to 3.

Inventors: ADACHI HIROSHI, BABA FUMIAKI, FUKUYA KAZUNORI, HIROI OSAMU, KATO HAZUHARU, KATSUBE SHUNICHI, MITSUHASHI TAKAO, MURAKAMI TADAKI, NISHINA KENICHI, NISHIYAMA ITSUO, TAKAHASI MITUGU, YAMAGATA SHINJI, YAMAGUCHI SHOJI, ADACHI, HIROSHI, BABA, FUMIAKI, FUKUYA, KAZUNORI, HIROI, OSAMU, KATO, HAZUHARU, KATSUBE, SHUNICHI, MITSUHASHI, TAKAO, MURAKAMI, TADAKI, NISHINA, KENICHI, NISHIYAMA, ITSUO, TAKAHASI, MITUGU, YAMAGATA, SHINJI, YAMAGUCHI, SHOJI, ADACHI, HIROSHI, C/O MITSUBISHI DENKI K. K., BABA, FUMIAKI, C/O MITSUBISHI DENKI K. K., FUKUYA, KAZUNORI, C/O MITSUBISHI DENKI K. K., HIROI, OSAMU, C/O MITSUBISHI DENKI K. K., KATO, HAZUHARU, C/O MITSUBISHI DENKI K. K., KATSUBE, SHUNICHI, C/O MITSUBISHI DENKI K. K., MITSUHASHI, TAKAO, C/O MITSUBISHI DENKI K. K., MURAKAMI, TADAKI, C/O MITSUBISHI DENKI K. K., NISHINA, KENICHI, C/O MITSUBISHI DENKI K. K., NISHIYAMA, ITSUO, C/O MITSUBISHI DENKI K. K., TAKAHASI, MITUGU, C/O MITSUBISHI DENKI K. K., YAMAGATA, SHINJI, C/O MITSUBISHI DENKI K. K., YAMAGUCHI, SHOJI, C/O MITSUBISHI DENKI K. K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1995
Main classification: H01H33
IPC codes: H01H33, H01H9