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 plate material, a preparation method therefor and a switch having an arc extinguishing chamber of which side plate comprises the arc extinguishing plate material. More specifically, the present invention relates to an arc extinguishing plate material exhibiting excellent heat resistance, arc resistance, heat impact resistance and a like characteristic, which can readily be prepared and is capable of extinguishing an arc generated in an arc extinguishing chamber upon an opening or closing operation of the contacts of electrodes in a switch such as an electromagnetic contactor, circuit breaker or current-limiting device by absorbing the energy of the arc and cooling down, thereby protecting the devices or components installed in such a switch from the heat of the arc, while at the same time exhibiting the effect of preventing the electrical resistance of the switch from decreasing by insulating a metal vapor and molten metal droplets that are generated from the electrodes, contacts and other metal components located adjacent thereto upon an opening or closing operation of the electrodes. The invention also relates to a preparation method for such arc extinguishing plate material and a switch having an arc extinguishing chamber of which arc extinguishing side plate comprises the arc extinguishing plate material. A typical arc extinguishing chamber will be illustrated by way of Fig. 3 showing, in schematic perspective, one example of a conventional arc extinguishing chamber. The arc extinguishing chamber shown in Fig. 3 includes a plurality of arc extinguishing magnetic plates 201 each defining a U-shaped notch 201a in the central portion thereof and formed of, for example, an iron plate, and a pair of arc extinguishing side plates 207 to which the both sides of each magnetic plate 201 are secured at caulking portions 203. Fig. 4 is a partially cutaway side view of one example of a conventional switch for illustrating the arc extinguishing operation of an arc extinguishing chamber, wherein like numerals are used to denote like or corresponding parts of Fig. 3, and numerals 204 and 205 denote a fixed contact and a moving contact, respectively. Reference will be made to the operation of the switch. In the arc extinguishing chamber comprising the magnetic plates 201 and the arc extinguishing side plates 207, the fixed contact 204 and moving contact 205 assuming contact condition (closed condition) allows electric current to flow therethrough. When the electric current is to be interrupted, the moving contact 205 is moved toward the position indicated by dotted line (opened condition). At this time an arc is generated over the gap between the fixed contact 204 and moving contact 205. Such arc is drawn in the direction indicated by arrow so as to be extinguished. Conventionally, the arc extinguishing side plate forming part of the arc extinguishing chamber is usually formed of an organic-inorganic combined material such as a rigid fiber material, a combination of this rigid fiber material and asbestos paper attached onto the inner face of the rigid fiber material, a laminated plate comprising a glass base and a melamine resin and a laminated plate comprising glass mat and polyester resin (refer to Japanese Examined Patent Publication No. 54609/1990). There are also used as the material of the side plate a material formed only of inorganic substance such as a glass fiber sheet laminated plate using a boric acid-zinc oxide based binder (refer to Japanese Examined Patent Publication No. 9335/1988), and various sintered ceramic materials. The rigid fiber material, however, is prone to be decomposed by heat of an arc at arc extinguishing or to be carbonized by repeated exposure to arc and, hence, the insulation resistance thereof will be severely lowered. In addition, the rigid fiber material involves a problem of deformation by thermal shrinkage. With the combination of such a rigid fiber material and asbestos paper attached thereto, the asbestos is likely to scatter when subjected to the pressure of arc and to enter the gap between the contacts 204 and 205, thus resulting in the likehood of a conduction failure. The glass base-melamine resin laminated plate also presents the problem of susceptibility to decomposition or carbonization due to heat of arc at arc extinguishing. Further, the glass mat-polyester resin laminated plate in general is incorporated with an inorganic substance containing crystal water for an improvement in arc resistance (by utilizing the cooling action of latent heat of vaporization of moisture physico-chemically adhering thereto upon interruption of current, or the arc extinguishing action of free water, or by improving the heat release or heat conduction). Usually used as the inorganic filler is, for instance, alumina hydrate or aluminum hydroxide. This type of laminated plate, however, suffers non-uniform surface characteristics due to, for example, the formation of glass fiber and resin-excessive layer which is poor in arc resistance in the surface layer and hence cannot serve the purpose, resulting in a problem similar to that of the glass base-melamine resin laminated plate. It has heretofore been assumed that the insulation failure due to the generation of arc in a switch is caused by a decrease in electrical resistance attributed to carbon resulting from the decomposition of an organic substance and adhering to the surfaces of components accommodated within the switch as well as the inner walls of the arc extinguishing chamber. To prevent the decrease in electrical resistance, there have been proposed methods such as employing an organic substance which is free of any aromatic ring having many carbon atoms and is rich in hydrogen atom (as disclosed in Japanese Unexamined Patent Publication No. 310534/1988), and utilizing the generation of carbon monoxide or volatile hydrocarbon resulting from the reaction represented by: Al 2 O 3 ·3H 2 O → Al 2 O 3 + 3H 2 O organic group (HC) + 3H 2 O → CO(HC) wherein aluminum hydroxide (Al 2 O 3 ·3H 2 O) is contained in an arc extinguishing material and is used as a starting material, and the crystal water dissociated therefrom is allowed to react with the organic group (HC) (as disclosed in Japanese Unexamined Patent Publication No. 144844/1990). However, with increasing demand for a switch of smaller size and higher capacity in the recent trend toward electric devices of smaller sizes and lighter weights, the number of components made of an organic substance and used in a switch is increased and, hence, there is a high possibility that the amount of free carbon to be generated from such organic substance by arc is increased. For this reason the method proposed in, for example Japanese Unexamined Patent Publication No. 310534/1988 employs an organic substance which is free of any aromatic ring having many carbon atoms and is rich in hydrogen atom, and which is prepared by filling 5 to 30 % of a glass fiber material into an acrylic acid ester copolymer or an aliphatic hydrocarbon resin. Such a method, however, sometimes fails to satisfactorily prevent the electrical resistance from decreasing. Alternatively, in the method employing an arc extinguishing material formed from a resin filled with aluminum hydroxide as disclosed in Japanese Unexamined Patent Publication No. 144844/1990, although there is a certain effect in inhibiting the generation of free carbons by virtue of the reaction of the crystal water dissociated from aluminum hydroxide with the organic group of the organic material, it is possible that the organic material be cracked and broken by expansion of the crystal water due to rapid vaporization thereof when exposed to arc and hence be rendered unusable. The glass fiber sheet laminated plate of Japanese Examined Patent Publication No. 9335/1988 which uses a boric acid-zinc oxide binder and is formed only of inorganic substances is insusceptible to carbonization and decomposition and hence exhibits an excellent wear resistance, but is incapable of satisfactorily preventing the decrease in insulation resistance due to free carbon and is poor in applicability to mass production. Further, the ceramic material, though it does not generate carbon, is likely to be damaged by thermal shock when rapidly heated by arc and hence involves a danger of a severe accident. In addition, a molded product of the ceramic material needs to be baked at a high temperature, e.g. 1300°C or above. This causes energy loss and shrinkage in dimensions and hence leads to a lower yield for a product of more complicated shape. According to the detailed analysis on the deposit adhering to the inner surface of a switch by the inventors of the present invention, there was found that a metal layer is formed from metal vapor or molten metal droplets that are generated from the electrodes, contacts and other metal components located adjacent thereto by an arc generated upon an opening or closing operation of the contacts, and such a metal layer as well as free carbon greatly contributes to the decrease in electrical resistance. Consequently, the prior art, or only inhibiting the generation of free carbons cannot sufficiently prevent the decrease in electrical resistance. According to the document US-A-3 761 660 there is disclosed an arc extinguishing plate material comprising a combination of hydrated alumina and melamine in a binder which may be a thermosetting resin, preferably a thermoplastic resin. The composition may further contain conventional fillers, reinforcing fibers such as glas fiber, asbestos, and the like. In such a composition the binder vaporizes under arching conditions or decomposes violently and the hydrated alumina gives off water of hydration. This process renders the melamine more effective and creates improved arc interruption conditions. Document US-A-4 436 831 discloses another arc extinguishing calcinated material consisting of an inorganic composite material. This material is obtained by molding and heat treating a starting material under pressurized condition, the starting material comprising 10-50% by weight a mica powder as a base material, 10-50% by weight of a magnesium oxide, and 25-60% by weight of a mixture powder of boric acid, boric anhydrite and zinc oxide as a binder. According to JP-A-54 072 471 there is disclosed an arc extinguishing board produced by stacking inorganic material base sheets bonded together by two kinds of binders. One of the binders contains boric acid and zinc oxide or cadmium oxide, and the other binder contains boric acid and carbonate or hydroxide, said second binder being coated on a peripheral portion of the base sheet for providing extinguishing characteristics to the stacked sheets. In view of the foregoing, it is the object of the present invention to provide an arc extinguishing plate material and a switch comprising the same, said arc extinguishing plate material having excellent heat resistance, arc resistance, thermal shock resistance and the like which can be readily prepared and which is adapted to extinguish an arc to be generated within an arc extinguishing chamber of a switch upon an opening or closing operation of the contacts of the electrodes by absorbing the energy of the arc and cooling down thereby protecting the components of the switch from the heat of the arc, while satisfactorily preventing the electrical resistance of the switch from decreasing by insulating metal vapor and molten metal droplets produced from the electrodes, contacts and other metal components located adjacent thereto upon an opening or closing operation of the contacts. Further objects of the invention is to provide a switch provided with an arc extinguishing chamber of which arc extinguishing side plate comprises the arc extinguishing plate material. The object of the invention is achieved by an arc extinguishing plate material according to claims 1, 3 and 5 and by a switch according to claim 7. As defined in these claims, the arc extinguishing plate material according to the invention is substantially characterized in that it comprises a reinforcing inorganic element, like a fiber or sheet, an insulating imparting gas generating source compound, and an arc resistant inorganic powder, said compounds being provided in a certain amount defined in the claims. Preferable embodiments of the arc extinguishing plate material are defined in the subclaims. The above object is further achieved by a switch comprising the arc extinguishing plate material according to the invention. In the following, the invention is further illustrated by examples with reference to the accompanying drawings. Fig. 1 is a schematic perspective view showing one embodiment of an arc extinguishing chamber manufactured by using an arc extinguishing plate material according to the present invention; Fig. 2 is a partially cutaway explanatory side view showing one embodiment of a switch according to the present invention; Fig. 3 is a schematic perspective view showing one example of a conventional arc extinguishing chamber; and Fig. 4 is a partially cutaway explanatory side view showing one example of a conventional switch. Embodiments of the present invention are as follows. According to embodiment 1 of the present invention, there is provided an arc extinguishing plate material (I) comprising 35 to 50 % of a reinforcing inorganic material sheet and 50 to 65 % of an inorganic binder composition (B), wherein the arc extinguishing plate material is prepared by pressure molding and aging a sheet comprising the reinforcing inorganic material sheet and an inorganic binder composition (A). According to embodiment 2 of the present invention, the reinforcing inorganic material sheet in the arc extinguishing plate material (I) of embodiment 1 comprises a glass mat or glass fabric formed of a glass fiber having an insulating property or a ceramic paper prepared by papering of a ceramic fiber. According to embodiment 3 of the present invention, the inorganic binder composition (A) in the arc extinguishing plate material (I) of embodiment 1 is an inorganic binder composition (I) comprising 30 to 45 % of an insulation imparting gas generating source compound, 0 to 28 % of an arc resistant inorganic powder, 40 to 65 % of an aqueous solution of a primary metal salt of phosphoric acid, and 2 to 10 % of a curing agent for the primary metal salt of phosphoric acid. According to embodiment 4 of the present invention, the insulation imparting gas generating source compound in the arc extinguishing plate material (I) of embodiment 3 is aluminum hydroxide. According to embodiment 5 of the present invention, the primary metal salt of phosphoric acid in the arc extinguishing plate material (I) of embodiment 3 is aluminum primary phosphate or magnesium primary phosphate. According to embodiment 6 of the present invention, the concentration of the primary metal salt of phosphoric acid in the aqueous solution used in the arc extinguishing plate material (I) of embodiment 3 is from 25 to 55 %. According to embodiment 7 of the present invention, the curing agent for the primary metal salt of phosphoric acid in the arc extinguishing plate material (I) of embodiment 3 is wollastonite crystal or aluminum hydroxide. According to embodiment 8 of the present invention, the inorganic binder composition (A) in the arc extinguishing plate material (I) of embodiment 1 is an inorganic binder composition (II) comprising 30 to 50 % of an insulation imparting gas generating source compound, 0 to 20 % of an arc resistant inorganic powder, and 50 to 70 % of an aqueous solution of condensed alkali metal phosphate. According to embodiment 9 of the present invention, the insulation imparting gas generating source compound in the arc extinguishing plate material (I) of embodiment 8 is selected from the group consisting of magnesium hydroxide, magnesium carbonate and calcium carbonate. According to embodiment 10 of the present invention, the condensed alkali metal phosphate in the arc extinguishing plate material (I) of embodiment 8 is sodium metaphosphate or potassium metaphosphate. According to embodiment 11 of the present invention, the concentration of the condensed alkali metal phosphate in the aqueous solution used in the arc extinguishing plate material (I) of embodiment 8 is from 10 to 40 %. According to embodiment 12 of the present invention, the insulation imparting gas generating source compound in the arc extinguishing plate material (I) of embodiment 8 or 9 acts also as a curing agent for the aqueous solution of condensed alkali metal phosphate. According to embodiment 13 of the present invention, the arc resistance inorganic powder in the arc extinguishing plate material (I) of embodiment 3 or 8 is selected from the group consisting of aluminum oxide powder, zircon powder and cordierite powder. According to embodiment 14 of the present invention, there is provided a method for preparing an arc extinguishing plate material (I) comprising 35 to 50 % of a reinforcing inorganic material sheet and 50 to 65 % of an inorganic binder composition (B), and the method comprises the steps of: drying a sheet comprising the reinforcing inorganic material sheet and an inorganic binder composition (A) at 80° to 120°C and then subjecting the same to pressure molding; and aging the sheet at 120° to 200°C to remove moisture therefrom and cure the sheet, followed by cooling the sheet down to 80°C or below. According to embodiment 15 of the present invention, the sheet prior to undergoing the pressure molding in the method of embodiment 14 is prepared by the steps of: mixing 30 to 45 % of an insulation imparting gas generating source compound, 0 to 28 % of an arc resistant inorganic powder and 2 to 10 % of a curing agent for a primary metal salt of phosphoric acid; adding, to the resulting mixture, 40 to 65 % of the aqueous solution of primary metal salt of phosphoric acid, followed by kneading to prepare an inorganic binder composition (I); immersing the reinforcing inorganic material sheet into the inorganic binder composition (I) to form a sheet with the inorganic binder composition (I) adhering thereto; and drying the sheet at 80° to 120°C to adjust the concentration of the primary metal salt of phosphoric acid in the aqueous solution to 65 to 85 %. According to embodiment 16 of the present invention, in the method of embodiment 15, the insulation imparting gas generating source compound is aluminum hydroxide; the arc resistant inorganic powder is selected from the group consisting of aluminum oxide powder, zircon powder and cordierite powder; the curing agent for the primary metal salt of phosphoric acid is wollastonite crystal or aluminum hydroxide; and the aqueous solution of the primary metal salt of phosphoric acid is a 25 to 55 % aqueous solution of aluminum primary phosphate or magnesium primary phosphate. According to embodiment 17 of the present invention, the sheet prior to undergoing the pressure molding in the method of embodiment 14 is prepared by the steps of: mixing 30 to 50 % of an insulation imparting gas generating source compound and 0 to 20 % of an arc resistant inorganic powder; adding, to the resulting mixture, 50 to 70 % of an aqueous solution of condensed alkali metal phosphate, followed by kneading to prepare an inorganic binder composition (II); immersing the reinforcing inorganic material sheet into the inorganic binder composition (II) to prepare a sheet with the inorganic binder composition (II) adhering thereto; and drying the sheet at 80° to 120°C to adjust the concentration of the condensed alkali metal phosphate in the aqueous solution to 65 to 85 %. According to embodiment 18 of the present invention, in the method of embodiment 17, the insulation imparting gas generating source compound is selected from the group consisting of magnesium hydroxide, magnesium carbonate and calcium carbonate; the arc resistant inorganic powder is selected from the group consisting of aluminum oxide powder, zircon powder and cordierite powder; and the aqueous solution of condensed alkali metal phosphate is a 10 to 40 % aqueous solution of sodium metaphosphate or potassium metaphosphate. According to embodiment 19 of the present invention, in the method of any one of embodiments 14, 3-15 and 3-17, the proportion of the inorganic binder composition (I) or (II) adhering to the sheet is 200 to 350 parts relative to 100 parts of the reinforcing inorganic material sheet. According to embodiment 20 of the present invention, in the method of embodiment 14, the sheet prior to undergoing the pressure molding comprises a plurality of stacked sheets dried at 80° to 120°C. According to embodiment 21 of the present invention, the method of embodiment 14 or 20 comprises the step of applying, prior to pressure molding, an insulation imparting gas generating source compound onto either or both faces of a reinforcing inorganic material sheet containing the inorganic binder composition (A). According to embodiment 22 of the present invention, in the method of embodiment 21, the insulation imparting gas generating source compound is selected from the group consisting of magnesium hydroxide, magnesium carbonate and calcium carbonate. According to embodiment 23 of the present invention, the method of embodiment 20 comprises the steps of: preparing one of the sheets to be stacked on each other with use of an inorganic binder composition (I) as recited in embodiment 3-3 and the other with use of an inorganic binder composition (II) as recited in embodiment 3-8; drying at 80° to 120°C the one sheet to adjust the concentration of the primary metal salt of phosphoric acid in the aqueous solution contained therein to 65 to 85 % and the other sheet to adjust the concentration of the condensed alkali metal phosphate in the aqueous solution contained therein to 65 to 85 %; stacking the other sheet on either or both faces of the one sheet; further stacking the resulting stacked sheet on a stacked sheet of the same type to obtain a laminated sheet of a required thickness; subjecting the laminated sheet to pressure molding; aging the thus molded laminated sheet to facilitate removal of moisture therefrom and curing of the molded laminated sheet; and cooling the molded laminated sheet down to 80°C or below. According to embodiment 24 of the present invention, the method of any one of embodiments 14, 20, 21 and 23 further comprises the step of coating or impregnating the arc extinguishing plate material (I) with a coating material for preventing the arc extinguishing plate material (I) from dusting when subjected to a punching process. According to embodiment 25 of the present invention, the coating material used in the method of embodiment 24 is an organic metal compound (a metal alkoxide) or an organic resin. According to embodiment 26 of the present invention, there is provided an arc extinguishing plate material (I) which is obtained by pressure molding and aging an inorganic binder composition (C) comprising 40 to 55 % of an insulation imparting gas generating source compound, 25 to 40 % of an arc resistant inorganic powder, 8 to 18 % of a primary metal salt of phosphoric acid, 5 to 10 % of a curing agent for the primary metal salt of phosphoric acid, 2.6 to 12 % of water, and 2 to 10 % of a reinforcing inorganic fiber. According to embodiment 27 of the present invention, the insulation imparting gas generating source compound in the arc extinguishing plate material (II) of embodiment 26 is selected from the group consisting of magnesium hydroxide, aluminum hydroxide, magnesium carbonate and calcium carbonate. According to embodiment 28 of the present invention, the arc resistant inorganic powder in the arc extinguishing plate material (II) of embodiment 26 is selected from the group consisting of zircon powder, cordierite powder and mullite powder. According to embodiment 29 of the present invention, the primary metal salt of phosphoric acid in the arc extinguishing plate material (II) of embodiment 26 is selected from the group consisting of aluminum primary phosphate, magnesium primary phosphate and sodium primary phosphate. According to embodiment 30 of the present invention, in the arc extinguishing plate material (II) of any one of embodiments 26 to 28, the water is contained in such an amount as to afford a 60 to 75 % aqueous solution of the primary metal salt of phosphoric acid. According to embodiment 31 of the present invention, the curing agent for the primary metal salt of phosphoric acid in the arc extinguishing plate material (II) of embodiment 26 is selected from the group consisting of wollastonite crystal, magnesium hydroxide, aluminum hydroxide, magnesium carbonate and calcium carbonate. According to embodiment 32 of the present invention, the reinforcing inorganic fiber in the arc extinguishing plate material (II) of embodiment 26 is an inorganic short fiber. According to embodiment 33 of the present invention, the inorganic short fiber in the arc extinguishing plate material (II) of embodiment 32 is selected from the group consisting of a natural mineral fiber, a ceramic fiber and a ceramic whisker. According to embodiment 34 of the present invention, the natural mineral fiber in the arc extinguishing plate material (II) of embodiment 33 is wollastonite crystal which acts also as a curing agent for the primary metal salt of phosphoric acid. According to embodiment 35 of the present invention, there is provided a method for preparing an arc extinguishing plate material (II) comprising the steps of pressure molding in a mold an inorganic binder composition (C) comprising 40 to 55 % of an insulation imparting gas generating source compound, 25 to 40 % of an arc resistant inorganic powder, 8 to 18 % of a primary metal salt of phosphoric acid, 5 to 10 % of a curing agent for the primary metal salt of phosphoric acid, 2.6 to 12 % of water and 2 to 10 % of a reinforcing inorganc fiber; and aging the thus molded product at 120° to 200°C. According to embodiment 36 of the present invention, the insulation imparting gas generating source material in the method of embodiment 35 is selected from the group consisting of magnesium hydroxide, aluminum hydroxide, magnesium carbonate and calcium carbonate. According to embodiment 37 of the present invention, the arc resistant inorganic powder in the method of embodiment 35 is selected from the group consisting of zircon powder, cordierite powder and mullite powder. According to embodiment 38 of the present invention, the primary metal salt of phosphoric acid in the method of embodiment 35 is selected from the group consisting of aluminum primary phosphate, magnesium primary phosphate and sodium primary phosphate. According to embodiment 39 of the present invention, the curing agent for the primary metal salt of phosphoric acid in the method of embodiment 35 is selected from the group consisting of wollastonite crystal, magnesium hydroxide, aluminum hydroxide, magnesium carbonate and calcium carbonate. According to embodiment 40 of the present invention, there is provided a switch comprising electrodes, contacts provided to the electrodes, and an arc extinguishing chamber provided in the vicinity of the electrodes and contacts and having an arc extinguishing side plate formed of an arc extinguishing plate material as recited in any one of embodiments 1 to 13 and 26 to 34. The arc extinguishing plate material (I) of the present invention comprises, after curing, 35 to 50 % of the reinforcing inorganic material sheet and 50 to 65 % of the inorganic binder composition (B). Such a high content of the inorganic binder composition (B) imparts the arc extinguishing plate material (I) with excellent heat resistance, arc resistance, thermal shock resistance and the like. Further, the reinforcing inorganic material sheet contained in the proportion of 35 to 50 % allows the plate material (I) to exhibit excellent mechanical strength, punching quality and the like and to be readily produced. Such plate material (I) offers such merit as to absorb the energy of an arc generated in the arc extinguishing chamber of a switch upon an opening or closing operation of the electrodes thereof to extinguish the arc by absorbing the energy of the arc and cooling down, thereby protecting components of the switch from the heat of the arc. Where the reinforcing inorganic material sheet used in the arc extinguishing plate material (I) is formed of a glass mat or glass fabric, e.g. those made of a glass fiber having an excellent insulating property, or a ceramic paper made from a ceramic fiber, the plate material (I) exhibits higher mechanical strength and heat resistance. Where the inorganic binder composition (A) used in the arc extinguishing plate material (I) is the inorganic binder composition (I) comprising 30 to 45 % of an insulation imparting gas generating source compound, 0 to 28 % of an arc resistant inorganic powder, 40 to 65 % of an aqueous solution of primary metal salt of phosphoric acid and 2 to 10 % of a curing agent for the primary metal salt of phosphoric acid, combining the binder composition (I) integrally with the reinforcing inorganic material sheet affords the arc extinguishing plate material (I) with excellent mechanical strength, arc resistance, heat resistance and the like. When this plate material (I) is applied to a switch, it will demonstrate the effect of satisfactorily preventing a decrease in electrical resistance by insulating metal vapor and molten metal droplets which are generated from the electrodes, contacts and other metal components located adjacent thereto by an arc generated upon an opening or closing operation of the contacts. Where the insulation imparting gas generating source compound in the arc extinguishing plate material (I) is aluminum hydroxide, the compound will generate atomic oxygen and molecular oxygen (O and O 2 ) as the insulation imparting gas, resulting in a more potent effect in preventing the decrease in electrical resistance. Where the primary metal salt of phosphoric acid contained in the inorganic binder composition (A) in the arc extinguishing plate material (I) is aluminum primary phosphate or magnesium primary phosphate, the binder composition exhibits favorable properties required for a binder since aluminum primary phosphate or magnesium primary phosphate exhibits an excellent solubility in water and affords an aqueous solution of satisfactory viscosity and binding property, thus giving the inorganic binder composition (A) advantageously. Where the aqueous solution of primary metal salt of phosphoric acid contained in the inorganic binder composition (A) in the arc extinguishing plate material (I) has a concentration of the primary metal salt of phosphoric acid ranging from 25 to 55 %, the concentration of the primary metal salt of phosphoric acid in such solution can be easily adjusted to 65 to 85 %. In addition, it is possible to adjust the contents of the insulation imparting gas generating source compound and arc resistant inorganic powder to predetermined values and, hence, the inorganic binder composition (A) is possible to be favorably made to adhere to the reinforcing inorganic material sheet. This results in an easy preparation of the sheet. Where the curing agent for the primary metal salt of phosphoric acid in the arc extinguishing plate material (I) is wollastonite crystal or aluminum hydroxide, it is possible to impart the primary metal salt of phosphoric acid with water resistance by heating to about 150°C, thereby giving the plate material (I) with an excellent water resistance. Where the inorganic binder composition (A) in the arc extinguishing plate material (I) is the inorganic binder composition (II) comprising 30 to 50 % of an insulation imparting gas generating source compound, 0 to 20 % of an arc resistant inorganic powder and 50 to 70 % of an aqueous solution of condensed alkali metal phosphate, the plate material (I) containing the inorganic binder composition (II) is capable of more effectively preventing the decrease in electrical resistance than that containing the aforementioned inorganic binder composition (I). Where the insulation imparting gas generating source compound in the arc extinguishing plate material (I) is magnesium hydroxide, magnesium carbonate or calcium carbonate, the plate material (I) is capable of more effectively preventing the decrease in electrical resistance than that containing aluminum hydroxide. Where the condensed alkali metal phosphate contained in the inorganic binder composition (A) in the arc extinguishing plate material (I) is sodium metaphosphate or potassium metaphosphate, the binder composition exhibits favorable properties required for a binder such as an excellent solubility in water and affords an aqueous solution of satisfactory viscosity and binding property, thus giving the inorganic binder composition (A) advantageously. Where the aqueous solution of condensed alkali metal phosphate contained in the inorganic binder composition (A) in the arc extinguishing plate material (I) has a condensed alkali metal phosphate concentration of 10 to 40 %, it is possible to easily adjust the concentration of the condensed alkali metal phosphate in the aqueous solution to 65 to 85 %. In addition, the contents of the insulation imparting gas generating source compound and arc resistant inorganic powder can readily be adjusted to predetermined values and, hence, it is possible to make the inorganic binder composition (A) favorably adhere to the reinforcing inorganic material sheet, thereby facilitating the preparation of the sheet. Where the insulation imparting gas generating source compound in the arc extinguishing plate material (I) acts also as a curing agent for the condensed alkali metal phosphate, the compound reacts with the condensed alkali metal phosphate, thereby advantageously rendering the condensed alkali metal phosphate water resistant. Where the arc resistant inorganic powder in the arc extinguishing plate material (I) is aluminum oxide powder, it exhibits excellent arc resistance and electrical insulating property and serves also as a curing agent, while on the other hand when the arc resistant inorganc powder is zircon powder or cordierite powder, it exhibits excellent arc resistance and low expansibility. Accordingly, the plate material (I) obtained with use of such arc resistant inorganic powder exhibits improved thermal shock resistance and can be prepared with less raw material cost. The arc extinguishing plate material (I) of the present invention is prepared by the steps of: drying a sheet comprising a reinforcing inorganic material sheet and an inorganic binder composition (A) at 80° to 120°C and then subjecting the same to pressure molding; and aging the sheet, thus pressure molded at 120° to 200°C to remove moisture therefrom and cure the sheet, followed by cooling the sheet thus molded and cured down to 80°C or below. Such preparation method affords the aforementioned excellent arc extinguishing plate material (I) with ease. In the above preparation method, the sheet prior to undergoing the pressure molding is prepared by the steps of: mixing 30 to 45 % of an insulation imparting gas generating source compound, 0 to 28 % of an arc resistant inorganic powder and 2 to 10 % of a curing agent for a primary metal salt of phosphoric acid; adding, to the resulting mixture, 40 to 65 % of the aqueous solution of primary metal salt of phosphoric acid, followed by kneading to prepare the inorganic binder composition (I); immersing the reinforcing inorganic material sheet into the inorganic binder composition (I) to form a sheet with the inorganic binder composition (I) adhering thereto; and drying the sheet at 80° to 120°C to adjust the concentration of the primary metal salt of phosphoric acid in the aqueous solution to 65 to 85 %. The inorganic binder composition (I) can be well integrated with the reinforcing inorganic material sheet without being forced out thereof when the sheet is pressure molded, thereby giving the arc extinguishing plate material (I) of dense quality which offers an excellent mechanical strength and the like. In the method of the present invention, in case that the insulation imparting gas generating source compound is aluminum hydroxide; the arc resistant inorganic powder is selected from the group consisting of aluminum oxide powder, zircon powder and cordierite powder; the curing agent for the primary metal salt of phosphoric acid is wollastonite crystal or aluminum hydroxide; and the aqueous solution of primary metal salt of phosphoric acid is a 25 to 55 % aqueous solution of aluminum primary phosphate or magnesium primary phosphate, the arc extinguishing plate material (I) obtained by the method exhibits excellent arc resistance, heat resistance and thermal shock resistance and offers a favorable effect in preventing the decrease in electrical resistance. In the method of the present invention, in case that the sheet prior to undergoing the pressure molding is prepared by the steps of: mixing 30 to 50 % of an insulation imparting gas generating source compound and 0 to 20 % of an arc resistant inorganic powder; adding, to the resulting mixture, 50 to 70 % of an aqueous solution of a condensed alkali metal phosphate, followed by kneading to prepare the inorganic binder composition (II); immersing the reinforcing inorganic material sheet into the inorganic binder composition (II) to prepare a sheet with the inorganic binder composition (II) adhering thereto; and drying the sheet at 80° to 120°C to adjust the concentration of the condensed alkali metal phosphate in the aqueous solution to 65 to 85 %, the arc extinguishing plate material obtained by this method offers a more potent effect in preventing the decrease in electrical resistance than that employing the inorganic binder composition (I). In the method of the present invention, in case that the insulation imparting gas generating source compound is selected from the group consisting of magnesium hydroxide, magnesium carbonate and calcium carbonate; the arc resistant inorganic powder is selected from the group consisting of aluminum oxide powder, zircon powder and cordierite powder; and the aqueous solution of the condensed alkali metal phosphate is a 10 to 40 % aqueous solution of sodium metaphosphate or potassium metaphosphate, the resulting arc extinguishing plate material offers a more potent effect in preventing the decrease in electrical resistance than that employing the aqueous solution of primary metal salt of phosphoric acid. In the method of the present invention, in case that the proportion of the inorganic binder composition (I) or (II) adhering to the sheet is 200 to 350 parts relative to 100 parts of the reinforcing inorganic material sheet, the resulting arc extinguishing plate material exhibits excellent heat resistance, arc resistance and thermal shock resistance. In the method of the present invention, in case that the sheet prior to undergoing the pressure molding comprises a plurality of stacked sheets dried at 80° to 120°C, the resulting arc extinguishing plate material offers the effect of easily controlling the size (thickness) thereof and enjoys improved mechanical strength as compared to that comprising a single sheet. Where the method of the present invention comprises the step of applying an insulation imparting gas generating source compound onto either or both faces of a reinforcing inorganic material sheet containing the inorganic binder compostion (A), the resulting arc extinguishing plate material offers a potent effect in preventing the decrease in electrical resistance as compared to that resulting from the method excluding the application step. In the method of the present invention, in case that the insulation imparting gas generating source compound is selected from the group consisting of magnesium hydroxide, magnesium carbonate and calcium carbonate, the resulting arc extinguishing plate material offers a greater effect in preventing the decrease in electrical resistance than that empolying aluminum hydroxide. Where the method of the present invention comprises the steps of: preparing one of the sheets to be stacked on each other with use of an inorganic binder composition (I) as recited in embodiment 3-3 and the other with use of an inorganic binder composition (II) as recited in embodiment 3-8; drying at 80° to 120°C the one sheet to adjust the concentration of the primary metal salt of phosphoric acid in the aqueous solution contained therein to 65 to 85 % and the other sheet to adjust the concentration of the condensed alkali metal phosphate in the aqueous solution contained therein to 65 to 85 %; stacking the other sheet on either or both faces of the one sheet; further stacking the resulting stacked sheet on a stacked sheet of the same type to obtain a laminated sheet of a required thickness; subjecting the laminated sheet to pressure molding; aging the thus molded laminated sheet to facilitate removal of moisture therefrom and curing of the molded laminated sheet; and cooling the molded laminated sheet down to 80°C or below, the resulting arc extinguishing plate material enjoys a more potent effect in preventing the decrease in electrical resistance than that using the inorganic binder composition (I) singly. Where the method of the present invention further comprises the step of coating or impregnating the arc extinguishing plate material (I) with a coating material for preventing the plate material (I) from dusting when subjected to a punching process, the resulting plate material enjoys the effect of reducing the amount of fiber particles to be generated when the plate material is punched or cut in the punching process. In the method of the present invention, in case that the coating material is an organic metal compound (a metal alkoxide) or an organic resin, the binding property of the coating material with the underlying plate material (I) is satisfactory, thus resulting in a potent effect in preventing dusting. The arc extinguishing plate material (II) of the present invention is obtained by pressure molding and aging the inorganic binder composition (C) comprising 40 to 55 % of an insulation imparting gas generating source compound, 25 to 40 % of an arc resistant inorganic powder, 8 to 18 % of a primary metal salt of phosphoric acid, 5 to 10 % of a curing agent for the primary metal salt of phosphoric acid, 2.6 to 12 % of water, and 2 to 10 % of a reinforcing inorganic fiber. The plate material (II) of such constitution enjoys excellent heat resistance and arc resistance. Where the insulation imparting gas generating source compound used in the arc extinguishing plate material (II) is selected from the group consisting of magnesium hydroxide, aluminum hydroxide, magnesium carbonate and calcium carbonate, the plate material (II) offers an potent effect in preventing the decrease in electrical resistance, like the foregoing plate material (I) empolying the inorganic binder composition (II). Where the arc resistant inorganic powder used in the arc extinguishing plate material (II) is selected from the group consisting of zircon powder, cordierite powder and mullite powder, the plate material (II) exhibits excellent thermal shock resistance as well as excellent arc resistance. Where the primary metal salt of phosphoric acid used in the arc extinguishing plate material (II) is selected from the group consisting of aluminum primary phosphate, magnesium primary phosphate and sodium primary phosphate, the insulation imparting gas generating source compound acts also as a curing agent, thus leading to a favorable inorganic binder composition. Where the water is contained in the arc extinguishing plate material (II) in such an amount as to afford a 60 to 75 % aqueous solution of primary metal salt of phosphoric acid, the plate material (II) becomes plastic when subjected to the pressure molding and hence is turned into a dense molded product. Where the curing agent for the primary metal salt of phosphoric acid in the arc extinguishing plate material (II) is selected from the group consisting of wollastonite crystal, magnesium hydroxide, aluminum hydroxide, magnesium carbonate and calcium carbonate, there appears an effect such that heating up to 200°C affords a molded product with water resistance. Where the reinforcing inorganic fiber in the arc extinguishing plate material (II) is an inorganic short fiber, the reinforcing inorganic fiber is homogeneously dispersed in the plate material (II) and imparts the plate material (II) with an excellent heat resistance. Where the inorganic short fiber in the arc extinguishing plate material (II) is selected from the group consisting of a natural mineral fiber, a ceramic fiber and a ceramic whisker, the plate material (II) enjoys further enhanced mechanical strength and arc resistance. Where the natural mineral fiber in the arc extinguishing plate material (II) is wollastonite crystal which acts also as a curing agent for the primary metal salt of phosphoric acid, the unreacted fiber component thereof acts to improve the mechanical strength of the plate material while the reacted fiber component thereof acts to impart the plate material with water resistance. The arc extinguishing plate material (II) of the present invention is prepared by the steps of: pressure molding in a mold the inorganic binder composition (C) comprising 40 to 55 % of an insulation imparting gas generating source compound, 25 to 40 % of an arc resistant inorganic powder, 8 to 18 % of a primary metal salt of phosphoric acid, 5 to 10 % of a curing agent for the primary metal salt of phosphoric acid, 2.6 to 12 % of water and 2 to 10 % of a reinforcing inorganic fiber; and aging the thus molded product at 120° to 200°C. The arc extinguishing plate material thus prepared does, in most cases, not require finishing and hence can be a final product such as an arc extinguishing plate. In the above method of the present invention, in case that the insulation imparting gas generating source compound is selected from the group consisting of magnesium hydroxide, aluminum hydroxide, magnesium carbonate and calcium carbonate, the compound will generate an insulation imparting gas comprising atomic oxygen, molecular oxygen, carbon dixoide and carbon monoxide, which effectively prevents the decrease in electrical resistance. In the method of the present invention, in case that the arc resistant inorganic powder is selected from the group consisting of zircon powder, cordierite powder and mullite powder, the resulting arc extinguishing plate material (II) enjoys excellent arc resistance as well as excellent thermal shock resistance. In the method of the present invention, in case that the primary metal salt of phosphoric acid is selected from the group consisting of aluminum primary phosphate, magnesium primary phosphate and sodium primary phosphate, the inorganic binder composition (C) exhibits a strong binding power. In the method of the present invention, in case that the curing agent for the primary metal salt of phosphoric acid is selected from the group consisting of wollastonite crystal, magnesium hydroxide, aluminum hydroxide, magnesium carbonate and calcium carbonate, the water resistance of the plate material (II) is developed by heating up to 200°C and, in addition, the mechanical strength thereof is improved. The switch of the present invention comprises electrodes, contacts provided to the electrodes, and an arc extinguishing chamber provided in the vicinity of the electrodes and contacts, and the chamber has an arc extinguishing side plate formed of an arc extinguishing plate material (I) or (II) as recited in any one of the above embodiments. The switch of such arrangement enjoys superior interrupting property, durability and insulation resistance enhancing performance. The arc extinguishing plate material (I) of the present invention comprises 35 to 50 % of a reinforcing inorganic material sheet and 50 to 65 % of an inorganic binder composition (B), the arc extinguishing plate material resulting from pressure molding and aging of a sheet comprising the reinforcing inorganic material sheet and an inorganic binder composition (A). The reinforcing inorganic material sheet serves to impart the obtained arc extinguishing plate material with an excellent mechanical strength, and any reinforcing inorganic material sheets which have been conventionallly used in the production of arc extinguishing plate material can be used in the present invention without particular limitations. Examples of specific reinforcing inorganic material sheets are, for instance, glass mat and glass fabric, e.g. those made of a glass fiber having an excellent insulating property such as E glass, S glass, D glass or silica glass, and a ceramic paper of about 0.5 to 2.0 mm thickness which is obtained by papering a ceramic fiber such as alumina fiber or aluminosilicate fiber, which are all commercially available. The inorganic binder composition (A), which is used as integrated with the reinforcing inorganic material sheet, serves to afford a plate material with excellent mechanical strength, heat resistance, arc resistance, thermal shock resistance and the like. The inorganic binder composition (A) also serves, when an arc is generated in the arc extinguishing chamber of a switch upon an opening or closing operation of the electrodes of the switch, to absorb the energy of the arc for cooling down and extinguish it, thereby protecting the components of the switch from the heat of the arc, while at the same time insulating metal vapor and molten metal droplets that are generated from the electrodes, contacts and other metal components adjacent thereto by the arc, thereby preventing a decrease in the insulation resistance of the switch. The inorganic binder composition (A) used in the preparation of the aforementioned sheet may comprise any such composition which serves the aforementioned purposes without particular limitations. Examples of the binder composition (A) include inorganic binder composition (I) comprising 30 to 45 % of an insulation imparting gas generating source compound, 0 to 28 % of an arc resistant inorganic powder, 40 to 65 % of an aqueous solution of primary metal salt of phosphoric acid and 2 to 10 % of a curing agent for the the primary metal salt of phosphoric acid, and inorganic binder composition (II) comprising 30 to 50 % of an insulation imparting gas generating source compound, 0 to 20 % of an arc resistant inorganic powder and 50 to 70 % of an aqueous solution of condensed alkali metal phosphate. Detailed description will be made on the inorganic binder composition (I) for use as the inorganic binder composition (A). The insulation imparting gas generating source compound contained in the binder composition (I) is adapted to generate a gas by an arc generated upon an opening or closing operation of the electrodes of a switch, and the gas acts to insulate metal vapor and molten metal droplets which are generated from the electrodes, contacts and other metal components adjacent thereto of the switch by the arc. It is assumed that the insulation imparting gas generated from the insulation imparting gas generating source compound insulates the metal vapor and molten metal droplets produced from the metal components of the switch according to the following process. When the electrodes disposed within the arc extinguishing chamber of the switch is operated to be opened or closed, an arc is generated between the contacts of the electrodes and generates heat of about 4000° to about 6000°C. As a result, the electrodes, contacts and other metal components located adjacent thereto are heated and thereby scatter metal vapor and molten metal droplets therefrom. At this time, the insulation imparting gas generating source compound contained in the arc extinguishing side plate of the arc extinguishing chamber is heated by the arc as well as by the metal vapor and molten metal droplets to generate an insulation imparting gas. The insulation imparting gas herein is meant by a gas of the properties to insulate the metal vapor and molten metal droplets. The gas reacts with the metal vapor and molten metal droplets and thereby insulates the same. When the gas reactive with such metal vapor and molten metal droplets is generated, the gas reacts with the metal vapor and molten metal droplets and, hence, the reaction product is scattered together with unreacted insultion imparting gas generating source compound. Accordingly, the substance thus insulated and the substance inherently insulative are deposited onto walls of the arc extinguishing chamber as well as onto the surfaces of components accommodated within the switch. Thus, the metal vapor and molten metal droplets, which conventionally have greatly contributed to the decrease in electrical resistance, are insulated and, hence, the decrease in electrical resistance is prevented, thereby inhibiting the occurrence of insulation failure due to the generation of arc. It should be noted that when the metal vapor and molten metal droplets being forcibly scattered from the electrodes, contacts and other metal components located adjacent thereto by arc are insulated, the insulation imparting gas cannot approach the contacts because of the high pressure metal vapor expanding, so that a layer resulting from insulated metal vapor and molten metal droplets is not formed on the contacts and, hence, the electric conduction between the contacts will not be affected. Examples of the insulation imparting gas generating source compound for generating the aforementioned gas which is reactive with the metal vapor and molten metal droplets are, for instance, metal hydroxides and metal carbonates, which are advantageously used in view of their great insulation imparting effect. Representative examples of the metal hydroxides are zinc hydroxide (Zn(OH) 2 ), aluminum hydroxide (Al(OH) 3 ), calcium hydroxide (Ca(OH) 2 ) and magnesium hydroxide (Mg(OH) 2 ). Representative examples of the metal carbonates are calcium carbonate (CaCO 3 ), magnesium carbonate (MgCO 3 ) and dolomite (CaMg(CO 3 ) 2 ). Of these compounds, aluminum hydroxide is preferred, since it reacts with the aqueous solution of primary metal salt of phosphoric acid not rapidly, imparts the inorganic binder composition (I) with appropriate viscosity, and offers a potent insulation imparting effect. The above-mentioned insulation imparting gas generating source compounds which are reactive with the metal vapor and molten metal droplets may be used either alone or in combination. Where the insulation imparting gas generating source compound is in powder form, the average particle diameter thereof is not particularly limited, but is usually from about 0.6 to about 40 µm for metal hydroxides and from about 0.3 to about 20 µm for metal carbonates from the viewpoints of the mixing property thereof in the inorganic binder composition (A), the moldability of the resulting arc extinguishing plate material and cost. The arc resistant inorganic powder used in the inorganic binder composition (I) is a component for imparting the obtained arc extinguishing plate material (I) with an excellent arc resistance. Examples of the arc resistant inorganic powders are, for instance, aluminum oxide powder (alumina powder, Al 2 O 3 ), zircon powder (zirconium silicate, ZrO 2 ·SiO 2 ), cordierite powder (2MgO·2Al 2 O 3 ·5SiO 2 ), mullite powder (3Al 2 O 3 ·2SiO 2 ), magnesium oxide (MgO) and zirconium oxide (ZrO 2 ). These may be used either alone or in combination. Of these powders, aluminum oxide powder, zircon powder, cordierite powder and mullite powder are preferred in terms of the following features. Aluminum oxide powder is excellent in arc resistance and electrical insulating property and acts also as a curing agent for the primary metal salt of phosphoric acid and of condensed alkali metal phosphate to be described later and hence is advantageously used in the present invention. Zircon powder is excellent in arc resistance, has a low expansibility, and offers the effect of improving the thermal shock resistance of the resulting arc extinguishing plate material together with less raw material cost. Cordierite powder is excellent in arc resistance, has a low expansibility, and offers the effect of improving the thermal shock resistance of the resulting arc extinguishing plate material together with less raw material cost. Mullite powder is excellent in arc resistance, has a low expansibility, and offers the effect of improving the thermal shock resistance of the resulting arc extinguishing plate material together with less raw material cost. The average particle size of the arc resistant inorganic powder is not particularly limited herein but is usually about 0.3 to about 40 µm in terms of its mixing property, dispersibility and cost. The aqueous solution of primary metal salt of phosphoric acid used in the inorganic binder composition (I) is a component for serving as a binder of the reinforcing inorganic material sheet, insulation imparting gas generating source compound, arc resistant inorganic powder and curing agent for the primary metal salt of phosphoric acid. Examples of the primary metal salts of phosphoric acid are, for instance, aluminum primary phosphate, magnesium primary phosphate, zinc primary phosphate and calcium primary phosphate. Among these, aluminum primary phosphate and magnesium primary phosphate are advantageously used, since they offers favorable properties in the preparation of the inorganic binder composition (I) such as high solubility in water and suitable viscosity for a binder when in the aqueous solution thereof. The suitable viscosity herein is such a low viscosity as to facilitate the mixing of the aqueous solution with the other components of the inorganic binder composition (I) and as to provide the binder composition (I) with a suitable binding characteristic for making the binder composition (I) adhere to the reinforcing inorganic material sheet. The concentration of the primary metal salt of phosphoric acid in the aqueous solution is preferably not lower than 25 %, more preferably not lower than 30 %, since when it is too low, a prolonged time period is likely to be required to remove excessive moisture from the aqueous solution in adjusting the concentration of the primary metal salt of phosphoric acid to 65 to 85 % for the pressure molding of the sheet. When the concentration of the aqueous solution of primary metal salt of phosphoric acid is too high, such difficulties in preparing the plate material are likely to occur that: the aqueous solution comes to have an undesirably high viscosity; it is impossible for the binder composition (I) to contain the predetermined amounts of the insulation imparting gas generating source compound and arc resistant inorganic powder; and further the reaction of the aqueous solution with the curing agent proceeds too rapidly. Therefore, the concentration of the primary metal salt of phosphoric acid is preferably not higher than 55 %, more preferably not higher than 50 %. The aluminum primary phosphate represented by Al(H 2 PO 4 ) 3 remains water soluble when heated to a temperature lower than 500°C and hence is poor in water resistance and electrical insulating property. For this reason, the aluminum primary phosphate is required to be heated to 500°C or above so as to develop the water resistance thereof. The same is true for the magnesium primary phosphate (Mg(H 2 PO 4 ) 2 ). Accordingly, any of the following curing agents is needed to cause the primary metal salts to develop their water resistance. Examples of the curing agents for the primary metal salt of phosphoric acid for use in the inorganic binder composition (I) include, as well as conventionally known aluminum hydroxide, wollastonite crystal (CaO·SiO 2 ), magnesium oxide (MgO), calcium oxide (CaO) and zinc oxide (ZnO). Among these, wollastonite crystal and aluminum hydroxide are preferable. Aluminum hydroxide serves also as the insulation imparting gas generating source compound. Hence, where aluminum hydroxide is used for both the curing agent for the primary metal salt of phosphoric acid and the insulation imparting gas generating source compound, the amount thereof to be used is the total of the amounts required for the two. As a result of intensive study by the inventors on curing agents other than aluminum hydroxide which are applicable to the primary metal salt of phosphoric acid, wollastonite crystal is found to act as a curing agent which is capable of imparting the primary metal salt of phosphoric acid with water resistance by heating to about 150°C. The average particle diameter of the curing agent is not particularly limited but is usually less than about 60 µm, especially about 2 to about 40 µm in terms of its mixing property, dispersibility and cost. The content of the insulation imparting gas generating source compound in the inorganic binder composition (I) is usually within the range of 30 to 45 %, preferably 35 to 40 %. When it is too small, the insulation imparting gas generating source compound is consumed as the curing agent for the primary metal salt of phosphoric acid and hence is impossible to serve the inherent purpose, i.e., to generate the insulation imparting gas. On the other hand, when the content thereof is too large, it exceeds the range for assuring the effect of binding the primary metal salt of phosphoric acid and, hence, it is difficult to obtain a dense plate material but a bulky plate material with less strength hence susceptible to damage. When the content of the arc resistant inorganic powder in the inorganic binder composition (I) is too large, the resulting arc extinguishing plate material exhibits a degraded strength and hence is susceptible to damage though enjoying an enhanced arc resistance. For this reason, the content of the arc resistant inorganic powder is usually not greater than 28 %, preferably not greater than 25 %. In case that there is used no arc resistant inorganic powder, the insulation imparting gas generating source compound can replace the arc resistant inorganic powder, thereby suppressing the decrease in the arc resistance of the plate material. Therefore, there is no particular lower limit of the content of the arc resistant inorganic powder. Nevertheless, as far as it is used, the arc resistant inorganic powder is preferably contained in an amount of about 10 % or greater to serve its purpose. When the content of the aqueous solution of primary metal salt of phosphoric acid in the inorganic binder composition (I) is too small, it is difficult to obtain a dense arc extinguishing plate material. For this reason, the content of the aqueous solution is usually not smaller than 40 %, preferably not smaller than 45 %. On the other hand, when the content of the aqueous solution is too large, not only it is difficult for the curing agent to impart the plate material with water resistance but also the aqueous solution adheres to the reinforcing inorganic material sheet in a decreased amount, resulting in the plate material with degraded strength. For this reason the content of the aqueous solution is usually not greater than 65 %, preferably not greater than 60 %. When the content of the curing agent for the primary metal salt of phosphoric acid in the inorganic binder composition (I) is too small, there is little difference in the temperature at which the primary metal salt of phosphoric acid develops its water resistance between the case where the curing agent is used and the case where no curing agent is used and, hence, the heating to about 500°C is required for the development of the water resistance. For this reason, the content of the curing agent is usually not less than 2 %, preferably not less than 3 %. When the content of the curing agent is too large, the primary metal salt of phosphoric acid is cured too rapidly and, hence, the time period for required operation is shortened; for example, such a problem may arise that the inorganic binder composition (I) is solidified upon the preparation thereof, thereby rendering the subsequent operation impossible to be carried out. For this reason, the content of the curing agent in the inorganic binder composition (I) is usually not greater than 10 %, preferably not greater than 5 %. Where the curing agent is used within the above range, such benefits will result that: a sufficient time is assured for subsequent operations; the water resistance of the aqueous solution of primary metal salt of phosphoric acid is developed at about 150° to about 200°C; the preparation of the plate material is facilitated; and the resulting plate material is excellent in arc resistance, mechanical strength and thermal shock resistance. Where wollastonite crystal is used as the curing agent, there is no need to change the aforementioned content thereof, whereas when aluminum hydroxide is used which acts also as the insulation imparting gas generating compound, the amount thereof to be used has to be the total of the amount acting as the insulation imparting gas generating source compound and the amount acting as the curing agent. Where the arc extinguishing plate material is prepared by gradually increasing the amount of aluminum hydroxide in the inorganic binder composition (A), the amount of aluminum hydroxide for use as the curing agent is the minimum amount for sufficient curing, and the amount thereof for use as the insulation imparting gas generating source compound is the amount used as exceeding the amount for use as the curing agent. When wollastonite crystal and aluminum hydroxide are used in combination as the curing agent, the amount of aluminum hydroxide for use as the curing agent and that for use as the insulation imparting gas generating source compound can also be determined. In the present invention, it is preferable to use wollastonite crystal as the curing agent and aluminum hydroxide as the insulation imparting gas generating source compound for preventing the decrease in insulation resistance due to arc, for the purpose of imparting the plate material with insulating property and water resistance. Next, reference will be made to the aforementioned inorganic binder composition (II). The purposes and specific examples of the insulation imparting gas generating source compound in the inorganic binder composition (II) are the same as those of the inorganic binder composition (I) and, hence, the description thereon is herein omitted. Nevertheless, the use of the insulation imparting gas generating source compound comprising magnesium hydroxide, magnesium carbonate or calcium carbonate is advantageous in that the compound partially reacts with the condensed alkali metal phosphate in the drying step of the preparation process for the arc extinguishing plate material and further reacts therewith by 10 to 25 % in the aging step at 120° to 200°C after the pressure molding, thereby acting also as the curing agent which imparts the plate material with water resistance as in the inorganic binder composition (I). Magnesium hydroxide, magnesium carbonate and calcium carbonate are each insoluble in the aqueous solution of condensed alkali metal phosphate at room temperature but each assumes suspended condition therein. The purposes and specific examples of the arc resistant inorganic powder in the inorganic binder composition (II) are the same as those of the inorganic binder composition (I) and, hence, the description thereon is herein omitted. The aqueous solution of condensed alkali metal phosphate in the inorganic binder composition (II) serves as a binding agent as does the aqueous solution of primary metal salt of phosphoric acid in the inorganic binder composition (I). Examples of the condensed alkali metal phosphates are, for instance, sodium metaphosphate, potassium metaphosphate and lithium metaphosphate. Among these, sodium metaphosphate and potassium metaphosphate are advantageously used because they are less reactive with the aforementioned insulation imparting gas generating source compound at room temperature and have favorable characteristics for the preparation of the inorganic binder composition (II) such as to enjoy good solubility in water and to assure an aqueous solution thereof with suitable viscosity for a binding agent. The suitable viscosity of the aqueous solution of condensed alkali metal phosphate is meant by such a low viscosity as to facilitate the mixing thereof with other components of the inorganic binder composition (II) and as to impart the aqueous solution with a binding property suitable for making the solution adhere to the reinforcing inorganic material sheet. The concentration of the condensed alkali metal phosphate in the aqueous solution is preferably not lower than 10 %, more preferably not lower than 12 %, since when it is too low, a prolonged time period is likely to be required to remove excessive moisture from the aqueous solution in adjusting the concentration of the condensed alkali metal phosphate in the aqueous solution to 65 to 85 % for the pressure molding of the sheet. On the other hand, when the concentration of the condensed alkali metal phosphate is too high, such difficulties in preparing the plate material are likely to occur that: the aqueous solution comes to have an undesirably high viscosity; it is impossible for the binder composition (II) to contain the predetermined amounts of the insulation imparting gas generating source compound and arc resistant inorganic powder; and further the reaction of the aqueous solution with the curing agent proceeds too rapidly. Therefore, the concentration of the condensed alkali metal phosphate in the aqueous solution is preferably not higher than 40 %, more preferably not higher than 30 %. The content of the insulation imparting gas generating source compound in the inorganic binder composition (II) is usually within the range of 30 to 50 %, preferably 35 to 45 %. When it is too small, the effect of the insulation imparting gas generating source compound is degraded. On the other hand, when the content thereof is too large, it exceeds the range for assuring the effect of binding the condensed alkali metal phosphate and, hence, the resulting plate material becomes bulky with less strength and hence susceptible to damage; in some cases the inorganic binder composition (II) assumes a condition like undissolved lump of flour, resulting in a difficulty in the preparation of the binder composition (II), which makes the subsequent operations unabled. Where the insulation imparting gas generating source compound is used within the above range, such benefits will result that: a sufficient time is assured for required operations; the water resistance of the aqueous solution of condensed alkali metal phosphate is developed at about 150° to about 200°C; the preparation of the plate material is facilitated; and the resulting plate material is excellent in arc resistance, strength and thermal shock resistance. When the content of the arc resistant inorganic powder in the inorganic binder composition (II) is too large, the resulting arc extinguishing plate material exhibits degraded strength and hence is susceptible to damage though enjoying enhanced arc resistance. For this reason, the content of the arc resistant inorganic powder is usually not greater than 20 %, preferably not greater than 15 %. Where there is used no arc resistant inorganic powder, the insulation imparting gas generating source compound can replace the arc resistant inorganic powder, thereby suppressing the decrease in the arc resistance of the plate material. Therefore, there is no particular lower limit of the content of the arc resistant inorganic powder. Nevertheless, as far as it is used, the arc resistant inorganic powder is preferably contained in an amount of about 10 % or greater to serve its purpose. When the content of the aqueous solution of condensed alkali metal phosphate in the inorganic binder composition (II) is too small, it is difficult to obtain a dense arc extinguishing plate material. For this reason, the content of the aqueous solution is usually not smaller than 50 %, preferably not smaller than 55 %. On the other hand, when the content of the aqueous solution is too large, the aqueous solution adheres to the reinforcing inorganic material sheet in a decreased amount, resulting in the plate material with degraded strength. For this reason, the content of the aqueous solution is usually not greater than 70 %, preferably not greater than 65 %. The arc extinguishing plate material (I) of the present invention is obtained by preparing the sheet from the foregoing reinforcing inorganic material sheet and the inorganic binder composition (A) and pressure molding and aging the sheet. The details of the pressure molding and aging overlaps the preparation method for the arc extinguishing plate material (I) to be described later and hence will be described in the description on such method. In preparing the sheet, there may, as required, be incorporated, in addition to the aforementioned raw materials, a binder such as methyl cellulose or polyvinyl alcohol, a coloring agent such as glass frit or ceramic color, or the like within such a range as not to obstruct the purpose of the invention. The inorganic binder composition (B) contained in the arc extinguishing plate material (I), which is used as integrated with the reinforcing inorganic material sheet, serves to afford the plate material with excellent mechanical strength, heat resistance, arc resistance, thermal shock resistance and the like. The inorganic binder composition (B) also serves, when an arc is generated in the arc extinguishing chamber of a switch upon an opening or closing operation of the electrodes of the switch, to absorb the energy of the arc for cooling down and extinguish it, thereby protecting the components of the switch from the heat of the arc, while at the same time insulating metal vapor and molten metal droplets that are generated from the electrodes, contacts and other metal components adjacent thereto by the arc, thereby preventing the decrease in the insulation resistance of the switch. The inorganic binder composition (B) is prepared by drying, pressure molding and aging the inorganic binder composition (A) adhering to the reinforcing inorganic material sheet. Accordingly, the moisture originating from the aqueous solution of primary metal salt of phosphoric acid or condensed alkali metal phosphate in the inorganic binder composition (A) is removed, while all the solid contents of the composition (A) are retained as adhering to the reinforcing inorganic material sheet. When the arc extinguishing plate material (I) was heated to 200°C to examine whether or not the weight thereof was decreased, the plate material was found not to lose its weight. Therefore, the inorganic binder composition (B) has, for example, such a composition as approximately containing 40 to 55 % of the insulation imparting gas generating source compound, 0 to 34 % of the arc resistant inorganic powder and 26 to 45 % of the cured reaction product of the primary metal salt of phosphoric acid, when the inorganic binder composition (I) is used as the inorganic binder composition (A); a composition as approximately containing 42 to 65 % of the insulation imparting gas generating source compound, 0 to 28 % of the arc resistant inorganic powder and 34 to 40 % of the cured condensed alkali metal phosphate, when the inorganic binder composition (II) is used as the inorganic binder composition (A), or a like composition. It should be noted that although the curing agent for the primary metal salt of phosphoric acid does not necessarily 100 % react with the aqueous solution, the content of the curing agent is assumed to have entirely reacted therewith and hence is entirely included in the content of the cured reaction product of the primary metal salt of phosphoric acid. When the content of the reinforcing inorganic material sheet in the arc extinguishing plate material (I) is too small, the amount of the inorganic binder composition (B) adhering to the sheet is greater than required. This results in the arc extinguishing plate material having degraded moldability for an arc extinguishing side plate though exhibiting excellent arc resistance and insulation imparting gas generating effect. Further, when such plate material is incorporated in the arc extinguishing chamber and is subjected to interrupting operations, the plate material may be peeled off or released from the chamber by heat of arc, vibration and generation of the insulation imparting gas and hence cannot retain its arc extinguishing property. For this reason the content of the reinforcing inorganic material sheet is set to not less than 35 %, preferably not less than 37 %. On the other hand, when the content of the reinforcing inorganic material sheet is too large, the amount of the inorganic binder composition (B) adhering to the sheet is less than required. This results in the arc extinguishing plate material which exhibits poor arc resistance and insulation imparting gas generating effect and hence does not show the characteristics required for an arc extinguishing plate material. For this reason the content of the reinforcing inorganic material sheet is set to not greater than 50 %, preferably not greater than 45 %. The content of the inorganic binder composition (B), or 50 to 65 % in the arc extinguishing plate material (I) of the present invention is so high that it was conventionally difficult to make such content of a binder composition adhere to the reinforcing inorganic material sheet and that even if such content of the binder composition had been successfully made adhere to the reinforcing inorganic material sheet and then aged, the binder composition would have been likely to be released from the sheet upon exposure to arc. Since the present invention enables a large amount of the inorganic binder composition (B) to be contained in the arc extinguishing plate material, the plate material enjoys superior arc resistance and insulation imparting gas generating effect. The arc extinguishing plate material (I) may be a plate material having a thickness of 0.2 to 1.5 mm, preferably 0.4 to 1.2 mm which is obtained by pressure molding and aging a single sheet of the aforementioned type, or a plate material having a thickness of 0.5 to 3 mm, preferably 0.8 to 2.0 mm which is obtained by pressure molding and aging two or more, preferably two to five stacked sheets of the aforementioned type. Where the arc extinguishing plate material is to be prepared from a single sheet, an insulation imparting gas generating source compound may further be applied onto either or both faces of the sheet. Further, the sheet may be coated or impregnated with a coating material for preventing the resulting plate material (I) from dusting when the plate material (I) is subjected to punching. The above insulation imparting gas generating source compound to be applied is the same as the foregoing compound and preferably has an average diameter of about 0.3 to about 40 µm. Such insulation imparting gas generating source compound to be applied is preferably magnesium hydroxide, magnesium carbonate or calcium carbonate in terms of their potent insulation imparting effect. In the application of the source compound, any binder is usually unnecessary, but it is possible to use the aforesaid coating material as a binder. The amount of the insulation imparting gas generating source compound to be applied is usually about 200 to about 450 g/m 2 for one face of the sheet. The amount of the coating material for coating or impregnation is usually about 40 to about 100 g/m 2 for one face of the sheet. Examples of specific coating materials include organic metal compounds (metal alkoxides and the like) such as ethyl silcate, methyl silicate and tributoxy aluminum, and organic resins such as an acrylic resin, epoxy resin and polyester resin. Where a plurality of stacked sheets are used, preferably the sheet prepared using the inorganic binder composition (II) is stacked on either or both faces of the sheet prepared using the inorganic binder composition (I). Such stacked sheets usually having a thickness of 1.1 to 3.0 mm in total, and the total thickness thereof is preferably adjusted to the required thickness of 0.8 to 2.5 mm in view of the mechanical strength and punching quality of the resulting plate material. The plate material comprising a plurality of stacked sheets may also be applied with the insulation imparting gas generating source compound on either or both faces thereof, and further coated or impregnated with the coating material. Next, the preparation method for the arc extinguishing plate material (I) will be described. The arc extinguishing plate material (I) of the present invention is prepared by the steps of: preparing the sheet from the aforementioned reinforcing inorganic material sheet and the aforementioned inorganic binder composition (A); drying the sheet at 80° to 120°C and then pressure molding the sheet; aging the sheet at 120° to 200°C during or after the pressure molding to remove moisture therefrom so as to allow it to cure; and cooling the cured sheet to 80°C or below. The preparation of the inorganic binder composition (A) can be made by any of various methods without any particular limitations so far as the components of the composition are uniformly dispersed. For example, the solid components of the inorganic binder composition (A) are mixed using a mixer such as an agitation mortar, and then the liquid component (the aqueous solution of primary metal salt of phosphoric acid or of condensed alkali metal phosphate) is added to the mixture, followed by kneading to prepare the binder compostion (A). Such method is preferred because it permits the solid components of the inorganic binder composition (A) to be uniformly mixed and dispersed and the liquid component to be uniformly mixed with the solid components, with the solid components being prevented from partially reacting with the liquid components. If the inorganic binder composition (I) is prepared by, for example, mixing the solid components: 30 to 45 % of the insulation imparting gas generating source compound, 0 to 28 % of the arc resistant inorganic powder and 2 to 10 % of the curing agent for the primary metal salt of phosphoric acid, and adding the liquid component, 40 to 65 % of the aqueous solution of primary metal salt of phosphoric acid, to the resulting mixture, followed by kneading, the resulting binder composition (I) assumes a condition like a slurry as having the solid components thereof uniformly dispersed in the liquid component, i.e. the aqueous solution of primary metal salt of phosphoric acid and exhibiting a viscosity suitable for a binder. A representative example of the inorganic binder composition (I) comprises aluminum hydroxide as the insulation imparting gas generating source compound, aluminum oxide powder, zircon powder or cordierite powder as the arc resistant inorganic powder, wollastonite crystal or aluminum hydroxide as the curing agent for the aqueous solution of primary metal salt of phosphoric acid, 25 to 55 % aqueous solution of aluminum primary phosphate or magnesium primary phosphate as the aqueous solution of primary metal salt of phosphoric acid. If the inorganic binder composition (II) is prepared by, for example, mixing the solid components: 30 to 50 % of the insulation imparting gas generating source compound and 0 to 20 % of the arc resistant inorganic powder, and adding the liquid component, 50 to 70 % of the aqueous solution of condensed alkali metal phosphate, to the resulting mixture, followed by kneading, the resulting binder composition (II) assumes a condition like a slurry as having the solid components thereof uniformly dispersed in the liquid component, i.e. the aqueous solution of condensed alkali metal phosphate and exhibiting a viscosity suitable for a binder. A representative example of the inorganic binder composition (II) comprises aluminum hydroxide, magnesium carbonate or calcium carbonate as the insulation imparting gas generating source compound, aluminum oxide powder, zircon powder or cordierite powder as the arc resistant inorganic powder, and 10 to 40 % aqueous solution of sodium metaphosphate or potassium metaphosphate as the aqueous solution of condensed alkali metal phosphate. It should be noted that the concentration of the primary metal salt of phosphoric acid or condensed alkali metal phosphate in the aqueous solution contained in the inorganic binder composition (I) or (II) is the same as that before the kneading. The inorganic binder composition exhibiting the foregoing characteristics facilitates the subsequent preparation of the sheet and easily adheres to the voids or gaps and surface of the reinforcing inorganic material sheet. The sheet can be prepared from the inorganic binder composition and reinforcing inorganic material sheet by any method without any limitations. Examples of specific methods include one in which the reinforcing inorganic material sheet is immersed in the predetermined inorganic binder composition and removed therefrom to have the predetermined impregnation rate, a roll coating method in which the predetermined inorganic binder composition (A) is supplied to the reinforcing inorganic material sheet from between rolls, and a doctor blade method in which the predetermined inorganic binder composition is applied to the reinforcing inorganic material sheet through a blade having a thickness set to a predetermined value. The amount of the inorganic binder composition (I) or (II) adhering to the reinforcing inorganic material sheet is preferably 200 to 350 parts, more preferably 250 to 300 part relative to 100 parts of the reinforcing inorganic material sheet. The amount of the binder composition (I) or (II) within such range facilitates the transportation of the sheet in the preparation thereof, allows the arc extinguishing plate material (I) after undergoing the aging to have a suitable thickness, and results in the weight ratio between the reinforcing inorganic material sheet and the inorganic binder composition (B) after undergoing the aging falling within a proper range. The sheet thus formed, which retains moisture in the inorganic binder composition (A) and is in a soft and deformable condition, is then dried at 80° to 120 °C (in an over, for example) to adjust the concentration of the primary metal salt of phosphoric acid or condensed alkali metal phosphate in the aqueous solution to 65 to 85 %, preferably 75 to 80 %. This is because if the sheet not subjected to the drying is directly pressure molded, the inorganic binder composition (A) with which the reinforcing inorganic material sheet is impregnated will ooze out of the sheet, so that the resulting plate material (I) of the undesired composition results. If the concentration of the primary metal salt of phosphoric acid or condensed alkali metal phosphate in the aqueous solution exceeds 85 %, the following disadvantages may result: the sheet can hardly be deformed even when pressure molded; the inorganic binder composition (A) is not densely filled into voids or gaps of the reinforcing inorganic material sheet; and where a plurality of sheets are stacked, the adhesion between the sheets becomes insufficient. As will be described later, in the case of the sheets stacked, the concentration of the primary metal salt of phosphoric acid or condensed alkali metal phosphate in the aqueous solution contained therein is preferably adjusted to 70 to 80 % for providing suitable interlayer adhesion. In the preparation method of the present invention, the adjustment of the concentration of the primary metal salt of phosphric acid or condensed alkali metal phosphate is very important. The sheet dried at 80° to 120°C is then subjected to pressure molding. When the pressure in the pressure molding is too low, the arc extinguishing plate material is insufficiently pressurized and, hence, the plate material prior to undergoing the aging may suffer non-uniform denseness, or an unbonded portion may result at the interface of the stacked sheets. For this reason, the pressure in the pressure molding is preferably not lower than 100 kg/cm 2 . On the other hand, when such pressure is too high, the inorganic binder composition (I) or (II) is likely to flow out of the reinforcing inorganic material sheet and thereby to make the sheet exposed and, hence, the characteristics required for the plate material may be impaired. In view of this, the pressure in the pressure molding is preferably not higher than 200 kg/cm 2 . In the present invention, the pressure molding may be carried out at room temperature or with the surface table of a press machine appropriately heated. Further, the duration of the pressure molding can be appropriately adjusted. Devices for use in the pressure molding include press machines having surface table, such as a hand press, mechanical press and oil press. The arc extinguishing plate material prior to undergoing the aging is allowed to stand over a whole day and night, aged by heating at 120° to 200°C in, for example, an oven to cure with the moisture contained therein being removed, and then cooled down to 80°C or below. Thus, the arc extinguishing plate material is prepared. When the temperature at which the heat aging is carried out is too low, the curing of the plate material proceeds taking a very long time, or otherwise even when the curing is successfully carried out, the compound for imparting the primary metal salt of phosphoric acid or condensed alkali metal phosphate with water resistance is insufficiently produced. For this reason, such temperature has to be not lower than 120°C, preferably not lower than 150°C. When the temperature is too high, only the surface layer of the molded product is cured rapidly and, hence, the required reaction becomes non-uniform between the surface portion and the deep portion of the molded product, thereby causing warpage of the plate material. For this reason, the temperature for the heat aging has to be not higher than 200°C, preferably not higher than 180°C. Since rapid cooling of the molded product after the heat aging causes the product to be warped, the molded product is cooled to 80°C or below, preferably 50°C or below to prevent such warpage. The cooling may be natural gradual cooling or a program-controlled cooling for cooling the product stepwise. In pressure molding the sheet dried at 80° to 120°C, a suitable number of sheets may be stacked depending on the required thickness for the purpose of improving the mechanical strength and adjusting the size of the product. In this case, the insulation imparting gas generating source compound may further be applied onto either or both faces of the sheet to increase the amount of insulation imparting gas to be generated. The application is carried out by, for example, sieving the insulation imparting gas generating source compound with a 35-mesh sieve onto the sheet which has been dried at 80° to 120°C to such an extent that the sheet becomes tacky when touched with a finger, in such a manner that the layer of the sieved compound has a uniform thickness. Alternatively, to cause an increased amount of a more effective insulation imparting gas to be generated, the sheet containing the inorganic binder composition (II) may be stacked on either or both sides of the sheet containing the inorganic binder composition (I), and an appropriate number of sets of such stacks may further be stacked on top of another depending on the thickness of an intended product and then pressure molded. In these cases also, the molded product is aged at 120° to 200°C to remove the moisture contained therein and to cure, and then cooled to 80°C or below. Thus, the arc extinguishing plate material (I) of the present invention is prepared. The plate material thus formed may further be coated or impregnated with the coating material so as to prevent the plate material (I) from dusting when subjected to punching. The coating of the coating material can be carried out by roll coating, spray coating or brush coating. The impregnation can be carried out by filling a container sufficiently sized to accommodate the plate material (I) with the coating material and immersing the plate material (I) into the coating material with optional vacuum drawing process. The arc extinguishing plate material (I) thus prepared is then subjected to a machine work such as finishing or punching to form an arc extinguishing plate, which is in turn combined with a magnetic plate to construct an arc extinguishing chamber. The arc extinguishing plate material -(II) of the present invention is obtained by pressure molding and aging the inorganic binder composition (C) comprising 40 to 55 % of an insulation imparting gas generating source compound, 25 to 40 % of an arc resistant inorganic powder, 8 to 18 % of a primary metal salt of phosphoric acid, 5 to 10 % of a curing agent for the primary metal salt of phosphoric acid, 2.6 to 12 % of water, and 2 to 10 % of a reinforcing inorganic fiber. Unlike the inorganic binder composition (A), the inorganic binder composition (C) does not require the adjustment of the concentration of the primary metal salt of phosphoric acid in the aqueous solution. Further, the composition (C) advantageously has good moldability (the plate material can be molded directly into an arc extinguishing plate) and can afford the arc extinguishing plate material (II) with an excellent mechanical strength. The purpose of the insulation imparting gas generating source compound contained in the inorganic binder composition (C), the process of insulating metal vapor and the like with an insulation imparting gas generated from the insulation imparting gas generating source compound, examples of specific insulation imparting gas generating source compounds, and the average particle diameter of the source compound when in powder form are the same as described with respect to the arc extinguishing plate material (I) and, hence, the description thereof is herein omitted. It should be noted that among insulation imparting gas generating source compounds, there are preferred magnesium hydroxide, aluminum hydroxide, magnesium carbonate and calcium carbonate, since they are each capable of generating a sufficient amount of a highly effective insulation imparting gas. The purpose, examples, preferable examples with reasons therefor, and average particle diameter of the arc resistant inorganic powder contained in the inorganic binder composition (C) are the same as described with respect to the arc extinguishing plate material (I) and, hence, the description thereon is herein omitted. Nevertheless, although aluminum oxide powder is preferably used in the plate material (I), aluminum oxide powder which is poor in thermal shock resistance cannot be preferably used in the arc extinguishing plate material (II) not containing the reinforcing inorganic material sheet for fear of break of the plate material (II) due to thermal shock. The primary metal salt of phosphoric acid contained in the inorganic binder composition (C) acts to bind the insulation imparting gas generating source compound, arc resistant inorganic powder, curing agent for the primary metal salt of phosphoric acid and reinforcing inorganic fiber. Examples, preferred examples together with reasons therefor of the primary metal salt of phosphoric acid are the same as described with respect to the arc extinguishing plate material (I) and, hence, the description thereon is herein omitted. When the concentration of the primary metal salt of phosphoric acid in the aqueous solution is too low, the inorganic binder composition (C) has a decreased binding capacity and develops no plasticity, thereby providing a less densified molded product with degraded dimensional accuracy. For this reason the concentration is preferably not lower than 60 %, more preferably not lower than 65 %. When the concentration is too high, the aqueous solution exhibits an increased viscosity and rapidly reacts with the curing agent, thereby rendering the preparation of the composition (C) difficult, and even if the composition (C) is prepared, the resulting composition (C) readily adheres to a mold and hence can hardly be released therefrom, resulting in a molded product with lessened dimensional accuracy. For this reason, the concentration is preferably not higher than 75 %, more preferably not higher than 72 %. Examples of the curing agents for the aqueous solution of primary metal salt of phosphoric acid for use in the inorganic binder composition (C) are, for instance, wollastonite crystal (CaO·SiO 2 ), magnesium hydroxide, aluminum hydroxide, magnesium carbonate and calcium carbonate. Among these, wollastonite crystal is found to serve as a curing agent which is capable of imparting the primary metal salt of phosphoric acid with water resistance by heating to about 150°C from the intensive study on curing agents applicable to the primary metal salt of phosphoric acid made by the inventors, as described earlier. Wollastonite crystal effectively acts also as the reinforcing inorganic fiber of the arc extinguishing plate material (II), as will be described later. Among the above curing agents, there are preferably used magnesium hydroxide, aluminum hydroxide, magnesium carbonate and calcium carbonate, since they act also as the insulation imparting gas generating source compound. The average particle diameter of the curing agent is not particularly limited but is usually smaller than about 60 µm, preferably about 2 to about 40 µm in terms of mixing property, dispersibility and cost. The water is used in the inorganic binder composition (C) for purposes of affording the aqueous solution of the primary metal salt of phosphoric acid having an appropriate concentration, imparting the binder composition (C) with excellent moldability and causing the arc extinguishing plate material (II) to develop a mechanical strength. The reinforcing inorganic fiber contained in the inorganic binder composition (C) is a component which imparts the resulting plate material (II) with an excellent mechanical strength. The reinforcing inorganic fiber is preferably an inorganic short fiber which is excellent in arc resistance and electrical insulating property and can be readily mixed uniformly with other materials. Examples of the short fibers are natural mineral fibers such as wollastonite crystal, ceramic fibers such as silica-alumina glass fiber (amorphous aluminum silicate fiber, Al 2 O 3 :SiO 2 = 47:53, 56:44, or the like), and ceramic whiskers such as aluminum borate whisker (9Al 2 O 3 ·2B 2 O 3 ), silicon carbide whisker (SiC), silicon nitride whisker (Si 3 N 4 ) and calcium carbonate whisker. These may be used either alone or in combination. The natural mineral fibers, ceramic fibers and ceramic whiskers are preferred, since they exhibit excellent arc resistance and electrical insulating property and are readily uniformly mixed with the other components of the inorganic binder composition (C). The average fiber diameter and average fiber length of the reinforcing inorganic fiber are not particularly limited, and commercially-available ones are usable in the present invention. Nevertheless, the average fiber diameter and average fiber length of the reinforcing inorganic fiber are preferably about 1 to about 10 µm and about 20 to about 50 µm, respectively, for wollastonite crystal; about 1 to about 15 µm and about 2 to about 100 µm, respectively, for silica-alumina glass fiber; about 1 to about 10 µm and about 30 to about 100 µm, respectively, for alumina fiber; about 0.5 to 1 µm and about 10 to 30 µm, respectively, for aluminum borate whisker; about 0.05 to about 10 µm and about 5 to about 40 µm, respectively, for silicon carbide whisker; about 0.2 to about 1 µm and about 5 to about 200 µm, respectively, for silicon nitride whisker; and about 0.5 to about 1 µm and about 20 to about 30 µm, respectively, for calcium carbonate whisker. When the content of the insulation imparting gas generating source compound in the inorganic binder composition (C) is too small, the compound is consumed as the curing agent for the primary metal salt of phosphoric acid and hence cannot serve its inherent purpose or generate the insulation imparting gas, as described earlier. For this reason, the content of the insulation imparting gas generating source compound is set to usually not less than 40 %, preferably not less than 45 %, more preferably not less than 50 %. On the other hand, when the content thereof is too large, it exceeds the range for assuring the effect of binding the primary metal salt of phosphoric acid and, hence, it is difficult to obtain a dense plate material but a bulky plate material with less strength hence susceptible to damage. The content of the compound is set to usually not greater than 55 %, preferably not greater than 52 %. When the content of the arc resistant inorganic powder in the inorganic binder composition (C) is too small, the resulting arc extinguishing plate material (II) exhibits degraded arc resistance and loses the characteristics required for the arc extinguishing plate material. For this reason, the content thereof is set to usually not less than 25 %, preferably not less than 30 %. When the content of the powder is too large, the resulting arc extinguishing plate material (II) exhibits degraded strength and hence is susceptible to damage though enjoying enhanced arc resistance. For this reason, the content of the arc resistant inorganic powder is set to usually not greater than 40 %, preferably not greater than 35 %. When the content of the primary metal salt of phosphoric acid in the inorganic binder composition (C) is too small, it is difficult to obtain a dense arc extinguishing plate material (II). For this reason, the content thereof is set to usually not less than 8 %, preferably not less than 10 %. When the content thereof is too large, it is difficult for the curing agent to impart the plate material with water resistance. For this reason, the content of the primary metal salt of phosphoric acid is set to usually not greater than 18 %, preferably not greater than 15 %. When the content of the curing agent for the primary metal salt of phosphoric acid in the inorganic binder composition (C) is too small, there is little difference in the temperature, at which the primary metal salt of phosphoric acid develops its water resistance, whether or not the curing agent be used and, hence, the heating to about 500°C is required for the development of the water resistance. For this reason, the content of the curing agent is set to usually not less than 5 %, preferably not less than 7 %. When the content of the curing agent is too large, the primary metal salt of phosphoric acid is cured too rapidly and, hence, the time period available for necessary operations is shortened; for example, such a problem may arise that the inorganic binder composition (C) is solidified upon the preparation thereof, thereby rendering the subsequent operation impossible to be carried out. For this reason the content of the curing agent is set to usually not greater than 10 %, preferably not greater than 9 %. Where the curing agent is used within the above range, such benefits will result that: a sufficient time is assured for operations; the water resistance of the aqueous solution of primary metal salt of phosphoric acid is developed at about 150°C to about 200°C; the preparation of the plate material (II) is facilitated; and the resulting plate material (II) is excellent in arc resistance, strength and thermal shock resistance. Where wollastonite crystal is used as the curing agent, there is no need to change the aforementioned content thereof, whereas when there is used aluminum hydroxide, magnesium hydroxide, magnesium carbonate or calcium carbonate, each of which acts also as the insulation imparting gas generating source compound, the amount thereof to be used should be the total of the amount acting as the insulation imparting gas generating source compound and the amount acting as the curing agent. Where the arc extinguishing plate material (II) is prepared by gradually increasing the amount of, for example, aluminum hydroxide in the inorganic binder composition (C), the amount of aluminum hydroxide for use as the curing agent is the minimum amount for sufficient curing, and the amount thereof for use as the insulation imparting gas generating source compound is the amount used as exceeding the amount for use as the curing agent. Where wollastonite crystal and aluminum hydroxide are used in combination, the amount of aluminum hydroxide for use as the curing agent and that for use as the insulation imparting gas generating source compound can also be determined. In the present invention, it is preferable to use wollastonite crystal as the curing agent, and aluminum hydroxide, magnesium hydroxide, magnesium carbonate or calcium carbonate as the insulation imparting gas generating source compound for preventing the decrease in insulation resistance due to arc, for the purpose of maximizing the inherent effect of the plate material (II). As described earlier, when the concentration of the aqueous solution of primary metal salt of phosphoric acid is adjusted to within the preferable range, particularly to 60 to 75 %, it is easy to obtain a dense molded product. From this point of view, the amount of water used in the inorganic binder composition (C) is at least 2.6 %, preferably not less than 5 %, more preferably not less than 6 %. When the amount of water is too large, the inorganic binder composition (C) comes to assume a slurry condition in the preparation thereof, thus rendering required operations difficult. For this reason, the amount of water to be used in the inorganic binder composition (C) is usually not greater than 12 %, preferably not greater than 10 %, more preferably not greater than 8 %. When the content of the reinforcing inorganic fiber in the inorganic binder composition (C) is too small, the resulting arc extinguishing plate material (II) exhibits degraded mechanical strength (flexural strength) and hence loses the characteristics required for the arc extinguishing plate material (II). For this reason the content thereof is set to usually not less than 2 %, preferably not less than 3 %. When the content thereof is too large, it exceeds the range for assuring the effect of binding the primary metal salt of phosphoric acid and, hence, it is difficult to obtain a dense plate material but a bulky plate material with less strength hence susceptible to damage. For this reason, the content of the reinforcing inorganic fiber is set to usually not greater than 10 %, preferably not greater than 8 %. If required, the inorganic binder composition (C) in the present invention may be incorporated with, in addition to the foregoing components, a binder such as methyl cellulose or polyvinyl alcohol, a coloring agent such as glass frit or ceramic color, or the like within such a range as not to obstruct the purpose of the invention. The arc extinguishing plate material (II) of the present invention is obtained by pressure molding and aging the inorganic binder compostion (C) thus described. Details of the pressure molding and aging will be described in the preparation method for the arc extinguishing plate material (II). Since the inorganic binder composition (C) becomes free of water, the obtained plate material (II) approximately comprises 46 to 55 % of the insulation imparting gas generating source compound, 33 to 45 % of the arc resistant inorganic powder, 18 to 35 % of the cured reaction product of the primary metal salt of phosphoric acid and 3 to 12 % of the reinforcing inorganc fiber. It should be noted that although the curing agent for the primary metal salt of phosphoric acid does not necessarily 100 % react with the primary metal salt of phosphoric acid, the content of the curing agent is assumed to have entirely reacted therewith and hence is entirely included in the content of the cured reaction product of the primary metal salt of phosphoric acid. When the arc extinguishing plate material (II) was heated to 200°C to examine whether or not the weight thereof was decreased, the plate material was found not to lose its weight. The arc extinguishing plate material (II) has a thickness of, for example, 0.5 to 2.5 mm, preferably 0.8 to 2.0 mm. Next, the preparation method for the arc extinguishing plate material (II) is to be described. The arc extinguishing plate material (II) of the present invention is prepared by the steps of: preparing the inorganic binder composition (C); pressure molding the composition (C) with a mold; and aging the molded product at 120° to 200°C. The preparation of the inorganic binder composition (C) can be made by any of various methods without particular limitations so far as the components thereof are uniformly dispersed. For example, the solid components (insulation imparting gas generating source compound, arc resistant inorganic powder, primary metal salt of phosphoric acid, curing agent and reinforcing inorganic fiber) of the composition (C) are mixed using a mixer such as an agitation mortar, and the resulting mixture is kneaded while adding thereto the predetermined amount of water dropwise, to prepare the inorganic binder composition (C). Such method is preferred since it permits the primary metal salt of phosphoric acid to be uniformly mixed with and dispersed in the solid components and the water to be evenly added to the mixture and hence is capable of affording the inorganic binder composition (C) which assures homogenized plate material (II). The inorganic binder composition (C) is in the form like granulates comprising primary particles which can be readily filled into a mold. The inorganic binder composition (C) in such form can be readily filled into the mold and plastically deformed within the mold in the pressure molding, thereby achieving a close packing. This assures a dense molded product. A representative example of the inorganic binder composition (C) comprises magnesium hydroxide, aluminum hydroxide, magnesium carbonate or calcium carbonate as the insulation imparting gas generating source compound, zircon powder, cordierite powder or mullite powder as the arc resistant inorganic powder, aluminum primary phosphate, magnesium primary phosphate or sodium primary phosphate as the primary metal salt of phosphoric acid, wollastonite crystal or magnesium hydroxide as the curing agent for the primary metal salt of phosphoric acid, water and the reinforcing inorganic fiber. Such representative composition (C) is preferred because it is excellent in filling property into a mold and moldability and assures the arc extinguishing material (II) in the form of a molded product or the like by heat aging, which exhibits excellent arc resistance and mechanical strength and serves as a favorable insulation imparting gas source. In turn, the inorganic binder composition (C) thus formed is filled into a mold defining a desired shape of arc extinguishing plate material and pressure molded. When the pressure in the pressure molding is too low, the composition (C) is insufficiently pressurized and, hence, the resulting molded product may suffer non-uniform denseness. For this reason, the pressure is preferably not lower than 400 kg/cm 2 , more preferably not lower than 500 kg/cm 2 . On the other hand, when the pressure is too high, the composition (C) is likely to penetrate into the clearance of the mold and thereby to cause the mold to be hardly opened. In view of this, the pressure in the pressure molding is preferably not higher than 800 kg/cm 2 , more preferably not higher than 750 kg/cm 2 . In the present invention, the pressure molding may be carried out at room temperature or with the surface table of a press machine appropriately heated. Further, the duration of the pressure molding can be appropriately adjusted. Devices for use in the pressure molding include press machines having surface table for molding to a uniform thickness, such as a hand press, mechanical press and oil press. The arc extinguishing plate material prior to undergoing the aging is allowed to stand over a whole day and night, aged by heating at 120° to 200°C in, for example, an oven to cure for removing the moisture contained therein. Thus, the arc extinguishing plate material (II) is prepared. When the temperature, at which the heat aging is carried out, is too low, the curing of the plate material requires a very long time, or otherwise even when the curing is successfully carried out, the compound for imparting the primary metal salt of phosphoric acid with water resistance is insufficiently produced. For this reason, such temperature has to be not lower than 120°C, preferably not lower than 150°C. When the temperature is too high, only the surface layer of the molded product is cured rapidly and, hence, a non-uniform reaction occurs between the surface portion and the deep portion thereof, thereby causing warpage of the plate material. For this reason, the temperature for the heat aging has to be not higher than 200°C, preferably not hihger than 180°C. The cooling following the heat aging may be a natural gradual cooling. Since the finishing, punching, or the like of the arc extinguishing plate material (II) can be achieved at the time of molding, no machine working is required. Accordingly, in most cases the arc extinguishing plate material (II) finished with the heat aging can be directly used as an arc extinguishing plate or arc extinguishing side plate. An arc extinguishing chamber can be contructed of, for example, two such arc extinguishing side plates and a magnetic plate. The description will be made on the switch of the present invention. The switch of the present invention comprises an arc extinguishing chamber disposed in the vicinity of electrodes and contacts, and the arc extinguishing chamber uses an arc extinguishing side plate formed of the arc extinguishing plate material (I) or (II). The switch of the present invention is similar to the conventional one in strcuture and shape, but is characterized by the arc extinguishing plate such as the arc extinguishing side plate being formed of the arc extinguishing plate material (I) or (II). The switch of the present invention is applicable to any kinds of switches without particular limitations so far as they generate an arc in the arc extinguishing chamber thereof when the contacts of the electrodes thereof is opened or closed. Examples of such switches are, for instance, an electromagnetic contactor, circuit breaker and current-limiting device. Reference is first made to the arc extinguishing chamber according to the present invention. Fig. 1 is a schematic perspective view of one embodiment of an arc extinguishing chamber according to the present invention. The chamber shown includes a plurality of arc extinguishing magnetic plates 201, each defining a U-shaped notch 201a in a central portion thereof and formed of an iron plate or a chrome-plated iron plate, and a pair of arc extinguishing side plates 202, each formed of the arc extinguishing plate material (I) or (II). The arc extinguishing plates 202 and the magnetic plates 201 are secured to each other at caulking portions 203. The electrodes and contacts are meant by those in, for example, an electromagnetic contactor, circuit breaker or current-limiting device, and are formed of, for example, an Ag-WC alloy or Ag-CdO alloy. The term "in the vicinity of the electrodes and contacts" as used herein is equivalent to the arc exposure position in a conventional switch and means a region spaced apart from the electrodes and contacts by about 5 to about 15 cm in the electromagnetic contactor, by about 5 to about 15 cm in the circuit breaker or by about 5 to about 30 cm in the current-limiting device. Fig. 2 is a schematic side view, partly in section, of one embodiment of a switch according to the present invention, and wherein the reference numerals 201 to 203 denote the same parts of Fig. 1, and numerals 204 and 205 denotes a fixed contact and a moving contact, respectively. The fixed and moving contacts 204 and 205 located within the arc extinguishing chamber constructed of the magnetic plates 201 and arc extinguishing side plates 202 permit electric current to flow therethrough when they contact each other (closed condition). To interrupt the current, the moving contact 205 is moved toward the position (opened condition) indicated by broken line. At this time an arc is generated over the gap between the fixed and moving contacts 204 and 205 and is drawn in the direction indicated by arrow so as to be extinguished. The arc extinguishing side plate formed of the arc extinguishing plate material (I) or (II) of the present invention is excellent in heat resistance, arc resistance, thermal shock resistance and the like, acts to absorb the energy of an arc, generated in the arc extinguishing chamber for cooling down and extinguish it, thereby protecting the components of the switch from the heat of the arc, and serves to insulate metal vapor and molten metal droplets that are generated from the electrodes, contacts and other metal components adjacent thereto by the arc, thereby overcoming the problems such as the decrease in electrical resistance. Therefore, the switch of the present invention using the plate material (I) or (II) also offers highly excellent effects. Where the arc extinguishing plate material (I) is the arc extinguishing plate material (I) of embodiment 2, it further enjoys enhanced electrical insulating property and mechanical strength. Where the arc extinguishing plate material (I) is the arc extinguishing plate material (I) of embodiment 3, it further enjoys such benefits as easy preparation, excellent heat resistance and arc resistance, and an enhanced effect in preventing the decrease in electrical resistance. Where the arc extinguishing plate material (I) is the arc extinguishing plate material (I) of embodiment 4, it further enjoys such benefits as excellent water resistance and a potent effect in preventing the decrease in electrical resistance, since aluminum hydroxide contained therein acts also as the curing agent for the primary metal salt of phosphoric acid. Where the arc extinguishing plate material (I) is the arc extinguishing plate material (I) of embodiment 5, it further enjoys such a benefit as a highly dense quality, since the material (I) has a water solubility and viscosity suitable as a binder and hence uniformly adheres to the reinforcing inorganic material sheet. Where the arc extinguishing plate material (I) is the arc extinguishing plate material (I) of embodiment 6, it further enjoys such a benefit that the inorganic binder composition (I) and the sheet can be prepared with ease. Where the arc extinguishing plate material (I) is the arc extinguishing plate material (I) of embodiment 7, it further enjoys such a benefit as enhanced water resistance. Where the arc extinguishing plate material (I) is the arc extinguishing plate material (I) of embodiment 8, it further enjoys such benefits as easy preparation, excellent heat resistance and arc resistance, and enhanced effect in preventing the decrease in electrical resistance. Where the arc extinguishing plate material (I) is the arc extinguishing plate material (I) of embodiment 9, it further enjoys such a benefit as an enhanced effect in preventing the decrease in electrical resistance. Where the arc extinguishing plate material (I) the arc extinguishing plate material (I) of embodiment 10, it further enjoys such a benefit that the insulation imparting gas generating source compound can be easily incorporated therein. Where the arc extinguishing plate material (I) is the arc extinguishing plate material (I) of embodiment 11, it further enjoys such benefits that the inbrganic binder composition (II) and the sheet can readily be prepared. Where the arc extinguishing plate material (I) is the arc extinguishing plate material (I) of embodiment 12, it further enjoys such a benefit that there is no need to blend the curing agent for imparting the plate material (I) with water resistance. Where the arc extinguishing plate material (I) is the arc extinguishing plate material (I) of embodiment 13, it further enjoys such benefits as excellent heat resistance and arc resistance. Where the arc extinguishing plate material (II) is the arc extinguishing plate material (II) of embodiment 26, it further enjoys such benefits as easy preparation, excellent heat resistance and arc resistance, and enhanced effect in preventing the decrease in electrical resistance. Where the arc extinguishing plate material (II) is the arc extinguishing plate material (II) of embodiment 27, it further enjoys such a benefit as a more enhanced effect in preventing the decrease in electrical resistance as compared with the case using the material (II) of embodiment 26. Where the arc extinguishing plate material (II) is the arc extinguishing plate material (II) of embodiment 28, it further enjoys such benefits as excellent arc resistance and thermal shock resistance. Where the arc extinguishing plate material (II) is the arc extinguishing plate material (II) of embodiment 29, it further enjoys such a benefit as a highly dense quality, since the material (II) has a water solubility and viscosity suitable as a binder. Where the arc extinguishing plate material (II) is the arc extinguishing plate material (II) of embodiment 30, it further enjoys such a benefit that it is possible to obtain a highly dense molded product, since the material of the product becomes plastic during the pressure molding. Where the arc extinguishing plate material (II) is the arc extinguishing plate material (II) of embodiment 31, it further enjoys such a benefit as excellent water resistance. Where the arc extinguishing plate material (II) is the arc extinguishing plate material (II) of embodiment 32, it further enjoys such a benefit as excellent heat resistance. Where the arc extinguishing plate material (II) is the arc extinguishing plate material (II) of embodiment 33, it further enjoys such benefits as excellent arc resistance and mechanical strength. Where the arc extinguishing plate material (II) is the arc extinguishing plate material (II) of embodiment 34, it further enjoys such benefits as developed water resistance and enhanced mechanical strength. The arc extinguishing plate material (I) of the present invention comprises, after aging, 35 to 50 % of the reinforcing inorganic material sheet and 50 to 65 % of the inorganic binder composition (B). Such a high content of the inorganic binder composition (B) imparts the plate material (I) with excellent heat resistance, arc resistance, thermal shock resistance and the like. Further, the reinforcing inorganic material sheet contained in the proportion of 35 to 50 % allows the plate material (I) to exhibit excellent punching quality and mechanical strength and to be readily produced. Such plate material (I) offers such a merit as to absorb the energy of an arc generated in the arc extinguishing chamber of a switch upon an opening or closing operation of the electrodes thereof for cooling down and extinguish the arc, thereby protecting components of the switch from the heat of the arc. Where the reinforcing inorganic material sheet used in the arc extinguishing plate material (I) is formed of a glass mat or glass fabric, e.g. those made of a glass fiber having an insulating property, or a ceramic paper made of a ceramic fiber, the plate material (I) exhibits higher mechanical strength and heat resistance. Arc extinguishing plate materials (I) and (II), preparation methods for the respective materials, and switch employing the material (I) or (II) according to the third group inventions of the present invention will be more fully described by way of specific examples thereof. The present invention will not be limited to such examples. EXAMPLES 3-1 to 3-10 An inorganic binder composition (I) was prepared by mixing solid materials of the ingredients thereof shown in Table 3-1, namely insulation imparting gas generating source compound, arc resistant inorganic powder and curing agent, for 30 minutes using an Ishikawa-type agitating mortar, and then adding an aqueous solution of primary metal salt of phosphoric acid to the mixture, followed by further kneading for 15 minutes. A reinforcing inorganic material sheet of 30 cm square and 0.2 mm (in the case of glass fabric) or 0.5 mm (in the case of glass mat or ceramic paper) thickness was immersed in the inorganic binder composition (I) to prepare a sheet impregnated with the inorganic binder composition (I) in an amount shown in Table 3-1. The impregnated sheet was placed in a vat and introduced into an oven where the sheet was heated to 80°C to remove the moisture thereof until the concentration of the aqueous solution of primary metal salt of phosphoric acid reached 65 % and to allow the curing of the sheet to proceed, thereby preparing a sheet before undergoing pressurization. The thus prepared sheet was pressure-molded under 150 kg/cm 2 -G at room temperature for one minute to afford a molded product. The molded product thus obtained was allowed to stand for one day and then heated from room temperature up to 200°C at a rate of 5°C/min in an oven, followed by aging therein at 200°C for one hour. The molded product was then allowed to be naturally cooled down to afford an arc extinguishing plate material (I). The composition and thickness of the thus obtained arc extinguishing plate material were as shown in Table 3-2. It was confirmed that only the moisture of the inorganic binder composition (I) adhering to the arc extinguishing plate material (I) was removed. Further, when the arc extinguishing plate material (I) was heated to 200°C to examine whether there was a loss of weight, there was found no loss of weight. Thereafter, both faces of the arc extinguishing plate material were coated with a dusting preventive coating material shown in Table 3-1 by means of brush and then dried. In any of Examples 3-1 to 3-10, the total amount of the coating material used per plate material was 9 g, 4.5 g for each face. Such an amount was determined by measuring the change in weight after the aging. The arc extinguishing plate material (I) thus obtained was punched and then finished into a predetermined form to afford an arc extinguishing side plate. Two such arc extinguishing side plates were combined to form an arc extinguishing chamber of 30 mm (length) x 20 mm (width) x 50 mm (height) as shown in Fig. 3-1. Using the arc extinguishing chamber thus constructed, a switch as shown in Fig. 3-2 was manufactured wherein the distance between the contacts and the chamber was 2 cm at the largest. Particulars of abbreviations, compounds and reinforcing inorganic material sheets including glass mat, glass fabric and ceramic paper are as follows and the same is true for Tables hereinafter. A: aluminum hydroxide, average particle size of 0.8 µm; Alumina powder: aluminum oxide powder, average particle size of 0.3 µm (350-mesh pass); Zircon powder: zirconium silicate powder, average particle size of 16 µm (350-mesh pass); Cordierite powder: average diameter of 7.5 µm, SS-200 (trade mark) of MARUSU YUYAKU KABUSHIKI KAISHA; Aluminum primary phosphate: produced by NACALAI TESQUE KABUSHIKI KAISHA, powdery reagent; Magnesium primary phosphate: produced by NACALAI TESQUE KABUSHIKI KAISHA, powdery reagent B: wollastonite crystal, 350-mesh pass, FPW-350 (trade mark) of KINSEI MATEC KABUSHIKI KAISHA; Glass mat: formed of E glass, weight per square meter: 455 g/m 2 , CM455FA (trade mark) of ASAHI FIBER KABUSHIKI KAISHA; Glass fabric: formed of silica glass, 7628 STYLE (trade mark) of ASAHI SCHWEBEL KABUSHIKI KAISHA, 0.2 mm thick, 44 x 33 filaments/in.; Ceramic paper: formed of aluminosilicate, FIBER FLUX NO. 300 (trade mark) of TOSHIBA MONOFRAX KABUSHIKI KAISHA, 0.5 mm thick; Dusting preventive coating material (a): ethyl silicate containing 20 % of Si, TSB4200 (trade mark) of YUGEN KAISHA TSB; Dusting preventive coating material (b): acrylic resin, MASACO (trade mark) of MITSUBISHI KASEI KABUSHIKI KAISHA Note that amounts of aluminum hydroxide represented by the character A are separately shown in Table 3-1, one acting as a curing agent and the other acting as an insulation imparting gas generating source compound (hereinafter the same). The switch thus manufactured was subjected to the following interrupting test, durability test and insulation resistance test (megohm measurement). The results are as shown in Table 3-2. Overload interrupting test In accordance with the measuring method for molded case circuit breakers provided in JIS C8370, the switch in closed condition is applied with a current six times as high as a rated current (for example, if the rated current is 100 A, the current to be applied is three-phase 550 V/600 A) and the movable contact is separated away from the fixed contact to generate an arc current. If the switch successfully interrupts the arc current predetermined times (50 times), the switch is regarded as passed the test. Durability test The switch in closed condition is applied with a current of three-phase 550 V/100 A and the movable contact is mechanically separated away from the fixed contact to generate an arc current. If the switch successfully interrupts the arc current predetermined times (6000 times) and the arc extinguishing side plate used therein exhibits a consumption-by-arc resistance, specifically to such a degree that a hole is not formed in the side plate by arc, the switch is regarded as passed the test. Insulation resistance test The switch in closed condition is applied with an overcurrent of three-phase 460 V/25 kA and the movable contact is separated away from the fixed contact to generate an arc current. If the switch successfully interrupts the arc current, the switch is regared as passed a short circuit test. Thereafter, the insulation resistances between terminals are measured using the insulation resistance tester provided in JIS C1302. The results shown in Table 3-2 are the lowest values of phase-to-phase insulation resistances (MΩ ) on the load side. As can be understood from Table 3-2, any of the switches according to these Examples succeeded in interrupting an arc 50 times in the interrupting test and 6000 times in the durability test and, therefore, was found to exhibit excellent interrupting performance. This means that the arc extinguishing plate materials (I) prepared in these Examples were excellent. Visual observation of the portion, contacted by arc, of the arc extinguishing side plate after the tests revealed that the portion had remained in a satisfactory condition with little damage. Further, as can be understood from the results of the insulation resistance test, the arc extinguishing side plate formed from the arc extinguishing plate material (I) of the present invention exhibited a potent effect in enhancing the insulation resistance, the enhanced insulation resistance being higher than the required value, 0.5 MΩ. EXAMPLES 3-11 to 3-20 Arc extinguishing plate materials (I) were prepared in the same manner as in Examples 3-1 to 3-10 except that the impregnated sheet was dried at 120°C and that two sheets before undergoing pressurization were laid on top of the other and pressure-molded under 200 kg/cm 2 -G at room temperature for one minute and the resultant molded product was allowed to be aged at 180°C over a whole day and night. Each of the arc extinguishing plate materials (I) thus obtained was coated with a dusting preventive coating material and then dried. The thus obtained arc extinguishing material (I) was formed into an arc extinguishing side plate, which was in turn used to construct arc extinguishing chamber and switch similar to those of Examples 3-1 to 3-10. In Table 3-3 are shown the inorganic binder compositions (I) used in Examples 3-11 to 3-20, amount of each inorganic binder composition (I) applied relative to 100 parts of the reinforcing inorganic material sheet and the kind of dusting preventive coating material used, and in Table 4 are shown the composition and thickness of each of the arc extinguishing plate materials (I) obtained. The switches thus constructed were subjected to the same evaluation tests as in Examples 3-1 to 3-10. The results are as shown in Table 3-4. As can be understood from Table 3-4, the arc extinguishing plate materials (I) and switches obtained in Examples 3-11 to 3-20 exhibited excellent performance. Visual observation of the portion, contacted by arc, of the arc extinguishing side plate after the tests revealed that the portion had remained in a satisfactory condition with little damage. EXAMPLES 3-21 to 3-26 Arc extinguishing plate materials (I), arc extinguishing side plates, arc extinguishing chambers and switches were manufactured in the same manner as in Examples 3-4 and 3-7 except that the insulation imparting gas generating source compound of Table 3-5 was applied onto either or both of the faces of the sheet before undergoing pressurization. The application of the insulation imparting gas generating source compound was achieved by sieving the compound onto the entire face of the sheet to an even thickness with use of a 35-mesh sieve. The amount of the applied compound was calculated by subtracting the amount of the compound not adhering to the sheet from the total amount of the compound used. Table 3-5 shows the kind of the sheet before undergoing pressurization (represented by the number of Example where the corresponding sheet was prepared), the kind and amount of the applied insulation imparting gas generating source compound, and the kind of dusting preventive coating material. Particulars of the compounds shown in Table 3-5 are as follows: Magnesium hydroxide: average particle size 0.6 µm, powdery reagent of NACALAI TESQUE KABUSHIKI KAISHA; Magnesium carbonate: average particle size 0.4 µm, powdery reagent of NACALAI TESQUE KABUSHIKI KAISHA; Calcium carbonate: average particle size 0.3 µm, special grade chemical made by NACALAI TESQUE KABUSHIKI KAISHA. The thickness of each of the prepared arc extinguishing plate materials (I) and the results of the evaluation tests, same as in Examples 3-1 to 3-10, on each of the switches constructed in Examples 3-21 to 3-26 are shown in Table 3-6. As can be understood from Table 3-6, the arc extinguishing plate materials (I) and switches obtained in Examples 3-21 to 3-26 of the present invention exhibited excellent performance, like those obtained in Examples 3-1 to 3-10. Visual observation on the portion, contacted by arc, of each of the arc extinguishing side plates after the tests revealed that the portion had remained in satisfactory condition with little damage. EXAMPLES 3-27 to 3-32 Arc extinguishing plate materials (I), arc extinguishing side plates, arc extinguishing chambers and switches were manufactured in the same manner as in Examples 3-21 to 3-26 except that two sheets applied with the insulation imparting gas generating source compound used in each of Examples 3-21 to 3-26 were laid on top of the other (in Example 3-27 such two sheets were laid on top of the other with their compound-free faces opposed to each other). Table 3-7 shows the kind of the sheet before undergoing pressurization (the number of Example where the corresponding sheet was prepared), the kind and amount of the applied insulation imparting gas generating source compound, and the kind of dusting preventive coating material. The thickness of each of the prepared arc extinguishing plate materials (I) and the results of the evaluation tests, same as in Examples 3-1 to 3-10, on each of the switches constructed in Examples 3-27 to 3-32 are shown in Table 3-8. As can be understood from Table 3-8, the arc extinguishing plate materials (I) and switches obtained in Examples 3-27 to 3-32 of the present invention exhibited excellent performance, like those obtained in Examples 3-1 to 3-10. Visual observation on the portion, contacted by arc, of each of the arc extinguishing side plates after the tests revealed that the portion had remained in satisfactory condition with little damage. EXAMPLES 3-33 to 3-42 Arc extinguishing plate materials (I) were manufactured in the same manner as in Examples 3-1 to 3-10 except that solid materials, i.e., insulation imparting gas generating source compound and arc resistant inorganic powder, of inorganic binder composition (II) shown in Table 3-9 were mixed for 30 minutes by the use of an Ishikawa-type agitating mortar and further kneaded together with an additional aqueous solution of condensed alkali metal phosphate (referred to as "aqueous solution of condensed metal phosphate" in Table, and hereinafter the same) for 15 minutes to give inorganic binder composition (II), and then the moisture of the aqueous solution of condensed alkali metal phosphate was removed until the concentration thereof reached 65 % to afford a sheet before undergoing pressurization. Each of the arc extinguishing plate materials (I) thus manufactured was punched and finished into a predetermined form to obtain an arc extinguishing side plate. In this case the arc extinguishing side plate was not applied with a dusting preventive coating material. Using the thus obtained arc extinguishing side plate were obtained an arc extinguishing chamber and then a switch. Particulars of the compounds and abbreviations in Table 3-9 are as follows: Sodium metaphosphate: powdery reagent, produced by NACALAI TESQUE KABUSHIKI KAISHA; Potassium metaphosphate: powdery reagent, produced by NACALAI TESQUE KABUSHIKI KAISHA; C: magnesium hydroxide (same as used in Examples 3-21 to 3-26); D: magnesium carbonate (same as used in Examples 3-21 to 3-26); E: calcium carbonate (same as used in Examples 3-21 to 3-26). The thus obtained switches were subjected to the same evaluation tests as in Examples 3-1 to 3-10. The results of the tests together with the composition and thickness of each arc extinguishing plate material (I) are shown in Table 3-10. As can be understood from Table 3-10, the arc extinguishing plate materials (I) and switches obtained in Examples 3-33 to 3-42 of the present invention exhibited excellent performance, like those obtained in Examples 3-1 to 3-10. Visual observation on the portion, contacted by arc, of each of the arc extinguishing side plates after the tests revealed that the portion had remained in satisfactory condition with little damage. EXAMPLES 3-43 to 3-52 Arc extinguishing plate materials (I) were manufactured in the same manner as in Examples 3-33 to 3-42 except that two sheets before undergoing pressurization as manufactured in Examples 3-33 to 3-42 were laid on top of the other and pressure-molded under 200 kg/cm 2 -G at room temperature for one minute. Note that the abbreviations and compounds shown in Table 3-11 are the same as in Table 3-9. Using each of the thus manufactured arc extinguishing plate materials (I) were prepared an arc extinguishing side plate, arc extinguishing chamber and switch which were similar to those of Examples 3-1 to 3-10. The thus obtained switches were subjected to the same evaluation tests as in Examples 3-1 to 3-10. The results of the tests together with the composition and thickness of each arc extinguishing plate material (I) are shown in Table 3-12. As can be understood from Table 3-12, the arc extinguishing plate materials (I) and switches obtained in Examples 3-43 to 3-52 of the present invention exhibited excellent performance, like those obtained in Examples 3-1 to 3-10. Visual observation on the portion, contacted by arc, of each of the arc extinguishing side plates after the tests revealed that the portion had remained in satisfactory condition with little damage. EXAMPLES 3-53 to 3-60 Sheets (4), (7), (33) and (39) before undergoing pressurization were prepared in the same manner as in Examples 3-4, 3-7, 3-33 and 3-39, respectively, except that the moisture of the aqueous solution of primary metal salt of phosphoric acid or aqueous solution of condensed alkali metal phosphate was removed until the concentration thereof reached 85 %. Arc extinguishing plate materials (I) of Examples 3-53 to 3-56 were manufactured in the same manner as in Examples 3-1 to 3-10 except that a sheet (I) comprising the thus prepared sheet (4) or (7) and a sheet (II) comprising the thus prepared sheet (33) or (39), shown in Table 3-13, were laid on top of the other and then pressure-molded under 200 kg/cm 2 -G at 200°C for one minute. Further, arc extinguishing plate materials (I) of Examples 3-57 to 3-60 were manufactured in the same manner as in Examples 3-53 to 3-56 except that a sheet (I) shown in Table 3-13 was sandwiched between sheets (II) shown in Table 3-13. In this case three sheets were used in total. Using each of the thus manufactured arc extinguishing plate materials (I) were prepared an arc extinguishing side plate, arc extinguishing chamber and switch which were similar to those of Examples 3-1 to 3-10. The thus obtained switches were subjected to the same evaluation tests as in Examples 3-1 to 3-10. The results of the tests together with the thickness of each arc extinguishing plate material (I) are shown in Table 3-13. As can be understood from Table 3-13, the arc extinguishing plate materials (I) and switches obtained in Examples 3-53 to 3-60 of the present invention exhibited excellent performance, like those obtained in Examples 3-1 to 3-10. Visual observation on the portion, contacted by arc, of each of the arc extinguishing side plates after the tests revealed that the portion had remained in satisfactory condition with little damage. EXAMPLES 3-61 to 3-77 Solid contents, i.e., insulation imparting gas generating source compound, arc resistant inorganic powder, primary metal salt of phosphoric acid, curing agent and reinforcing inorganic fiber, of each of inorganic binder compositions (C) shown in Tables 3-14 and 3-15 were mixed for 30 minutes by the use of an Ishikawa-type agitating mortar, followed by further mixing the mixture for 15 minutes while adding thereto water dropwise using an injector, to give a material before undergoing pressurization. The abbreviations used in Tables 3-14 and 3-15 represent the compounds as follows: F: zircon powder (same as used in Examples 3-1 to 3-10); G: cordierite powder (same as used in Examples 3-1 to 3-10); H: mullite powder, average particle size of 4 µm (350-mesh pass); I: aluminum primary phosphate (same as used in Examples 3-1 to 3-10); J: magnesium primary phosphate (same as used in Examples 3-1 to 3-10); K: sodium primary phosphate, powdery reagent, produced by NACALAI TESQUE KABUSHIKI KAISHA; L: aluminum borate whisker, average fiber diameter: 0.6 µm, average fiber length: 25 µm, ALBOREX (trade mark) of SHIKOKU KASEI KABUSHIKI KAISHA; M: SiC whisker, average fiber diameter: 0.08 µm, average fiber length: 7 µm, SCW (trade mark) of TATEHO KAGAKU KOGYO KABUSHIKI KAISHA; N: calcium carbonate whisker, average fiber diameter: 0.6 µm, average fiber length: 25 µm, WHISCAL (trade mark) of SHIKOKU KASEI KABUSHIKI KAISHA; O: silica alumina glass fiber, average fiber diameter: 10 µm, average fiber length: 60 µm, KAOWOOL (trade mark) MILLED FIBER of ISOLITE KOGYO KABUSHIKI KAISHA; and P: Si 3 N 4 whisker, average fiber diameter: 0.5 µm, average fiber length: 130 µm, SNW (trade mark) of TATEHO KAGAKU KOGYO KABUSHIKI KAISHA. In Tables 3-14 and 3-15, the amount of each of the compounds represented by abbreviations A and C (same as in the foregoing Table) is divided into an amount acting as a curing agent and an amount acting as an insulation imparting gas generating source compound, and the amount of the material (wollastonite crystal) represented by abbreviation B is also divided into an amount acting as a curing agent and an amount acting as a reinforcing inorganic fiber. The thus prepared material before undergoing pressurization was filled into a mold of the shape of an arc extinguishing side plate having dimensions of 40 mm (length) x 50 mm (width) x 5 mm (depth) and pressure-molded under 700 kg/cm 2 -G at room temperature for one minute to afford a molded product in the form of arc extinguishing side plate. This molded product was allowed to stand for one day, then heated from room temperature up to 200°C at a rate of 5°C/min in an oven and allowed to be aged at the temperature maintained at 200°C for three hours, followed by allowing it to cool naturally to afford an arc extinguishing side plate (arc extinguishing plate material (II)). Further, using the thus prepared arc extinguishing side plates were manufactured arc extinguishing chambers and switches which were similar to those obtained in Examples 3-1 to 3-10. The thus manufactured switches were subjected to the same evaluation tests as in Examples 3-1 to 3-10. The results were as shown in Tables 3-16 and 3-17. As can be understood from Tables 3-16 and 3-17, the arc extinguishing plate materials (II) and switches obtained in Examples 3-61 to 3-77 of the present invention exhibited excellent performance, like those obtained in Examples 3-1 to 3-10. Visual observation on the portion, contacted by arc, of each arc extinguishing side plate after the tests revealed that the portion had remained in satisfactory condition with little damage. COMPARATIVE EXAMPLE 3-1 In accordance with Japanese Unexamined Patent Publication No. 310534/1988, a 1 mm-thick lamination plate having dimensions of 300 mm x 300 mm was prepared using an organic material which was free of any aromatic ring having many carbon atoms and abundant in hydrogen and which comprised an acrylic acid ester copolymer (polymethylmethacrylate) and 30 % of a glass fiber filled therein The lamination plate was then shaped into an arc extinguishing side plate having the same dimensions and thickness as those of Example 3-1. Using the arc extinguishing side plate thus obtained were manufactured an arc extinguishing chamber and a switch in the same manner as in Examples 3-1 to 3-10. The switch was subjected to the same evaluation tests as in Examples 3-1 to 3-10. The results are as shown in Table 3-17. COMPARATIVE EXAMPLE 3-2 A molded product (GLASSMER of NIKKO KASEI KABUSHIKI KAISHA) formed from a glass fabric-polyester resin composite plate in which the polyester resin contained 30 % of alumina as a filler was shaped into an arc extinguishing side plate having the same dimensions and thickness as those of Example 3-1. Using the arc extinguishing side plate thus obtained were manufactured an arc extinguishing chamber and a switch in the same manner as in Examples 3-1 to 3-10. The switch thus manufactured was subjected to the same tests as in Examples 3-1 to 3-10. The results are as shown in Table 3-17. As can be understood from Table 3-17, Comparative Examples 3-1 and 3-2 contributed to an insulation resistance much lower than the required value, i.e., 0.5 Ω in the insulation resistance test. As has been described, the present invention provides an arc extinguishing material and a switch using the same which are applicable to a switch generating an arc upon interruption of electric current flowing therethrough such as a circuit breaker, current-limiting device or electromagnetic contactor and which is capable of immediately extinguishing the arc and inhibiting the decrease in insulation resistance within and around an arc extinguishing chamber and at inner wall surfaces of the switch case. 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.

Claims: An arc extinguishing plate material which is obtained by pressure molding and aging an inorganic binder composition (C) comprising 40 to 55% by weight of an insulation imparting gas generating source compound, 25 to 40% by weight of an arc resistant inorganic powder, 8 to 18% by weight of a primary metal salt of phosphoric acid, 5 to 10% by weight of a curing agent for the primary metal salt of phosphoric acid, 2.6 to 12% by weight of water, and 2 to 10% by weight of a reinforcing inorganic fiber. The arc extinguishing plate material of claim 1, wherein said insulation imparting gas generating source compound is selected from the group consisting of magnesium hydroxide, aluminum hydroxide, magnesium carbonate and calcium carbonate; said arc resistant inorganic powder is selected from the group consisting of zircon powder, cordierite powder and mullite powder; said primary metal salt of phosphoric acid is selected from the group consisting of aluminum primary phosphate, magnesium primary phosphate and sodium primary phosphate; said water is used in such an amount as providing a 60 to 75% by weight aqueous solution of the primary metal salt of phosphoric acid; said curing agent for said primary metal salt of phosphoric acid is selected from the group consisting of wollatonite crystals, magnesium hydroxide, aluminum hydroxide, magnesium carbonate and calcium carbonate; said reinforcing inorganic fiber is an inorganic short fiber. An arc extinguishing plate material comprising 35 to 50% by weight of a reinforcing inorganic material sheet, and 50 to 65% by weight of an inorganic binder composition (B), said arc extinguishing plate material being prepared by pressure-molding and aging of a sheet comprising said reinforcing inorganic material sheet and an inorganic binder composition (A), wherein said inorganic binder composition (A) is an inorganic binder composition comprising 30 to 45% by weight of an insulation imparting gas generating source compound, 0 to 28% by weight of an arc resistant inorganic powder, 40 to 65% by weight of an aqueous solution of a primary metal salt of phosphoric acid, and 2 to 10% by weight of a curing agent for said primary metal salt of phosphoric acid. The arc extinguishing plate material of claim 3, wherein said insulation imparting gas generating source compound is aluminum hydroxide; said primary metal salt of phosphoric acid is aluminum primary phosphate or magnesium primary phosphate; the concentration of the primary metal salt of phosphoric acid in said aqueous solution is 25 to 55% by weight; said curing agent for the aqueous solution of primary metal salt of phosphoric acid is wollastonite crystals or aluminum hydroxide; and said arc resistant inorganic powder is selected from the group consisting of aluminum oxide powder, zircon powder and cordierite powder. An arc extinguishing plate material comprising 35 to 50% by weight of a reinforcing inorganic material sheet, and 50 to 65% by weight of an inorganic binder composition (B), said arc extinguishing plate material being prepared by pressure-molding and aging of a sheet comprising said reinforcing inorganic material sheet and an inorganic binder composition (A), wherein said inorganic binder composition (A) is an inorganic binder composition (II) comprising 30 to 50% by weight of an insulation imparting gas generating source compound, 0 to 20% by weight of an arc resistant inorganic powder and 50 to 70% by weight of an aqueous solution of condensed alkali metaphosphate phosphate. The arc extinguishing plate material of claim 5, wherein said insulation imparting gas generating source compound which serves also as a curing agent for said primary metal salt of phosphoric acid is selected from the group consisting of magnesium hydroxide, magnesium carbonate and calcium carbonate; said condensed alkali metal phosphate is sodium metaphosphate or potassium metaphosphate; the concentration of said condensed alkali metal phosphate in said aqueous solution is from 10 to 40% by weight; and said arc resistant inorganic powder is selected from the group consisting of aluminum oxide powder, zircon powder and cordierite powder. A switch comprising electrodes, contacts provided to said electrodes, and an arc extinguishing chamber disposed in the vicinity of said electrodes and contacts and having an arc extinguishing side plate formed of the arc extinguishing plate material recited in one of claims 1 to 6.

Inventors: ADACHI HIROSHI, BABA FUMIAKI, FUKUYA KAZUNORI, HIROI OSAMU, KATO KAZUHARU, 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, KAZUHARU, 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, KAZUHARU, 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: H01H9
IPC codes: H01H9