JPH1150176A - Contact material for vacuum circuit breaker, its production and vacuum circuit breaker - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a contact material capable of improving voltage resistance in the case of being used as the contact of a vacuum circuit breaker in particular by regulating the total content of Li, Na, K, Rb, Cs, Be, Mg, Ca, Sr and Ba to a specified value or below. SOLUTION: The contact material for a vacuum circuit breaker is produced as follows. A Cu-Cr material composed of Cu as high conductive component and Cr as arc resistant component is formed into an electrode rod, and this electrode rod is melted to be dispersed by a rotary electrode method in which it is rotated while being electrically heated in a high vacuum. In this way, Cu-Cr alloy powder in which the total content of Li, Na, K, Rb, Cs, Be, Mg, Ca, Sr and Ba is regulated to <=20 ppm is prepd. Next, this Cu-Cr alloy powder is compacted to obtain a compacted body, and this composed body is sintered at 900 to 1050 deg.C for 0.5 to 3 hr in a nonoxidizing atmosphere such as a hydrogen atmosphere.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a contact material for a vacuum circuit breaker, a method of manufacturing the same, and a vacuum circuit breaker, and more particularly to an improvement in withstand voltage when used as a contact (contact) of a vacuum circuit breaker. The present invention relates to a contact material for a vacuum circuit breaker, a method of manufacturing the same, and a vacuum circuit breaker.

[0002]

2. Description of the Related Art Circuit breakers open and close electric circuits in a normal state,
Widely used in power equipment, substation equipment, high-speed railway vehicles, etc. to automatically and instantaneously cut off the electric circuit when combined with an overcurrent relay that detects a failure state when an abnormality such as a grounding accident or short circuit accident occurs. Have been. In particular, vacuum circuit breakers
Container (vacuum valve) maintained at a high vacuum of about 10 ^-4 Pa
The electric circuit is opened and closed by opening and closing a pair of contact members disposed opposite to each other.

FIG. 1 is a sectional view showing an example of the structure of a general vacuum circuit breaker. In FIG. 1, a shut-off chamber 1 in which contact opening and closing operations are performed includes an insulating container 2 made of an insulating material and formed in a substantially cylindrical shape, and sealing metals 3 a at upper and lower ends of the insulating container 2.
It is partitioned and formed by metal lids 4a and 4b provided via the base 3b, and is formed in a vacuum-tight manner. A pair of conductive rods 5 and 6 are arranged in the blocking chamber 1 so as to face each other in the axial direction, and a pair of electrodes 7 and 8 are attached to opposing ends of the conductive rods 5 and 6. . In the figure, the upper electrode 7 is a fixed electrode, while the lower electrode 8 is a movable electrode. A telescopic bellows 9 is mounted on the conductive rod 6 of the movable electrode 8 to enable the movable electrode 8 to reciprocate in the axial direction while the interior of the shut-off chamber 1 is maintained in a vacuum-tight manner. An arc shield 10 made of metal is provided on the bellows 9 to prevent the bellows 9 from being covered with the arc vapor by the arc shield 10.

[0004] A metal arc shield 11 is provided in the cut-off chamber 1 so as to cover the pair of electrodes 7 and 8 facing each other. The arc shield 11 covers the insulating container 2 with arc vapor. Is prevented.

As shown in FIG. 2 in an enlarged manner, the electrode 8 is fixed to a brazing portion 12 formed at the end of the conductive rod 6 by heat bonding or crimped by crimping. The contact member 13a is integrally fixed to the center of the end face of the electrode 8 via a brazing material 14. The fixed-side contact member 13b shown in FIG. 2 is also integrally joined to the end face of the fixed electrode 7 via a brazing material.

According to the vacuum circuit breaker having the above structure, since a high dielectric strength in a high vacuum can be utilized, the opening and closing stroke of the opposed contact member can be shortened.

[0007] The characteristics generally required as a contact material are that the contacts are frequently opened and closed.
Large breaking capacity, (2) high withstand voltage,
(3) small contact resistance, (4) small welding force, (5) small contact wear, (6) small cutting current value, (7) good workability, (8) Have sufficient mechanical strength, and so on. However, in an actual contact material, it is difficult to satisfy all of these characteristics at the same time, and in general, a material that satisfies particularly important characteristics depending on the application and uses some material at the expense of other characteristics is used. That is the current situation.

In particular, the contact material must have arc resistance (arc resistance) and welding resistance so as not to be welded by an arc generated when the contacts are frequently opened and closed, while maintaining low contact resistance. Therefore, a contact material having arc resistance (arc resistance) and welding resistance is actually required since it is essential to have high conductive properties. Specific contact forming materials satisfying both arc resistance and high conductivity include, for example, Ag-based materials, Ag-Cu-based materials, and A-based materials.
g-CdO material, 30% Cu-W material, 50% Cu-
There are Cr-based materials and the like. Particularly, Cu-W-based contact materials have excellent conductivity, while Cu-Cr-based contact materials have excellent withstand voltage characteristics. Therefore, Cu-W-based contact materials have become widespread particularly as contact materials for high-output electrical equipment.

This Cu-Cr contact material is mainly composed of Cu, which has high conductivity, and Cr, which is inferior in conductivity to Cu but has a high melting point and excellent arc resistance and voltage resistance. Thus, both the high withstand voltage and the large current interrupting property required for the contact material are compatible. The Cu-Cr-based contact material having both high withstand voltage and large current interrupting properties is expected to continue to be used in power equipment, substation equipment, railway vehicles, etc., while the vacuum circuit breaker itself is expected. There is also a technical demand for miniaturization, and further improvement in performance is required.

Among the above-mentioned contact materials, a Cu-Cr-based contact material particularly suitable as a contact material for high-power equipment is, for example, a mixed powder of Cu powder and Cr powder or an atomized powder which is formed at a high temperature of about 1050 ° C. Sintering powder metallurgy,
A method of infiltrating Cu into a porous Cr calcined body, or Cr
And Cu at a predetermined composition ratio.

Particularly, in the powder metallurgy method, it is possible to form into a shape close to the final product, there is an advantage that the raw material cost can be reduced, and it is possible to increase the mixing accuracy of the composition ratio of the Cu component and the Cr component. There are advantages. On the other hand, in the powder metallurgy method, since a sintered body is formed by solid-phase sintering, the melting point of the material having the lowest melting point (Cu in the case of Cu-Cr-based material) among the constituent materials of the contact member is used. There is a restriction that heating is not possible. Therefore, compared to the infiltration method and the dissolution method, it is more difficult to remove impurities by volatilization. As a result, there is a drawback that the purity of the contact member is greatly affected by the purity of each raw material powder. Therefore, in a conventional manufacturing process using a general powder metallurgy method, in order to remove impurities mixed into the material due to recontamination of the contact material with oxygen or metal impurities, the compact is sintered in a hydrogen reducing atmosphere. Or sintering of the compact in a vacuum.

[0012]

However, in the conventional manufacturing method, raw materials containing impurities are inevitably used. Therefore, even if the compact is sintered in a reducing atmosphere or in a vacuum, the impurity content is low. However, there was a problem that it could not be reduced sufficiently. Particularly, alkali metals (group 1A elements) such as Li, Na, K, Rb, and Cs, Be, Mg, Ca, Sr, and B
It has been very difficult to produce a contact material with a sufficiently reduced content of alkaline earth metals (group 2A elements) such as a. For example, in the powder metallurgy method, the impurity element that has entered the inside of the raw material powder cannot be sufficiently removed. Further, the technically acceptable limit values of the amounts of these light metal elements contained in the contact material were not clear.

[0013] The main reason why the light metal elements of the 1A group and 2A group as impurities are not sufficiently reduced is that the sintering start temperature is not sufficiently reduced in the sintering step of the compact while oxygen and metal impurities are not sufficiently removed. Since the heating rate is set to be large so as to reach the temperature, it is conceivable that the densification of the compact is started early and a large amount of oxygen and metal impurities remain in the sintered compact. In particular, since the firing temperature is as low as the melting point of Cu or lower and the degree of vacuum is generally about 10 ⁻² to 10 ⁻⁴ Pa, Be and the like cannot be sufficiently volatilized and removed under these conditions. At the end of the sintering step, it is extremely difficult to completely remove oxygen and metal impurities remaining inside the sintered body. Furthermore, when sintering is performed in a hydrogen atmosphere, if a large amount of pores remain in the sintered body, the ratio of trapped hydrogen in the pores after sintering increases, and the hydrogen component becomes vacuum. There is also a problem that the breaking performance of the circuit breaker is deteriorated.

Also, it is difficult to remove impurities in Cr by the infiltration method. On the other hand, although the metal impurities can be volatilized and removed by the melting method, recontamination from a generally used magnesia or calcia crucible may occur. In addition, it is difficult to control the composition, and there is a problem of segregation, etc., and there are many disadvantages in the case of manufacturing by a conventional dissolution method.

In any case, it is extremely difficult to sufficiently reduce oxygen and metal impurities in a conventional method of manufacturing a contact member by powder metallurgy, infiltration or melting.
When a vacuum circuit breaker is formed using this contact member, it has been difficult to obtain sufficient breaking characteristics and pressure resistance.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and particularly, when used as a contact (contact) of a vacuum circuit breaker, a contact for a vacuum circuit breaker capable of improving withstand voltage. It is an object to provide a material, a method for manufacturing the same, and a vacuum circuit breaker.

[0017]

In order to achieve the above object, the present inventors have studied the effects of the type and amount of metal impurities remaining in the contact member on the breaking characteristics of the contact material, and have studied the effect of the contact material. As a result of examining a method of reducing the amount of metal impurities mixed in, the following findings were obtained. That is, gaseous impurity components such as oxygen, Na, K, B
It is extremely effective for stabilizing the arc and improving the breaking performance by reducing as much as possible the content of metal impurities such as a, which have a small work function and degrade the breakdown voltage performance due to electron emission. found. As a result of further investigation, it was found that in order to particularly improve the withstand voltage among the breaking performances, it is necessary to improve the group 1A and 2 as impurities contained in the contact material.
It has been found that it is important to sufficiently reduce the group A light metal elements and to minimize electron emission from the contact material surface. That is, an element which emits a lot of electrons, for example, an alkali metal (Group 1A (Li, Na, K, Rb, Cs)) or an alkaline earth metal (Group 2A (Be, Mg, Ca, Sr, B)
It is important to reduce the content of elements having a small work function such as a)), and these Group 1A and 2A elements are effectively removed at the stage of preparing a raw material powder such as a Cu—Cr alloy powder. It turned out to be possible. That is, by using the rotating electrode method and the centrifugal spraying method, a raw material powder in which the elements of the group 1A and group 2A elements as impurities are more effectively reduced is obtained. It has been found that a contact material having a high purity and excellent breaking characteristics can be obtained for the first time. The present invention has been completed based on the above findings.

That is, the contact material for a vacuum circuit breaker according to the present invention comprises Cu as a highly conductive component and C as an arc resistant component.
r, Li, as impurities,
The total content of Na, K, Rb, Cs, Be, Mg, Ca, Sr and Ba is not more than 20 ppm. Li, Na, K, Rb, C as impurities
It is preferable that each content of s, Be, Mg, Ca, Sr and Ba is 2 ppm or less.

The method for producing a contact material for a vacuum circuit breaker according to the present invention is characterized in that Cu as a highly conductive component and C as an arc resistant component are used.
The electrode rod is made of a Cu—Cr material composed of R and melted and dispersed by a rotating electrode method in which the electrode rod is rotated while being energized and heated in a high vacuum, and Li, Na, K, and R as impurities are dispersed.
a step of preparing a Cu—Cr alloy powder having a total content of b, Cs, Be, Mg, Ca, Sr, and Ba of 20 ppm or less; and a step of molding the Cu—Cr alloy powder to form a compact. Sintering the obtained molded body in a non-oxidizing atmosphere.

In another method for producing a contact material for a vacuum circuit breaker according to the present invention, a molten metal of Cu as a highly conductive component and Cr as an arc resistant component is injected into a rotating cooling medium,
The molten metal is dispersed by a centrifugal force and, at the same time, treated by a centrifugal separation method in which the molten metal is cooled and solidified.
a, K, Rb, Cs, Be, Mg, Ca, Sr and B
a) a step of preparing a Cu—Cr alloy powder having a total content of 20 ppm or less, a step of molding the Cu—Cr alloy powder to form a molded body, and a step of subjecting the obtained molded body to a non-oxidizing atmosphere. And sintering.

Further, the vacuum circuit breaker according to the present invention is a vacuum circuit breaker, which opens and closes an electric circuit by opening and closing a pair of contacts arranged in a vacuum vessel so as to face each other. And Cr as an arc resistant component, and Li, Na, K, Rb, and C as impurities.
It is characterized by comprising a contact material having a total content of s, Be, Mg, Ca, Sr and Ba of 20 ppm or less.

Here, the rotating electrode method used in the above manufacturing method is such that a Cu-Cr material manufactured by powder metallurgy or the like is used as an electrode rod, and the electrode rod is rotated and energized in a high vacuum while being dispersed. This is a method for producing a Cu—Cr alloy powder.
That is, an alloy rod of a Cu-Cr material having a material composition of a powder to be produced is mounted as one electrode rod on the consuming rotary electrode, and is rotated at a high speed while being electrically heated in a high vacuum. The other end of the rotating electrode is a W (tungsten) electrode, and C
When the u-Cr electrode rod approaches the W electrode, an arc causes C
The tip of the electrode rod made of a u-Cr material is melted and scattered by rotation, and the scattered molten metal is solidified and formed into a powder before falling into the tank. Since the tank is filled with He, there is no need to worry about oxidation, and high-purity powder can be obtained. Also,
In the powder thus produced, the impurities are sufficiently reduced and the composition is less segregated. In addition, Cu-Cr
The composition of the alloy powder has the advantage that it can be easily controlled by changing the mixing ratio of Cu and Cr during electrode rod fabrication.

On the other hand, the centrifugal spray method is a method in which a molten metal is subdivided by various mechanical means into fine droplets, which are dispersed in a gas phase and cooled to prepare a raw material powder having a predetermined composition. . Specifically, a method of injecting a molten metal of Cu and Cr onto a rotating drum, cooling the molten metal and simultaneously dispersing the molten metal using centrifugal force to produce a Cu—Cr alloy powder. According to this centrifugal spray method, droplets having a particle size in the range of 50 to 100 μm are generated. According to this centrifugal spraying method, since the cooling time is extremely short, by producing sufficiently fine dispersed particles, the segregation is small and the Cu—C
Multi-component type fine particles such as r type can be obtained. In addition, the composition of the fine particles can be precisely controlled in a wide range, and the diameter, shape, structure, and the like of the fine particles can be controlled by operating conditions. Furthermore, since the preparation temperature is relatively low, there is little reaction with the spray device wall, and a high-purity alloy powder can be obtained. Therefore, this method is excellent as a precise preparation method of multi-component type fine particles such as Cu-Cr type.

Here, Cr as an arc-resistant component is a component that is excellent in arc resistance and welding resistance and extends the life of the contact, and is contained in the raw material mixture in the range of 30 to 70% by weight. Is done. If the content is less than 30% by weight, it is difficult to prolong the service life of the contact due to reduced arc resistance. On the other hand, when the content exceeds 70% by weight, the content of Cu as a high conductive component described later is relatively reduced, and the contact function is reduced due to an increase in contact resistance.

Cu as a high conductive component has a high conductivity and is contained in an amount of about 70 to 30% by weight (wt%) as a residual component excluding the Cr component in order to reduce the contact resistance value of the contact. If the Cu content is less than 30% by weight, the conductivity decreases, the contact resistance increases, and the function as a contact material decreases. On the other hand, when the content exceeds 70% by weight, the content of the arc resistant component relatively decreases, and the arc (electric arc) generated at the time of the contact opening / closing operation causes the contacts to be easily welded, resulting in reduced wear resistance. I will.

The contact material for a vacuum circuit breaker according to the present invention is manufactured, for example, by the following procedure. First, a Cr powder is blended with a Cu powder in a ratio of 30 to 70% by weight, and a Cu-Cr alloy powder is obtained by the above-mentioned rotary electrode method or centrifugal spraying method using a Cu-Cr material having these raw material compositions. .

Next, the prepared Cu-Cr alloy powder is filled into a mold of a press molding machine, and press-molded under a pressure of about 600 to 1000 MPa to prepare a Cu-Cr molded body having a predetermined shape. The formed body is heated in a non-oxidizing atmosphere such as a hydrogen atmosphere at a temperature of 900 to 1050 ° C. for 0.5 to 3 hours.
By sintering for a time, a Cu—Cr sintered body is formed.

The Cu—Cr sintered body thus formed is processed into a predetermined shape to form a contact (contact member), and this contact is made of a brazing material on the end face of the opposing electrode as shown in FIGS. Then, the electrodes to which the respective contacts are joined are joined to the ends of the conductive rods to form a vacuum circuit breaker.

[0029]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, embodiments of the present invention will be described with reference to the following examples.

Example 1 An alloy rod of a Cu—Cr material containing 50 wt% of Cr was used as a rotating electrode for consumption, and was rotated at a high speed while being electrically heated in a high vacuum. A W (tungsten) electrode is arranged at one end of this electrode, and the Cu-Cr electrode rod is brought close to the W electrode, and the tip of the electrode rod of the Cu-Cr material is melted out by an arc to be scattered by rotation. Was. Thereafter, the droplets are allowed to solidify by dropping the droplets into a tank filled with He, thereby forming Cr.
A Cu-Cr alloy powder having a content of 50 wt% was prepared.
A mold is filled with the Cu-Cr alloy powder and press-molded with a pressing force such that the relative density of the finally obtained contact material is 94%, and a Cu-Cr compact (a green compact) having a predetermined shape is formed.
Was prepared. Then, the obtained molded body is heated to a temperature of 1050 ° C.
In a hydrogen atmosphere. Next, the Cu—Cr sintered body is processed into a predetermined shape to obtain contacts (contact members) 13 a and 13 b shown in FIGS. 1 and 2, and the brazing material 14 is attached to the end surfaces of the opposed electrodes 7 and 8. , And the electrodes 7, which are respectively joined to the contacts 13 a, 13 b,
8 was joined to the ends of the conductive rods 5 and 6 in the vacuum valve to assemble the vacuum circuit breaker.

Example 2 A melt of Cu and Cr containing 50% by weight of Cr was dropped on a rotating body (rotary drum), and simultaneously cooled to fly around using a centrifugal force to obtain a Cu-Cr alloy powder. A mold is filled with the Cu-Cr alloy powder and press-molded with a pressing force such that the relative density of the finally obtained contact material is 94%, and a Cu-Cr compact (a green compact) having a predetermined shape is formed. Was prepared. Then, the obtained molded body is heated to a temperature of 105
It was sintered in a hydrogen atmosphere at 0 ° C. Next, this Cu-Cr
The sintered body is processed into a predetermined shape to form contacts (contact members) 13a and 13b shown in FIGS. 1 and 2, and these contacts are integrally joined to the end faces of the opposed electrodes 7 and 8 using a brazing material 14. And
Further, the electrodes 7 and 8 to which the contacts 13a and 13b were respectively joined were joined to the ends of the conductive rods 5 and 6 in the vacuum valve, thereby assembling into a vacuum circuit breaker.

Comparative Example 1 Cu powder and Cr powder produced by the atomizing method were weighed at a weight ratio of 1: 1 and uniformly mixed by a ball mill mixer. This Cu-Cr mixed powder is filled in a mold of a press molding machine, and press-molded with a pressing force such that the relative density of the finally obtained contact material becomes 94%, to form a Cu-Cr molded body having a predetermined shape. Produced. Then, the obtained molded body was sintered in a hydrogen atmosphere at a temperature of 1050 ° C. next,
This Cu—Cr sintered body is processed into a predetermined shape to obtain contacts (contact members) 13 a and 13 b shown in FIGS. 1 and 2, and the contacts are made of brazing material 14 on the end surfaces of the electrodes 7 and 8 facing each other. The electrodes 7 and 8 to which the contacts 13a and 13b were respectively joined were joined to the ends of the conductive rods 5 and 6 in the vacuum bulb, thereby assembling a vacuum circuit breaker.

In order to compare and evaluate the breaking characteristics of each contact material, a Group 1A element (Li, Na, K, Rb, C) contained in the contact materials according to Example 1, Example 2, and Comparative Example 1 was used.
s) and the content of group 2A elements (Be, Mg, Ca, Sr, Ba) were determined by ICP emission spectroscopy (manufactured by Seiko Instruments Inc .:
(SPS-1100 type) and a withstand voltage test of each vacuum circuit breaker. Table 1 below shows the measurement test results. The withstand voltage shown in the table is shown as a relative value with the characteristic value of the vacuum circuit breaker according to Comparative Example 1 as a reference value 1.00.

[0034]

[Table 1]

As is clear from the results shown in Table 1 above,
In the contact member according to Example 1 manufactured according to the manufacturing method using the rotating electrode method, the contents of Li, Na, and K of Group 1A and Be and Ca of Group 2A were particularly reduced, and the light metal element was used. Is also 20 ppm or less. From this, it was found that the withstand voltage was improved as compared with Comparative Example 1 because the impurities contained could be effectively reduced by the rotating electrode method.

On the other hand, in the contact member according to Example 2 manufactured according to the manufacturing method using the centrifugal spray method, no remarkable effect was shown in reducing the content of the group 1A element, but the group 2A group Be, The contents of Mg, Sr, and Ba are particularly effectively reduced, and the total content of light metal elements is also 20 ppm or less, as in Example 1. From this, it was found that the withstand voltage was improved as compared with Comparative Example 1 as in Example 1 because the impurities contained could be effectively reduced by the centrifugal spray method.

[0037]

As described above, according to the contact material for a vacuum circuit breaker according to the present invention, a light metal of Group 1A or 2A consisting of Cu as a highly conductive component and Cr as an arc resistant component is used. Since the total content of the elements is reduced to 20 ppm or less, a vacuum circuit breaker excellent in withstand voltage can be obtained.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a structure of a vacuum circuit breaker to which a contact material according to the present invention is applied.

FIG. 2 is an enlarged sectional view showing a contact and an electrode unit shown in FIG. 1;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Shutoff room 2 Insulating container (vacuum container, vacuum valve) 3a, 3b Sealing metal 4a, 4b Lid 5 Conductive rod 6 Conductive rod 7 Electrode (fixed electrode) 8 Electrode (movable electrode) 9 Bellows 10 Arc shield 11 Arc shield 12 Brazing portions 13a, 13b Contact member 14 Brazing material (Ag brazing material)

Claims (5)

[Claims] 1. A contact material for a vacuum circuit breaker comprising Cu as a highly conductive component and Cr as an arc resistant component, wherein Li, Na, K, Rb, Cs, Be, M as impurities are contained.
A contact material for a vacuum circuit breaker, wherein the total content of g, Ca, Sr and Ba is 20 ppm or less. 2. Li, Na, K, Rb,
2. The content of each of Cs, Be, Mg, Ca, Sr and Ba is 2 ppm or less.
The contact material for a vacuum circuit breaker according to the above. 3. An electrode rod made of a Cu--Cr material comprising Cu as a highly conductive component and Cr as an arc-resistant component, and melt-dispersed by a rotating electrode method in which the electrode rod is rotated while being heated while being energized in a high vacuum. At least, Li, Na as impurities,
A step of preparing a Cu—Cr alloy powder having a total content of K, Rb, Cs, Be, Mg, Ca, Sr, and Ba of 20 ppm or less; and a step of molding the Cu—Cr alloy powder to form a compact And a step of sintering the obtained compact in a non-oxidizing atmosphere. 4. A centrifugal separation method of injecting a molten metal of Cu as a highly conductive component and Cr as an arc resistant component into a rotating cooling medium, dispersing the molten metal by centrifugal force and simultaneously cooling and solidifying the molten metal, Li, Na,
A step of preparing a Cu—Cr alloy powder having a total content of K, Rb, Cs, Be, Mg, Ca, Sr, and Ba of 20 ppm or less; and a step of molding the Cu—Cr alloy powder to form a compact And a step of sintering the obtained compact in a non-oxidizing atmosphere. 5. A vacuum circuit breaker which opens and closes an electric circuit by opening and closing a pair of contacts opposed to each other in a vacuum vessel, wherein the contacts are made of Cu as a highly conductive component and Cr as an arc resistant component. And Li and N as impurities
a, K, Rb, Cs, Be, Mg, Ca, Sr and B
A vacuum circuit breaker comprising a contact material having a total content of a of 20 ppm or less.