EP1655749B1 - Contact pastille for a movable electrical contact of a circuit breaker, movable electrical contact with such a pastille and circuit breaker with such a contact - Google Patents

Description

TECHNICAL FIELD OF THE INVENTION

The invention relates to a contact pad intended to be fixed on a low voltage circuit breaker movable contact intended to withstand short-circuit peak currents of between 200 and 600 amperes per square millimeter of pellet. The pellet consists of an alloy of conducting material based on silver or copper, a fraction of refractory particles such as tungsten carbide, tungsten or titanium nitride and a fraction of carbon fibers.

STATE OF THE ART

The presence of electrical faults within cut-off devices such as electrical circuit breakers causes their electrical contacts to open abruptly. This rapid opening is generally accompanied by electric arcs giving rise to significant stresses at said contacts, more particularly at their surfaces or contact areas.

In order to increase the life of the electrical contacts used in such devices, it is known to modify the structure of the surface or contact area of the electrical contacts. Solutions include adding at the surface or contact area composite materials in the form of pellets of different thicknesses and different materials.

The pellets are usually made of silver or copper alloy. They are made in the usual manner by sintering a powder composed of a silver-based alloy, in particular silver and tungsten or silver and nickel. These contact materials have a high electrical conductivity, a high sufficient resistance to oxidation and good properties with regard to contact resistance.

However, the known silver-based contact materials show an undesirable tendency to weld. In addition, they tend to cause adhesion of the contact surfaces and / or migration of material between the contact elements. Finally, their use is generally associated with excessive wear of the contacts.

Many solutions consist in adding to the alloy powder a conductive material such as graphite. Such materials are commonly used to make the contact pads of the circuit breakers. The graphite present in the metal matrix makes it possible to reduce the risk of welding of the contacts. However, the presence of graphite causes increased mechanical erosion of the pellet.

Carbon, in the form of fibers, can also be added to the alloy powder (document US4699763 ). The resistance to erosion is then improved, but this improvement is achieved at the cost of an alteration of the behavior of the contact at the weld. The carbon fibers are mixed with the metal powder with a wetting agent, lubricant and wet solvent, followed by drying, compression and sintering. The disadvantage of such a method is that it involves complications inherent to a wet process.

To find an acceptable compromise between the behavior of the material with erosion and its anti-welding behavior, it is proposed according to the document DE4111683 mixing graphite particles with carbon fibers, this mixture being incorporated into the metal powder. This hybrid carbon contribution in the metal matrix makes it possible to obtain intermediate erosion and solder behaviors between those which it would have shown with a contribution of graphite particles alone or with a contribution of carbon fibers. alone. But it turns out that, under heavy loads and in particular under strong short-circuit currents, the graphite particles, because of their perfect crystalline structure, show a tendency to be expelled from the surface. of the material. This expulsion deteriorates the surface of the material in such a way that the carbon fibers are also expelled in turn. This results in an enrichment of the silver surface and thus an alteration of the qualities initially sought by the contribution of graphite.

In the document EP0171339 is described a method of manufacturing electrical contacts by impregnating a carbon fiber substrate with a pressurized liquid metal and then hot-spinning the mixture thus obtained. It can be seen that the resulting contacts show an excessive propensity for welding.

In the document EP0729162 is described a method of manufacturing powder-based electrical contacts of a good electrically conductive metal such as silver and crushed carbon fibers of average length less than 20 .mu.m. In common breaking applications, it is found that the contact surface keeps, after cutting under a short-circuit current, its fine, homogeneous and isotropic structure of fiber residues of variable length and any orientation in the matrix of money. The weight proportions of crushed carbon fiber residues are between 2 and 5% with the silver powder. For limiting circuit breakers used for applications in which the arc stagnates little on the contacts, the material thus obtained shows a low post contact resistance, good anti-weld qualities and acceptable erosion resistance. However, the use of this material finds its limits in applications for which there is presence of selective circuit breakers where the fault current appearing on the chips, has the value of the presumed short-circuit current; that's several tens of kilo amps. The erosion resistance of the contact pads described above is no longer sufficient and micro-welds are observed when current passes several tens of kilo amperes for 1 second without opening the device.

Moreover, when the size of this type of circuit breaker is miniaturized and when the addition of filter elements drastically reduces the external manifestations, the optimum choice of contact material is even more draconian. Indeed, the cutoff energy similar to that released in conventional circuit breakers must be absorbed in a miniaturized volume and almost closed. This results in a large deposit of pollutants on the contacts. This deposit consists in particular of molten steel resulting from the ablation of the fins of the arc extinction chamber.

SUMMARY OF THE INVENTION

The invention therefore aims to overcome the disadvantages of the state of the art, so as to provide a contact pad having a low contact resistance post cut, high erosion resistance and good anti-welding properties.

A contact pad according to the invention comprises a weight percentage of carbon fibers in the pellet strictly less than 2% of the total mass of said contact patch.

The weight percentage of carbon fibers in the contact pad is between 0.5% and 1.9% of the total mass of said contact pad.

Advantageously, the carbon fibers are crushed fibers with a length of less than 20 μm.

Preferably, the carbon fibers consist of carbonized fibers containing at least 90% carbon, less than 10% nitrogen, substantially 1% oxygen and less than 1% hydrogen.

According to one embodiment of the invention, the contact patch is composed of 79% of the silver alloy, 20% of tungsten carbide (CW) and 1% of carbon fiber.

A mobile circuit breaker contact according to a development mode of the invention comprises a movable contact finger connected to a contact pad as defined above, the chip being connected to the movable contact finger via a thin layer of conductive material based on silver alloy.

A circuit breaker according to a development mode of the invention comprises an interrupting chamber comprising filtering means intended to attenuate external manifestations of the gases present in the breaking chamber at the time of the openings, and comprises an opening mechanism acting on at least a movable contact as defined above and placed opposite a fixed contact, said contacts being placed in the interrupting chamber.

BRIEF DESCRIPTION OF THE FIGURES

• la figure 1 représente une vue en coupe d'un disjoncteur comportant un contact mobile possédant une pastille de contact selon un mode de réalisation de l'invention.

Other advantages and features will emerge more clearly from the following description of a particular embodiment of the invention, given by way of non-limiting example, and represented in the accompanying drawing in which:

• Figure 1 shows a sectional view of a circuit breaker having a movable contact having a contact pad according to one embodiment of the invention.

DETAILED DESCRIPTION OF AN EMBODIMENT

According to the preferred embodiment of the invention, the contact pad 3 is intended to be fixed on a moving contact 2 low-voltage circuit breaker 1 for supporting short-circuit currents of several kilo amperes. By expressing the electrical current density in amperes per pellet surface unit, a contact pad 3 according to the invention is intended to withstand short-circuit peak currents of between 200 and 600 amperes per square millimeter of pellet.

The effects generated by the passage of these strong short-circuit currents are even more felt when the cut-off energy is absorbed in a breaking chamber 10 having a miniaturized internal volume and almost closed or sealed.

The contact pad 3 is composed of a conductive material such as silver (Ag) or copper (Cu) in which is inserted a fraction of carbon fibers and a fraction of refractory particles such as for example carbide tungsten (WC), tungsten (W), titanium nitride (TiN). These refractory particles have an average diameter of 1 to 10 microns.

The carbon fibers consist of carbonized fibers containing at least 90% carbon, less than 10% nitrogen, substantially 1% oxygen and less than 1% hydrogen.

In the embodiment shown, carbon fibers with an average length L1 of between approximately 100 μm and 800 μm and a diameter of between 4 and 20 μm are chosen. These fibers undergo a cold and dry mechanical treatment in a mechanical grinder. The conditions of intensity and duration of grinding make it possible to obtain fibers the length of which is statistically distributed around a mean value which is much lower than the initial average value. From the initial fibers, crushed fiber residues having an average length of less than 20 μm are thus obtained. The crushed fibers are then added to the conductive material containing the refractory fractions. The powder mixture of conductive material and the crushed carbon fibers is carried out dry in a mechanical mixer until a homogeneous mixture is obtained. Said mixture then undergoes a unitary compression and a sintering so as to obtain an isotropic material structure

Such a manufacturing process is described in detail in the document EP-B-0729162 , whose description is on these points incorporated herein by reference.

In order to resist the specific stresses observed in selective circuit breakers capable of withstanding currents under the conditions described above, it is necessary to increase the erosion resistance of the pellet thus obtained. To do this, the weight percentage of carbon fibers in the pellet is strictly less than 2%. It is preferably between 0.5% and 1.9% of the total mass of the pellet.

In an exemplary application of the invention, an ideal compromise in the choice of the proportions of materials for this type of application consists in using a pellet comprising 79% silver (Ag), 20% tungsten carbide ( CW) and 1% of crushed carbon fibers.

Finally, this material having an appropriate homogeneous structure shows a low and stable post-cut contact resistance as well as a good resistance to erosion under a short circuit of several tens of kiloamperes. In addition, this material develops anti-welding characteristics with respect to the steel balls present in the environment during the cut especially in the electric arc column. An explosive phenomenon due to the degradation of the carbon fiber explains the anti-adhesion characteristics of this material with respect to molten steel balls.

A movable contact 2 circuit breaker 1 comprising a movable contact finger moving relative to a fixed contact 4 of the circuit breaker, has a contact pad 3 as defined above. Said contact pad is connected to the movable contact finger by means of a thin layer of conductive material based on silver or a mixture of copper and silver. This intermediate layer makes it possible to block the propagation of cracks in the interface zone between the contact pad 3 and the finger.

A circuit breaker 1 comprising an opening mechanism 6 acting on at least one electric moving contact 2 as defined above. Said at least one moving contact 2 is placed opposite a fixed contact 4. Said fixed contact is made of an alloy, generally comprising a silver metal matrix in which a fraction of particles of graphite powder is inserted. Said fraction of particles constitutes 3 to 5% of the mass of the alloy. The alloy may also comprise a certain amount of refractory elements (W, WC, Ni) of between 2 and 30% of the total mass. The circuit breaker 1 according to a particular embodiment, is intended to accept nominal currents of 1600 amps and short-term currents of 42000 amps for one second. The breaking chambers 10 comprise, by pole, five movable contacts 2 associated with a fixed contact area 4. Filtering means 5 placed on the walls of the breaking chambers 10 are intended to strongly attenuate external manifestations of the gases present in said rooms at the time of opening contacts. By way of non-limiting example, the filtering means 5 comprise filters having characteristics specific techniques as described in the applicant's patents ( EP-A-1115132 , EP-A-1251533 ).

Claims (5)

1. A contact pad (3) designed to be fixed onto a movable contact (2) of a low-voltage circuit breaker (1) designed to withstand peak short-circuit currents comprised between 200 and 600 amperes per square millimeter of pad, formed by an alloy of silver (Ag) or copper (Cu) based conducting material, a fraction of refractory particles such as tungsten carbide (CW), tungsten (W) or titanium nitride (TiN), and a fraction of carbon fibres, characterized in that the weight percentage of carbon fibres in the contact pad (3) is strictly less than 2% of the total weight of said contact pad, the carbon fibres being crushed fibres with a mean length of less than 20 µm and containing at least 90% of carbon, less than 10% of nitrogen, substantially 1% of oxygen and less than 1% of hydrogen.
2. The contact pad (3) according to claim 1 characterized in that the weight percentage of carbon fibres in the contact pad (3) is comprised between 0.5% and 1.9% of the total weight of said contact pad.
3. The contact pad (3) according to one of the foregoing claims characterized in that it is composed of 79% of silver, 20% of tungsten carbide (CW) and 1% of carbon fibre.
4. A movable contact (2) of a circuit breaker (1) comprising a movable contact finger characterized in that a contact pad (3) according to the foregoing claims is connected to the movable contact finger by means of a thin layer of conducting material the base whereof is silver or a mixture of copper and silver.
5. A circuit breaker (1) comprising an arc chute (10) comprising filtering means (5) designed to attenuate external manifestations of the gases present in the arc chute when openings take place, an opening mechanism (6) acting on at least one movable contact (2), characterized in that said movable contact (2) is a contact according to claim 6 placed facing a stationary contact (4), said contacts being placed in the arc chute.

Images