Electricity: circuit makers and breakers – Electric switch details – Contact
Reexamination Certificate
2000-09-22
2002-08-20
Donovan, Lincoln (Department: 2832)
Electricity: circuit makers and breakers
Electric switch details
Contact
C200S275000
Reexamination Certificate
active
06437266
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to a contact arm assembly having an electrical contact for making and breaking an electrical current in an electrical circuit breaker. Contacts and contact arm assemblies are well known in the art of circuit breakers. An example of an electrical contact suitable for circuit breaker applications is described in U.S. Pat. No. 4,162,160 entitled “Electrical Contact material and Method for Making the same.” An example of a method of making an electrical contact material suitable for circuit breaker applications is described in U.S. Pat. No. 4,249,944 entitled “Method of Making Electrical Contact Material.” Examples of contact arm assemblies suitable for circuit breaker applications is described in U.S. Pat. No. 4,999,464 entitled “Molded Case Circuit Breaker Contact and Contact Arm Arrangement”.
Contact arm assemblies having electrical contacts for making and breaking an electrical current are not only employed in electrical circuit breakers, but also in other electrical devices, such as rotary double break circuit breakers, contactors, relays, switches, and disconnects. The applications that these electrical devices are used in are vast, and include, but are not limited to, the utility, industrial, commercial, residential, and automotive industries. The primary function of a contact arm assembly is to provide a carrier for an electrical contact that is capable of being actuated in order to separate the contact from a second contact and contact arm arrangement, thereby enabling the making and breaking of an electrical current in an electric circuit. Electrical contacts suitable for the noted applications are typically made of a silver impregnated material, such as, but not limited to; silver-tungsten, silver-tungsten-carbide, silver-nickel, silver-tin oxide, silver-cadmium oxide, silver-graphite, silver-molybdenum, silver-nickel-graphite, and silver-iron. However, the use of copper in place of silver may also be suitable for some lower current applications. The contact must be bonded to the contact arm, which is typically, but not necessarily, a copper alloy, in such a manner that the assembly will not disassemble during operation of the host device. The bonding method that is typically employed is brazing. The process of brazing electrical contacts to contact arms is well know to one skilled in the art and is fully described in Advanced Metallurgy's article entitled “Brazing Electrical Contacts” by Peter C. Murphy, published by Advanced Metallurgy, Inc., 1028 E. Smithfield Street, McKeesport, Pa. 15135 (July, 1987).
To facilitate the brazing process, contacts have been known to be manufactured with serrated detail on the back. The serrated detail on the back of the contact serves to retain the excess silver infiltrant and braze alloy that results during contact manufacturing, thereby providing a silver rich layer and a layer of braze alloy on the back of the contact for brazing. The resulting finished contact is substantially void of any serration pockets on the back since the silver infiltrant and braze alloy have substantially filled them in. Thus, the purpose of the serrated detail on the back of the contact is for contact manufacturing purposes and not for influencing current distribution during brazing. Serrated contacts are described in Advanced Metallurgy's article entitled “Serrated Backed Contacts” in their publication entitled “Advanced Metallurgy, Inc., Electrical Contacts and Assemblies”, published by Advanced Metallurgy, Inc., 1028 E. Smithfield Street, McKeesport, Pa. 15135 (1987). Various contact manufacturing methods are also described in the aforementioned publication entitled “Advanced Metallurgy, Inc., Electrical Contacts and Assemblies”.
In order to accommodate thermal limitations within an electrical device, the cross-sectional areas of the contact, contact arm, and bond area between contact and contact arm, typically increase as the ampacity rating of the contact arm assembly increases. While the cross-sectional areas of the contact and contact arm are readily determined by geometric measurements, the cross-sectional area of the bond surface between contact and contact arm is not so readily determined. Factors such as brazing temperature, brazing time, surface oxidation, brazing electrode geometry variations, and braze alloy geometry variations, can effect the percentage of bond area that is actually brazed, thereby effecting the ability of the brazed joint to withstand adiabatic heating at short circuit, and to withstand shear forces during mechanical opening and closing of the contacts. Thus, it would be beneficial to have an improved method of bonding an electrical contact to a contact carrier and an improved contact arm assembly resulting therefrom.
SUMMARY OF THE INVENTION
In an exemplary embodiment of the present invention, a contact arm assembly and method of making the same are provided having an improved bond between contact and contact arm, thereby enabling the contact arm assembly to withstand increased adiabatic heating and shear forces than would be possible without the improved bond. Also provided is an improved contact arm assembly in accordance with the present invention that also includes nickel metal arranged intermediate a silver-impregnated contact and a copper contact arm, thereby preventing intermixing between the copper and silver when the contact is bonded to the contact arm. Further provided is an electric circuit breaker having an improved contact arm assembly in accordance with the present invention, which enables the circuit breaker to perform according to specification when the contact arm assembly is subjected to increased adiabatic heating and shear forces. An alternative benefit of the present invention is to provide an improved contact arm assembly of a reduced size that is capable of withstanding the same adiabatic heating and shear forces as a contact arm assembly of normal size but with less effective bonding between contact and contact arm.
The improved bond between contact and contact arm is accomplished by conditioning the bond surface of the contact arm to produce a serrated finish. While there are many arrangements of serrated finishes that produce satisfactory results, the exemplary embodiment having a plurality pyramid-shaped serrations, or solid geometric saw-like projections, has been s improve the brazed connection between contact and contact arm. The serrated finish on the bond surface of the contact arm serves to more uniformly distribute the electrical current during brazing, provide multiple areas of localized current constriction during brazing, and provide collector pockets for accumulating the molten braze alloy during brazing. A more uniform distribution of electrical current across the contact-to-contact-arm interface during brazing produces a more uniform heat profile throughout the cross-sectional area of the braze alloy, thereby resulting in more uniform melting of the braze alloy. The multiple areas of localized current constriction across the contact-to-contact-arm interface serve to rapidly increase the interface temperature without excessively overheating the contact or contact arm, thereby resulting in rapid melting of the braze alloy while minimizing the degree of annealing experienced by the contact and contact arm. In normal contact-to-contact-arm brazing operations, where annealing of the copper contact arm occurs, the softened copper of the contact arm can result in deformation of the contact arm after the contact arm experiences repeated mechanical on-off impact loads, thereby reducing the term of usability of the contact arm and host device. Minimizing the degree of annealing experienced by the copper contact arm will avoid premature deformation of the contact arm, thereby enhancing the term of usability of the contact arm and host device as compared to a normal contact-to-contact-arm assembly employing a less effective brazing technique. Collector pockets created by the serration pattern provide the molten braz
Criniti Joseph
Larranaga Javier Ignacio
Pannenborg Erich John
Rosario Gerardo
Arnold David
Donovan Lincoln
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