Failure analysis of electrical contacts
Common control electrical products such as AC contactors, electromagnetic relays, electrical switches and air switches all contain electrical contacts, and contacts are one of the most important components of these products. The reliability and service life of products largely depend on the contact performance, which is affected by such factors as contact material, load type, contact pressure, working frequency and service environment. When these factors cannot meet the requirements, there will be problems such as electrochemical corrosion of contacts, fusion welding between contacts, fire, contact wear, contact resistance increase, which will lead to unreliable operation, functional failure and even potential safety hazards.
In the energized state, if the current of the disconnected circuit exceeds a certain limit, or if the voltage on the contact gap after disconnection exceeds a certain limit, an arc will appear in the contact gap. Arc is the discharge phenomenon of gas between contacts under the action of strong electric field. Sparks and strong light are generated and emitted with high temperature. The electric arc will corrode the metal surface of the contact, shorten the service life of the electrical appliance, extend the breaking time of the circuit, and cause contact failure of the contact. In serious cases, it can lead to fire or gas accidents.
When moving contact and static contact contact, external pressure must be applied to ensure good electrical contact between moving contact and static contact, which is usually called contact pressure of contact.
The contact pressure has the following functions:
Ensure the good contact between the moving contact and the static contact, and make the contact resistance as small as possible.
And (b) reduce the generation of surface oxide film and the pollution of contact surfaces.
Suppresses contact bounce.
The contact pressure of the contact is an important parameter of the contact of electrical products, which can only be selected after many tests in the initial design of the product. If the contact pressure of the contact is relatively small, it cannot meet the reliability requirements of the relay. If the contact pressure of the selected contact is relatively large, it is necessary to increase the operating power of the relay, and the requirements for the reaction spring also need to be improved, which is technically uneconomical.
Disconnectable contacts are an indispensable part of switching appliances. According to their different structures, they can be divided into the following types: (1) Knife contact: it has a simple structure and can be divided into surface contact and line contact, which are widely used in low-voltage switches and high-voltage disconnectors. (2) Butt joint contact: It has the characteristics of simple structure and fast action, but the contact surface is unstable and varies greatly with the pressure. It is easy to rebound during operation, without self-cleaning effect, and the contact is easy to be burned by electric arc. This contact is usually used for distribution circuit breakers with rated currents below 1000 A and below 500 A
With the development of high voltage, high current, miniaturization and long life of switching devices, the electrical properties of contact materials are required to be higher and higher. Nanometer contact materials have become a hot spot in the research and preparation of contact materials.
The material property of contact is one of the key factors affecting the working characteristics and electrical life of vacuum switchgear. Almost all early vacuum switches used tungsten, molybdenum and other refractory pure metals as contact materials. This contact material has a series of advantages, such as appropriate breaking performance, good compressive strength and welding resistance, appropriate thermal conductivity and conductivity, low ablation rate during arc combustion, long contact life, etc. It is widely used in breaking small current vacuum switch. However, high melting point pure metal contact materials have many shortcomings, such as breaking current and high cut-off level, which limit the application range of vacuum switches.