Contacts are the places where arcs are generated and extinguished, which require high materials and structures.
4.1 The requirements for contact materials are as follows:
(1) High interruption capability. The material itself is required to have high conductivity, low thermal conductivity, large thermal capacity and low thermal electron emission capability.
(2) High breakdown voltage. If the breakdown voltage is high, the dielectric recovery strength will be high, which is beneficial to arc extinguishing.
(3) High resistance to electric corrosion. That is to say, it can withstand the ablation of arc and has less metal evaporation.
(4) Anti-welding ability.
(5) Low cut-off current, expected to be less than 2.5A.
(6) Low gas content. Low gas content is the requirement of all materials used in vacuum interrupter. Especially for copper, oxygen-free copper must be treated by special process with low gas content. The solder is made of silver and copper alloys.
At present, most contact materials of vacuum interrupter for circuit breaker are copper-chromium alloy, with copper and chromium accounting for 50% respectively. A copper-chromium alloy sheet is welded on the butt surfaces of the upper and lower contacts with a thickness of 3 mm. The rest is called contact base, which can be made of oxygen-free copper.
4.2 Contact Structure
The contact structure has a great influence on the interruption ability of the isolation chamber. The arc-extinguishing effect of different structure contacts is different. In the early stage, simple cylindrical contacts were used. Although the structure was simple, the breaking ability could not meet the requirements of circuit breakers. Only the current below 10 kA could be interrupted. At present, only vacuum switches such as vacuum load switches and high-voltage vacuum contactors were used. At present, three kinds of contacts are commonly used: spiral groove type contacts, cup type contacts with inclined grooves and cup type contacts with longitudinal magnetic field. Among them, cup type contacts with longitudinal magnetic field are mainly used.
