Switching capacity of electrical contacts

The material properties of contacts are one of the key factors that affect 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 suitable breaking performance, good compressive strength and welding resistance, suitable thermal conductivity and electrical conductivity, low ablation rate during arc combustion, contact, long life, etc. 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 limits the application range of vacuum switches.

Switching capacity of electrical contacts:
According to the theory of arc and electrical contact, vacuum switches with high switching capacity should generally have:
1. When the current is cut off, there is no arc concentration, and the arc diffusion is distributed on the entire contact surface;
2. After high current breakdown, there is no local overheating area on the contact surface, which is conducive to the rapid recovery of dielectric strength;
3. It has a high breaking speed, especially the initial breaking speed, to avoid the initial stagnation time of the arc is too long.

Type of vacuum contact material and alloying principle:
It is difficult for a single pure metal to meet all the requirements of the contact. With the development of vacuum switchgear to large capacity, contact materials have experienced the development process from single metal to binary, and then to multi-component composite materials. On different occasions, the requirements for contact materials are also emphasized to varying degrees. There are many contradictions in the requirements of the electrical characteristics of the contact on the physical properties of the material.
1. Low cut-off level requires materials with low electrical and thermal conductivity and low melting point. However, high fracture ability, solder resistance and electrical wear resistance require opposite properties.
Materials with welding resistance are generally brittle and poor in strength, which contradicts the requirements of contact for mechanical strength, deformation resistance and electrical wear resistance.