(1) In the closing operation should be a good conductor, not only can pass the load current for a long time, even through the short-circuit current, should also have sufficient thermal stability and dynamic stability.2. There should be good insulation in the tripping state.(3) There should be sufficient break-off capacity and as short a period as possible.(4) There should be as long as possible mechanical life and electrical life, and require simple structure, small size, light weight, easy installation and maintenance.