Why should the electrical alloy contact be inlaid with a piece of pure silver block or silver based alloy block?

The contact surface of the contact will oxidize when contacted with the atmosphere. If the copper is oxidized, the surface will produce copper oxide or cuprous oxide film. The conductivity of copper oxide film is very poor, and its resistivity is more than 1016 times larger than that of pure copper. Therefore, the contact resistance of copper contacts is very large, and the temperature rise is not allowed when passing through the current.
Electrical alloy contact
Although the silver oxide film resistivity is also great, but compared with the copper oxide film, in its 1/1012, and in not too high temperature will disintegrate, decomposition, therefore, unless can use mechanical friction rotating structure to remove the oxide film, usually without copper contact. Taking into account the silver arc resistance, anti wear and anti welding ability is low, so the electrical contact current over 40A using silver alloy (such as silver nickel, silver tungsten, silver cadmium oxide, silver tin oxide contact, etc.) and to insert form Brazing on the connecting plate. In the relay type of weak current apparatus, the precious metal, such as gold, molybdenum and platinum, is also used to make contact insert.
Understood the above reason, also knew why, when the block has been seriously worn, the contact will not allow work.