10 Contact materials

Abstract

Transmission of electric power and information involves the passage of electric current through numerous devices and components that are joined to one another directly or via wires and leads. The path for the power transmission from the generator to the consumer contains about 1000 contact points, of which app. 100 are switching contacts. These switching contacts must switch the current on (make operation), conduct the current without loss when switched on (current carrying) and must insulate the current paths from one another when switched off (break operation). The currents to be switched range from a few milliamps to 100,000 A. Contact components on power engineering in most cases consist of silver-based composite materials. Those materials are used in numerous electrical switching devices and switchgear [67H, 73H, 74E, 84K, 94S, 95F, 96F, 97S1, 99S, 02V] such as high-voltage switchgear, industrial and installation switchgear, switches for domestic equipment, switchgear for railways and electrically powered vehicles and switches for motor vehicles. For low current applications in power engineering AgCuand AgNi-alloys will be used. Those materials are manufactured by melting and casting and therefore will not be described here. Apart from materials used in power engineering even precious metal alloys based on gold and platinum are used for contact components in communication and information technology. However those materials are in general produced by conventional melting metallurgy and will also not be described here. Switching contacts have to satisfy many requirements. 1. During make operation an arc may be formed after springback of the contacts, this is termed bouncing. As a result the contact surfaces heat up extremely and may weld together when they finally close. This welding tendency of contact materials should be minimised. 2. In current carrying the contact surfaces only touch one another at a few points, resulting in a current constriction and thus in an electrical contact resistance that causes heating of the contact components. The contact resistance and thus the heating effect should be minimised. 3. During break operation an arc is formed at the contacts resulting in a high thermal load of the contact material. The contact material must have a low erosion rate and thus long service life. 4. After the break operation and the extinguishing of the arc there must not occur an arc reignition. Thus the contact material must not have constituents unfavourable for good arc quenching properties.