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The interrupter of a circuit breaker is responsible for extinguishing the arc. It is typically comprised of a fixed contact and a moving contact.
The main contact in a circuit breaker is the mechanical part within the interrupter that carries the load when closed and provides the electrical isolation in the open position. When in the closed position, a very low resistance [typically less than 300 µΩ on transmission breakers or less than 10 µΩ (!) on generator circuit breakers] is essential for a long service lifetime.
An arcing contact can be a separate contact or an integral part of the main contact. In SF6 circuit breakers the arcing and main contact are two different contacts whereas in vacuum CBs you have only one contact that serves as both the arcing and the main (current carrying) contact. For SF6 CBs where you have separate main and arcing contacts the arcing contact is the first to make on a closing operation and the last to part on an opening operation. The purpose of the arcing contact is to withstand the energy during switching and protect the main contact from damage during arcing. If the arcing contact is too short or otherwise in bad condition, the breaker becomes unreliable. The main contact surfaces can be degraded by arcing, resulting in increased resistance, excessive heating and in extreme cases, explosion.
According to a 2005 CIGRE CB survey, out of all faults identified in circuit breaker components, 14 percent were associated with the interrupter. (See recommended circuit breaker tests for testing designed to assess the condition of the main/arcing contacts.)