page_title: Electrical contacts
section_title: Arc snuffing
hierarchical_section_title: Electrical contacts / Arc snuffing
caption_attribution_description: Deutsch: Ein Hochleistungs- Freistrahlschalter vom Typ II von AEG. Baujahr 1970. Ausgestellt in Kempten im Allgäu.
context_page_description: An electrical contact is an electrical circuit component found in electrical switches, relays, connectors and circuit breakers. Each contact is a piece of electrically conductive material, typically metal. When a pair of contacts touch, they can pass an electrical current with a certain contact resistance, dependent on surface structure, surface chemistry and contact time; when the pair is separated by an insulating gap, then the pair does not pass a current. When the contacts touch, the switch is closed; when the contacts are separated, the switch is open. The gap must be an insulating medium, such as air, vacuum, oil, SF₆. Contacts may be operated by humans in push-buttons and switches, by mechanical pressure in sensors or machine cams, and electromechanically in relays. The surfaces where contacts touch are usually composed of metals such as silver or gold alloys that have high electrical conductivity, wear resistance, oxidation resistance and other properties.
context_section_description: When relay contacts open to interrupt a high current with an inductive load, a voltage spike will result, striking an arc across the contacts.  If the voltage is high enough, an arc may be struck even without an inductive load. Regardless of how the arc forms, it will persist until the current through the arc falls to the point too low to sustain it.  Arcing damages the electrical contacts, and a sustained arc may prevent the open contacts from removing power from the system being controlled.
  In AC systems, where the current passes through zero twice for each cycle, all but the most energetic arcs are extinguished at the zero crossing. The problem is more severe with DC where such zero crossings do not occur. This is why contacts rated for one voltage for switching AC frequently have a lower voltage rating for DC.
caption: Contacts of a high-voltage switch employing open air as an insulating medium
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