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Concept
Electrical contact
Summary
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.
A normally closed () contact pair is closed (in a conductive state) when it, or the device operating it, is in a deenergized state or relaxed state.
A normally open () contact pair is open (in a non-conductive state) when it, or the device operating it, is in a deenergized state or relaxed state.
The National Association of Relay Manufacturers and its successor, the Relay and Switch Industry Association define 23 distinct forms of electrical contact found in relays and switches. Of these contact forms, the following are particularly common:
Form A contacts ("make contacts") are normally open contacts. The contacts are open when the energizing force (magnet or relay solenoid) is not present. When the energizing force is present, the contact will close. An alternate notation for Form A is SPST-NO.
Form B contacts ("break contacts") are normally closed contacts. Its operation is logically inverted from Form A. An alternate notation for Form B is SPST-NC.
Official source
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