Electrical contact

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, usually metal. When a pair of contacts touch, they can pass an electrical current; 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 or other electrically insulating fluid. 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. Beyond the contact surface and immediate area, the electrical contact is usually composed of other metals for mechanical and cost reasons.

Even when contacts are separated, there may be conduction due to insulation resistance, arcing or a glow discharge.

A normally closed (N.C.) contact pair is closed (in a conductive state) when it, or the device operating it, is in a de-energized state or relaxed state.

A normally open (N.O.) contact pair is open (in a non-conductive state) when it, or the device operating it, is in a de-energized state or relaxed state. An example is the common door-bell push button.

In some devices there is not a specific de-energized or relaxed state. Latching relays and toggle switches have two relaxed states. For such devices, the meaning of N.C. and N.O. is not well defined.

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