Electrical contacts
An electrical contact is an electrical circuit component found in electrical switches, relays and circuit breakers. Each contact is a piece of electrically conductive 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. Contact materials are usually composed of superior conduction materials such as silver or gold. Cheaper metals may be used to reduce costs for the main contact bump and plated with superior metals.
Even when contacts are separated, there may be a conductive path due to 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 of a normally open contact usage is the common door-bell push button.
Where the state of the operating device can be latched in either state and therefore the normal state is not clearly defined a different style of contact definitions may be used.
A 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.
A Form B contacts ("break contacts") are normally closed contacts. Its operation is logically inverted from Form A.
A Form C contacts ("change over" or "transfer" contacts) are composed of a normally closed contact pair and a normally open contact pair that are operated by the same device; there is a common electrical connection between a contact of each pair that results in only three connection terminals. These terminals are usually labelled as normally open, common, and normally closed (NO-C-NC).
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