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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.
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.
There are several common arrangements of electrical contacts.
Form A contacts
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
Form C contacts
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). An alternate notation for Form C is SPDT.
These contacts are quite frequently found in electrical switches and relays as the common contact element provides a mechanically economical method of providing a higher contact count.
Make break order
The order in which make and break occurs in a group of N.O. and N.C. contacts, particularly Form C contacts, is usually non-overlapping. That is, the N.O. and N.C. contacts are never simultaneously closed during the transition between states. This is called break-before-make or BBM. The less common configuration, when the N.O. and N.C. contacts are simultaneously closed during the transition, is make-before-break or MBB.
Contacts are rated for the current carrying capacity while closed and the voltage breaking capacity when opening (due to arcing) or while open. Opening voltage rating may be an A.C. voltage rating, D.C. voltage rating or both. Some manufacturers offer guaranteed minimum operation counts based on accelerated testing. If contact life is specified, it is usually a number of operations at a specific electrical load. This is because electrical contact wear is associated with the contacts opening and closing.
In high current and high voltage applications a secondary contact may be used to transfer load to a load break mechanism in order to avoid arcing wear and tear of the main contact.
Many other methods to avoid arcing of contacts are used including air blasts, magnetic forces, elongation of arc, alternate resistive current paths and various combinations of these methods.
Electrical contact theory
Ragnar Holm contributed greatly to electrical contact theory and application.
Macroscopically smooth and clean surfaces are microscopically rough and, in air, contaminated with oxides, adsorbed water vapor, and atmospheric contaminants. When two metal electrical contacts touch, the actual metal-to-metal contact area is small compared to the total contact-to-contact area physically touching. In electrical contact theory, the relatively small area where electrical current flows between two contacts is called the a-spot where "a" stands for asperity. If the small a-spot is treated as a circular area and the resistivity of the metal is homogeneous, then the current and voltage in the metal conductor has spherical symmetry and a simple calculation can relate the size of the a-spot to the resistance of the electrical contact interface. If there is metal-to-metal contact between electrical contacts, then the electrical contact resistance (as opposed to the bulk resistance of the contact metal) is mostly due to constriction of the current through a very small area, the a-spot. Contact force or pressure increases the size of the a-spot which decreases the constriction resistance and the electrical contact resistance.
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