An electromagnetic relay with a pair of contacts|thumb|upright
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; that have high electrical conductivity, wear resistance, corrosion resistance, oxidation resistance and other properties.
Materials
Contacts can be produced from a wide variety of materials. Typical materials include:
- Copper
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.
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, or ECR (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. For contact spots of radii smaller than the mean free path of electrons <math>\lambda</math>, ballistic conduction of electrons occurs, resulting in a phenomenon known also as Sharvin resistance. Contact force or pressure increases the size of the a-spot which decreases the constriction resistance and the electrical contact resistance. When the size of contacting asperities becomes larger than the mean free path of electrons, Holm-type contacts become the dominant transport mechanism, resulting in a relatively low contact resistance.
Relay contacts
thumb|Schematic for an electromechanical relay showing a coil, four pair of [[normally open and one pair of normally closed contacts]]
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.
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.
Contact form
The National Association of Relay Manufacturers and its successor, the Relay and Switch Industry Association define 23 distinct electrical contact forms found in relays and switches. The following contact forms are particularly common:
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.