HOME

TheInfoList



OR:

An electrical contact is an electrical circuit component found in electrical
switch In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type of ...
es,
relay A relay Electromechanical relay schematic showing a control coil, four pairs of normally open and one pair of normally closed contacts An automotive-style miniature relay with the dust cover taken off A relay is an electrically operated switch ...
s, connectors and
circuit breaker A circuit breaker is an electrical safety device designed to protect an electrical circuit from damage caused by an overcurrent or short circuit. Its basic function is to interrupt current flow to protect equipment and to prevent the ris ...
s. Each contact is a piece of electrically conductive material, typically
metal A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...
. When a pair of contacts touch, they can pass an electrical current with a certain
contact resistance The term contact resistance refers to the contribution to the total resistance of a system which can be attributed to the contacting interfaces of electrical leads and connections as opposed to the intrinsic resistance. This effect is describe ...
, 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 Currents, Current or The Current may refer to: Science and technology * Current (fluid), the flow of a liquid or a gas ** Air current, a flow of air ** Ocean current, a current in the ocean *** Rip current, a kind of water current ** Current (stre ...
. 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 In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type of ...
, 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 alloysMatsushita Electronics, "Relay Techninal Information: Definition of Relay Terminology", § Contact, http://media.digikey.com/pdf/other%20related%20documents/panasonic%20other%20doc/small%20signal%20relay%20techincal%20info.pdf that have high electrical conductivity, wear resistance, oxidation resistance and other properties.


Contact states

A ''
normally closed In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common typ ...
'' (') 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 In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type of ...
'' (') 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 forms of electrical contact found in relays and switches.Section 1.6, Engineers' Relay Handbook, 5th ed, Relay and Switch Industry Association, Arlington, VA; 3rd ed, National Association of Relay Manufacturers, Elkhart Ind., 1980; 2nd Ed. Hayden, New York, 1966; large parts of the 5th edition are on lin
here
.
Of these contact forms, the following 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.


Form B contacts

''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.


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.


Form D contacts

''Form D'' contacts ("continuity transfer" contacts) differ from ''Form C'' in only one regard, the make-break order during transition. Where ''Form C'' guarantees that, briefly, both connections are open, ''Form D'' guarantees that, briefly, all three terminals will be connected. This is a relatively uncommon configuration.


Form K contacts

''Form K'' contacts (center-off) differ from ''Form C'' in that there is a center-off or normally-open position where neither connection is made. SPDT toggle switches with a center off position are common, but relays with this configuration are relatively rare.


Form X contacts

''Form X'' or double-make contacts are equivalent to two ''Form A'' contacts in series, mechanically linked and operated by a single actuator, and can also be described as SPST-NO contacts. These are commonly found in
contactor :''In semiconductor testing, contactors can also be referred to as the specialized socket that connects the device under test.'' :''In process industries, a contactor is a vessel where two streams interact, for example, air and liquid. See Gas ...
s and in toggle switches designed to handle high power inductive loads.


Form Y contacts

''Form Y'' or double-break contacts are equivalent to two ''Form B'' contacts in series, mechanically linked and operated by a single actuator, and can also be described as SPST-NC contacts.


Form Z contacts

''Form Z'' or double-make double-break contacts are comparable to ''Form C'' contacts, but they almost always have four external connections, two for the normally open path and two for the normally closed path. As with forms ''X'' and ''Y'', both current paths involve two contacts in series, mechanically linked and operated by a single actuator. Again, this is also described as an SPDT contact.


Make break order

Where a switch contains both normally open (NO) and normally closed (NC) contacts, the order in which they make and break may be significant. In most cases, the rule is ''
break-before-make In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common typ ...
'' or ''B-B-M''; that is, the NO and NC contacts are never simultaneously closed during the transition between states. This is not always the case, ''Form C'' contacts follow this rule, while the otherwise equivalent ''Form D'' contacts follow the opposite rule, make before break. The less common configuration, when the NO and NC contacts are simultaneously closed during the transition, is '' make-before-break'' or ''M-B-B''.


Electrical ratings

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.


Arc snuffing

When relay contacts open to interrupt a high current with an
inductive load In electrical engineering, the power factor of an AC power system is defined as the ratio of the '' real power'' absorbed by the load to the ''apparent power'' flowing in the circuit. Real power is the average of the instantaneous product of ...
, a
voltage spike In electrical engineering, spikes are fast, short duration electrical transients in voltage (voltage spikes), current (current spikes), or transferred energy (energy spikes) in an electrical circuit. Fast, short duration electrical transients ( ...
will result, striking an arc across the contacts. If the voltage is high enough, an arc may be struck even without an inductive load. Regardless of how the arc forms, it will persist until the current through the arc falls to the point too low to sustain it. Arcing damages the electrical contacts, and a sustained arc may prevent the open contacts from removing power from the system being controlled. 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.


Materials

Contacts can be produced from a wide variety of materials. Typical materials include: *
Silver Silver is a chemical element with the symbol Ag (from the Latin ', derived from the Proto-Indo-European ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, white, lustrous transition metal, it exhibits the highest electrical ...
alloys *
Gold Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile me ...
* Platinum-group metals *
Carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon mak ...


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 Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current. A low resistivity indicates a material that readily allows ...
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 The term contact resistance refers to the contribution to the total resistance of a system which can be attributed to the contacting interfaces of electrical leads and connections as opposed to the intrinsic resistance. This effect is describe ...
, 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 \lambda,
ballistic conduction In mesoscopic physics, ballistic conduction (ballistic transport) is the unimpeded flow (or transport) of charge carriers (usually electrons), or energy-carrying particles, over relatively long distances in a material. In general, the resistivity ...
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.


See also

*
Contact bounce In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common typ ...
* Latching relay *
Wetting current In electrical and electronics engineering, wetting current is the minimum electric current needing to flow through a contact to break through the surface film resistance at a contact. It is typically far below the contact's nominal maximum curr ...
* Wetting voltage * Electrical splice


References


Further reading

* (NB. Free download after registration.) * * (NB. A rewrite of the earlier "''Electric Contacts Handbook''".) *
https://web.archive.org/web/20180520151548/http://www.gbv.de/dms/ilmenau/toc/176968075.PDF -->
(NB. A rewrite and translation of the earlier "''Die technische Physik der elektrischen Kontakte''" (1941) in German language, which is available as reprint under .) * {{cite book , editor-first1=Eduard , editor-last1=Vinaricky , editor-first2=Karl-Heinz , editor-last2=Schröder , editor-first3=Josef , editor-last3=Weiser , editor-first4=Albert , editor-last4=Keil , editor-first5=Wilhelm A. , editor-last5=Merl , editor-first6=Carl-Ludwig , editor-last6=Meyer , author-first1=Manfred , author-last1=Huck , author-first2=Eugeniucz , author-last2=Walczuk , author-first3=Isabell , author-last3=Buresch , author-first4=Josef , author-last4=Weiser , author-first5=Lothar , author-last5=Borchert , author-first6=Manfred , author-last6=Faber , author-first7=Willy , author-last7=Bahrs , author-first8=Karl E. , author-last8=Saeger , author-first9=Reinhard , author-last9=Imm , author-first10=Volker , author-last10=Behrens , author-first11=Jochen , author-last11=Heber , author-first12=Hermann , author-last12=Großmann , author-first13=Max , author-last13=Streuli , author-first14=Peter , author-last14=Schuler , author-first15=Helmut , author-last15=Heinzel , author-first16=Ulf , author-last16=Harmsen , author-first17=Imre , author-last17=Györy , author-first18=Joachim , author-last18=Ganz , author-first19=Jochen , author-last19=Horn , author-first20=Franz , author-last20=Kaspar , author-first21=Manfred , author-last21=Lindmayer , author-first22=Frank , author-last22=Berger , author-first23=Guenter , author-last23=Baujan , author-first24=Ralph , author-last24=Kriechel , author-first25=Johann , author-last25=Wolf , author-first26=Günter , author-last26=Schreiner , author-first27=Gerhard , author-last27=Schröther , author-first28=Uwe , author-last28=Maute , author-first29=Hartmut , author-last29=Linnemann , author-first30=Ralph , author-last30=Thar , author-first31=Wolfgang , author-last31=Möller , author-first32=Werner , author-last32=Rieder , author-first33=Jan , author-last33=Kaminski , author-first34=Heinz-Erich , author-last34=Popa , author-first35=Karl-Heinz , author-last35=Schneider , author-first36=Jakob , author-last36=Bolz , author-first37=L. , author-last37=Vermij , author-first38=Ursula , author-last38=Mayer , title=Elektrische Kontakte, Werkstoffe und Anwendungen: Grundlagen, Technologien, Prüfverfahren , language=de , edition=3 , location=Berlin / Heidelberg / New York / Tokyo , publisher=
Springer-Verlag Springer Science+Business Media, commonly known as Springer, is a German multinational publishing company of books, e-books and peer-reviewed journals in science, humanities, technical and medical (STM) publishing. Originally founded in 1842 ...
, date=2016 , orig-year=1984 , isbn=978-3-642-45426-4 Electrical components