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Electrical contact
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An electrical contact is an electrical circuit component found in electrical switches, relays, connectors and circuit breakers.[1] 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;[2] 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. 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[3][4] that have high electrical conductivity, wear resistance, oxidation resistance and other properties.[5]
Materials
[edit]Contacts can be produced from a wide variety of materials. Typical materials include:[5]
- Silver alloys
- Gold
- Platinum-group metals
- Carbon[6]
Electrical ratings
[edit]Contacts are rated for the current carrying capacity while closed, breaking capacity when opening (due to arcing) and voltage rating. Opening voltage rating may be an AC voltage rating, DC voltage rating or both.[citation needed]
Arc suppression
[edit]
When relay contacts open to interrupt a high current with an inductive load, a voltage spike 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.[7]
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.[8]
Electrical contact theory
[edit]Ragnar Holm contributed greatly to electrical contact theory and application.[9]
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 , ballistic conduction of electrons occurs, resulting in a phenomenon known also as Sharvin resistance.[10] Contact force or pressure increases the size of the a-spot which decreases the constriction resistance and the electrical contact resistance.[11] 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.[2]
Relay contacts
[edit]
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.[12]
A normally closed (NC) 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 (NO) contact pair is open (in a non-conductive state) when it, or the device operating it, is in a deenergized state or relaxed state.[citation needed]
Contact form
[edit]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.[13] The following contact forms are particularly common:
Form A contacts
[edit]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.[12]
Form B contacts
[edit]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.[12]
Form C contacts
[edit]
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.[12]
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.[12]
Form D contacts
[edit]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.[12]
Form E contacts
[edit]Form E is a combination of form D and B.
Form K contacts
[edit]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.[12]
Form X contacts
[edit]
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 contactors and in toggle switches designed to handle high power inductive loads.[12]
Form Y contacts
[edit]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.[12]
Form Z contacts
[edit]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.[12]
Make-break order
[edit]
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 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.[citation needed]
See also
[edit]References
[edit]- ^ Relay Basics; Omron.
- ^ a b Zhai, C.; Hanaor, D.; Proust, G.; Gan, Y. (2015). "Stress-Dependent Electrical Contact Resistance at Fractal Rough Surfaces" (PDF). Journal of Engineering Mechanics. 143 (3): B4015001. doi:10.1061/(ASCE)EM.1943-7889.0000967.
- ^ Matsushita 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
- ^ "Mech Eng Term" (PDF). Panasonic.biz.
- ^ a b "Electrical Contact Materials". PEP Brainin. 2013-12-13. Archived from the original on 2017-03-05. Retrieved 2017-03-04.
- ^ Beurskens, Jack. "Contacts - Shin-Etsu Polymer Europe B.V." www.shinetsu.info. Archived from the original on 2019-11-13. Retrieved 2017-03-04.
- ^ "Contact Arc Phenomenon" (PDF). PickerComponents.com. Picker Components.
- ^ Chapter 4, Volume IV, Lessons in Electric Circuits, EETech Media, retrieved June 2017.
- ^ "IEEE Holm Conferences on Electrical Contacts". ieee-holm.org. Retrieved 2017-03-04.
- ^ Zhai, C; et al. (2016). "Interfacial electro-mechanical behaviour at rough surfaces" (PDF). Extreme Mechanics Letters. 9: 422–429. doi:10.1016/j.eml.2016.03.021.
- ^ Holm, Ragnar (1999). Electric Contacts: Theory and Applications (4th ed.). Springer. ISBN 978-3540038757.
- ^ a b c d e f g h i j 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 line here Archived 2017-07-05 at the Wayback Machine.
- ^ 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 line here Archived 2017-07-05 at the Wayback Machine.
Further reading
[edit]- Pitney, Kenneth E. (2014) [1973]. Ney Contact Manual - Electrical Contacts for Low Energy Uses (reprint of 1st ed.). Deringer-Ney, originally JM Ney Co. ASIN B0006CB8BC.[permanent dead link] (NB. Free download after registration.)
- Slade, Paul G. (2014-02-12) [1999]. Electrical Contacts: Principles and Applications. Electrical engineering and electronics. Vol. 105 (2 ed.). CRC Press, Taylor & Francis, Inc. ISBN 978-1-43988130-9.
{{cite book}}:|work=ignored (help) - Holm, Ragnar; Holm, Else (2013-06-29) [1967]. Williamson, J. B. P. (ed.). Electric Contacts: Theory and Application (reprint of 4th revised ed.). Springer Science & Business Media. ISBN 978-3-540-03875-7. (NB. A rewrite of the earlier "Electric Contacts Handbook".)
- Holm, Ragnar; Holm, Else (1958). Electric Contacts Handbook (3rd completely rewritten ed.). Berlin / Göttingen / Heidelberg, Germany: Springer-Verlag. ISBN 978-3-66223790-8.
{{cite book}}: ISBN / Date incompatibility (help) [1] (NB. A rewrite and translation of the earlier "Die technische Physik der elektrischen Kontakte" (1941) in German language, which is available as reprint under ISBN 978-3-662-42222-9.) - Huck, Manfred; Walczuk, Eugeniucz; Buresch, Isabell; Weiser, Josef; Borchert, Lothar; Faber, Manfred; Bahrs, Willy; Saeger, Karl E.; Imm, Reinhard; Behrens, Volker; Heber, Jochen; Großmann, Hermann; Streuli, Max; Schuler, Peter; Heinzel, Helmut; Harmsen, Ulf; Györy, Imre; Ganz, Joachim; Horn, Jochen; Kaspar, Franz; Lindmayer, Manfred; Berger, Frank; Baujan, Guenter; Kriechel, Ralph; Wolf, Johann; Schreiner, Günter; Schröther, Gerhard; Maute, Uwe; Linnemann, Hartmut; Thar, Ralph; Möller, Wolfgang; Rieder, Werner; Kaminski, Jan; Popa, Heinz-Erich; Schneider, Karl-Heinz; Bolz, Jakob; Vermij, L.; Mayer, Ursula (2016) [1984]. Vinaricky, Eduard; Schröder, Karl-Heinz; Weiser, Josef; Keil, Albert; Merl, Wilhelm A.; Meyer, Carl-Ludwig (eds.). Elektrische Kontakte, Werkstoffe und Anwendungen: Grundlagen, Technologien, Prüfverfahren (in German) (3 ed.). Berlin / Heidelberg / New York / Tokyo: Springer-Verlag. ISBN 978-3-642-45426-4.
Electrical contact
View on Grokipediafrom Grokipedia
An electrical contact is a conductive interface that establishes electrical continuity between two or more conductors, enabling the flow of current while facilitating functions such as making, breaking, or transferring circuits in devices like switches, relays, connectors, and circuit breakers.
These components are essential in electrical and electronic engineering for controlling power distribution, signal transmission, and system reliability across applications ranging from consumer electronics to high-voltage power systems. Key design considerations include minimizing contact resistance, which arises from constriction at the interface and surface films, to ensure efficient current transfer under varying loads and environmental conditions.[1]
Electrical contacts are typically classified by their operation, such as make contacts that close circuits, break contacts that open them, transfer contacts that switch between paths, and multiple contacts for complex switching.[2] Materials selection is critical, with high-conductivity options like silver and copper for low-resistance paths, often alloyed or plated with gold or tin for corrosion resistance and durability against wear from arcing or mechanical stress. Common failure modes, including fretting corrosion from vibration and arc-induced erosion during switching, underscore the need for robust engineering to maintain long-term performance.[3]
where is initial conductivity and is a power-related time constant. The Cassie model complements this for high-current phases, assuming constant arc voltage. These black-box models capture the dynamic transition from high to low conductivity during extinction.
Fundamentals
Definition and Principles
An electrical contact serves as a physical interface between two or more conductive parts, enabling the flow of electric current when the parts are connected and interrupting it upon separation. This interface is fundamental to devices such as switches, relays, connectors, and circuit breakers, where it functions to complete or break electrical circuits as needed.[4][5] The operational principles of electrical contacts rely on mechanical closure to establish connection, typically involving the application of force that causes elastic deformation of the contacting surfaces for intimate engagement. Due to inherent surface roughness on even polished conductors, the actual electrical contact does not occur across the entire apparent area but rather at discrete points known as asperities—the microscopic peaks and valleys on the surfaces. These asperities deform under pressure to form the real contact spots, which conduct current while the surrounding regions may be separated by thin insulating films or air gaps.[6][7][8] In electrical contacts, Ohm's law governs the relationship between voltage, current, and resistance at the interface, stating that the current through the contact is , where is the applied voltage and is the contact resistance. This equation highlights how contact resistance limits current flow, distinguishing point contacts at individual asperities from the broader area contact observed macroscopically, as the effective conducting area is significantly smaller—often 1-10% of the nominal surface area.[9][10][8] Electrical contacts are categorized by their motion relative to each other: stationary contacts, which remain fixed and are suited for low-power applications like signal relays handling currents below 10 A; sliding contacts, where one surface moves over the other to maintain connection; and wiping contacts, which incorporate a scrubbing motion during engagement to remove oxides or contaminants. Sliding and wiping types are particularly useful in high-power scenarios, such as motor controls or circuit breakers managing loads over 100 A, to ensure reliable conductivity under demanding conditions.[11][12][13][14]Historical Development
The invention of the electromagnetic relay in 1835 by American physicist Joseph Henry marked a pivotal early advancement in electrical contacts, enabling reliable signal transmission over long distances in telegraph systems through the use of mechanical armatures that made and broke contact.[15] This primitive relay design relied on basic mechanical contacts to control larger currents, laying the foundation for subsequent developments in switching technology. Mercury-wetted contacts, which improved speed and reduced arcing by coating the contact surfaces with mercury, were later patented in the 1930s by Charles Hatay for General Electric, enhancing performance in high-speed applications.[16] In the 1870s, Thomas Edison's innovations in arc lighting systems introduced more practical electrical contacts, utilizing carbon electrodes to sustain arcs for illumination, which influenced the design of early switches and relays.[17] By the late 19th century, silver emerged as a preferred material for contacts in the first practical electrical switches due to its superior conductivity and resistance to oxidation, enabling safer and more efficient operation in emerging power distribution networks.[18] The 1920s saw the introduction of tungsten and platinum alloys for electrical contacts in high-reliability relays, providing greater hardness, higher melting points, and better arc resistance compared to pure metals, which was critical for industrial and telecommunication uses. Reed relays, developed in 1936 by Walter B. Ellwood at Bell Telephone Laboratories, further advanced the field with hermetically sealed ferromagnetic reed contacts that offered compact size and low power consumption for telephone switching.[19] Post-World War II, the emergence of solid-state relays in the late 1960s, utilizing semiconductors to eliminate moving parts, significantly reduced the need for mechanical contacts in low- to medium-power electronics, though mechanical designs endured in high-power and high-voltage applications due to their robustness. During the 1950s, the International Electrotechnical Commission (IEC) played a key role in standardizing electrical contact configurations and terminology, promoting uniformity in relay and switch designs across international markets.[20] Up to 2025, innovations in nanomaterial coatings, such as those incorporating carbon nanotubes or nanoparticles, have been applied to electrical contacts to minimize wear, enhance conductivity, and extend lifespan in demanding environments.[21] Concurrently, graphene-based contacts have gained traction for their ultra-low resistance—often achieving values below 10 Ω·μm²—enabling efficient performance in next-generation flexible electronics and high-speed devices.[22]Materials
Key Properties
Electrical properties of materials for electrical contacts are paramount for reliable performance, with high electrical conductivity—measured in siemens per meter (S/m)—being essential to minimize contact resistance and enable efficient current flow without significant energy loss.[23] This property ensures that the contact interface supports high current densities while keeping voltage drops low. Additionally, sufficient dielectric strength is required to prevent electrical breakdown across the open contact gap, avoiding unintended arcing or flashover under high-voltage conditions.[24] Mechanical properties directly influence the longevity and reliability of contacts under operational stresses. Hardness, typically evaluated on the Vickers scale, provides resistance to surface deformation and wear from mating forces and cycling. Elasticity supports spring-back mechanisms that maintain consistent contact pressure over time, while fatigue resistance allows the material to withstand millions of open-close cycles without cracking or permanent deformation. Thermal properties are critical for managing heat generated during operation, particularly from arcing events. A high melting point prevents material erosion or fusion at localized hot spots, preserving the contact's integrity. Complementing this, high thermal conductivity facilitates rapid heat dissipation away from the interface, reducing the risk of overheating and thermal runaway.[25] Environmental properties ensure sustained performance in diverse conditions. Strong corrosion resistance and oxidation prevention are vital to avoid the formation of insulating layers that increase resistance or cause intermittent failures. Materials must also demonstrate compatibility with vacuum or gaseous atmospheres, where reduced oxygen can alter wear patterns but may exacerbate issues like welding if not properly addressed.[26] Material selection involves inherent trade-offs, such as balancing relative softness—which promotes low initial resistance and conformability—for optimal electrical performance against sufficient hardness for durability and resistance to mechanical wear. Failure to achieve this balance can result in fretting corrosion, a common degradation mode where minute relative motions at the interface remove protective oxide films, leading to accelerated oxidation and rising contact resistance.[27] These properties are rigorously evaluated through standardized testing protocols from bodies like ASTM and IEC. For instance, ASTM E1004 outlines electromagnetic methods for assessing electrical conductivity in nonmagnetic metals, while IEC 60947 series provides guidelines for evaluating contact performance in low-voltage devices, including endurance and environmental exposure tests.[28]Common Materials and Alloys
Electrical contacts are commonly fabricated from precious metals, base metal alloys, refractory materials, and composites, each selected for specific performance needs in switching devices, relays, and circuit breakers. Silver (Ag) is the most widely used precious metal for electrical contacts due to its superior electrical and thermal conductivity, making it ideal for high-current applications where low resistance is critical. Pure silver or alloys like silver-copper (Ag-Cu) enhance mechanical strength while maintaining high conductivity, often employed in relays and switches. However, silver is susceptible to tarnishing in humid environments, which can increase contact resistance over time. Gold (Au) is preferred for low-voltage, low-current contacts, such as in electronics and connectors, owing to its exceptional corrosion resistance and stable performance even in harsh atmospheres; it is typically applied as a thin plating (3-10 µm) over base metals to minimize costs. Despite these benefits, gold's high cost and tendency to wear under frequent cycling limit its use to signal-level applications. Base metal alloys like copper-tungsten (CuW) are engineered for high-current arcing environments, combining copper's conductivity with tungsten's high melting point and arc erosion resistance; typical compositions range from 50-75% tungsten, used in circuit breakers and welding equipment. Silver-cadmium oxide (AgCdO), with 10-15% cadmium oxide, was historically favored for its excellent arc quenching and weld resistance in inductive loads like motors, but due to its toxicity and restrictions imposed by the RoHS regulations (effective 2006), it has been increasingly replaced by alternatives like silver-tin oxide (AgSnO₂) with 8-12% tin oxide, which offers comparable arc resistance without environmental hazards, although exemptions have permitted continued use in certain applications as of 2025.[29] AgSnO₂ is now standard in contactors and relays for high inrush currents, though it may exhibit higher wear under DC loads. Refractory materials such as tungsten (W) and molybdenum (Mo) provide durability in heavy-duty applications involving severe arcing, like welding electrodes and high-power switches; pure tungsten or molybdenum contacts withstand temperatures exceeding 3,000°C but have lower conductivity, necessitating higher contact forces. Composites expand these properties: silver-graphite (AgC), typically 95-97% silver with graphite, is used in slip rings and stationary contacts for its anti-welding characteristics and lubricity, reducing friction in rotating applications. Silver-nickel (AgNi), with 10-40% nickel, suits general-purpose relays under resistive loads, balancing conductivity, erosion resistance, and cost. The primary limitations of precious metals include their elevated cost, which drives the use of alloys and thin coatings, while cadmium-based materials face regulatory bans due to toxicity, accelerating adoption of eco-friendly options like AgSnO₂. Manufacturing techniques tailor these materials: cladding bonds thin layers of precious metals (e.g., gold or silver) to base substrates via rolling or welding for cost efficiency; sintering, often via press-sinter-repress or liquid-phase methods, produces dense composites like CuW or AgW by compacting powders at high temperatures below melting points; infiltration fills porous refractory skeletons (e.g., tungsten) with molten copper or silver for enhanced conductivity; and coating via electroplating or vacuum deposition applies protective layers like gold to prevent corrosion.| Material/Alloy | Typical Composition | Key Advantages | Primary Applications | Limitations |
|---|---|---|---|---|
| Silver (Ag) or Ag-Cu | 90-100% Ag, balance Cu | High conductivity, arc erosion resistance | Relays, switches | Tarnishing, cost |
| Gold (Au) plating | 3-10 µm Au over base | Corrosion resistance, low-voltage stability | Connectors, signal contacts | High cost, wear |
| Copper-Tungsten (CuW) | 50-75% W, balance Cu | Arc resistance, thermal management | Circuit breakers, arcing contacts | Oxide formation in air |
| Silver-Tin Oxide (AgSnO₂) | 88-92% Ag, 8-12% SnO₂ | Weld resistance, eco-friendly | Contactors, motors | DC wear |
| Tungsten (W) | Pure W or Mo | High melting point, durability | Welding equipment | Low conductivity |
| Silver-Nickel (AgNi) | 60-90% Ag, 10-40% Ni | Erosion resistance, durability | General relays | Oxidation at high temps |
| Silver-Graphite (AgC) | 95-97% Ag, balance C | Anti-welding, lubricity | Slip rings, stationary contacts | Higher erosion |
Electrical Theory
Contact Resistance
Contact resistance in electrical contacts arises primarily from the constriction of current flow through a limited number of small contact spots, known as a-spots, on otherwise rough mating surfaces, where the actual conducting area is much smaller than the apparent contact area. This phenomenon, termed constriction resistance, occurs because the current lines must crowd together to pass through these microscopic spots, increasing the effective path length and thus the resistance. The foundational theory for calculating this resistance was developed by Ragnar Holm in the mid-20th century, building on earlier work by James Clerk Maxwell. For a single circular a-spot of radius between two semi-infinite conductors of the same material with resistivity , the total constriction resistance is given by where the factor of accounts for the spreading resistance in each conductor ( per side). This formula derives from solving Laplace's equation for the electric potential, subject to boundary conditions of uniform current far from the spot and equipotential on the spot, as originally posed by Maxwell in 1904 for the spreading resistance problem. Holm extended this to practical contact scenarios by considering the geometry of metallic contacts.[30] For real contacts involving multiple a-spots, the total resistance is lower due to parallel conduction paths, but interactions between spots complicate exact calculation. J.A. Greenwood provided a seminal approximation for a large number of randomly distributed, identical a-spots, yielding assuming the spots are sufficiently separated relative to their size; this reflects the statistical averaging of constriction effects across the cluster. More precise models account for spot interactions via elliptic integrals, but the square-root dependence captures the scaling for typical rough surfaces. Several factors influence contact resistance. The radius of each a-spot depends on the applied contact force through Hertzian contact mechanics for elastic deformation of asperities, where for spherical asperities on a flat surface, leading to . Surface films, such as oxide layers, add a film resistance component; thick films (>10 nm) block conduction, while thin films on noble metals like gold enable electron tunneling, maintaining low resistance via quantum mechanical transmission through the barrier. Temperature affects resistance through the material's resistivity , which typically increases linearly with temperature () for metals, thus raising . Material conductivity, governed by , directly scales the resistance magnitude.[31] Contact resistance is commonly measured using the four-point probe method, which applies a known current through outer probes and measures voltage drop across inner probes directly at the contact to eliminate lead and bulk resistances. For clean metal contacts under moderate pressure (e.g., 10-100 N/cm²), typical values range from 1 to 10 mΩ, depending on material and force; higher values indicate contamination or poor contact. Arcing can temporarily alter surface conditions and thus resistance, as explored in related theories.[32][9]Arcing Phenomena
Arcing in electrical contacts occurs when the voltage across separating or separating contacts exceeds the breakdown voltage of the intervening medium, leading to a plasma discharge that sustains current flow despite the physical gap. This phenomenon is a primary failure mode in switching devices, as it dissipates significant energy and degrades contact performance. The arc initiates as a transition from metallic conduction through the contact bridge to a gaseous phase discharge, where ionized gas or vapor maintains the current path. During contact opening under current, a molten metal bridge initially sustains conduction; its rupture releases metal vapor, facilitating arc formation even at low circuit voltages through vapor ionization rather than pure gas breakdown.[33] Arc types in electrical contacts include cathode spot arcs, anodic arcs, and glow discharges, distinguished by their current density, voltage characteristics, and electrode interactions. Cathode spot arcs feature localized, high-current-density regions on the cathode surface, typically 1-10 μm in diameter, where intense heating causes explosive electron emission and metal vaporization. Anodic arcs involve similar spots on the anode but with different heat distribution due to ion bombardment. In contrast, glow discharges exhibit lower current densities and higher voltages, serving as precursors to full arcs in low-current scenarios. These types arise during the transition from metallic to gaseous phase, where initial bridge rupture releases vapor that ionizes under the electric field. Arc formation is governed by the breakdown voltage in the gap, particularly for small separations. According to Paschen's law, the breakdown voltage in air at atmospheric pressure reaches a minimum of approximately 300–350 V for pressure-distance products (pd) around 0.75 torr·cm, corresponding to gaps of about 10 μm. For initial small gaps during contact opening, pure gas breakdown is rare in typical low-voltage applications; instead, arcs initiate via metal vapor from the rupturing molten bridge.[34][35] The energy in an arc is characterized by its power , where typically ranges from 10-20 V, depending on contact material and arc type. For instance, arcs on fine silver contacts sustain at approximately 12 V, while those on cadmium reach 10 V. This low voltage, combined with circuit current , generates intense heat flux (up to W/m² at spots), causing localized melting, pitting on the cathode, and material transfer to the anode due to ion and electron flows. Consequences of arcing include contact erosion, welding, and electromagnetic interference. Erosion results from uneven heat distribution, where the Wiedemann-Franz law dictates that thermal conductivity scales with electrical conductivity (, with Lorenz number W Ω K⁻²), leading to preferential heating and vaporization at high-current spots. Welding occurs when molten material from arcing bridges the gap upon closure, fusing contacts and preventing reliable operation. Arcing also generates broadband electromagnetic interference through rapid voltage transients and plasma oscillations, potentially disrupting nearby electronics. Key factors influencing arcing include current level, atmosphere, and contact bounce. A minimum arc current of approximately 0.1 A is required to sustain the discharge in air, below which it extinguishes due to insufficient ionization. In vacuum environments, arcs differ fundamentally, relying on metal vapor from electrode evaporation rather than ambient gas, resulting in more diffuse plasmas and higher erosion rates without external medium support. Contact bounce during closure induces short arcs (bounces lasting 0.1-10 ms), amplifying erosion through repeated low-energy discharges. Arc behavior is often modeled using the Mayr and Cassie equations to describe voltage-time characteristics via conductivity . The Mayr model, suitable for post-current-zero decay, is given bywhere is initial conductivity and is a power-related time constant. The Cassie model complements this for high-current phases, assuming constant arc voltage. These black-box models capture the dynamic transition from high to low conductivity during extinction.