===<!--6.1 --> Application Ranges for Switching Contacts===

===<!--6.1.1 -->Low and Medium Electrical Loads===Switching processes at low and medium electrical loads are experienced forexample in relays and switches for the measuring technology, telecommunications,automotive usage, and appliances. The switching voltage ranges fromμV to 400V with currents between μA and about 100A.

Guided by empirically developed arc-limiting graphs typical switchingprocesses can be distinguished. As Fig. 6.1 illustrates, voltage Main Article: [[Low and currentdetermine if switching occurs without arcing, results in a glow discharge, shortinstable arcs are generated, or a fully developed electrical arc is created. Themore exact current-voltage curve characte-ristics are depending on theelectrical contact material used. They also depend on the contact gap Medium Electrical Loads| Low and theatmosphere the switching occurs in; an ambient air atmosphere is assumed inthe shown schematic example.Medium Electrical Loads]]

Fig. ===<!--6.1:Arc.2--limiting graphs (schematic)>High Electrical Loads===1With high electrical loads, as usually occuring in power engineering equipment, the switching phenomena are usually due to arcing. Arc-less For most applications the management of the switching2arc is the key problem. Short instable arcs3Depending on the device type, different requirements are dominant, which influence the selection of the contact material. Glow discharge4Similar to those in communications engineering, issues related to the switching characteristics and current path have to be considered. Full electrical arcs

For the different requirements on the electrical contacts in various applications itis useful to differentiate across the broad spectrum of possible load conditionsguided by the arc-limiting graphs between four different partial ranges which result intypical physical effectsMain Article:[[High Electrical Loads| High Electrical Loads]]

U < 80mVfigtable id="tab:Material Selection and Contact Component Design">I < 10mAcaption>'''<!--Table 6.1:-->Material Selection and Contact Component Design'''</caption>

*Intermediate Level {| class="twocolortable" style="text-align: left; font-size: 12px"|-!Type of Contacts or Devices !Characteristic Requirements for Contacts!Contact Material!Design Form of Contacts|-|Automatic staircase lighting switches|High arc erosion and weld resistance|Ag/Ni, Ag/SnO₂, (Ag/CdO), Ag/C against Ag/SnO₂|Rivets, welded contact parts|-|Miniature Circuit breakers|Extremely high weld resistance, low temperature rise in use, sufficient arc erosion resistance|I< 50 A: Ag/C97/3 (Cu/C) against Cu, I> 50 A : Ag/C97/3 o. 95/5 against AgCu3, Ag/Ni90/10 o. 80/20, Ag/W, Ag/WC (USA)|Welded contact parts (Ag/C), clad stamped parts|-|Fault current circuit breakers|Extremely high weld resistance, low contact resistance, high arc erosion resistance|Stationary contact: Ag/C96/4 o. 95/5 Movable contact: Ag/Ni, Ag/MeO, Ag/W, Ag/WC, Ag/WC/C|Welded and brazed contact parts|-|Micro snap switches|Low contact resistance, no sticking during make operation|AgNi 0,15, Ag/Ni, Ag/SnO₂, (Ag/CdO)|Rivets, clad or welded contact parts|-|Control and auxiliary switches|Low contact resistance over extended life span|Ag, AgNi 0,15, AgCu, Ag/Ni|Rivets, clad stamped parts, (gold plated rivets), welded contact parts|-|Auxiliary and control relays|High reliability over extended life span, low contact resistance|AgNi 0,15, Ag/Ni|Rivets, clad profile parts, welded contact parts|-|Cam switches (higher loads)|High arc erosion and weld resistance, low contact resistance|AgCu, Ag/Ni, Ag/SnO₂, Ag/ZnO, (Ag/CdO)|Rivets, welded contact parts|-|Contactors|High arc erosion and weld resistance, low contact resistance|I< 20A : Ag/Ni, Ag/SnO₂ I>20A : Ag/SnO₂, (AgCdO)|Welded and brazed contact tips|-|Motor -protective circuit breakers|Extremely high weld resistance, low contact resistance|Ag/ZnO, Ag/C against Ag/Ni|Welded contact parts, toplay stamping parts|-|Power switches and circuit breakers|Extremely high arc erosion and weld resistance, low contact resistance|Ag/ZnO, Ag/SnO₂ , Ag/C against Ag/Ni o. Ag/W, Ag/W, Ag/WC/C, Ag/W against Ag/CdO|Brazed and welded contact tips and formed parts|-|Power switches with arcing and main contacts|High weld resistance, low contact resistance, high arc erosion resistance|Arcing contacts: W/Ag, W/Cu, (Cu) Main contacts: Ag/Ni, Ag/ZnO, Ag/W, Ag/WC|Brazed and welded contact tips and formed parts|-|Disconnect switches|Low contact resistance, sufficient mechanical strength|AgNi 0,15, Ag/Ni, Ag (electroplated)|Electroplated coatings, brazed contact parts|-|High voltage circuit breakers|Arcing contacts: highest arc erosion resistance Main contacts: low contact resistance|Arcing contacts: W/Cu-infiltrated Main contact CuCrZr silver plated,|Cast-on, electron-beam welded (or brazed) formed parts, percussion welded pins|-|Loaddisconnect switches (medium and high voltage) Contacts|Low contact resistance, sufficient mechanical strength, high arc erosion resistance of precontacts|Arcing contact: W/Cu, Cu, Ag/C Main contact: Cu, CuCrZr silver plated, Ag/Ni, AgNi0,15, Ag/C|Arcing contacts: brazed or welded parts Main contacts: silver plated, brazed or welded parts|-|Vacuum contactors|Low chopping current, high arc erosion resistance, low contact resistance|Low gas content W/Cu, W/CuSb, WC/Ag, CuCr|Contact discs, shaped rings|-|Vacuum circuit breakers|High switching capacity, low contact resistance|Low gas content CuCr|Contact discs|-|Transformer tab changers|High arc erosion resistance in oil environment|W/Cu in filtrated with approx. 70%|Brazed contact tips|-|Disconnect switches in high voltage circuits|Low contact resistance, low mechanical wear, sufficient arc erosion resistance during current commutation|Ag (electroplated), AgNi0,15, Ag/SnO₂|Electroplated coatings, brazed parts, Toplay profile segments|}'''Notes:'''<xr id="tab:Material Selection and Contact Component Design"/><!--Table 6.1--> is meant to give suggestions for the use of contact materials for the specified devices. For most of the contact materials, we deliberately did not indicate the exact composition and, as for Ag/SnO₂ and AgZnO, did also not include specific additives. The final material composition depends on specific design parameters of the electrical device. Advise on the special properties of specific contact materials can be found in chapter 2 [[Contact Materials for Electrical Engineering| Contact Materials for Electrical Engineering ]].

U = 300mV – 10V=<!--6.3-->Design Technologies for Contacts==I A multitude of technologies is available and used for the actual manufacturing of contact components (see chapter 3 [[Manufacturing Technologies for Contact Parts|Manufacturing Technologies for Contact Parts]]). The desired contact shape however, requires specific material properties like formability and weldability, which cannot be fulfilled by all materials in the same way. In addition, the design of the contact part must be compatible with the stresses and requirements of each switching device. The following <xr id= 10mA – 100mA"tab:Design Technologies for Contacts"/><!--table 6.2--> combines contact design, contact material and specific applications.

U <figtable id="tab:Design Technologies for Contacts"> 10VI <caption>'''<!--Table 6.2:-->Design Technologies for Contacts'''</caption> 300mA

*Dry Circuit Contacts{| class="twocolortable" style="text-align: left; font-size: 12px"|-!Contact Parts, Semi-finished Materials!Typical Contact Materials and Dimensions!Main Areas of Application!Remarks|-|Contact rivets solid, inserted wire segments|Ag, Ag alloys, Au alloys, Pd alloys, Ag/Ni, Ag/C97/3, Ag/MeO (1.2 – 8 mm Ø)|All types of switches in the communications, automotive or power distribution technology simple contact component, universally applied, selection through economic aspectsThis load range is characterized |Secure rivet attachment only with sufficiently thick shank (shank Ø = 1⁄2 head Ø); change-over contacts by the fact that the voltage is below theforming secondary head from longer shanks|-|Contact rivets, clad (Composite Rivets)|Ag, Ag alloys, Ag/Ni, Ag/MeO on Cu base (2 ~ 10 mm Ø)softening voltage |All types of switches in the respective communications, automotive or power engineering|Secure rivet attachment only with sufficiently thick shank (shank Ø = 1⁄2 head Ø)|-|Contact rivets with brazed surface layer|Tungsten and difficult to form powder metallurgical materials (i.e. Ag/C) on Cu or Fe bases (1 ~ 12 mm Ø)|Switches for power engineering, W layers mostly for controls|Tungsten contact to be staked (riveted) with moderate force or using orbital riveting; for Fe bases also warm-forming|-|Contact screws|Any contact material on Fe and CuZn screws, brazed, (1 ~ 10 mm Ø, M 2 ~ M 10)|Adjustable contacts for controls and horns|During brazing carrier may get soft|-|Vertically welded wire segments|Ag, Ag alloys, Ag/Ni, AgPd, Au alloys (< approxwire 0. 80mV6 ~ 5 mm Ø) |Contact parts for control functions and thepower engineering; economical manufacturing at higher quantitiescurrent stays below 10mA. Because |Welding and subsequently heading or orbital forming of this low electrical load the switchinghead shape|-occurs without any electrical discharge |Horizontally welded wire and profile segments|Au alloys, Pd alloys, Ag, Ag alloys, Ag/Ni, Ag/MeO, Ag/C in strip or profile form, Miniature profiles - also without any significant thermalmulti-layered (profile width 0.2 ~ 5 mm)stress |Contact parts for communication, measurement, controls and power engineering; very economical with respect to precious metal usage|Welding synchronized to stamping / forming on special equipment|-|Weld buttons|Ag, Ag alloys, Ag/Ni, Ag/MeO on the Steel, Ni, Monel; Ag/W, Ag/Mo (1.5 ~ 10 mm Ø)|Welded for example to steel springs or thermostatic bimetals for temperature controls|Metallurgical bond through simple projection welding remains strong in temperature cycling applications|-|Tungsten weld buttons|W on Ni or Ni-plated Fe, (2 ~ 6 mm) with weld projections|Contacts for controls, ignition points and horns; arcing contacts in special relays|For change-over contact spot. The main influences welded on the both sides of carrier|-|Brazed contact behavior aretipstherefore chemical |All materials and mechanical in naturedimensions, oxide and graphite containing materials with brazable backing, such as contaminationcarrier parts from Fe, Cu and dustCu alloys, at higher strength requirements also CuCrZr or CuBeor abrasion particles |Medium and higher load switching devices forpower engineering|Braze alloy layer with low meting point, carriers may soften during brazing|-med |Clad contact materials (Contact Bimetals), totally covered or with inlayed strips|Ductile precious metals on Cu and Cu alloys, minimum precious metal layer 2% of total strip thickness for Ag and Ag alloys, 0.5% of total strip thickness for Au alloys (with Ni intermediate layer), max. inlayed thickness 50% of total, strip width starting at 2 mm|Clad contact springs; stamped and formed parts for communications and power engineering; aluminum clad for bonding capability|Metallurgical bond; inlayed strip stamped perpendicular or at angle to strip direction; avoid bends at the cladding edges|-|Strips or profiles with brazed contact surfacesmaterial layers (Toplay material)|Ag, Ag alloys, Ag/Ni, Ag/MeO on Cu and Cu alloy carriers, total width 10 ~ 100mm, carrier thickness 0.3 – 5 mm, Ag strip cross section from 0.3 x 3 mm², strip thickn. to be &le; carrier thickn. The required highreliability can only be reached by using highly corrosion resistant |Stationary and moving contactbridges for power engineering switching devicesmaterials. Since dust particle contamina|Contact layers brazed with Ag brazing alloys; strips re-hardened during profile rolling|-|Seam-tions play a major role in determiningwelded contact strips or profilesthe failure rate of these contacts|Wire, strip, double miniature profiles (bifurcatedsolid or clad) welded to Cu alloy carrier strip (0.3 – 3 mm Ø or multiple up to 5 mm width)|Switches, pushbuttons, relays, auxiliary contactors, sliding contacts are|Broad usability, highly economical, thin spring hard carriers can be used frequently|-|Miniature profiles (Weld tapes)|Mostly high precious contact materials, double or multi layer, Ni, Monel, or Cu alloy carrier; miniature-profile width 0.2 – 2 mm|Welded profile segments for contact parts in communication, measurement and control engineering|Manufacturing of cross-directional contact spots; most economical precious metal usage|}

*Intermediate Level Table 2: '''Design Technologies for ContactsThis load range is characterized by a voltage below the minimum arc voltageand a current below 300mA. In this range discharges occur between thecontacts which can electrically or thermally destroy at least partiallycontamination layers on the contact surfaces. At lower electrical load organicfilms may not be thermally destroyed completely which may lead to a steepincrease in contact resistance. In DC circuits short arcs may result in materialtransfer. Contact materials for this load range need to be resistant againstcorrosion and the tendency to material transfer.(Fortsetzung)'''

*{| class="twocolortable" style="text-align: left; font-size: 12px"|-!Contact Parts, Semi-finished Materials!Typical Contact Materials and Dimensions!Main Areas of Application!Remarks|-|Clad profiles|Ag, Ag alloys, Ag/Ni, Ag/MeO, on Cu or Cu alloy carriers, all cross-sectional areas that can be drawn or rolled; Profile width: 2 ~ 10 mm|Profile segments as contact areas for low and high voltage switching devices|More complex shapes require costly tooling|-|Sintered and infiltrated parts|W-, WC-, Mo-based materials, in almost any contact shapes|Contact parts for low and high voltage switching devices|Single parts pressing; mostly with weld projec- tions and braze alloy coating on underside|-|Formed arc erosion parts|W/Cu infiltration materials, parts in almost any shapes|Arcing contacts for extreme duty switching devices, i.e. SF₆ circuit breakers|Attachment to Cu carriers by cast-on, percussion welding, electron-beam welding; rarely by brazing|-|Low Power gas content contact parts|W/Cu-, WC/Ag-, CuCr-based materials, rings and discs in almost any shape|Shaped contact parts for vacuum switches (Loadcontactors, power switches, circuit breakers) ContactsThe main characteristic of this load range is the presence of stable electrical|Brazing to Cu carriers requires special brazing alloys|-arcs. Caused by the interaction between |Cast-on contact material parts|W/Cu cast on with Cu, shaped parts and electrical arcs therings up to 100 mm Øelectrical life of |Arcing contacts is limited by arc erosion in high voltage switchgear|Seamless bond interface, carriers get hardened through subsequent forming|-|Electron-beam welded contact parts|W/Cu on Cu or material transfer CuCrZr contact rods, tubes, tulips|Arcing contacts in high voltage circuit breakers|Seamless bond interface, withstands high mechanical and thermal stresses|-|Silver electroplating|Layer thickness up to 20 μm, mostly on Cu and Cu alloys|Connecting areas and no-load switching contacts in thepower engineering; rotary switches, sliding contacts, connectors|For switching contacts only under very low loads|-|Gold electroplatingcase of higher make currents also by weld failures|Flash plating 0.1 – 0.2 μm on Ag alloys, and Cu alloys; contact layers 0. For 5 – 5 μm mostly with intermediate Ni layer|Contacts with low current and voltage loads, connectors, rotary and sliding switches, contact materialareas on printed circuit boards|Flash plating only limited effective as corrosion resistant layer on silver contacts|-|Selectively electroplated stripsselection the type of electrical load|Stripe coatings: Tin plating 1- 10 μm, Ag plating 1 – 20 μm, iAu plating 0.e2 – 5 μm; stripe width 2 mm min, stripe distance > 2 mm; carrier material: Cu and Cu alloys, Ni alloys, stainless steel; strip thickness: 0. resistive1 ~ 1 mm; strip width: 5 ~ 100 mm|Contact parts for connectors, inductivekeyboard switches, capacitiverotary and sliding switches; bondable areas (Au) for electronic components|Economic manufacturing for partially plated parts; hard gold with Ni intermediate layer possible but has limited formability|-|Selectively electroplated pre-stamped strips, motorSpot gold platingload|Continuous partial electroplating of pre-stamped and coined contact spots; allprecious metals; intermediate layers of Cu or Ni; selective tinning of connector contact areas and terminal ends; carrier materials up to 1 mm thick, which determine the time function strip width up to ~ 80 mm|Precious metal plating of switching contacts, connector parts, and terminal pins in the electrical currentcommunication technology|Crack-free and wear resistant layers possible since contact areas are already formed to final shape|-|Sputtered profiles|Au, is most criticalAu alloys in any composition; layer thickness 0.1 – 5 μm|Contact profiles for relays, switches and keyboard contacts in the information and measuring technology|High purity contact layers for high reliability|-|Hot-dip tinned strips|All around or stripe tinning 1 ~ 15 μm|Connectors for automotive and consumer technology; screw and crimp connectors|Economic coating method; does not form (Sn) whiskers|}</figtable>

Fig. ==<!--6.2 gives an overview for commonly used electrical contact materials fordifferent load ranges in switches used in the information technology up to thetransition range towards power switching applications. The ranges areillustrated as a function of switching current 4-->Formulas and voltage.Design Rules==

Fig. 6.2:Application ranges (switching currentand voltage) of contact materials forinformation technology andtransitioning into the power switchingdevices For lower electrical loads mainly high precious materials based on Au and Pt areused because of their high corrosion resistance, the latter materials howeverused only in limited quantities because of the high price of platinum metals. Agbased materials cover the medium load range and are alloyed with Pd forcurrents <1A and voltages > 24V, and for loads above these levels Agcomposite materials with additions of Ni, or the metal oxides SnO₂, ZnO, or CdOare used. While the Pd addition reduces the silver sulfide formation in sulfurcontaining environments, adding metal oxides increases the resistance againstwelding and arc erosion at higher make currents. At high switching currents andswitching frequency tungsten containing contacts are used, mainly as switchingpre-contacts which absorb the electrical arcs at high make and break currentswhile parallel contacts mainly produced from silver containing materials such asAgNi0.15 (Fine-Grain Silver) are employed for current carrying in the closedcondition. Primarily the specific stresses on the contact assemblies must be consideredduring the selection of contact materials: *During make of bouncing contacts mechanical wear, arc erosion,and material transfer occur, the latter mostly in DC switching circuits. *In the closed condition the value and consistency of the contactresistance must be considered. Both are affected by the resistance tocorrosion and changes in composition caused by the effects of arcing. *During off-switching (break) the frictional wear leads to material loss;besides this material transfer and arc erosion effect contact life. ===6.1.2 High Electrical Loads===At high electric loads that usually occur in power engineering devices theswitching phenomena are mostly related to arc formation. For most applicationsthe management of the switching arc is the key problem. Depending on thedevice type different require<!-ments are dominant which influence the selectionof the contact material. Similar to those in communications engineering, issuesrelated to the switching characteristics and current path have to be considered. *Make operationMake erosion caused by pre-close and bounce arcsWelding mainly during bounce arcMechanical wear mainly through bounce and relative motion *Current carrying through closed contactsIncreased contact resistance and temperature rise duringnominal loadWelding through high contact resistance during overload andshort circuit loadWelding during dynamic separation of the contacts with arcing *Break operationArc erosion during openingArc movementArc extinguishingMechanical wear als bilder? The typical application ranges for different contact materials in devices forpower engineering are illustrated in Figs. 6.3 and 6.4. In the lower load rangesmostly silver and fine grain silver (AgNi0.15) are used because of their highelectrical and thermal conductivity. With increasing currents the more arcerosion resistant AgCu alloy materials are used. For the medium current rangeup to 100A Ag/Ni composite materials are advantageous because of their lowerand consistent contact resistance and their favorable re1--solidificationproperties. If higher welding and at the same time arc erosion resistance arerequired, such as for example in motor contactors for switching currents up to5,000A, silver – metal oxide materials are superior. In protective switches(mainly circuit breakers) which are required to handle high short circuit energies,asymmetrical contact pairings are used where the fixed contact is made fromAg/C materials and the moving ones consist, depending on the devicecharacteristics, of Cu, Ag/Ni, or Ag/W. For UL rated and certified circuitbreakers (UL = Underwriters Laboratories) which are mainly used in NorthAmerican power distribution networks symmetrical pairings of Ag/W or Ag/WCare the preferred contact materials. For very high loads in main power switches and power circuit breakers formedium and high voltage power engineering applications the most suitablematerials are tungsten based infiltration materials such as W/Cu. Fig. 6.3:Typical application rangesfor contact materials inpower engineeringswitching devices as afunction of switchingcurrent and voltage Fig. 6.4:Application ranges forcontact materials in powerengineering switchingdevices as a function ofswitching current andnumbers of operation  ===6.2 Contact Materials and Design of Contact Components===The highest reliability and electrical life of electromechanical components andswitching devices can only be achieved if both, the material selection and thedesign of the actual contact parts, are optimized. Economic considerationsmust of course also be applied when selecting the most suitable contactmaterial and its way of application as an electrical contact. In the following Table6.1 recommendations are made for selected application examples for contactmaterials and contact shape or configuration. Table 6.1: Material Selection and Contact Component Design Table 6.1: Material Selection and Contact Component Design  ===Notes:===Table 6.1 is meant to give suggestions for the use of contact materials for thespecified devices. For most of the contact materials we deliberately did notindicate the exact composition and, as for Ag/SnO₂ and AgZnO, did also notinclude specific additives. The final material composition depends on specificdesign parameters of the electrical device. Advise on the special properties ofspecific contact materials can be found in chapter 2. ===6.3 Design Technologies for Contacts===A multitude of technologies is available and used for the actual manufacturing ofcontact components (see chapter 3). The desired contact shape howeverrequires specific material properties like for example formability and weldabilitywhich cannot be fulfilled by all materials in the same way. In addition the designof the contact part must be compatible with the stresses and requirements ofeach switching device. The following table 6.2 combines contact design,contact material, and specific applications.   Manufacturing of Conductive Preparationsbild Table 6.2: Design Technologies for Contacts Table 6.2: Design Technologies for Contacts  ===6.4 Formulas and Design Rules=== ===6.4.1 Definition of Terms and Symbols===

*'''Electrical contact''' is a property which is generated through the touching Main Article: [[Definition oftwo electrically conducting surfaces. *'''Contact part''' is a metallic component which is designed to create or interruptan electrical contact (is frequently replaced by the term “contact” if it is clearlyunderstandable that a physical piece or item is meant). *'''Contact area''' is the whole area on a contact part that may be used forcontacting. *'''Apparent contact area A_s''' is the part of the contact area on contact parts thatcan make physical contact during the touching of two contacts. *'''Load bearing contact area A_t''' is the part of the apparent contact area whichis affected by the contact force. It is the sum of all microscopic actual touchingpoints. *'''Effective contact area A_w''' is the part of the load bearing contact area throughwhich current is flowing and therefore the sum of all current carrying touchingareas (a-spots), A_w< A_t< A_s. *'''Contour area A_n''' is the contiguous area which includes all effectivea-spots, A_w< A_n< A_s; A_n≠ A_t. *'''Contact resistance R_k''' is composed of the constriction resistance and the filmresistance. *'''Constriction resistance R_e''' is the incremental electrical resistance generatedby the constriction of the currents paths in the touching area(a-spot). *'''Film resistance R_f''' is generated by a foreign matter layer, which for ex. isformed by a reaction of the contact material surface with the surroundingatmosphere (a surface film is a substance on the contact surface withdifferent properties than those of the actual contact material). *'''Path resistance R_d''' is the total electrical resistance between referencepoints (usually the device terminals) which can be freely chosen but mustbe defined. It is the sum of the conductor resistance R_b Terms and the contact resistance R_k. *'''Contact force F_k''' is the force that is exerted between the two contactparts in the closed position. *'''Frictional wear''' is the loss of material caused by mechanical wearbetween contact parts. *'''Bounce''' is the single or multiple interruption of conduction betweencontact parts during the make operation caused by alternatingtransformation of kinetic to potential energy. *'''Contact wear''' includes all changes on a contact surface. Mechanicaland electrical wear must be distinguished. *'''Material transfer''' is the transfer of contact material from one contactpart to the other. It occurs mainly during switching of DC loads. Thedirection of the transfer depends on the load circuit properties and thecontact materials used. *'''Arc erosion''' is the loss of material into the surrounding of the contactspot which is generated by electrical arcing. It occurs during contact makeas well as break operations. *'''Contact welding''' occurs when melt-liquefied touching areas of thecontact parts come in contact with each other. The melting occurs duringhigh current carrying through these areas. During make operations thisoccurs through bounce arcs, on closed contacts a too high contactresistance or dynamic separation of the contacts due to high short circuitcurrents can cause the welding of the contacts. The welding then maycause a device failure if the device specific opening forces cannot breakthe weld connection. *'''Arc movement''' happens when during the break operation a sufficiently highmagnetic field is generated which exerts a force on the electrical arc which isthen moved from the originating spot towards an arc chute (or arc splittingplates). *'''Arc extinguishing''' means the process of letting the current go to zero andtransferring the arcing gap from a conducting to the non-conducting stage.Selecting the most effective extinguishing measures depend mostly on thecurrent characteristics, the current value and the circuit voltage. *'''Recovery''' of an arc gap during contact opening is defined as the process ofthe electrically conducting plasma of an arc losing its electrical conductivity afterreaching current-zero. *'''Symbole used''' bild  ===6.4.2 Contact Physics – Formulas=== *'''Constriction resistance''' Re = D/2a(Single spot contact according to Holm; circular touching spot between cleancontact surfaces)Re = D/2Na(Multi-spot contact according to Holm without influence between the Nindividual spots)Re = D/2 x E ai + 3B D/32N² x E E (sij) i = j(Multi-spot contact according to Greenwood considering the influence betweenthe spots) als bild?  *'''Contact resistance'''RK = Re + Rf *'''Path resistance'''Rd = Rb + RK *'''Contact resistance and contact force'''R = 280D ³ E(F · r) K K(According to Holm model for film-free spherical contact surfaces with plasticdeformation of the contact material; F < 1 N for typical contact materials) kRK = 9000 D H/ FK(According to Holm model for film-free spherical contact surfaces with plasticdeformation of the contact material; F > 5 N for typical contact materials)  *'''Dynamic contact separation''' (without considering magnetic fields caused by the current path) FA 0,8 x I²(Rule Symbols| Definition of thumb with F in N Terms and I in kA)Symbols]]

*'''===<!--6.4.2-->Contact voltage and max. contact temperature'''T kmax 3200 UKPhysics – Formulas===

*'''Main Article: [[Contact resistance at higher contact forces (according to Babikow)'''R = cF -m K KFor F between 10 and 200 N Kc = material dependent proportionality factorm = shape dependent exponent of the contact forcePhysics – Formulas| Contact Physics – Formulas]]

<div class===6.4.3 Closed Contacts==="multiple-images">

Fig<figure id="fig:Rough flat surface">[[File:Rough flat surface. 6.5jpg|left|thumb|Figure1: Rough flat surface. a) before and b) during making contact with an ideally

contact areas (not to scale; dashed lines are elevation lines)]] Fig. 6.6:Contact resistance of crossed rodsas a function of the contact force for gold, silverand silver-palladium alloys</figure>

Table 6.3<figure id="fig: ThermoContact-resistance-electrical Voltage of -crossed-rods">[[File:Contact Materials (against Copper)-resistance-of-crossed-rods.jpg|right|thumb|Figure 2: Contact resistance of crossed rods as a function of the contact force for gold, silver and silver-palladium alloys]]</figure></div><div class="clear"></div>

<figtable id==="tab:Thermo-electrical Voltage of Contact Materials (against Copper)"><caption>'''<!--Table 6.4.4 Switching Contacts===3:-->Thermo-electrical Voltage of Contact Materials (against Copper)'''</caption>

Fig===<!--6. 64.7 Contact opening with arc formation (schematic)4-->Switching Contacts===

*'''Influence of out-gasing from plastics'''Fig. 6.9Main Article:Histogram of the contactresistance R of an electroplated Kpalladium layer (3 μm) with andwithout hard gold flash plating(0.2 μm) after exposure withdifferent plastic materials[[Switching Contacts| Switching Contacts]]

Fig===<!--6. 64.10: Contact resistance with exposure to out-gasing from plastics as a function of numbers ofoperations at 6 V ,100 mA: 1 Silicon containing plastic; 2 Plastics with strongly out5-gasing DCcomponents; 3 Plastics with minimal out-gasing components>Physical Effects in Sliding and Connector Contacts===

Fig===<!--6.4. 6.11: Distribution of cumulative frequency H of the contact resistance -->General Rules for solid contact rivetsafter 10 days exposure in a three-component test environment with 400 ppb each Dimensioning of H₂S, SO₂ andNO₂ at 25°C, 75% RH; Contact force 10cN; Measuring parameters: ≤ 40 mV_DC,10 mA; Probingcontact: Gold rivetContacts===

Fig. 6.8Main Article: Influences on contact areas in relays[[General Rules for Dimensioning of Contacts| General Rules for Dimensioning of Contacts]]

*'''Main Article: [[Contact Phenomena under the influence of arcing Matertia'''*'''Material transfer'''Fig. 6.12: Material transfer under DC load a) Cathode; b) Anode.6 Material: AgNi0.15; Switching parameters: 12VDC, 3 A, 2x10 operationsSpring Calculations| Contact Spring Calculations]]

*'''Arc erosion''' Fig. 6.13 Arc erosion of a Ag/SnO₂ contact pair after extreme arcing conditionsa) Overall view; b) Partial detail view *'''Contact welding'''Fig. 6.14: Micro structure of a welded contact pair (Ag/SnO₂ 88/12 - Ag/CdO88/12) after extremely high current load. a) Ag/SnO₂ 88/12; b) Ag/CdO88/12  ===6.4.5 Physical Effects in Sliding and Connector Contacts=== *'''Mechanical wear of sliding contacts''' dV/dx = k x FK /3 HW3 dV/dx Wear volume in mm per slide path length in mmk Coefficient of frictional wearHW Hardness of the softer material(Brinell or Vickers units)FK Contact force in cNWear coefficient k during material transfer-4 Silver – Silver 120 x 10-4 Platinum – Platinum 400 x 10-4 Silver – Platinum 1.3 x 10Coefficient of fractional wear k during wear loss-4 Silver – silver 8 x 10-4 Gold – gold 9 x 10-4 Platinum – platinum 40 x 10-4 Silver – gold 9 x 10-4 Silver – platinum 5 x 10 Fig. 6.15: Coefficient of frictional wear for the wear loss of sliding contacts Silver/Silverand hard gold/hard gold as a function of the contact force *'''Contact behavior of connectors'''Fig. 6.16: Contact resistance R as a function kof the contact force F for different surface kcoating materials. Measured against aspherical gold probe; I = 10 mA, U < 20 mV Fig. 6.17: Contact resistance R as a function kof the fretting wear cycles for different surfacecoating materials Tab.6.4: Surface Coating Materials for Connectors  ===6.4.6 General Rules for Dimensioning of Contacts===*'''Recommended Minimum Contact Forces at Slightly SlidingContact Make:''' Gold 0.03 NSilver 0.1 NTungsten 0.5 N *'''Contact Force Recommendations:''' Signal relays >3 cNAC power relays > 20 cNAutomotive relays > 20 cNMotor switches (Contactors) 0.05 - 0.08 N/A(Silver – Metal oxide contacts)Power switches 0.1 - 0.2 N/AConnectors > 30 cN/contact element(Gold coating)Connectors > 50 cN/contact element(Silver coating)Connectors > 1 N/contact element(Tin coating) *'''General Rules for Dimensioning of Contact Rivets'''bild *'''Head diameter for electrical loads''' For AC currents: approx. 1 – 1.5 A/mm²For 1 A min. 2 mm head diameter10 A approx. 3 – 3.5 mm head diameter20 A approx. 5 mm head diameterFor DC currents: approx. 0.5 – 0.8 A/mm² *'''Head radius R for electrical loads''' for I < 1 A R 1,5 mmI = 6 A R 5 mmI = 10 A R 10 mmI = 20 A R 15 mm *'''Failure Probability of Single and Double (Bifurcated) Contacts''' (according to Thielecke) Fig. 6.18: Failure probability of a contact as afunction of the voltage (according to Kirchdorfer);Ag/Ni10; 10 mA Fig. 6.19: Failure probability of a contact as afunction of the current (according toKirchdorfer); Ag/Ni10; F = 0.45 N; U = 24 V ===6.4.7 Contact Spring Calculations===Fig. 6.20:One side fixed contact bending springL = Length of springE = Modulus of elasticityB = Width of springF = Spring forceD = Thickness of springx = Deflectionmax = maximum bending force The influence of the dimensions can be illustrated best by using the single sidefixed beam model (Fig. 6.20). For small deflections the following equation is valid: F= x3 x E x JL³ where J is the momentum of inertia of the rectangular cross section of the beam J=B x D³12 For springs with a circular cross-sectional area the momentum of inertia is J=BD4/64D= Durchmesser To avoid plastic deformation of the spring the max bending force σ cannot be maxexceeded Fmax= 3 x E x D xmax2L² The stress limit is defined through the fatigue limit and the 0.2% elongation limitresp. xmax= 2 x L ² Rp0,23 x D x E and/or Fmax= B x D ² Rp0,26L  *'''Special Spring Shapes''' *'''Triangular spring''' Deflectionx= L³F2 x E x J = x L³D³6 x FE x B Max. bending forceFmax= 1 8 x F x LB x D² *'''Trapezoidal spring''' Deflectionx= x L³E x JF(2 + B /B ) x= x L³E x B x D³12 x F(2 + B /B ) min ma Max. bending force Fmax= 1 8 x F x L(2 + B /B ) x B x D² min max max  ===Referencens=References==