Application Tables and Guideline Data for Use of Electrical Contact Design

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Application Ranges for Switching Contacts

Low and Medium Electrical Loads

Switching processes at low and medium electrical loads are experienced for example in relays and switches for the measuring technology, telecommunications, automotive usage, and appliances. The switching voltage ranges from μV to 400V with currents between μA and about 100A.

Main Article: Low and Medium Electrical Loads

High Electrical Loads

At high electric loads that usually occur in power engineering devices the switching phenomena are mostly related to arc formation. For most applications the management of the switching arc is the key problem. Depending on the device type different require-ments are dominant which influence the selection of the contact material. Similar to those in communications engineering, issues related to the switching characteristics and current path have to be considered.

Main Article: High Electrical Loads

Contact Materials and Design of Contact Components

The highest reliability and electrical life of electromechanical components and switching devices can only be achieved if both, the material selection and the design of the actual contact parts, are optimized. Economic considerations must of course also be applied when selecting the most suitable contact material and its way of application as an electrical contact. In the following Table 1 recommendations are made for selected application examples for contact materials and contact shape or configuration.

Table 1: Material Selection and Contact Component Design
Type of Contacts or Devices Characteristic Requirements for Contacts Contact Material Design Form of Contacts
Contacts for dry circuits Reliable contacting at very low currents and voltages and mostly at also
low contact forces
AuAg alloys, (AuPt), Au Contact rivets, welded miniature profiles (tapes), electroplated Au, sputtered Au layers
Switching contacts in measuring devices Reliable switching at low voltages and currents at low contact forces Au and Pt alloys, (AgPd alloys) Contact rivets, welded tips, clad parts
Keyboard contacts Defined contacting, close to bounce-free make, high reliability at low switching loads Au alloys, (AgPd), Au on Ni substrate Au plated snap discs, Au clad wires and stamped parts, hard gold electroplated contact spots on printed circuit boards
Rotary switches on printed circuit boards Good frictional wear resistance, low contact résistance Sliding track: hard gold on Ni substrate Slider: AgPd alloy, (Hard silver) Electroplated coatings on slide tracks; clad, welded, or riveted stamping parts
Slip rings with high reliability Low and consistent contact resistance at low contact forces Brushes: Au alloys, AgPd, AgPdCu; Slip rings: Au alloys, Ag alloys (Rh); For higher currents: Ag/C brushes against Ag slip rings Brush wires, stamped brushes; solid, clad, or electroplated slip rings, Ag/C formed parts
Sliding contacts in miniature motors Very high frictional wear resistance, sure contacting even at very low contact forces Ag and Au alloys, Pd alloys, Au multi component alloys Brushes from flat rolled wire or stamped; collector hard gold electroplated or clad, or made from miniature profile segments
Centrifugal controllers for small motors Little shape changes, defined contacting at very low contact forces and high frequency of operation Pd alloys Contact rivets, contact screws, welded parts
Connectors Low contact resistance, corrosion resistance, sufficient frictional wear resistance, good sliding capabilities Ag and Au alloys, Pd, PdNi; For automotive and consumer electronic at low operation numbers: Sn and Sn alloys Electroplated layers or clad, often Au flash plated, mostly with Ni substrate layer, stamped parts from hot tin dipped strip
Telecommunication relays Reliable contacting even at high operational frequency Ag, AgPd, Au alloys, PdRu Rivets, welded profile segments
Reed relay contacts High reliability at low currents independent of atmospheric environment Au, (Rh) Switch paddles FeNi with partially diffused Au, (electroplated Rh)
Relays in electronic circuits High reliability at low switching loads and compact device design Au alloys, AgPd, Ag alloys Stamped springs from seam-welded profiles, welded miniature profile (tape) segments, contact rivets
GP relais (Elementary relays) Low arc erosion, high weld resistance, low and consistent contact resistance Ag/Ni, Ag/SnO2, (Ag/CdO), Ag/ZnO,AgNi0.15, (Ag) Solid and composite contact rivets, welded miniature profile (tape) segments
Automotive relays Low material transfer, low contact resistance, high weld resistance AgNi0.15, Ag/SnO2, Ag/Ni Contact rivets, welded miniature profile (tape) segments
Flasher relays (automotive, > 3 Mio operations) Low material transfer, high arc erosion resistance, low contact resistance PdCu15 and 40 (Anode) vs. AgNi0.15, AgCu3 (Cathode), Ag/ZnO, Ag/SnO2 Contact rivets, welded miniature profile (tape) and strip segments
Breaker points (automotive ignition) Very high arc erosion resistance, high switching frequency W Tips or discs welded to formed parts or Fe supports
Automotive horn contacts High arc erosion resistance at extremely high number of switching operations W, Ag/SnO2 Contact rivets, W weld buttons, springs or formed parts with brazed or welded tips
Appliance switches Low contact resistance, reasonable arc erosion and weld resistance AgNi0.15, Ag/Ni, Ag/SnO2, (Ag/CdO) Contact rivets, welded contact parts
Temperature controllers (Thermostats) Defined contacting point even at slow motion make, high operating temperatures AgNi0.15, Ag/Ni, Ag/SnO2, (Ag/CdO) Contact rivets, welded contact parts, weld buttons
Wiring devices (Light switches) Low contact resistance, reasonable arc erosion and weld resistance AgNi0.15, AgCu, Ag/Ni, with make peaks also Ag/ZnO, (Ag/CdO) Contact rivets, welded contact parts


Table 1: Material Selection and Contact Component Design (Fortsetzung)

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
Load disconnect switches (medium and high voltage) 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: Table 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 .

Design Technologies for Contacts

A multitude of technologies is available and used for the actual manufacturing of contact components (see chapter 3 Manufacturing Technologies for Contact Parts). The desired contact shape however requires specific material properties like for example 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 Table 2 combines contact design, contact material, and specific applications.


Table 2: Design Technologies for Contacts
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 aspects Secure rivet attachment only with sufficiently thick shank (shank Ø = 1⁄2 head Ø); change-over contacts by forming secondary head from longer shanks
Contact rivets, clad (Composite Rivets) Ag, Ag alloys, Ag/Ni, Ag/MeO on Cu base (2 ~ 10 mm Ø) All types of switches in the 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 (wire 0.6 ~ 5 mm Ø) Contact parts for control functions and power engine- ering; economical manufacturing at higher quantities Welding and subsequently heading or orbital forming of head shape
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 multi-layered (profile width 0.2 ~ 5 mm) 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 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 welded on both sides of carrier
Brazed contact tips All materials and dimensions, oxide and graphite containing materials with brazable backing, carrier parts from Fe, Cu and Cu alloys, at higher strength requirements also CuCrZr or CuBe Medium and higher load switching devices for power engineering Braze alloy layer with low meting point, carriers may soften during brazing
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 material 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 ≤ carrier thickn. Stationary and moving contact bridges for power engineering switching devices Contact layers brazed with Ag brazing alloys; strips re-hardened during profile rolling
Seam-welded contact strips or profiles Wire, strip, miniature profiles (solid or clad) welded to Cu alloy carrier strip (0.3 – 3 mm Ø or up to 5 mm width) Switches, pushbuttons, relays, auxiliary contactors, sliding contacts Broad usability, highly economical, thin spring hard carriers can be used
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


Table 2: Design Technologies for Contacts (Fortsetzung)

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 gas content contact parts W/Cu-, WC/Ag-, CuCr-based materials, rings and discs in almost any shape Shaped contact parts for vacuum switches (contactors, power switches, circuit breakers) Brazing to Cu carriers requires special brazing alloys
Cast-on contact parts W/Cu cast on with Cu, shaped parts and rings up to 100 mm Ø Arcing contacts in high voltage switchgear Seamless bond interface, carriers get hardened through subsequent forming
Electron-beam welded contact parts W/Cu on Cu or 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 power engineering; rotary switches, sliding contacts, connectors For switching contacts only under very low loads
Gold electroplating Flash plating 0.1 – 0.2 μm on Ag alloys, and Cu alloys; contact layers 0.5 – 5 μm mostly with intermediate Ni layer Contacts with low current and voltage loads, connectors, rotary and sliding switches, contact areas on printed circuit boards Flash plating only limited effective as corrosion resistant layer on silver contacts
Selectively electroplated strips Stripe coatings: Tin plating 1- 10 μm, Ag plating 1 – 20 μm, Au plating 0.2 – 5 μm; stripe width 2 mm min, stripe distance > 2 mm; carrier material: Cu and Cu alloys, Ni alloys, stainless steel; strip thickness: 0.1 ~ 1 mm; strip width: 5 ~ 100 mm Contact parts for connectors, keyboard switches, rotary 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, Spot gold plating Continuous partial electroplating of pre-stamped and coined contact spots; all

precious metals; intermediate layers of Cu or Ni; selective tinning of connector contact areas and terminal ends; carrier materials up to 1 mm thick, strip width up to ~ 80 mm

Precious metal plating of switching contacts, connector parts, and terminal pins in the communication technology Crack-free and wear resistant layers possible since contact areas are already formed to final shape
Sputtered profiles Au, Au 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

Formulas and Design Rules

Definition of Terms and Symbols

Note: The symbols for electrical contact specific terms (i.e. contact area, contact resistance, etc. have been retained from the german version of the Data Book. In related English literature some of them may vary using subscript symbols related to the language used – for example “contact resistance”: as used here from german Rk, in english mostly Rc.

Main Article: Definition of Terms and Symbols

Contact Physics – Formulas

Main Article: Contact Physics – Formulas

Closed Contacts

Figure 1 Rough flat surface a) before and b) during making contact with an ideally smooth flat surface; c) Schematic of the apparent, load bearing and effective contact areas (not to scale; dashed lines are elevation lines)

Figure 2 Contact resistance of crossed rods as a function of the contact force for gold, silver and silver-palladium alloys

Rough flat surface. a) before and b) during making contact with an ideally smooth flat surface; c) Schematic of the apparent, load bearing and effective contact areas (not to scale; dashed lines are elevation lines)
Contact resistance of crossed rods as a function of the contact force for gold, silver and silver-palladium alloys


Table 3: Thermo-electrical Voltage of Contact Materials (against Copper)
Contact Materials Thermo-electric Voltage (0 - 100°C) [mV]
Pure metals Ag
Au
Pt
Ir
Pd
Rh
Re
Cu
W
Mo
+ 0.04
+ 0.06
+ 0.78
+ 0.13
+ 1.35
+ 0.08
+ 0.78
0
- 0.46
- 0.73
Alloys/Composite materials AgCu 3
AgPd 30
AgPd 40
AgPd 50
AgPd 60
Ag/Ni 10
Ag/Ni 20
Ag/W 65
AuNi 5
AuAg 20
AuPt 10
PtW 5
Ptlr 10
Ptlr 20
PtRu 5
PtRu 10
PdCu 15
PdCu 40
+ 0.026
+ 0.125
+ 0.198
+ 0.321
+ 0.412
+ 0.23
+ 0.27
+ 0.01
+ 4.7
+ 2.76
+ 1.11
+ 0.67
+ 0.56
+ 0.60
+ 0.32
+ 0.13
+ 0.180
+ 0.247

Switching Contacts

Main Article: Switching Contacts

Physical Effects in Sliding and Connector Contacts

Main Article: Physical Effects in Sliding and Connector Contacts

General Rules for Dimensioning of Contacts

Main Article: General Rules for Dimensioning of Contacts

Contact Spring Calculations

Main Article: Contact Spring Calculations

References

Vinaricky, E. (Hrsg): Elektrische Kontakte-Werkstoffe und Anwendungen. Springer-Verlag, Berlin, Heidelberg 2002

Schröder, K.-H.: Grundlagen der Werkstoffauswahl für elektrische Kontakte. Buchreihe „Kontakt & Studium“, Band 366:zit. in „Werkstoffe für elektrische Kontakte und ihre Anwendungen“, Expert Verlag, Renningen, Bd. 366, (1997) 1-30

Horn, J.: „Steckverbinder“. zit. in Vinaricky, E. (Hrsg): „Elektrische Kontakte- Werkstoffe und Anwendungen“, Springer-Verlag, Berlin, Heidelberg 2002, 401- 419

Holm, R.: Electric Contacts, Springer-Verlag, Berlin, Heidelberg, New York 1967

Sauer, H. (Hrsg): Relais-Lexikon. 2. Aufl. Hüthig-Verlag, Heidelberg 1985

Greenwood J.A.: Constriction Resistance and the Area of Contact, Brit.J.Appl.Phys. 17 (1966) 1621

Biefer, H.: Elektrische Kontakte, Technische Rundschau (Bern) (1954/10) 17

Thielecke, K.: Anwendung von Kontakten in Schwachstromschaltern, in “Kontaktwerkstoffe in der Elektrotechnik”, Akademie-Verlag Berlin 1962, 107

Kirchdorfer, J.: Schalter für elektrische Steuerkreise, Blaue TR-Reihe, Heft 91, Verlag Hallwag, Bern und Stuttgart 1969