Application Tables and Guideline Data for Use of Electrical Contact Design

Contents

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 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 Articel: Low and Medium Electrical Loads

6.1.2 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 Articel: High Electrical Loads

6.2 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 6.1 recommendations are made for selected application examples for contact materials and contact shape or configuration.

Table 6.1: Material Selection and Contact Component Design (2 teile!)

Table 6.1: Material Selection and Contact Component Design (Fortsetzung - 2 Teile!)

Notes:

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.

6.3 Design Technologies for Contacts

A multitude of technologies is available and used for the actual manufacturing of contact components (see chapter 3). 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 6.2 combines contact design, contact material, and specific applications.

Table 6.2: Design Technologies for Contacts (2 Teile!)

Table 6.2: Design Technologies for Contacts (Fortsetzung) - (2 Teile!)

6.4 Formulas and Design Rules

6.4.1 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 R_k, in english mostly R_c.

Main Articel: Definition of Terms and Symbols

6.4.2 Contact Physics – Formulas

Main Articel: Contact Physics – Formulas

6.4.3 Closed Contacts

Fig. 6.5: 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)

Fig. 6.6: Contact resistance of crossed rods as a function of the contact force for gold, silver and silver-palladium alloys

Table 6.3: Thermo-electrical Voltage of Contact Materials (against Copper)

6.4.4 Switching Contacts

Main Articel: Switching Contacts

6.4.5 Physical Effects in Sliding and Connector Contacts

• Mechanical wear of sliding contacts

dV/dx = k x FK /3 HW 3 dV/dx Wear volume in mm per slide path length in mm k Coefficient of frictional wear HW Hardness of the softer material (Brinell or Vickers units) FK Contact force in cN Wear coefficient k during material transfer -4 Silver – Silver 120 x 10 -4 Platinum – Platinum 400 x 10 -4 Silver – Platinum 1.3 x 10 Coefficient 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/Silver and hard gold/hard gold as a function of the contact force

• Contact behavior of connectors

Fig. 6.16: Contact resistance R as a function k of the contact force F for different surface k coating materials. Measured against a spherical gold probe; I = 10 mA, U < 20 mV

Fig. 6.17: Contact resistance R as a function k of the fretting wear cycles for different surface coating materials

Tab.6.4: Surface Coating Materials for Connectors

6.4.6 General Rules for Dimensioning of Contacts

• Recommended Minimum Contact Forces at Slightly Sliding

Contact Make:

Gold 0.03 N Silver 0.1 N Tungsten 0.5 N

• Contact Force Recommendations:

Signal relays >3 cN AC power relays > 20 cN Automotive relays > 20 cN Motor switches (Contactors) 0.05 - 0.08 N/A (Silver – Metal oxide contacts) Power switches 0.1 - 0.2 N/A Connectors > 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/mm² For 1 A min. 2 mm head diameter 10 A approx. 3 – 3.5 mm head diameter 20 A approx. 5 mm head diameter For DC currents: approx. 0.5 – 0.8 A/mm²

• Head radius R for electrical loads

for I < 1 A R 1,5 mm I = 6 A R 5 mm I = 10 A R 10 mm I = 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 a function of the voltage (according to Kirchdorfer); Ag/Ni10; 10 mA

Fig. 6.19: Failure probability of a contact as a function of the current (according to Kirchdorfer); Ag/Ni10; F = 0.45 N; U = 24 V

6.4.7 Contact Spring Calculations

Fig. 6.20: One side fixed contact bending spring L = Length of spring E = Modulus of elasticity B = Width of spring F = Spring force D = Thickness of spring x = Deflection max = maximum bending force

The influence of the dimensions can be illustrated best by using the single side fixed beam model (Fig. 6.20). For small deflections the following equation is valid:

F= x 3 x E x J L³

where J is the momentum of inertia of the rectangular cross section of the beam

J= B x D³ 12

For springs with a circular cross-sectional area the momentum of inertia is

J=BD4/64 D= Durchmesser

To avoid plastic deformation of the spring the max bending force σ cannot be max exceeded

Fmax= 3 x E x D xmax 2L²

The stress limit is defined through the fatigue limit and the 0.2% elongation limit resp.

xmax= 2 x L ² Rp0,2 3 x D x E

and/or

Fmax= B x D ² Rp0,2 6L

• Special Spring Shapes

• Triangular spring

Deflection x= L³ F 2 x E x J

= x L³ D³ 6 x F E x B

Max. bending force Fmax= 1 8 x F x L B x D²

• Trapezoidal spring

Deflection x= x L³ E x J F (2 + B /B )

x= x L³ E x B x D³ 12 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

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