Application Tables and Guideline Data for Use of Electrical Contact Design

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.

Guided by empirically developed arc-limiting graphs typical switching processes can be distinguished. As Fig. 6.1 illustrates, voltage and current determine if switching occurs without arcing, results in a glow discharge, short instable arcs are generated, or a fully developed electrical arc is created. The more exact current-voltage curve characte-ristics are depending on the electrical contact material used. They also depend on the contact gap and the atmosphere the switching occurs in; an ambient air atmosphere is assumed in the shown schematic example.

Fig. 6.1: Arc-limiting graphs (schematic) 1. Arc-less switching 2. Short instable arcs 3. Glow discharge 4. Full electrical arcs

For the different requirements on the electrical contacts in various applications it is useful to differentiate across the broad spectrum of possible load conditions guided by the arc-limiting graphs between four different partial ranges which result in typical physical effects:

• Dry Circuit Contacts

U < 80mV I < 10mA

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• Low Level (Load) Contacts

U = 80 to 300mV I < 10mA

• Intermediate Level (Load) Contacts

U = 300mV – 10V I = 10mA – 100mA

• Low Power (Load) Contacts

U > 10V I > 300mA

• Dry Circuit Contacts

This load range is characterized by the fact that the voltage is below the softening voltage of the respective contact material (< approx. 80mV) and the current stays below 10mA. Because of this low electrical load the switching occurs without any electrical discharge and also without any significant thermal stress on the contact spot. The main influences on the contact behavior are therefore chemical and mechanical in nature, such as contamination, and dust or abrasion particles for-med on the contact surfaces. The required high reliability can only be reached by using highly corrosion resistant contact materials. Since dust particle contamina-tions play a major role in determining the failure rate of these contacts, double (bifurcated) or multiple contacts are used frequently.

• Low Level Contacts

In this load range the voltage is between the softening and melting voltage of the contact material and the current is below 10mA. Because of the higher voltage compared to dry circuits a temperature induced softening of the contacting surface area occurs which increases the contacting area. Besides high corrosion resistance a higher hardness of the contact materials is required for this load range.

• Intermediate Level Contacts

This load range is characterized by a voltage below the minimum arc voltage and a current below 300mA. In this range discharges occur between the contacts which can electrically or thermally destroy at least partially contamination layers on the contact surfaces. At lower electrical load organic films may not be thermally destroyed completely which may lead to a steep increase in contact resistance. In DC circuits short arcs may result in material transfer. Contact materials for this load range need to be resistant against corrosion and the tendency to material transfer.

• Low Power (Load) Contacts

The main characteristic of this load range is the presence of stable electrical arcs. Caused by the interaction between contact material and electrical arcs the electrical life of contacts is limited by arc erosion or material transfer and in the case of higher make currents also by weld failures. For contact material selection the type of electrical load, i.e. resistive, inductive, capacitive, motor load, which determine the time function of the electrical current, is most critical.

Fig. 6.2 gives an overview for commonly used electrical contact materials for different load ranges in switches used in the information technology up to the transition range towards power switching applications. The ranges are illustrated as a function of switching current and voltage.

Fig. 6.2: Application ranges (switching current and voltage) of contact materials for information technology and transitioning into the power switching devices

For lower electrical loads mainly high precious materials based on Au and Pt are used because of their high corrosion resistance, the latter materials however used only in limited quantities because of the high price of platinum metals. Ag based materials cover the medium load range and are alloyed with Pd for currents <1A and voltages > 24V, and for loads above these levels Ag composite materials with additions of Ni, or the metal oxides SnO₂, ZnO, or CdO are used. While the Pd addition reduces the silver sulfide formation in sulfur containing environments, adding metal oxides increases the resistance against welding and arc erosion at higher make currents. At high switching currents and switching frequency tungsten containing contacts are used, mainly as switching pre-contacts which absorb the electrical arcs at high make and break currents while parallel contacts mainly produced from silver containing materials such as AgNi0.15 (Fine-Grain Silver) are employed for current carrying in the closed condition.

Primarily the specific stresses on the contact assemblies must be considered during 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 contact

resistance must be considered. Both are affected by the resistance to corrosion 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 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.

• Make operation

Make erosion caused by pre-close and bounce arcs Welding mainly during bounce arc Mechanical wear mainly through bounce and relative motion

• Current carrying through closed contacts

Increased contact resistance and temperature rise during nominal load Welding through high contact resistance during overload and short circuit load Welding during dynamic separation of the contacts with arcing

• Break operation

Arc erosion during opening Arc movement Arc extinguishing Mechanical wear

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The typical application ranges for different contact materials in devices for power engineering are illustrated in Figs. 6.3 and 6.4. In the lower load ranges mostly silver and fine grain silver (AgNi0.15) are used because of their high electrical and thermal conductivity. With increasing currents the more arc erosion resistant AgCu alloy materials are used. For the medium current range up to 100A Ag/Ni composite materials are advantageous because of their lower and consistent contact resistance and their favorable re-solidification properties. If higher welding and at the same time arc erosion resistance are required, such as for example in motor contactors for switching currents up to 5,000A, 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 from Ag/C materials and the moving ones consist, depending on the device characteristics, of Cu, Ag/Ni, or Ag/W. For UL rated and certified circuit breakers (UL = Underwriters Laboratories) which are mainly used in North American power distribution networks symmetrical pairings of Ag/W or Ag/WC are the preferred contact materials.

For very high loads in main power switches and power circuit breakers for medium and high voltage power engineering applications the most suitable materials are tungsten based infiltration materials such as W/Cu.

Fig. 6.3: Typical application ranges for contact materials in power engineering switching devices as a function of switching current and voltage

Fig. 6.4: Application ranges for contact materials in power engineering switching devices as a function of switching current and numbers of operation

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

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 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.

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Table 6.2: Design Technologies for Contacts

Table 6.2: Design Technologies for Contacts

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.

• Electrical contact is a property which is generated through the touching of

two electrically conducting surfaces.

• Contact part is a metallic component which is designed to create or interrupt

an electrical contact (is frequently replaced by the term “contact” if it is clearly understandable that a physical piece or item is meant).

• Contact area is the whole area on a contact part that may be used for

contacting.

• Apparent contact area A_s is the part of the contact area on contact parts that

can make physical contact during the touching of two contacts.

• Load bearing contact area A_t is the part of the apparent contact area which

is affected by the contact force. It is the sum of all microscopic actual touching points.

• Effective contact area A_w is the part of the load bearing contact area through

which current is flowing and therefore the sum of all current carrying touching areas (a-spots), A_w< A_t< A_s.

• Contour area A_n is the contiguous area which includes all effective

a-spots, A_w< A_n< A_s; A_n≠ A_t.

• Contact resistance R_k is composed of the constriction resistance and the film

resistance.

• Constriction resistance R_e is the incremental electrical resistance generated

by 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. is

formed by a reaction of the contact material surface with the surrounding atmosphere (a surface film is a substance on the contact surface with different properties than those of the actual contact material).

• Path resistance R_d is the total electrical resistance between reference

points (usually the device terminals) which can be freely chosen but must be defined. It is the sum of the conductor resistance R_b and the contact resistance R_k.

• Contact force F_k is the force that is exerted between the two contact

parts in the closed position.

• Frictional wear is the loss of material caused by mechanical wear

between contact parts.

• Bounce is the single or multiple interruption of conduction between

contact parts during the make operation caused by alternating transformation of kinetic to potential energy.

• Contact wear includes all changes on a contact surface. Mechanical

and electrical wear must be distinguished.

• Material transfer is the transfer of contact material from one contact

part to the other. It occurs mainly during switching of DC loads. The direction of the transfer depends on the load circuit properties and the contact materials used.

• Arc erosion is the loss of material into the surrounding of the contact

spot which is generated by electrical arcing. It occurs during contact make as well as break operations.

• Contact welding occurs when melt-liquefied touching areas of the

contact parts come in contact with each other. The melting occurs during high current carrying through these areas. During make operations this occurs through bounce arcs, on closed contacts a too high contact resistance or dynamic separation of the contacts due to high short circuit currents can cause the welding of the contacts. The welding then may cause a device failure if the device specific opening forces cannot break the weld connection.

• Arc movement happens when during the break operation a sufficiently high

magnetic field is generated which exerts a force on the electrical arc which is then moved from the originating spot towards an arc chute (or arc splitting plates).

• Arc extinguishing means the process of letting the current go to zero and

transferring the arcing gap from a conducting to the non-conducting stage. Selecting the most effective extinguishing measures depend mostly on the current characteristics, the current value and the circuit voltage.

• Recovery of an arc gap during contact opening is defined as the process of

the electrically conducting plasma of an arc losing its electrical conductivity after reaching current-zero.

• Symbole used

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6.4.2 Contact Physics – Formulas

• Constriction resistance

Re = D/2a (Single spot contact according to Holm; circular touching spot between clean contact surfaces) Re = D/2Na (Multi-spot contact according to Holm without influence between the N individual 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 between the spots)

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• 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 plastic deformation of the contact material; F < 1 N for typical contact materials) k RK = 9000 D H/ FK (According to Holm model for film-free spherical contact surfaces with plastic deformation 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 of thumb with F in N and I in kA)

• Contact voltage and max. contact temperature

T kmax 3200 UK

• Contact resistance at higher contact forces (according to Babikow)

R = cF -m K K For F between 10 and 200 N K c = material dependent proportionality factor m = shape dependent exponent of the contact force

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

• Effects during switching operations

Fig. 6.7 Contact opening with arc formation (schematic)

• Influence of out-gasing from plastics

Fig. 6.9: Histogram of the contact resistance R of an electroplated K palladium layer (3 μm) with and without hard gold flash plating (0.2 μm) after exposure with different plastic materials

Fig. 6.10: Contact resistance with exposure to out-gasing from plastics as a function of numbers of operations at 6 V ,100 mA: 1 Silicon containing plastic; 2 Plastics with strongly out-gasing DC components; 3 Plastics with minimal out-gasing components

• Influence of corrosive gases on the contact resistance

Fig. 6.11: Distribution of cumulative frequency H of the contact resistance for solid contact rivets after 10 days exposure in a three-component test environment with 400 ppb each of H₂S, SO₂ and NO₂ at 25°C, 75% RH; Contact force 10cN; Measuring parameters: ≤ 40 mV_DC,10 mA; Probing contact: Gold rivet

Fig. 6.8: Influences on contact areas in relays

• 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 operations

• Arc erosion

Fig. 6.13 Arc erosion of a Ag/SnO₂ contact pair after extreme arcing conditions a) 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 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

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• 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

Referencens

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