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===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.
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
als bild?
*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 SnO<sub>2</sub>, 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
als bilder?
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/SnO<sub>2</sub> 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.
Manufacturing of Conductive Preparations
bild
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===
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<sub>k</sub>, in english mostly R<sub>c</sub>.
*'''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<sub>s</sub>''' 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<sub>t</sub>''' 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<sub>w</sub>''' 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<sub>w</sub>< A<sub>t</sub>< A<sub>s</sub>.
*'''Contour area A<sub>n</sub>''' is the contiguous area which includes all effective
a-spots, A<sub>w</sub>< A<sub>n</sub>< A<sub>s</sub>; A<sub>n</sub>≠ A<sub>t</sub>.
*'''Contact resistance R<sub>k</sub>''' is composed of the constriction resistance and the film
resistance.
*'''Constriction resistance R<sub>e</sub>''' is the incremental electrical resistance generated
by the constriction of the currents paths in the touching area
(a-spot).
*'''Film resistance R<sub>f</sub>''' 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<sub>d</sub>''' 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<sub>b</sub> and the contact resistance R<sub>k</sub>.
*'''Contact force F<sub>k</sub>''' 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'''
bild
===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)
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 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 H<sub>2</sub>S, SO<sub>2</sub> and
NO<sub>2</sub> at 25°C, 75% RH; Contact force 10cN; Measuring parameters: ≤ 40 mV<sub>DC</sub>,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/SnO<sub>2</sub> 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/SnO<sub>2</sub> 88/12 - Ag/CdO88/12)
after extremely high current load. a) Ag/SnO<sub>2</sub> 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'''
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
===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
===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
als bild?
*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 SnO<sub>2</sub>, 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
als bilder?
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/SnO<sub>2</sub> 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.
Manufacturing of Conductive Preparations
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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===
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<sub>k</sub>, in english mostly R<sub>c</sub>.
*'''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<sub>s</sub>''' 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<sub>t</sub>''' 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<sub>w</sub>''' 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<sub>w</sub>< A<sub>t</sub>< A<sub>s</sub>.
*'''Contour area A<sub>n</sub>''' is the contiguous area which includes all effective
a-spots, A<sub>w</sub>< A<sub>n</sub>< A<sub>s</sub>; A<sub>n</sub>≠ A<sub>t</sub>.
*'''Contact resistance R<sub>k</sub>''' is composed of the constriction resistance and the film
resistance.
*'''Constriction resistance R<sub>e</sub>''' is the incremental electrical resistance generated
by the constriction of the currents paths in the touching area
(a-spot).
*'''Film resistance R<sub>f</sub>''' 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<sub>d</sub>''' 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<sub>b</sub> and the contact resistance R<sub>k</sub>.
*'''Contact force F<sub>k</sub>''' 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)
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 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 H<sub>2</sub>S, SO<sub>2</sub> and
NO<sub>2</sub> at 25°C, 75% RH; Contact force 10cN; Measuring parameters: ≤ 40 mV<sub>DC</sub>,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/SnO<sub>2</sub> 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/SnO<sub>2</sub> 88/12 - Ag/CdO88/12)
after extremely high current load. a) Ag/SnO<sub>2</sub> 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
