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Joint resistance of bolted copper - copper busbar joints depending on joint force at temperatures beyond 105 °C

S. Schlegel, S. Großmann, H. Löbl, M. Hoidis*, U. Kaltenborn**, T. Magier*** Technische Universität Dresden, Institut für Elektrische Energieversorgung und

Hochspannungstechnik, Germany *ABB Schweiz AG, Baden-Dättwil, Switzerland

**AREVA Energietechnik GmbH, Regensburg, Germany***Siemens AG, Berlin, Germany

Abstract The long-term behaviour of bolted joints used in high current systems is influenced by different ageing mechanisms. One of these mechanisms is the degradation of the joint force depending on temperature and time. If the joint force falls below a critical value, the joint resistance, the thermal dissipation and thereby the temperature of the joint can increase to a critical level. According to IEC 61439-1 the highest accepted temperature of joints is 140 °C. The influence of the force reduction to the ageing of joints is tested on current-carrying, bolted Cu-ETP (CW004A) busbar joints utilizing washers und spring washers at temperatures up to 160 °C. The joint force and the joint resistance are measured time-dependent. The possible physical mechanisms are discussed regarding to the results of the long-term tests and analyses of the busbar material by microscopic investigations. Based on this experimental data an extrapolation of the lifetime of such electrical joints is discussed in relation to the practical operating lifetime of more than 50 years.

Key words: Ageing, copper, copper joints, force reduction, joint resistance, long-term behaviour,

1 Introduction In general, electrical power systems contain electrical joints, whose are important parts of a safe and reliable power transmission. Especially in high current applications the long-term behaviour of the joints depends on different ageing mechanisms, which influence the joint resistance. If the joint resistance increases during the lifetime, the power loss and the temperature can rise up to a critical level and the joints might fail [1]. Today the mechanisms like chemical reactions, fretting, interdiffusion, electromigration and force reduction are known [2], [3].

One of the most important mechanisms in bolted copper busbar joints at high current load i. e. at high temperatures is the force reduction of the joints. The force reduction at cold worked busbar material is influenced by setting processes, dynamic recovery, dynamic recrystallisation and grain coarsening. All of them can contribute to the decrease of joint force depending on temperature and time.

For practical applications and standardization it is important to know the physical limits of the joints to operate them safely over the expected lifetime (> 50 years).

The influence of the reduction of the joint force Fj on the joint resistance Rj can be measured in long-term tests. For static compositions the minimum joint force can be derived from the hysteresis-curve of the joint resistance (Rj=f(Fj)) [1] as a threshold. The results of these tests are used to extrapolate the decrease of joint force with mathematical models like the Larson-Miller method. Additionally to the long-term tests, microscopic examinations can be conducted to describe the physical mechanisms, which might lead to the force reduction.

In the present paper the force reduction in bolted busbar joints consisting of Cu-ETP (CW004A) at two defined material conditions (MC) i. e. two degrees of cold work are analysed.

2 Material properties The electrolytic tough pitch copper (Cu-ETP) is an oxygen containing copper which has a very high electrical and thermal conductivity (Tab. 1) [4].

Tab. 1: Chemical composition of Cu-ETP [4] Impurities in weight-%

Cu1) Bi O Pb Other2)

≥ 99.9 max. 0.0005

max. 0.04

max. 0.005 0.03

1) Including Ag up to 0.015 % 2) Excluding of Ag

The chemical composition of the used copper in the long-term tests was analysed by inductive coupled plasma optical emission spectrometry (ICP-OES) (Tab. 2).

Tab. 2: Material composition of the used Cu-ETP Impurities in

weight-% Cu-ETP MC 1 Cu-ETP MC 2

Ag 0.0011 0.0007 Fe 0.0011 0.0007 Ni 0.0002 < 0.0001 Al, Be, Cd, Co, Cr, Mg, Mn, Ti, Zr < 0.0001

As, Se, Zn < 0.0002 Bi, P, Pb, S, Sb, Si, Sn, Te < 0.0005

MC…Material condition (i. e. degree of cold work)

978-1-4244-8177-4/10/$26.00 ©2010 IEEE

The mechanical properties at annealed (soft) condition are insufficient for the most practical applications. Cold forming processes are used to improve these characteristics (Fig. 1).

Elas

ticlim

itR

p0,2

; Ten

sile

stre

nght

Rm

in N

/mm

²

Cold forming degree in %El

onga

tion

A10

in %

Har

dnes

sH

V

Elas

ticlim

itR

p0,2

; Ten

sile

stre

nght

Rm

in N

/mm

²

Cold forming degree in %El

onga

tion

A10

in %

Har

dnes

sH

V

Fig. 1: Mechanical properties of Cu-ETP depending on cold forming degree (standard values) [5]

Cold forming processes increase the density of dislocations and blank positions in the texture of the material. The grains are stretched towards the cold hardening direction and show a cold forming structure. [6]

3 Theory of force reduction in pure copper busbar joints

In cold deformed pure metal bolted busbar joints different physical mechanisms are responsible for decrease of the joint force dependent on time e. g. mechanical stress σ as a function of time (Fig. 2).

mec

hani

cal s

tress

/

Fig. 2: Schematic of the mechanical stress reduction in bolted busbar joints (cold deformed material)

Within the first few hours setting processes dominate the force reduction processes: the setting between different parts of the joint like washers or spring washers and the basic copper busbar or the mechanical structure of the bolt itself. Surface roughness of the bolt thread and different thermal properties (thermal expansion coefficients) of the used materials are responsible for this processes. At the same time a dynamic recovery takes place as

well. Gliding of dislocations ( 0.15⋅Tm1) and the

cutting and cross slipping of screw dislocations ( 0.2⋅Tm) might happen due to a thermal activation [6]. Furthermore, the external mechanical stress generates new dislocations. So the primary area describes the generation (hardening) and annihilation (recovery) of dislocations up to achieving equilibrium conditions (Eq. 1). [7]

dtt

d∂σ∂−=ε

ε∂σ∂ (1)

Hardening Recovery

After this primary phase, the secondary area characterizes the lifetime of the joint. In case that the force reduction is initiated under stationary conditions, the strain rate sε can be described as:

ε∂σ∂

∂σ∂−=ε=ε /tdt

d

ss (2)

By excluding the initiation of any other process, the strain rate can be calculated with the phenomenological creep law. [7], [8], [9]

TRQ

ns

c

eC ⋅−

⋅σ⋅=ε (3)

C…Proportionality factor; n…Norton exponent Qc…Activation energy

Should another physical process start during the test time, the activation energy Qc is changed and will increase the complexity for the calculation the strain rate. Therefore extrapolation methods are a suitable alternative to calculate the force reduction based on the test results depending on time (Chapter 4.2).

In cold deformed metals the main secondary physical mechanism is the dynamic primary recrystallisation. The influence of this process is seen as a superimposed function in the force reduction-curve (Fig. 2). Recrystallisation processes need an incubation time and start at major defective locations. This process is dependent on the degree of deformation, temperature, time, grain size and solute foreign atoms [6]. If the dynamic primary recrystallisation is finished, the mechanical properties constituted by the cold forming process are lost resulting in a reduced joint force. Subsequent to this process the grain coarsening (secondary recrystallisation) can influence the mechanical properties of the metal and thereby the joint force. Grain coarsening describes the change of the grain sizes dependent on time and temperature. Small grains are incorporated by bigger grains. The mean grain size increases, whereas the number of grains decreases. The mechanical strength is also impaired. [6]

1) Melting temperature Tm of the material in K

4 Experiments

4.1 Long-term tests The joint force and joint resistance are measured at bolted current-carrying busbar joints made from Cu-ETP (40 mm x 10 mm) dependent on temperature and time. Two material conditions were used. The joint system was realized with two washers (DIN 125A) or spring washers (DIN 6795) (Tab. 3).

Tab. 3: Experimental design Material Test temperature in °CCu-ETP 105 140 160

Number of joints (washers / spring washers)MC 1 5/5 5/5 5/5MC 2 5/5 5/5 5/5

MC 1 < MC 2 degrees of cold work

The contact areas were cleaned by ethyl alcohol and brushed by a steel wire brush to remove the oxidation layer from the surface. The joint force at initial conditions and over time is measured by a micrometer bolt M12 (8.8) coupled by a MACOR2)-base with a micrometer dial (Fig. 3).

Fig. 3: Joint force measuring system

The temperature of the joints is regulated by the applied current and measured by three thermocouples evenly distributed at three joints of the test circuit. The thermocouples are fixed by a punch mark directly at the busbar. The power input was realized by a high current transformer controlled by a variable transformer (Fig. 4).

Fig. 4: Experimental setup

2) MACOR® - machinable glass ceramic

The joint forces in relation to the starting value are measured after heating up over time by fixing a constant joint temperature (± 4 K). Periodically the joint resistances are measured by a micro-ohmmeter (LoRe3)) after cooling down to room temperature (Fig. 5 to 7).

0

0,2

0,4

0,6

0,8

1

1,2

0 2000 4000 6000 8000 10000 12000 14000 16000time t / h

join

t for

ce F

j / F

j0

0

1

2

3

4

5

6

7

8

join

t res

ista

nce

Rj /

Rj0

Spring washer Cu - MC 1Washer Cu - MC 1Spring washer Cu - MC 2Washer Cu - MC 2

30 %

Rj

Fj

Fig. 5: Related joint force and resistance over time at 105 °C

0

0,2

0,4

0,6

0,8

1

1,2

0 2000 4000 6000 8000 10000 12000 14000 16000time t / h

join

t for

ce F

j / F

j0

0

1

2

3

4

5

6

7

8

join

t res

ista

nce

Rj /

Rj0


63 %

Rj

Fj


0

0,2

0,4

0,6

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1,2

0 2000 4000 6000 8000 10000 12000 14000 16000time t / h

join

t for

ce F

j / F

j0

0

1

2

3

4

5

6

7

8

join

t res

ista

nce

Rj /

Rj0


68 %

Rj

Fj


The maximum force reduction at all temperatures was identified for joints with washers and the material MC 2. After 11600 h at 105 °C the joint force of this contact system was decreased to 70 % of the initial value. After 11600 h at 140 °C (Fig. 6) and 13900 h at 160 °C (Fig. 7) the joint force is reduced to 37 % and 32 %, respectively. The force reduction at joints with washers and material MC 1

3) LoRe micro-ohmmeter by the company Werner Electronics

Micrometer bolt Nut

Washer / Spring washerCopper busbar

Macor-Base

Micrometer dial

Thermocouple Measuring of the joint temperature

Force measuring system Measuring of joint force

Insulator Electrical isolation of the high current circle

High current transformer Power input

showed a lower decrease in joint force. At 105 °C (13500 h), 140 °C (11300 h) and 160 °C (13900 h) the joint force is reduced to 75 %, 60 % and 51 %. The explanations for the different behaviour between the two material conditions are found in the physical mechanisms, which are influencing the force reduction process (Chapter 3 and 4.2).

At all temperatures the differences between the two material conditions (MC 1 and MC 2) were less for joints with spring washers. The maximum force reduction to 71 % was reached at 160 °C after 13900 h. First tests with the spring washers clarified, that the spring characteristic reduces the decrease of the joint force.

At all tests the joint resistance did not change, as the minimum joint force was not reached (Chapter 4.3). It can be concluded:

The material with the higher mechanical properties at initial conditions shows a worse long-term behaviour with respect to the force reduction.

4.2 Interpretation of the test results The test results are evaluated by using the Larson-Miller method [6]. The Larson Miller parameter is derived from the Mont-Grant law [10]

ms

mKtε

= (3)

K, m…Constant, sε …Strain rate, tm…Time to failure

and the Arrhenius-relationship

TRQ

s

c

eC ⋅−

⋅=ε . (4)

C…Proportionality factor T…Temperature R…Gas constant Qc…Activation energy

Solving the equations system of Eq. 3 and 4 leads to the Larson Miller parameter:

s1m lgmKtlg ε⋅−= KlgK1 = (5)

T1BBlg 21s ⋅−=ε ClgB1 = ;

RQ43,0B c

2 ⋅= (6)

T1BmBmKtlg 211m ⋅⋅+⋅−= (7)

T1'PKtlg 3m ⋅+= 113 BmKK ⋅−= ; 2Bm'P ⋅= (8)

)tlg'C(T'P m+⋅= 3K'C −= (9)

The parameter P’ is a function of the temperature T in K, the material specific parameter C’ and the time to failure tm in h. C’ can be calculated out of the test results at different temperatures and same stress conditions [6], [11]. In case of these tests the calculated value of C’ equals to 10.

21

1122

TTtlgTtlgT'C

−⋅−⋅= σ1(t1)T1=σ2(t2)T2 (10)

T1…First test temperature T2…Second test temperature t1,2…Time to reach the defined stress σ for T1 and T2

The result of the measured joint force versus the initial value of the long-term tests is plotted against the Larson-Miller parameter (Fig. 8 to 11). The curves are expected to follow a linear function. In case that a nonlinear behaviour appears, it has to be concluded that the activation energy QC has changed due to a change of the physical mechanism. As the superimposed setting processes will falsify the results (see also Chapter 3 and Fig. 2) the first 24 h of the long-term tests were not considered.

0

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0,4

0,6

0,8

1

1,2

8000 8500 9000 9500 10000 10500Larson-Miller parameter P' (C'=10)

join

t for

ce F

j / F

j0

105 °C - CS Washer140 °C - CS Washer160 °C - CS Washer

Fig. 8: Related joint force over Larson-Miller parameter (MC 1)

0

0,2

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1

1,2


join

t for

ce F

j / F

j0

105 °C - CS Spring washer140 °C - CS Spring washer160 °C - CS Spring washer


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1,2


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j / F

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105 °C - CS Washer140 °C - CS Washer160 °C - CS Washer Dynamic recristallisation

Start 300 h

End 3300 hStart 2860 h


0

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1,2


join

t for

ce F

j / F

j0

105 °C - CS Spring washer140 °C - CS Spring washer160 °C - CS Spring washer

Dynamic recristallisation


The results from the joints of the material MC 1 show a clear linear dependency over P’. Therefore only one physical mechanism (dynamic recovery) exists. At the temperatures 140 °C and 160 °C the results for copper joints of the material MC 2 shows a non-linear behaviour. The physical mechanism of the force reduction processes changed over time. Firstly, dynamic recovery processes identical to those seen for MC 1 started. After 2860 h at 140 °C and 300 h at 160 °C the dynamic primary recrystallisation is superimposed as a second mechanism. At the end of the dynamic primary recrystallisation process, an additional process started at temperature of 160 °C (Fig. 10 to 11). As a consequence of this process the texture created by the cold forming process was lost together with the improved mechanical strength. At 140 °C the necessary activation level for this process was not reached.

Some microstructure examinations and micro-hardness measurements were conducted to validate the physical mechanism of the dynamic primary recrystallisation. Therefore, test samples were aged in heating cabinets at the temperatures of 105 °C, 140 °C and 160 °C (Fig. 12).

40 m

m 22,5

mm

°…Points of micro-hardness measurement …Microsection examinations

Fig. 12: Test sample for microscopic examinations

Microscopic examinations of the test samples are performed after 3668 h at 160 °C. The results of the microscopic examinations showed, that no recrystallisation processes in the test sample of MC 1 were found (Fig. 13 to 14). The micro-structure and the micro-hardness did not change compared to the initial status. Only near the hole the cold forming

structure was lost and the micro-hardness decreased from 105 HV to 88 HV.

0

20

40

60

80

100

120

140

a b cMeasuring points

Har

dnes

s / H

V0,2

Cu - ETP MC 1 - Initial conditionCu - ETP MC 1 - 3668 h / 160 °C

Fig. 13: Micro-hardness of MC 1 over time in comparison to the initial condition

Fig. 14: Structural examination at MC 1 a) Initial condition b) 3668 h at heating cabinet

The microstructure and micro-hardness of the material MC 2 changed (Fig. 15 to 16). The micro-hardness is reduced from 107 HV to 54 HV, what can be expected as the hardness of annealed copper (appr. 50 HV - Fig. 2). The high mechanical strength incorporated by the cold forming process was lost completely.

0

20

40

60

80

100

120

140

a cMeasuring points

Har

dnes

s / H

V0,2

Cu - ETP MC 2 - Initial conditionCu - ETP MC 2 - 3668 h / 160 °C

Fig. 15: Micro-hardness of MC 2 over time in comparison to the initial condition

a) b)

No dynamic recristallisation processes proceeded

100 μm100 μm

1 mm 1 mm

Mirosectionac

b

Fig. 16: Structural examination of MC 2 a) Initial condition b) 3668 h at heating cabinet

Presumably, the same effect as seen in Fig. 15 and 16 exists at a temperature of 140 °C but would need a longer time to start and proceed.

4.3 Minimum joint force It is important to know the minimum joint force of the contact system to allow a correct interpretation of the results of the long-term tests. Therefore, a special test setup was utilized to determine the minimum contact force (Fig. 17).

Fig. 17: Experimental setup for measuring the minimum joint force

The stainless steel frame contains a load cell, a steel rod and the test joint. The steel pusher is surrounded by a DC current heater. Heating the steel pusher will lead to its expansion and generates a force to the joint. The joint is connected to the steel pusher and the steel frame. The electrical insulation of the joint towards the frame and pusher is provided by two MACOR-bases (Fig. 17). The power input is realized by a toroidal transformer. The current through the joint is measured by a high-precision current converter. Synchronously to the current, the voltage drop at the joint is measured.

With these sampled data the joint resistance is calculated. The joint force is measured by a HBM4)-load cell.

0

2

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10

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16

0 0,2 0,4 0,6 0,8 1 1,2 1,4joint force Fj / Fj0 long-term tests

join

t res

ista

nce

Rj /

μoh

m

Measuring 1 Measuring 2Measuring 3 Measuring 4 (2nd busbar pair)

Fig. 18: Joint resistance Rj over the related joint force Fj at 105 °C

The hysteresis-curve was measured at two joints with different Cu-ETP MC 1 busbar pairs (three measurements from one joint and one from the second joint). The plotted results show a small hysteresis between joint force and joint resistance during load and unload sequence (Fig. 18). These results are in good compliance to the values from the literature [1].

The diagram shows an unstable initial area followed by a stable region. That means, that in case the joint force falls below a minimum joint force of approx. 20 % of the initial value, the joints becomes unstable and the joint resistance will increase.

4.4 Lifetime-estimations of the joints For practical electrical applications lifetimes longer than 50 years are intended. Utilizing the Larson-Miller parameter (see also Chapter 4.2); the extrapolation of the lifetime up to 50 years was done (Fig. 19 to 21). As for 140 °C the recrystallization process is not finished, the final extrapolation might change (Fig. 10, 11 and 20).

0

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1

1,2

0 10 20 30 40 50time t / years

join

t res

ista

nce

F j /

F j0


Minimal joint force (static)

Fig. 19: Related joint force extrapolated (105 °C)

4) Measurement technique by the company Hottinger Baldwin

Messtechnik

a) b)

HBM load cell Measurement of the joint force

Stainless steel pusher Expansion of the pusher

force to the joint

Stainless steel frame

Toroidal transformer Power input measured by precision converter

I~U~

unstable stable

Dynamic recristallisation processes proceeded

Voltage drop

Arithmetic mean Confidence interval

100 μm100 μm

1 mm 1 mm

Load

Unload

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1,5

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0 0,2 0,4 0,6 0,8 1 1,2 1,4joint force Fj / Fj0 long-term tests

join

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Rj /

μoh

m

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j / F

j0




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1,2

0 10 20 30 40 50time t / years

join

t for

ce F

j / F

j0




Considering the force reduction and based on the extrapolated results, an operating temperature up to 160 °C for both material conditions and the joint system with spring washer is acceptable (Fig. 19 to 21). At joint systems with washers and the material MC 2 the minimum joint force will be under-run during the lifetime of 50 years at a temperature of 160 °C (Fig. 21) and likely for 140 °C (Fig. 20), too. Up to now for 140 °C the correct equations for extrapolation are not existent because the recristallisation process is still in progress (Fig. 10, 11 and 20).

5 Conclusion In pure copper joints the ageing mechanism of joint force reduction is influenced by different physical mechanisms: setting process, dynamic recovery, dynamic primary recrystallisation and grain coarsening (secondary recrystallisation). The acting mechanisms will depend on time, temperature, grain size at initial conditions, degree of cold work and foreign atoms. [6]

The result of the long-term tests showed that the force reduction is critical at temperatures ≥ 140 °C in joint systems with washers and material compositions allowing large deforming degrees of the Cu-ETP.

For Cu-ETP it was shown that the material condition having higher mechanical properties at initial state showed a worse long-term behaviour. For practical applications at high temperatures it is recommended to use the less cold-deformed material.

In further tests the recristallisation process at 140 °C will be verified. Furthermore, the influence of foreign atoms to the recrystallisation temperature and grain coarsening will be analysed. Besides, similar tests will be conducted with silver and tin coated copper busbar joints.

6 References [1] Böhme H.: Mittelspannungstechnik: Schalt-

anlagen berechnen und entwerfen; Huss-Medien, Verl. Technik; 2005; ISBN 3-341-01495-0

[2] Slade P. G.: Electrical Contacts: Principles and Applications; Cutler-Hammer Horsehead; New York; 1999; ISBN 0-8247-1934-4

[3] Braunovic M.: Electrical Contacts: Fundamentals, applications and technology; CRC Press, 2007; ISBN 978-1-57444-727-9

[4] Deutsches Kupferinsitut: Werkstoff-Datenblatter: Cu-ETP; DKI e. V; Düsseldorf; 2005

[5] Deutsches Kupferinstitut: Kupfer in der Elektrotechnik-Kabel und Leitungen; DKI e. V.; Düsseldorf; 2000

[6] Bürgel R.: Handbuch Hochtemperatur-Werkstofftechnik. Vieweg Verlag; 3. Auflage; 2006; ISBN 978-3-528-23107-1

[7] Ilschner B.: Werkstoffwissenschaften und Fertigungstechnik-Eigenschaften, Vorgänge, Technologien; 4. Auflage; Springer-Verlag; 2006; ISBN 3-540-21872-6

[8] Blumenroth F., Lücke N., Schlegel S., Großmann S., Löbl H.: Untersuchungen zum Langzeitverhalten von ruhenden Verbindungen in der Elektroenergietechnik; 20. Albert-Keil Kontaktseminar; VDE Verlag, pp.187-197; 2009

[9] Blumenroth, F., Löbl, H., Großmann, S., Kudoke, M.: Spannungsrelaxation an Kontaktelementen in Steckverbindungen der Elektroenergietechnik. Metall 62. Jahrgang, Ausgabe 11, S. 536-541, 2008

[10] Monkman F. C, Grant N. J.: An Emperial Relationship between Rupture Life and Minimum Creep Rate in Creep Rapture Tests, Proc. ASTM, S. 593, 1956

[11] Larson F., Miller J.: A Time-Temperature Relationship between for Rupture and Creep Stresses, Trans. ASME, S.765-775, 1952

Extrapolated with equations before recrystallisation process finish!

Minimum joint force under-run

Stephan Schlegel, (1983), VDE, received his diploma-degree in electrical engineering from the laboratory of electrical power system and high-voltage engineering (IEEH) at Technische Universität Dresden. Since 2008 he is employed as research assistant at the chair of high-voltage and high-current-engineering at Technische Universität Dresden and is working PhD-degree. His research interests are set on the ageing behaviour of high current copper contacts caused by the interdiffusion processes at coated surfaces and the reduction of joint force.

Steffen Großmann, (1954), VDE, received his diploma Technische Universität Dresden in 1976 and 1988. Between 1990 and 1996 he worked in the development department where he dealed with electrical and mechanical behaviour of fittings for substations and overhead transmission lines. From 1997 he worked as product team manager for substations and low voltage materials at Richard Bergner GmbH+Co, Radebeul, Germany. Since 2004 he is a full professor in the field of electrical power systems and high voltage engineering (IEEH) at Technische Universität Dresden.

Helmut Löbl, (1943), VDE, is Priv.-Doz. and senior lecturer at the laboratory of electrical power and high-voltage engineering (IEEH) at Technische Universität Dresden. He received his PhD-degree in electrical engineering from the Technische Universität Dresden in 1972 and 1985. He is dealing with the thermal stress of high-voltage devices and the aging of electrical joints.

Markus Hoidis, (1969) received his diploma-degree in material engineering from the Furtwangen University. Since 1993 he is employed as scientist at the ABB Corporate Research Center in Switzerland. One of his work fields is on contact materials, their stability and long term behaviour.

Uwe Kaltenborn, (1965), graduated at Dresden University of Technology and hold’s a PhD in EE from Darmstadt University of Technology. He is R&D Director of AREVA T&Ds global activities at Primary Distribution Switchgears. His research focus is on the thermal behaviour of MV switchgear and the properties of polymeric insulating materials for UHVAC and DC applications.

Tomasz Magier, (1975), received his diploma degree in electrical engineering form Politechnika Wroclawska (Poland) in 2001 and his PhD-degree from Technische Universität Dresden in 2007. Between 2007 and 2008 he was employed as Product Portfolio Manager at the Siemens AG, Power Transmission and Distribution, High Voltage Substations in Erlangen. Since end of 2008 he is employed as Development Engineer / Project Manager at Siemens AG, Energy Transmission Division in Concepts / Basics – Technology & Innovation Department in Berlin. His main research topic is worming optimization of High Voltage Gas Insulated Switchgears.

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