Chapter 4 Contact Resistance Data Acquisition
4.5 Test Results
The results show a general structure and certain boundaries may be categorised in Figure 4.22, showing the ensemble of test results. Firstly at a, for high voltage and currents (in excess of 1 A), the initial contact resistance will in most cases be high. This is due in part to the build-up of oxides and deposits on the surface of the contacts during manufacture and within storage/non-operation. This initial high resistance is quickly reduced by the effect of electrical cleaning due to fretting and the thermal destruction of the oxide layers, which sees the contact resistance drop back to a normal range. Hence manufacturers often give a value of maximum contact resistance
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when the device is new, this is often way in excess to the normal operating contact resistance until the device has bedded in.
Section b depicts the climb in contact resistance up to the threshold of 100,000 cycles, this is the minimum number of cycles the manufacturer states contacts will operate for under a resistive loading. In general, a steady climb in the contact resistance may be observed up to this point. The effects of wear due to the physical effects discussed in chapter 2 become more apparent as the cycles increase.
The contact it makes is subject to bounce causing multiple interruption of the current which leads to erosion on the contact surface.
Figure 4.22. Montage of results, showing the various identified sections of contact degradation.
This severity of the contact erosion and how much material is transferred is dependent upon the load, current level, voltage, contact material, surface condition and bounce characteristics such as the gap, frequency, duration and timing. When this bounce subsides and tends to zero, the contacts close onto a molten liquid spot and weld together (known as dynamic welding).
The second part of the effects occur in section c when the contacts breaks. The moving contact opens to interrupt the current flow and when the contacts separate a sequence of two events occur on the surface contact. High current density heating of localized spots, spot melting, metal bridge transfer, formation of metal vapour arc and gaseous arc, arc transfer of contact material and finally arc extinction. This break
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erosion depends on device characterisation, arc time and arc voltage and contact material, (Leung and Lee, 1991).
Section b & c therefore is made up of the superposition of both effects and gradual degradation of the contact will occur. In section c the damage becomes a lot more apparent, with the surface damage causing greater changes in contact resistance. However the underlying trend, despite the oscillating peaks and troughs is an increase in general and to some extent this may be characterised as fitting a linear best fit. In section d, the eventual failure of all the relays under test was due to the contacts no longer opening due to welding. The oscillations in the resistance are more pronounced, with the eventual failure happening in most cases as the contact resistance takes a downturn. Failure is sudden and total, with complete loss of functionality. It can be seen from the number of cycles however, this failure is in all cases at least twice that of the stated minimum specified by the manufactures.
This coincides with the processes described in literature. The arc melts the surface of the contact for a period of time and after that the process of solidification begins, this can cause changes in the surface topography. (Leung, 2006) shows the effects for the arc in atmospheric air for Ag–SnO2 and Ag–SnO2–InO2.
For contacts opening in air, the various gases present can make the change in surface structure very complex, this is in part due to the reaction permutations between the arc, the contacts surfaces and the constitute chemicals in the air. This may lead to the formation of oxides, nitrides and carbonates. Additional complication may arise due to additional pollutants such as dust, oils and greases, sulphides and chlorides that are present.
This change of surface topology shows as a change in the contact resistance. The resistance may increase, decrease or remain the same depending on the contact material, arc characteristics, atmosphere and mechanics, such as force on the contacts and if there is enough pressure or sliding action to rupture the surface film. The above failure modes due to impurities may be reduced by using hermetically sealed relay cases, filled with an inert gas such as helium or argon.
Even with hermetically sealed contacts, erosion however is still present, depending on the arc varicosity on the surface, this erosion may reveal new, un-oxidized material on each operation, and this will cause the contact resistance to remain at a low value. As
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soon as a film forms, the closed contact will exhibit an increased temperature, leading to additional film formation. The process is summarized in figure 4.23 below.
In the above results, this becomes particularly evident, where as one may expect a general exponential rise to the end of life, the reality is far from this and can be explained from the above discussion as well as the section by section discussion.
Figure 4.23. Summarizing the variation in Rc in an electrical contact.