Experimental Derivation

From Chapter 2 it becomes apparent that contact failure is the primary failure mode and is in the main part due to arc erosion and material transfer between one contact and another. This is due to the discharge of the arc causing contact surface damage, degradation of the performance, as well as eventual total failure of the contact. Failure modes may be dependent upon numerous factors; including the arcing time, loading, contact material, bounce and atmosphere and thus affects the amount of material transfer, shape and direction.

Three main categories of contact failures can be identified; failure from material loss, insulation contamination failure and gap bridging (Balme, 1990), the failure mode is also related to changes in contact gap. Contact surfaces are, relatively speaking smooth to the naked eye when under normal conditions and the contact gap travel is consistent. Once material transfer starts to occur, there is a loss of material from one contact causing the surface to be eroded, with material being deposited onto the opposite contact over a wide area. This may now cause an increase in the contact resistance as well as the size of the contact gap, which inevitably leads to reduction in the over travel time. (Morin, Laurent, et al., 2000), (Morin and Laurent, 2000).

Eventually, a point is reached where the over travel time becomes zero and the pair of contacts hardly touch one another, resulting in failure due to non-closing caused by the material loss. This mode of failure is defined as material loss failure. The other

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extent of this is where the material transfer is concentrated over a small area as opposed to the whole contact, resulting in the formation of a small pip in one contact (usually the anode). This leads to the gap between the contacts eventually becoming zero and causes a failure mode known as bridging, where the contacts remain electrically closed even when opening.

In the third case, the situation may occur where the pip and the corresponding crater due to the material removal are more or less axially symmetrical and therefore the contact is gaining material at the same extent that its counterpart is losing material. This results in the equal shifting of the contact surfaces and thus the over travel distance remains constant. (Leung et al., 1991)

Lastly, a failure mode called insulation contamination failure can also occur. Contact resistance as the surface insulation contamination due to arc erosion increases also increase and can cause non-closing failure. The complexity of the how the contacts fail presents the question of what is the best degradation parameter to use in order to provide a reliable feature vector for condition monitoring. Numerous authors have examined various strategies which will be described below.

(Hammerschmidt et al., 2004) looked at the interaction between material transfer and contact kinetics yielding actual switching failures. This paper suggested that detailed information about the development of failure processes caused by material transfer may be gained by measuring the contact force in relays during life-tests. However, to measure contact forces the relay has to be modified, e.g. the fixed contact has to be removed and mounted on a movable force sensor. No modification is required if merely the values of both opening time and closing time are measured.

Arc duration and subsequent erosion measurements for contacts made of pure silver, silver alloys (AgNi, AgCu), and silver metal-oxides (AgCdO, AgSnOa) were carried out by (Jemaa, 1996) for switches and relays connected to complex circuits (motor, resistance, and lamp) used in automobile field. The actual parameters of the experiments such as the contact material, the environment, voltage and current (50 V, 0.1-3 A DC) and mechanics (opening speed) were controlled. The accurate voltage values of the consecutive arc plateaus included in arc phases are determined by statistical measurements and voltage histogram drawings.

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The contact voltage drop, closing time and opening time were monitored by (Li, Kui, et al., 2000). In the end of test, the reliability of relay was estimated according to test data and the failure mode analysed.

Other measurements looked at are over-travel and contact gap (Xuerong et al. 2010), which was deemed extremely difficult to measure at present during the life of the test. The degradation and failure mechanism could only be analysed by testing time parameters of contacts, such as over-travel time and rebound duration. According to above mentioned analysis, different failure mechanisms will lead to different trends of contact gap and over-travel. The paper integrates over-travel time and rebound duration together to describe the degradation of contacts performance.

The most universally used metric for contact degradation in literature is contact resistance. This is also adopted by the manufacturers who often quote a contact resistance figure in their literature for a healthy contact and one deemed to have reached a point where failure is impending. This is useful, as it gives a threshold that may be used when a prognostic solution is proposed.

(Zhai, 2006) states contact resistance is an important parameter that directly reflects relay performance and it is also a basic datum to evaluate relay reliability. Based on theories of electromagnetic field, kinetics, contact force and electric contact, a full simulation analysis scheme of relay dynamic characteristics was presented by using the method of coupling finite element analysis and kinetic computation.

(Rieder and Strof., 1991) stated modern reliability requirements for relays cannot be satisfied unless the contact resistance is measured after each operation of the life test. A test device was developed to execute these measurements, and a special method was applied to reduce the resulting amount of data effectively without losing information. Commercial relays were investigated at intermediate and low power levels. Characteristic contact resistance patterns during the life of a relay were recorded depending on both the contact material and the electrical stress. Typical statistical patterns characterizing homogeneous and heterogeneous materials, erosion of contact plating, contact contamination were examined. (Chen, 1993), examined the contact morphology, surface composition, and contact resistance of DC relay contacts for two levels of low-current (less than 1 A) arc has shown that a similar contact erosion mechanism and similar contact resistance degradation exist for 0.5 A

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and 0.75 A switching a resistive load. Material transfer is attributed to ion sputtering during arcing, as is the degradation of contact resistance determined at different operating cycles, which appears to be influenced by both the contact morphology and surface contamination. A simple model is introduced and used to explain the process of arc erosion and contact resistance degradation during testing.