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Change in Electric Contact Resistance of Low-Voltage Relays Affected By materials Article Change in Electric Contact Resistance of Low-Voltage Relays Affected by Fault Current Andrzej Ksiazkiewicz, Grzegorz Dombek * and Karol Nowak Institute of Electric Power Engineering, Poznan University of Technology, Piotrowo 3A, 60965 Poznan, Poland * Correspondence: [email protected]; Tel.: +48-61-665-2192 Received: 6 June 2019; Accepted: 3 July 2019; Published: 5 July 2019 Abstract: Contact resistance is an important maintenance parameter for electromagnetic switches, including low-voltage relays. The flow of significant current through electric contacts may influence the contact surface and thus the value of the electric contact resistance (ECR). The change in ECR is influenced not only by the value of current but also by the current phase. Therefore, the impact of the switching short-circuit current’s phase on the ECR was analyzed in this paper. Significant changes in the resistance after each switching cycle were observed. The ECR decreased significantly after each make operation, and a correlation with current amplitude, total let-through energy, and short-circuit time was not observed. Keywords: relays; contact materials; electric contact resistance; contact welding 1. Introduction Low-voltage relays are commonly used to connect circuits with moderate switching currents. They have found an application mainly as executive elements in building automation systems [1,2] that are becoming more and more popular, as well as in programmable controllers [3,4]. The relays available on the market differ not only in technical parameters, but also in construction and purpose. An analysis of the literature on relays shows a certain research gap in the field of low- and medium-current AC switches. Research presented in referenced literature focuses either on different materials, like Ag-W, or higher test currents, over 1 kA, and contact force up to 50 N. It seldom touches upon the issues related to lower static contact force, in the range of centinewtons together with voltages up to 230 VAC and test currents reaching 300 A. In order to describe contact resistance, it is common to apply a one-point model with ellipsoidal, equipotential dimensioning currents [5]. There are also attempts to make new models to describe the value of contact resistance for two conducting surfaces [6]. The microstructure of a constant surface is presented on Figure1. The conducting contact area Ac is much smaller than the nominal (apparent) surface AC. The difference is significant and the actual contact area may constitute about 5% of the apparent contact surface [7,8]. The contact resistance of an electro-energetic switchgear is its significant parameter of use. It is important that the resistance in the maintenance period of a relay reach the smallest possible values and, at the same time, that it not change over time. Its value influences the acceptable working load of a relay which is related to its heating. The value of contact resistance depends on the shape and resistance of the thin film layer [5]. Resistance of the thin film layer Rn is difficult to establish analytically, as it depends on many factors, sometimes random, including ambient temperature, humidity, and contact material. Shape resistance Rk mainly depends on contact material and the clamping force of contacts. Materials 2019, 12, 2166; doi:10.3390/ma12132166 www.mdpi.com/journal/materials Materials 2019, 12, 2166 2 of 11 Materials 2019, 12, x FOR PEER REVIEW 2 of 11 FigureFigure 1. Electrical 1. Electrical surface surface contact contact model model withwith aspe asperitiesrities and and conducting conducting contact contact area area [8]. [8]. The valueThe value of the of the relay’s relay’s contact contact resistance resistance isis influencedinfluenced by by the the material material being being used. used. The Thecontact contact surface may be made of pure metals, including copper, silver, gold, platinum, palladium, wolfram, surface may be made of pure metals, including copper, silver, gold, platinum, palladium, wolfram, or molybdenum. However, alloys and sinters are more often used, such as silver-copper, silver- or molybdenum. However, alloys and sinters are more often used, such as silver-copper, silver-cadmium, cadmium, silver-palladium, silver-cadmium oxide, silver-wolfram, silver-nickel, and silver-tin oxide silver-palladium,[9]. The contact silver-cadmium surface may be covered oxide, with silver-wolfram, an additional layer silver-nickel, of material in and order silver-tin to enhance oxide its [9]. The contactproperties surface (e.g., resistance may be to covered material with transfer). an additional Coatings made layer of tin, of materialsilver, or gold in order are also to applied. enhance its propertiesThe coating (e.g., resistanceof a contact to point material with a transfer). layer of tin Coatings leads to a made minimal of tin, increase silver, of or contact gold areresistance also applied. in The coatingcomparison of a to contact a material point that with is not a layercoated. of The tin laye leadsr of to silver a minimal causes the increase reverse of effect contact of decreasing resistance in comparisonthe value to of a transition material thatcontact is notresistance coated. [10]. The layer of silver causes the reverse effect of decreasing the valuePresently, of transition the contactmost common resistance contact [10 materials]. for low-voltage relays of alternating currents of Presently,average power the mostare sinters common of silver contact with materialsnickel (AgNi), for low-voltagecadmium oxide relays (AgCdO), of alternating and tin oxide currents (AgSnO2). Materials that are made of silver-metal or silver-metal oxide typically present higher of average power are sinters of silver with nickel (AgNi), cadmium oxide (AgCdO), and tin oxide resistance to welding [11]. (AgSnO ). Materials that are made of silver-metal or silver-metal oxide typically present higher 2 The frequency of occurrence and the force of the welding of contacts increase proportionally to resistancethe value to welding of amperage [11]. of the electric arc, whereas the time of ignition in the arc does not have the Thesame frequency impact [12]. of The occurrence force of the and welding the force of cont of theacts welding does not ofshow contacts any dependence increase proportionally on the static to the valueforce ofof amperageclamping between of the electriccontact surfaces. arc, whereas However, the timeit is dependent of ignition onin the the travelling arc does pace not of have the the samemoveable impact [ 12contact.]. The These force considerations of the welding are of adequate contacts mainly does notfor those show contacts any dependence made of pure on silver the static force[12]. of clamping Some contact between materials contact present surfaces. a higher However,probability itof is welding dependent than others. on the If travelling a contact material pace of the moveableis characterized contact. These by a considerationshigh tendency to are welding, adequate then mainly the joints for thosewill be contacts strong. madeIn this of regard, puresilver pure [12]. Somesilver contact has materialsthe worst properties, present a highershowing probability a higher ten ofdency welding to the thanwelding others. of contacts If a contact [13], and material it is is one of the reasons why it is not used as contact material. Slightly better parameters are presented by characterized by a high tendency to welding, then the joints will be strong. In this regard, pure silver AgCdO. It has a lower tendency to welding but it is of higher resistance. Contact materials such as has the worst properties, showing a higher tendency to the welding of contacts [13], and it is one of the AgSnO2 and AgNi, used for the analyzed relays, are characterized by similar properties in terms of reasonswelding why and it is the not resistance used as contactto tearing material. apart [14]. Slightly In this case, better the parametersfirst one creates are stronger presented welds, by AgCdO.but It hasit a proves lower tendencythat it has a to lower welding tendency but it for is oftheir higher occurrence. resistance. Contact materials such as AgSnO2 and AgNi, usedThis for article the analyzed describes relays, the influence are characterized of the short- bycircuit similar current properties phase on in termsthe change of welding in electric and the resistancecontact to resistance. tearing apart The [contact14]. In thismaterial case,s tested the first were one AgNi, creates AgCdO, stronger and welds, AgSnO but2. The it provesAgSnO that2 was it has a lowertested tendency in two forvariations. their occurrence. For the first one, contact rivets were made using the internal oxidation Thisprocess article and are describes referred theto in influence the article as of simply the short-circuit AgSnO2. For current the second phase one, onthe therivet change was designed in electric to withstand higher inrush currents (up to 80 A for 20 ms) and are referred to in the paper as AgSnO2 contact resistance. The contact materials tested were AgNi, AgCdO, and AgSnO2. The AgSnO2 was P. In previous work [15], the experiments were carried out only for current switching at phase equal tested in two variations. For the first one, contact rivets were made using the internal oxidation process to zero and the AgCdO material was not tested. The results of this study are helpful in assessing and arecontact referred materials to in used the in article low-voltage as simply relays AgSnO as they2 .indicate For the how second the contact one, the resistance rivet was may designed change to withstandwhile higherswitching inrush fault current. currents This (up is to an 80 important A for 20 factor ms) andfor the are long-term referred exploitation to in the paper of relays as AgSnO used 2 P. In previousin modern work electrical [15], the installations. experiments were carried out only for current switching at phase equal to zero and the AgCdO material was not tested. The results of this study are helpful in assessing contact materials used in low-voltage relays as they indicate how the contact resistance may change while Materials 2019, 12, 2166 3 of 11 switching fault current.
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