Low voltage relay contact resistance change influence by short-circuit current

Abstract

IntroductionElectromagnetic relays are commonly used to connect circuitry with moderate values of the switching current (i. e. the medium current circuits) at a voltage not exceeding 1000 V. They are used inter alia as actuators in the building automation systems (eg. KNX, LCN, LonWorks) or in drivers and the programmable relays (Easy, NEED). They differ among themselves with a structure, purpose, and technical parameters. In regard to the contacts of connectors made of different materials, in literature, there are often presented results of research carried out as well in normal operation as in terms of specific exposures. These studies, however, are often focused on the low voltage (<50 V) direct current circuits [8]. Similar research were performed by Morin [16], Neuhaus [17] or Doublet [6], who independently conducted similar work for contacting materials for low voltage direct current circuits. Research related to processes of switching in circuits of medium voltage alternating current are focused on current ranges from a few to several kA [1,9]. A noticeable is the lack of research in the field of low-and medium current AC switches at currents close to normal and short-circuit working conditions which may occur in low voltage electrical installations, that usually do not exceed 1 kA. Within the scope of modeling simulation studies are being conducted on the heating rail and contact connections with complex shapes, configurations and utilizing a variety of conductive materials [10].Relays for connecting the receiving circuits are exposed to unfavorable processes. These phenomena may include closing overload and short-circuit currents, which may lead to shortening the life ofrelays or, in extreme cases, their total damage. The article describes the impact of switching short-circuit on the change of the contact resistance. Relays with three different contacting materials have been investigated.Contact resistance of the connector is the important performance parameter. It is important that during the utilisation of the relay this resistance reaches values as small as possible and simultaneously do not differed significantly in time. Among other things, associated with its heating the safe working load of the relay depends on its value [15]. The contact resistance value is dependent on [7,14]:shape resistance -The resultant value of contact resistance (transition) is equal to:The tarnish resistance R n is difficult to determine analytically, since it depends on multiple, sometimes random, factors including: temperature, humidity, contact material. The shape resistance R k mainly depends on the resistivity and hardness of the contact material. To its description the single-point model with elliptical, equipotential current flowlines is often used [5,7]. The actual contact area is much smaller than the apparent (nominal) surface of the contact point. This model can be considered correct, at low contact pressure forces used in relays.The value of the contact resistance is affected by the material used for the con...

Change in Electric Contact Resistance of Low-Voltage Relays Affected by Fault Current

“…Since the circuit was of resistant nature (power ratio j ≈ 0.99), the current in the circuit was in phase of supply voltage. The results obtained for the switching-on at zero of the supplying current for contacts made of AgNi and AgSnO 2 have already been presented in [15]. The expected amperage of short-circuit current was limited with a resistor R lim (resistance value set is 0.729 Ω) to the value of 320 A ( I m = 453 A).…”

“…Testing circuit diagram: R , tested relay; Z , protection of the circuit (installation switch or fuse); R lim , limiting resistor; SYNCH, synchronizing device; SP, current probe; SN, voltage probe; CH1, CH2, oscilloscope channels [15]. …”

“…For the first one, contact rivets were made using the internal oxidation process and are referred to in the article as simply AgSnO 2 . For the second one, the rivet was designed to withstand higher inrush currents (up to 80 A for 20 ms) and are referred to in the paper as AgSnO 2 P. In previous work [15], the 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 switching fault current.…”

Analysis of Cable Screen Currents for Diagnostics Purposes

“…It is, however, noted that there is no international standard regarding cable screen connections. Literature presents general information about contact resistance, e.g., about contact resistance of electromagnetic relays [9,10], but information about cable screen resistance is limited to a few publications, e.g., [11]. Problems with cable screen connection exist due to a lack of standards, bad design, non-clear installation procedures, unskilled personnel, and a lack of information about the installation of power cables.…”

Accelerated storage degradation testing and failure mechanisms of aerospace electromagnetic relay

“…They differ among themselves with a structure, purpose, and technical parameters [3]. Aerospace EMR is the key mechanical and electrical component in the national defence weapon system or industry to realize the functions of signal transmission, load switching, circuit protection and control, and system isolation.…”