Electrical Contacts

Fundamentals, Applications and Technology

Milenko Braunovic, Nikolai K. Myshkin, Valery V. Konchits

• Publisher: CRC Press
• Year: 2017
• Print ISBN:
  9781574447279
• eBook ISBN:
  9781351838085
• Topic: Technology & Engineering
• Subtopic: Electrical Engineering & Telecommunications
• Index: Technology & Engineering

Various factors affect the performance of electrical contacts, including tribological, mechanical, electrical, and materials aspects. Although these behaviors have been studied for many years, they are not widely used or understood in practice. Combining approaches used across the globe, Electrical Contacts: Fundamentals, Applications, and Technology integrates advances in research and development in the tribological, material, and analytical aspects of electrical contacts with new data on electrical current transfer at the micro- and nanoscales. Taking an application-oriented approach, the authors illustrate how material characteristics, tribological behavior, and loading impact the degradation of contacts, formation of intermetallics, and overall reliability and performance. Coverage is divided broadly into three sections, with the first focused on mechanics, tribology, materials, current and heat transfer, and basic reliability issues of electrical contacts. The next section explores applications, such as power connections, electronic connections, and sliding contacts, while the final section presents the diagnostic and monitoring techniques used to investigate and measure phenomena occurring at electrical contact interfaces. Numerous references to current literature reflect the fact that this book is the most comprehensive survey in the field. Explore an impressive collection of data, theory, and practical applications in Electrical Contacts: Fundamentals, Applications, and Technology, a critical tool for anyone investigating or designing electrical equipment with improved performance and reliability in mind.

Part I

Fundamentals of Electrical Contacts

Milenko Braunović, Valery V. Konchits, and Nikolai K. Myshkin

1	Introduction to Electrical Contacts

1.1 INTRODUCTION

An electrical contact is defined as the interface between the current-carrying members of electrical/electronic devices that assure the continuity of electric circuit, and the unit containing the interface. The current-carrying members in contact, often made of solids, are called contact members or contact parts. The contact members connected to the positive and negative circuit clamps are called the anode and cathode, respectively.

Electrical contacts provide electrical connection and often perform other functions. The primary purpose of an electrical connection is to allow the uninterrupted passage of electrical current across the contact interface. It is clear that this can only be achieved if a good metal-to-metal contact is established. The processes occurring in the contact zone are complex and not fully explained within the limits of present knowledge. Although the nature of these processes may differ, they are all governed by the same fundamental phenomena, the most important being the degradation of the contacting interface and the associated changes in contact resistance, load, temperature, and other parameters of a multipoint contact.

Electrical contacts can be classified according to their nature, surface geometry, kinematics, design and technology features, current load, application, and by others means.^1,2,3 In general, electrical contacts can be divided into two basic categories: stationary and moving. Figure 1.1 represents the most general classification of electrical contacts according to contact kinematics, functionality, and design features.

In stationary contacts, contact members are connected rigidly or elastically to the stationary unit of a device to provide the permanent joint. Stationary contacts are divided into nonseparable or all-metal (welded, soldered, and glued), and clamped (bolted, screwed, and wrapped). Nonseparable (permanent) joints have a high mechanical strength and provide the stable electrical contact with a low transition resistance. A nonseparable joint is often formed within one contact member. For example, in commutating devices, only materials with a complex composition and arc-resistant working layers are used as the contact members. They are made by contact welding, soldering, coating, deposition, electrospark alloying, and mechanical methods of joining.

Clamped contacts are made by mechanically joining conductors directly with bolts or screws or using intermediate parts, specifically, clamps. These contacts may be assembled or disassembled without damaging the joint integrity. The simplest case of a clamped contact is the joint of two massive conductors with flat contact surfaces, such as busbars. A more complex joint configuration is a contact comprising several conductors, such as joints of a multistrand wire and clamp that are used for joining wire conductors in transmission lines.

The nature of clamped and all-metal contacts is different. This is because in the all-metal contacts there is no physical interface between conductors, whereas in clamped contacts the interface is controlled by the contact pressure and the ability of the material to undergo plastic deformation. The lower the specific resistance and hardness of a material, the higher its corrosion resistance and, consequently, the lower the contact transition resistance. For this reason, contact surfaces are usually covered with soft, corrosion-resistant materials such as tin, silver, cadmium, or similar materials. Different surface cleaning techniques are often used to improve the joint connectability.

FIGURE 1.1 Classification of electrical contacts.

In moving contacts, at least one contact member is rigidly or elastically connected to the moving unit of a device. Depending on their operating conditions, these contacts are divided into two categories: commutating and sliding. Commutating contacts intermittently control the electric circuit. They fall into two categories: separable (various plug connectors, circuit breakers) and breaking. The latter are used for a periodical closing and opening of an electrical circuit, such as in different switches, contactors, relays, and similar devices. Because of differences in breaking power, current, and voltage, there is a great variety of breaking contacts. The breaking contacts can be classified as light-, medium- and heavy-duty:

• Light-duty contacts carry very low currents, operate at voltages up to 250 V, and display no appreciable arc-related electrical wear. The successful operation of these devices depends mainly on maintaining relatively low and stable contact resistance and also on the selection of the contact materials. The factors that must be taken into account are tendency to oxidize (tarnish); presence of dirt, dust or other contaminants on the contact surface; and contact design (form, size, contact pressure, and finish). Light-duty contacts are intended for use in instrument controls, general automation, radio and data communication, and telecommunication systems.

• Medium-duty contacts carry appreciably higher currents (see 5A above) and operate at voltages up to 1000 V. For this group, electrical wear is of prime importance. The factors governing contact material selection to meet the very severe operating conditions include tendency to welding, material transfer, and erosion (pitting). Applications of medium-duty contacts are control devices for industrial, domestic, and distribution network applications.

• Heavy-duty contacts carry very high currents (tens of kA) and operate at very high voltages (hundreds of kV). The most common types of these connectors are contactors, starters, and circuit breakers.

In sliding contacts, the contacting parts of the conductors slide over each other without separation. Current passage through the contact zone is accompanied by physical phenomena (electrical, electromechanical, and thermal) that produce changes in the state (characteristics) of surface layers of the contacting members that differ when operating without current (see Figure 1.2). The severity of the processes occurring at contact interface depends on the magnitude and character of the current passing through the contact, the appli...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Part I Fundamentals of Electrical Contacts
• Part II Applications of Electrical Contacts
• Part III Diagnostic and Monitoring Technologies
• Appendix 1: Methods of Description of Rough Surface
• Appendix 2: Shape-Memory Materials
• Appendix 3: Electrical Contact Tables
• References
• Index