Contacts are one of the critical components of switching devices; the primary performance characteristics and service life of such devices depend, to a large extent, on the quality of the contact materials used. Contact materials are typically required to possess excellent electrical conductivity, low contact resistance, high resistance to contact welding, high resistance to arc erosion, and strong resistance to material transfer. For vacuum contact materials, additional requirements include a low chopping current value, high dielectric strength, and high interrupting capacity. The microstructure of a contact material exerts a significant influence on its macroscopic properties; the electrical performance of contact materials-such as resistance to welding, arc ablation, and dielectric breakdown-depends not only on the chemical composition of the material but also on the grain size of its constituent crystals.
Contact Classification
Interruptible contacts are an indispensable part of switching devices. Based on their structural design, they can be broadly classified into the following types:
(1) Knife-edge Contacts: Characterized by a simple structure, these are classified into surface-contact and line-contact types. They are widely utilized in low-voltage switches and high-voltage disconnectors.
(2) Butt Contacts: These feature a simple structure and rapid actuation speed; however, their contact surfaces are relatively unstable-varying significantly with changes in contact pressure-and they are prone to contact bounce during operation. Furthermore, they lack a self-cleaning mechanism, making the contacts susceptible to damage from arc erosion. This type of contact is commonly employed in distribution circuit breakers with rated currents below 1000 A, and particularly in those rated under 500 A.
(3) Wedge-type (Finger) Contacts: These consist of pairs of contact fingers-spring-loaded and secured to a conductive base via double-ended studs-and a central wedge-shaped contact block. Typically, the wedge-shaped block serves as the moving contact; however, reverse configurations-where the wedge acts as the stationary contact and the fingers clamped to the base act as the moving contacts-also exist. During the engagement of the moving and stationary contacts, mutual friction occurs, resulting in a self-cleaning action on the contact surfaces. This design offers high electrodynamic stability and inherent self-cleaning capabilities. While increasing the number of contact finger-and-wedge sets can raise the rated current capacity, it also increases the lateral dimensions of the assembly, potentially complicating the installation process. The operating current is generally limited to below 5000 A, though it can reach up to 12,000 A in certain applications. Since the working surfaces of these contacts are susceptible to damage from arc erosion, they typically serve solely as main contacts and are not utilized as arc-extinguishing contacts.
(4) Plug-in (Petal-type) Contacts: The stationary contact consists of multiple trapezoidal contact fingers. These are classified into two types: those with flexible conductive strips and those without. In sockets featuring flexible conductive strips, each contact finger contains a groove fitted with an insulating sleeve and a helical spring; this arrangement ensures that the contact finger exerts sufficient pressure against the conductive rod. The other end of the spring is supported by a retaining ring, allowing for slight positional adjustment of the contact fingers around the circumference of the conductive rod (the moving contact). The contact fingers are connected to the contact base via the flexible conductive strips. In sockets without flexible conductive strips, the structurally complex and potentially unstable conductive strips are eliminated; instead, springs are used to press the contact fingers directly against the conductive base. The moving contact consists of a circular copper conductive rod. To enhance the contact's arc-resistance capabilities, a protective ring made of copper-tungsten alloy is often fitted to the end of the contact base housing, while an arc-resistant tip-also made of copper-tungsten alloy-is attached to the end of the conductive rod. During the closing operation, the conductive rod is inserted into the socket, and the trapezoidal contact fingers are pressed against the rod by the springs. Through the precise fit between the inner diameter of the socket and the conductive rod, each contact finger establishes two lines of contact with the rod, ensuring reliable electrical connection. Furthermore, the direction of the contact pressure between the moving and stationary contacts is perpendicular to the direction of motion, resulting in minimal contact bounce during closing. Relative motion between the moving and stationary contacts generates friction, which provides a self-cleaning effect. When a short-circuit current flows through the contacts, the current direction within the contact fingers-as well as between the fingers and the conductive rod-is aligned; consequently, the resulting electromagnetic forces tend to press the contact fingers more firmly against the conductive rod, thereby ensuring excellent dynamic stability. However, this contact structure is relatively complex, the permissible current capacity is limited, and the circuit-breaking time tends to be longer. Consequently, this type of contact is predominantly utilized in power distribution networks operating at voltages below 35 kV. Sliding contacts are designed to maintain a continuous electrical connection between the moving and stationary contacts while simultaneously allowing for relative motion between them without separation. They are classified into two types: Z-shaped finger-type sliding contacts and roller-type sliding contacts.
(5) Z-shaped Finger-type Sliding Contacts: The structural design of this type is similar to that of the plug-in (socket-type) contacts. It is constructed by housing Z-shaped contact fingers within a conductive base; springs are used to maintain the position of the fingers, pressing their opposing sides against the conductive rod and the conductive base, respectively. Its advantages include a compact profile, simple assembly, the absence of separate contact strips, stable contact performance, and a self-cleaning effect; consequently, it enjoys widespread application.
(6) Rolling-Type Sliding Contacts: In this configuration, the moving contact consists of a circular conductive rod, while the stationary contact comprises a conductive base formed by two circular rods, along with pairs of red copper rollers mounted between the conductive rod and the base. Springs are installed on both sides of the rollers; through the pressure exerted by these springs, continuous contact is maintained between the rollers and the conductive rod, as well as between the rollers and the conductive base. Current is conducted along the path formed by the conductive rod, the rollers, and the conductive base. Since the moving contact is in motion, the rolling friction resistance is low; however, the self-cleaning capability of the contact is relatively poor. This type of contact is frequently utilized as an intermediate contact within high-voltage circuit breakers.
