What Is A Load Switch?

A switch between a circuit breaker and a disconnector
It is generally equipped with a simple arc extinguishing device, but its structure is relatively simple. The figure shows a compressed air type high-voltage load switch. Its working process is: when opening the switch, under the action of the opening spring, the main shaft rotates clockwise. On the one hand, the piston moves upward through the crank slider mechanism to compress the gas; on the other hand, through the transmission system composed of two sets of four-bar mechanisms, the main switch knife is opened first, and then the arc extinguishing knife is pushed to open the arc contact, and the compressed air in the cylinder blows out the arc through the nozzle.

1. Basic concept
The load switch is a switch between a circuit breaker and a disconnector. It has a simple arc extinguishing device and can cut off the rated load current and a certain overload current, but cannot cut off the short-circuit current.
2. Classification of load switches
According to the operating voltage, it can be divided into high-voltage load switches and low-voltage load switches.
2.1 High-voltage load switch
2.1.1 There are 6 main types of high-voltage load switches.
① Solid gas-generating high-voltage load switch: The energy of the arc itself is used to make the gas-generating material in the arc chamber produce gas to blow out the arc. Its structure is relatively simple and suitable for products of 35 kV and below.
② Compressed gas high-voltage load switch: The compressed gas of the piston is used to blow out the arc during the breaking process. Its structure is also relatively simple and suitable for products of 35 kV and below.
③ Compressed air high-voltage load switch: The arc is blown out by compressed air, which can break a large current. Its structure is relatively complex and suitable for products of 60 kV and above.
④ SF6 high-voltage load switch: The arc is extinguished by SF6 gas. Its breaking current is large and its breaking capacitance current performance is good, but its structure is relatively complex and suitable for products of 35 kV and above.
⑤ Oil-immersed high-voltage load switch: The energy of the arc itself is used to decompose and gasify the oil around the arc and cool it to extinguish the arc. Its structure is relatively simple, but its weight is large and suitable for outdoor products of 35 kV and below.
⑥ Vacuum type high voltage load switch: uses vacuum medium to extinguish arc, has long electrical life, relatively high price, and is suitable for products of 220 kV and below.

2.1.2 Working principle
The working principle of high voltage load switch is similar to that of circuit breaker. Generally, it is equipped with a simple arc extinguishing device, but its structure is relatively simple. The figure shows a compressed air type high voltage load switch, and its working process is: when opening, under the action of the opening spring, the main shaft rotates clockwise, on the one hand, the piston moves upward through the crank slider mechanism to compress the gas; on the other hand, through the transmission system composed of two sets of four-bar mechanisms, the main switch knife is opened first, and then the arc extinguishing knife is pushed to open the arc contact, and the compressed air in the cylinder blows out the arc through the nozzle. When closing, through the main shaft and transmission system, the main switch knife and arc extinguishing knife rotate clockwise at the same time, and the arc contact is closed first; the main shaft continues to rotate, so that the main contact is closed later. During the closing process, the opening spring stores energy at the same time. Since the load switch cannot break the short-circuit current, it is often used in combination with a current-limiting high-voltage fuse. The current-limiting function of the current-limiting fuse can not only complete the task of breaking the circuit, but also significantly reduce the heat and electromotive force caused by the short-circuit current.

2.2 Low-voltage load switch
Low-voltage load switches are mainly divided into the following categories: knife switches, combination switches and compound switches.
2.2.1 Knife switch: The knife switch is the simplest and most widely used electrical appliance in the low-voltage switch. It is often used in lighting circuits and control circuits that are not frequently started. It mainly plays the role of switching on and off and isolating the circuit. Common knife switch models include rubber-covered knife switches, iron-shell switches and fuse-type switches, which are suitable for different circuit requirements.
2.2.2 Combination switch: The combination switch is also called a transfer switch. It has good seismic resistance and operational flexibility, but has low switching capacity and does not have overload protection and short-circuit protection. Common combination switch models include HZ5 series and HZ10 series, which are mainly used to connect or disconnect circuits and are suitable for forward and reverse control of small-power motors and other circuit controls.
2.2.3 ‌Compound switch‌: The compound switch combines the characteristics of knife switch and combination switch, has good arc extinguishing performance and convenient operation, and is suitable for various low-voltage electrical lighting circuits and branch distribution switches. Its structure includes porcelain base, static socket, incoming terminal, outgoing terminal, contact knife, porcelain handle (handle), fuse and other components. These classified low-voltage load switches have their own characteristics and are suitable for different circuit requirements and scenarios, ensuring the safe and stable operation of the power system. ‌

2.3 Low-voltage load switch is also called switch fuse group. It is suitable for manually switching on and off loaded circuits in AC power frequency circuits infrequently; it can also be used for overload and short-circuit protection of lines. The circuit is switched on and off by the contact knife, and the overload and short-circuit protection is completed by the fuse. The rubber-covered knife switch and iron-shell switch used before the 1970s are both low-voltage load switches. The contact opening and closing speed of small-capacity low-voltage load switches is related to the handle operation speed. The operating mechanism of the low-voltage load switch with larger capacity adopts the spring energy storage action principle. The opening and closing speed has nothing to do with the speed of the handle operation. The structure is relatively simple and is equipped with a reliable mechanical interlocking device. The switch cannot be closed after the cover is opened, and the cover cannot be opened after the switch is closed, which can ensure work safety.