Switch, Electric

a device for connecting and disconnecting electrical equipment such as lamps, motors, furnaces, transformers, and transmission lines. Electric switches may be of the low-voltage type (up to 1 kilovolt [kV]) or the high-voltage type (more than 1 kV). The basic structural elements of an electric switch are a contact system, which consists of movable and fixed contacts, and an actuator, which may be manual, spring-type, electromagnetic, or pneumatic. Electric switches for disconnecting heavy currents (hundreds or thousands of amperes) are equipped with arc extinguishers.

Low-voltage electric switches are made for household and industrial use. Household switches are used for connecting and disconnecting electrical appliances and devices operating on alternating current (50 hertz) at up to 220 volts and 10 amperes. Their construction depends on the materials used, the purpose of the unit, and the artistic and design considerations of the appliance or device. Household electric switches are manufactured with manual control or, much less frequently, with automatic controls, mainly for protection against current overloads. Single-pole, double-pole, and triple-pole industrial switches with current ratings ranging from several dozen amperes to thousands of amperes are used in electric power distribution circuits, mainly for protection against current overloads and short circuits; these switches are manufactured with both manual and automatic control. Automatically operated switches may also have protection against voltage drops: the switch disconnects automatically when the voltage drops below a specified limit. Knife switches, rotary switches, contactors, and magnetic starters are often regarded as industrial switches.

High-voltage electric switches are designed for manual or remote switching of high-voltage devices under normal operating conditions and for automatic disconnection of these devices under emergency conditions in case of current overloads or short circuits. The basic parameters of high-voltage switches are the rated voltage (3-750 kV and higher); the rated current (100 amperes to 12 kiloamperes and higher), which is the current that can flow through the switch for a long period of time without heating its parts above the permissible temperature; and the disconnect current (up to several hundred kiloamperes), which characterizes the disconnect capacity of the switch and which is determined by the maximum short-circuit current that the switch is able to dis-connect at a given voltage. In some cases the disconnect capacity of an electric switch is determined conventionally by the disconnect power, which can be as high as several dozen gigavolt-amperes.

Another important feature of the electric switch is the dis-connect time—that is, the elapsed time between the disconnect signal and the shutoff of the short-circuit current. The shorter this time, the less impairment and damage will result from the short circuit. The disconnect time usually ranges between 0.06 and 0.1 second, and in the best switch models it is reduced to 0.03 second. The basic structural elements and the classification of high-voltage switches are determined by the arc-extinguishing method or, more precisely, by the medium within which the arc is extinguished. Each type of electric switch has a specific range of applications that is standardized and based on maximum efficiency.