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electric switch[i¦lek·trik ′swich]
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.
REFERENCEChunikhin, A. A. Elektricheskie apparaty. Moscow, 1967.
A. M. BRONSHTEIN and V. T. NEZHDANOV
A device that makes, breaks, or changes the course of an electric circuit. Basically, an electric switch consists of two or more contacts mounted on an insulating structure and arranged so that they can be moved into and out of contact with each other by a suitable operating mechanism.
The term switch is usually used to denote only those devices intended to function when the circuit is energized or deenergized under normal manual operating conditions; as contrasted with circuit breakers, which have as one of their primary functions the interruption of short-circuit currents. Although there are hundreds of types of electric switches, their application can be broadly classified into two major categories: power and signal.
In power applications, switches function to energize or deenergize an electric load. On the low end of the power scale, wall switches are used in homes and offices for turning lights on and off; dial and push-button switches control power to electric ranges, washing machines, and dishwashers. On the high end of the scale are load-break switches and disconnecting switches in power systems at the highest voltages (several hundred thousand volts).
For power applications, when closed, switches are required to carry a certain amount of continuous current without overheating, and in the open position they must provide enough insulation to isolate the circuit electrically.
Load-break switches are required also to have the capability of interrupting the load current. Although this requirement is easily met in low-voltage and low-current applications, for high-voltage and high-current circuits, arc interrupters, similar to those used in circuit breakers are needed. In medium-voltage applications the most popular interrupter is the air magnetic type, in which the arc is driven into an arc chute by the magnetic field produced by the load current in a blowout coil. See Circuit breaker
Some load-break switches may also be required to have the capability of holding the contacts in the closed position during short-circuit conditions so that the contacts will not be blown open by electromagnetic forces when the circuit breaker in the system interrupts the short-circuit current.
For signal applications, switches are used to detect a specified situation that calls for some predetermined action in the electrical circuit. For example, thermostats detect temperature; when a certain limit is reached, contacts in the thermostat energize or deenergize another electrical switching device to control power flow.
Switches for signaling purposes are often required to have long life, high speed, and high reliability. Contaminants and dust must be prevented from interfering with the operation of the switch. For this purpose, switches are usually enclosed and are sometimes hermetically sealed.
Switches frequently are composed of many single circuit elements, known as poles, all operated simultaneously or in a predetermined sequence by the same mechanism. Switches are often typed by the number of poles and referred to as single-pole or double-pole switches, and so on. It is also common to express the number of possible switch positions per pole, such as a single-throw or double-throw switch.