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A switching device is any mechanical, electrical, pneumatic or hydraulic device designed to open or close an electrical circuit. This is generic description though, and the term switching devices can be applied correctly to a number of devices and applications that fall outside those boundaries. When applied to electrical circuits, the term switching device typically refers to any device that makes or breaks an electrical circuit. These devices can be as diverse in their operation and appearance as a simple switch on a bedside lamp to a captive air circuit breaker weighing several tons. They all fulfill the same basic function, however.
Electrical switching devices all consist of sets of contacts that open and close when the device is activated. There are one set of contacts for each line, phase or part of the circuit they are designed to switch. For instance, a simple circuit breaker in a domestic distribution board will have a single set of internal contacts similar to the bedside lamp switch. The earth leakage unit in the same board would typically have two, one for the live line and one for the neutral line.
The small, low voltage relays that are commonly used in the automotive industry to switch driving lights, indicators and fog lamps on are classic examples of multi-contact switching devices as they typically have between six and eight sets of contacts. Larger, heavy duty examples are common in industrial applications where they are used as starters for electric motors and other heavy, high current drain equipment.
These devices are operated in a variety of ways, the simplest of which are manual switches such as the domestic light switch. Others are remotely operated using an electromagnetic coil to activate the switch. Pneumatic or hydraulic switches are also fairly common, and use either compressed air or oil pressure to open or close the switch.
Switching devices are also common in the IT field and, although the basic principle still applies, their function is very different from their high tension electrical counterparts. Data switching is the process of channeling several data sources to a single output point or identifying and diverting data signals between points. These processes differ from the electrical varieties in that the actual switching is done at electronic component level and not with heavy mechanical switches.
Switching devices are found in many different common appliances. It is extremely unlikely that anyone will go through a day without using a least one of them without a second thought.