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A light switch is a switch, most commonly used to operate electric lights, permanently connected equipment, or electrical outlets. In torches (flashlight) the switch is often near the bulb, but may be in the tail, or even the entire head itself may constitute the switch (rotated to turn the light on and off).
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Wall-mounted switches
Switches for lighting may be in hand-held devices, moving vehicles and buildings. Residential and commercial buildings usually have wall-mounted light switches to control lighting within a room. Mounting height, visibility, and otehr design factors vary from country to country. Switches are often recessed within a finished wall. Surface mounting is also fairly common though is seen more in commercial industrial and outbuilding settings than in houses. Light switches boxes have plastic, ceramic or metal covers to prevent accidental contact with live terminals of the switch. Wall plates are available in different styles and colours to blend in with the style of a room.
History and culture
The first light-switch employing quick-break technology was invented by John Holmes in 1884 in the Shieldfield district of Newcastle-Upon-Tyne.[1] The toggle light switch was invented in 1916 by William J. Newton and Morris Goldberg.[2]
As a component of a building wiring system, installation of light switches will be regulated by some authority concerned with safety. The dimensions, mechanical designs, and even the general appearance of light switches changes very slowly with time. They frequently remain in service for many decades, often being changed only when a portion of a house is rewired. It is not extremely unusual to see century-old light switches still in functional use. Manufacturers introduce various new forms and styles, but for the most part decoration and fashion concerns are limited to the faceplates. Even the "modern" dimmer switch with knob is at least four decades old, and even in the newest construction the familiar toggle and rocker switch appearances predominate. The shape and size of the boxes and faceplates as well as what is integrated (for example in the UK it is normal to have the switch built into the plate) varies a lot by country.
Up or down
The direction which represents "on" also varies by country. In North America it is usual for the "on" position of a toggle switch to be "up", whereas in many other countries such as the UK, Ireland, Australia, and in New Zealand it is "down." In multiway switching, the correspondence between a single switch's state and whether lights are on or off depends on other switches in the circuit. In countries prone to earthquakes, such as Japan, most switches are positioned sideways to prevent the switch from inadvertently being turned on or off by falling objects[citation needed].
Design
The switches may be single or multiple, designed for indoor or outdoor use. Optional extras may include dimmer-controls, environmental protection, weather & security protection.
In the case of light switches, the circuit to be switched is within 10% of 230 volts at 5A 6A or 10A for all European and most of South American, African and Asian countries, whereas Japan and North America use a supply between 100 and 127 volts with maximum circuit currents of up to 15 or 20 amperes so the overall power per circuit is similar. In the US it was formerly considered acceptable to mix outlets and lighting on the same circuit; however, building codes in effect for the past three decade in most areas have required that lighting and receptacles be on separate circuits.[citation needed] In the UK putting normal 13A BS1363 sockets on a lighting circuit is frowned upon (though not explicitly prohibited) but 2A or 5A BS546 outlets are often put on lighting circuits to allow control of free-standing lamps from the room's light switches. In the U.S., this is very common in mobile homes. It is common in American site-built housing for living rooms and bedrooms to have a switched receptacle for a floor or table lamp.
Internal operation
A switch is most vulnerable when the contacts are opening or closing. As the switch is closed, the resistance of the switch changes from nearly infinite to nearly zero. At infinite resistance, no current flows and no power is dissipated. At zero resistance, there is no voltage drop and no power is dissipated. When the switch changes state however, there is a brief instant of partial contact when resistance is neither zero nor infinite and power is dissipated. During that transition the contacts heat up. If the heating is excessive, the contacts can be damaged or even weld themselves closed.
Thus the switch is designed to make the transition between effectively infinite resistance and effective zero resistance as swiftly as possible. This is achieved by the initial operation of the switch lever mechanism storing potential energy, usually as stress in a spring. When sufficient energy is stored, the mechanism in the switch "breaks over" driving the contacts through the transition from open to close, or close to open, without further input by the switch operator.
In addition, during the transition when the contact is broken there is an additional issue that if an inductive load is being switched, the stored energy in the inductor is dissipated as an arc within the switch, prolonging the transition and worsening the heating effect on the contacts. Thus switches are commonly rated by the current they are designed to break, as this is the most stringent constraint.
The arc that results when the switch operates corrodes the switch contacts, in time leading to erosion of the contact surface and fouling of the contact area by corrosion byproducts. A switch therefore has a finite life, again often being rated at a given number of cycles of disconnection at a specified current. Operation outside its design envelope will shorten the switch life very drastically.
To combat contact corrosion a switch is usually designed to have a wipe action such that the contact corrosion is cleaned off the area of the contact that forms the low resistance path when the switch is closed. It's also designed so that the initial point of contact, and thus the majority of the contact corrosion, occurs at a sacrificial part of the contact, rather than the face that is in contact when the switch is fully closed. Depending on the switch rating and price, the contact area of the switch is often a sophisticated construction of brass contact, silver contact button, and plated finish to minimize the amount of contact corrosion and thus extend the life of the switch.
Many higher current switch designs rely on the separation arc to assist in dispersing contact corrosion, and that a switch designed for high current/high voltage use may become unreliable if operated at very low currents and low voltages because the contact corrosion builds up excessively without an arc to disperse it.
When a pair of contacts are badly designed, the contacts themselves are visible, or the switch is overloaded in relation to its design then there are two kinds of "sparks" which may be seen. On closure, a few sparks like those from a flint-and-steel may appear as a tiny bit of metal is heated to incandescence, melted, and thrown off. On opening, a bluish arc may occur with a detectable "electrical" (ozone) smell; afterwards the contacts may be seen to be darkened and pitted. Damaged contacts have higher resistance, rendering them more vulnerable to further damage and causing a vicious circle in which the contacts soon fail completely.
To make a switch safe, durable, and reliable, it must be designed so that the contacts are held firmly together under positive force when the switch is closed. It should be designed so that regardless of how the person operating the switch manipulates it, the contacts always close or open quickly. Despite this, a switch should not be held between its two positions (on or off); this is especially true on older mechanisms.
The spring that stores the energy necessary for the snap action of the switch mechanism, in many small switch designs is made of a beryllium copper alloy, that is hardened to form a spring as part of the fabrication of the contact. The same part often also forms the body of the contact itself, and is thus the current path. Abusing the switch mechanism to hold the contacts in a transition state, or severely overloading the switch, will heat and thus anneal the spring, reducing or eliminating the "snap action" of the switch, leading to slower transitions, more energy dissipated in the switch, and progressive failure.
Variations on design
Push button
Prior to the toggle switch a popular design was the push-button switch, composed of a depressed button oriented below or beside a raised button. Pushing the raised button opens or closes the contacts while popping out the previously depressed button so the process can be reversed. Push button switch reproductions are available on the market today for vintage or authentic styling.
Toggle
The traditional light-switch mechanism is a toggle mechanism that provides "snap-action" through the use of an "overcenter" geometry. The design was patented in 1916 by William J. Newton and Morris Goldberg.[2] The switch handle does not control the contacts directly, but through an intermediate arrangement of springs and levers. Turning the handle does not initially cause any motion of the contacts, which in fact continue to be positively held open by the force of the spring. Turning the handle gradually stretches the spring. When the mechanism passes over the center point, the spring energy is released and the spring, rather than the handle, drives the contacts rapidly and forcibly to the closed position with an audible "snapping" sound. The snap-action switch is a mechanical example of negative resistance.
This mechanism is safe, reliable, and durable, but produces a loud snap or click. (Many people have at some point in their lives made an attempt to reduce this noise by operating the handle slowly or gingerly. Of course this is to no avail, since the very purpose of the mechanism is to ensure that the electrical portion of the switch always operates rapidly and forcefully — and noisily — regardless of how the handle is manipulated).
As of 2004[update] in the United States, the toggle switch mechanism was almost entirely supplanted by "quiet switch" mechanisms. "Quiet switch" mechanisms still possess a form of snap action, but which is very weak as compared to its predecessor. They are therefore equipped with larger, high-quality contacts that are capable of switching domestic loads without damage, despite the less-positive action.
Rocker
An alternative design is the rocker, commonly known as "decorator" in the United States. An example is the Leviton "Decora". This design sits flush to the wall, and is activated by "rocking" a paddle , rather than pushing a short protruding handle up and down. This type is near-universal in the UK, Ireland and India, where the toggle design would be considered old-fashioned.
Tamper resistant
Where lighting circuits must not be accidentally switched off (for example, school corridors), tamper-resistant light switches may be installed. These require a key to operate and so discourage casual or accidental operation of the switch.
Voltage class
In North American commercial and industrial lighting installations, lighting installed on 480Y/277 V 3 phase circuits uses voltages higher than the rating of common 120 V switches. Switches for these circuits are physically larger, so that a low-voltage switch is unlikely to be accidentally interchanged with one rated for 277 V - the low voltage switch will not align with the mounting holes in the switch box.
Mercury switches
Before the 1970s, mercury switches were popular. They cost more than other designs, but were totally silent in operation. The switch handle simply tipped a glass vial, causing a large drop of mercury to roll from one end to the other. As it rolled to one end, the drop of mercury bridged a pair of contacts to complete the circuit. Many of them also would glow faintly when they were "off" to aid people in finding them when the room was dark. The vial was hermetically sealed, but concerns about the release of toxic mercury when the switches were damaged or disposed of led to the abandonment of this design. In the U.S. there has never been any effort to recall or replace existing mercury switches, and millions of them remain in use.
Pull chain/pull cord
A light switch combined with a light socket is sometimes installed in basement or utility areas of homes. The switch is operated by a pull chain or cord. This type is particularly common in British bathrooms, on the basis that it prevents people with wet hands from making accidental electrical contact with the mains power supply.
Dimmer switch
A dimmer switch contains a solid-state circuit to allow changing the brightness by reducing the average voltage applied to the lamp.
Electronic switches
In principle, it is easy to design silent switches in which the mechanical contacts do not directly control the current, but simply signal a solid-state device such as a thyristor to complete the circuit. Many variations on this theme have been created and marketed. "Touch-plate" devices can be operated by touching or merely waving a hand near the switch. Touch switches have no moving parts and electronically switch the light circuit. Public buildings such as hospitals frequently save energy by using "motion-detector" switches. As of 2006[update] these remain specialty items. Electronic switches provide flexibility in terms of different interfaces for their operations, such as touch plates, soft-touch controls, pressure / light sensor based control, interactive touch-screens (which are widely used in aircraft for lighting control) and others.
A wireless light switch provides remote control of lighting using a hand-held transmitter. Wired remote control of lighting switches is possible using, for example, X10 signaling over the power wires.
Multiway switching
Two or more light switches can be interconnected to allow control of lighting from, for example, two ends of a long hallway or landings at the upper and lower landings of a flight of stairs.
See also
References
- ^ Electric Light Years 1878-1899 - Timeline of North East England History
- ^ a b "Electrical Wall Switch" (PDF). Google Patents. July 13, 1916. http://www.google.com/patents?id=eiVBAAAAEBAJ&printsec=abstract&zoom=4&dq=Morris+Goldberg+-+1916&source=gbs_summary_r&cad=0_0#PPA12,M1.pdf. Retrieved 2009-05-29.
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