Light switch

Light switch

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A light switch is a switch
Switch
In electronics, a switch is an electrical component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another....

, most commonly used to operate electric light
Electric light
Electric lights are a convenient and economic form of artificial lighting which provide increased comfort, safety and efficiency. Most electric lighting is powered by centrally-generated electric power, but lighting may also be powered by mobile or standby electric generators or battery systems...

s, permanently connected equipment, or electrical outlets. In torches
Flashlight
A flashlight is a hand-held electric-powered light source. Usually the light source is a small incandescent lightbulb or light-emitting diode...

 (flashlight
Flashlight
A flashlight is a hand-held electric-powered light source. Usually the light source is a small incandescent lightbulb or light-emitting diode...

) 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).

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 other 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. A light switch box (a pattress box) has a plastic, ceramic or metal cover 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 Henry Holmes in 1884 in the Shieldfield district of Newcastle-Upon-Tyne. The "quick-break" switch overcame the problem of a switch's contacts developing electric arcing whenever the circuit was opened or closed. Arcing would cause pitting on one contact and the build-up of residue on the other, and the switch's useful life would be diminished. Holmes' invention ensured that the contacts would fly apart or come together very quickly, however much or little pressure was exerted on the switch itself by the user. The action of this "quick break" meant that there was insufficient time for an arc to form, and the switch would thus have a long working life. This "quick break" technology is still in use in almost every ordinary light switch in the world today, numbering in the billions, as well as in many other forms of electric switch.

Holmes was also a prolific inventor of other electrical devices including the "Castle" dynamo, early electrical systems in trains and the generation, lighting and searchlight systems which first allowed the Suez Canal to be safely operated at night, thus boosting its profitability due to the increased operating hours.

The toggle light switch was invented in 1917 by William J. Newton.

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
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

 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.

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, North America and Liberia 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 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.

The switch is designed to make the transition 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. This quick-break action of the switch is essential to a long life for the switch contacts, as disclosed in Holmes' 1884 patent.

While the contacts are separating, the energy stored in the inductance
Inductance
In electromagnetism and electronics, inductance is the ability of an inductor to store energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit...

 of the circuit is dissipated as an arc within the switch, prolonging the transition and worsening the heating effect on the contacts. Switches are commonly rated by the current they are designed to break, under specified voltage and power factor
Power factor
The power factor of an AC electric power system is defined as the ratio of the real power flowing to the load over the apparent power in the circuit, and is a dimensionless number between 0 and 1 . Real power is the capacity of the circuit for performing work in a particular time...

 conditions, as this is the most stringent constraint.

The arc that results when the switch operates erodes the switch contacts. A switch therefore has a finite life, often rated at a given number of cycles of disconnection at a specified current. Operation outside its specified capacity will shorten the switch life very drastically.

To combat contact corrosion a switch is usually designed to have a wipe action so that the contacts are cleaned. Large switches may be designed with a supplemental contact that closes and opens before the main contact, protecting the main current-carrying contacts from wear due to arcing. The contact area of the switch is constructed of materials that resist corrosion and arcing.

Many higher current switch designs rely on the separation arc to assist in dispersing contact corrosion. 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.

There are two kinds of "sparks" which may be seen during switch operation. On closure, a few sparks like those from a flint
Flint
Flint is a hard, sedimentary cryptocrystalline form of the mineral quartz, categorized as a variety of chert. It occurs chiefly as nodules and masses in sedimentary rocks, such as chalks and limestones. Inside the nodule, flint is usually dark grey, black, green, white, or brown in colour, and...

-and-steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...

 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.

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.

Push button



The push-button light switch has two buttons that alternative close and open the contacts. Pushing the raised button opens or closes the contacts and pops 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 toggle mechanism provides "snap-action" through the use of an "overcenter" geometry. The design was patented in 1916 by William J. Newton and Morris Goldberg. 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
Negative resistance
Negative resistance is a property of some electric circuits where an increase in the current entering a port results in a decreased voltage across the same port. This is in contrast to a simple ohmic resistor, which exhibits an increase in voltage under the same conditions. Negative resistors are...

.

This mechanism is safe, reliable, and durable, but produces a loud snap or click.
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.

Illuminated switch



An internal light source allows the user to locate the switch in the dark. Most European illuminated switches are two pole requiring the live and neutral wires to pass into the switch which enables the neon to be powered directly from the mains via a resistor. The internal light source in a single pole illuminated switch derives its power when the switch is OFF from current passing through the external light bulb. Single pole illuminated switches work well with incandescent bulbs, non-electronic fluorescent light fixture
Light fixture
A light fixture, light fitting, or luminaire is an electrical device used to create artificial light and/or illumination, by use of an electric lamp...

 and halogen lighting, but they can cause a few compact fluorescent lamps to behave erratically. The current through the neon lamp in the switch can slowly charge the internal input capacitor in these lamps, until they begin to operate and produce a brief discharge. This cycle may repeat indefinitely in some cases, resulting in brief repetitive flashing while the switch is in the "off" position.

Rocker



An alternative design to the toggle switch is the rocker, commonly known as "decorator" in the United States. An example is the Leviton
Leviton
Leviton Manufacturing Company, Inc., is the largest privately held manufacturer of electrical wiring equipment in North America. It produces electrical products: light sockets, receptacles , dimmers and other lighting control systems, wire, power cables, power cords, wall and ceiling occupancy...

 "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.

In the UK, Ireland, Hong Kong, Singapore and India this type is near-universal, where the toggle design would be considered old-fashioned.

In Australia, a miniature rocker switch is near universally used, with a 1cm oval switch that slots into a backing plate. Clipsal
Clipsal
Clipsal is an Australian brand of electrical accessories. The main factory was located at Bowden, but it was announced in 2008 that a move would be made to a new site, located at Gepps Cross. The move took place in 2009. Clipsal also boasts smaller factories in Nuriootpa, Strathalbyn and Wingfield...

 make a large cover plate that clips over the assembly, and can be removed for painting, which is very common. The same switch is also used in sockets, and it is extremely rare to find any other type of switch in Australian homes, although the Australian Wiring Standard AS 3112
AS 3112
AS 3112 is an Australian standard for mains power plug. It is also known as a Type I plug.-Description:The plug matching the pictured socket, used in Australia, Argentina, New Zealand, Fiji, Papua New Guinea and Tokelau, has an earthing pin and two flat pins forming an inverted V-shape...

 does not forbid other types. Some Australian home renovators import UK designed switches when they are aiming for a high quality finish, due to the lack of Australian designs.

Tamper resistant


Where lighting circuits must not be accidentally switched off (for example, school corridors), a vandal resistant switch
Vandal resistant switch
Vandal resistant switches are electrical switches designed to be installed in a location and application where they may be subject to vandalism or tampering, as in the case of pedestrian crossing switches, or fraudulent or unauthorized use, as in the case of vending machine 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 switch
Mercury switch
A mercury switch is a switch whose purpose is to allow or interrupt the flow of electric current in an electrical circuit in a manner that is dependent on the switch's physical position or alignment relative to the direction of the "pull" of earth's gravity, or other inertia.Mercury switches...

es 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. It is also possible to have the cord-operated switch separate from the light socket, which is particularly common in British bathrooms. Until 2001 it was a requirement of the UK wiring regulations that all bathroom switches were operated by pull cords.

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
Thyristor
A thyristor is a solid-state semiconductor device with four layers of alternating N and P-type material. They act as bistable switches, conducting when their gate receives a current trigger, and continue to conduct while they are forward biased .Some sources define silicon controlled rectifiers and...

 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
Motion detector
A motion detector is a device for motion detection. That is, it is a device that contains a physical mechanism or electronic sensor that quantifies motion that can be either integrated with or connected to other devices that alert the user of the presence of a moving object within the field of view...

 switches. 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
Wireless light switch
A wireless light switch is a light switch that commands a light or home appliance to turn itself off or on, instead of interrupting the power line going to the light fixture...

 provides remote control of lighting using a hand-held transmitter. Wired remote control of lighting switches is possible using, for example, X10 signaling
X10 (industry standard)
X10 is an international and open industry standard for communication among electronic devices used for home automation, also known as domotics. It primarily uses power line wiring for signaling and control, where the signals involve brief radio frequency bursts representing digital information...

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.