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In building wiring, a light switch is a switch, most commonly used to operate electric lights, permanently connected equipment, or electrical outlets. Portable lamps such as table lamps will have a light switch mounted on the socket, base, or in-line with the cord. Manually operated on/off switches may be substituted by remote control switches, or light dimmers that allow controlling the brightness of lamps as well as turning them on or off. Light switches are also found in flashlights and automobiles and other vehicles.
- 1 Wall-mounted switches
- 2 History, culture and style
- 3 Design
- 4 Variations on design
- 5 Multiway switching
- 6 Materials
- 7 See also
- 8 References
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, culture and style
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.
The toggle light switch was invented in 1917 by William J. Newton.
As a component of a building wiring system, the installation of light switches will be regulated by some authority concerned with safety and standards. In different countries the standard dimensions of the wall mounting hardware (boxes, plates etc.) may differ. Since the face-plates used must cover this hardware, these standards determine the minimum sizes of all wall mounted equipment. Hence, the shape and size of the boxes and face-plates, as well as what is integrated, varies from country to country.
The dimensions, mechanical designs, and even the general appearance of light switches changes but 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 face-plates or wall-plates. Even the "modern" dimmer switch with knob is at least four decades old, and in even the newest construction the familiar toggle and rocker switch appearances predominate.
Up or down
The direction which represents "on" also varies by country. In the USA and Canada, 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 as well as rest of the world 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 and Mexico (as well as most of Latin America), most switches are positioned sideways to prevent the switch from inadvertently being turned on or off by falling objects.
The switches may be single or multiple, designed for indoor or outdoor use. Optional extras may include dimmer-controls, environmental protection, weather and 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.
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 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 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-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 cycle 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.
Variations on design
The push-button light switch has two buttons that alternatively 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.
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. This mechanism is safe, reliable, and durable, but produces a loud snap or click. 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.
Single pole illuminated switches derive their power from current passing through the external light bulb. Such switches work well with incandescent bulbs, non-electronic fluorescent fixtures, and halogen lighting. However, they can cause a few compact fluorescent lamps (CFLs) to behave erratically. Small amounts of current can trickle through the neon lamp in the switch, slowly charging the internal input capacitor in the CFL until it produces a brief discharge. This cycle may repeat indefinitely in some cases, resulting in brief repetitive flashing while the switch is in the "off" position. Most European illuminated switches are two-pole, requiring the hot and neutral wires to pass into the switch in order for it to illuminate.
An alternative design to the toggle switch is the rocker, commonly known as "decorator" in the United States. This design sits flush to the wall, and is activated by "rocking" a flat, broad lever, rather than pushing a short protruding handle up and down.
In Europe, 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 almost universally used, as a 16 millimetre switch mechanism, that slots from behind into a wall-plate. A slightly larger cover plate, supplied with the wall-plate or additional to it, then clips over the assembly, covering the wall-plate mounting screws. This cover plate can be removed for painting, without the use of tools. While larger "decorator" type switches are readily available, the advantage of the smaller mechanisms is that wall-plates are made to mount from one to six individual switch mechanisms or other correspondingly sized "mechanisms" - such as dimmers and indicator lights - in the same space as one (or two) switches of larger design can be mounted. All "mechanisms" have no external metal parts requiring earthing. Switches, wall-plates and cover plates from different manufacturers tend not to be interchangeable. (This mounting system is similar to the Keystone wall plate system for extra-low voltage electrical jacks). (A similar system, but with bigger switches, is used in Italy).
Below is shown typical Australian Rocker switches disassembled. Each Switch is actually a Single Pole Double Throw (SPDT) Switch – and is, thus, a two-way Switch. The “Actuator” pivots in two holes in the sides of the plastic Switch “Body”. An “Actuator Bar” slides in two grooves inside the “Actuator”, pressed down by a “Compression Spring” into a notch in the “Common Contact Bar”. The ”Common Contact Bar” is free to rock on a small diameter rod, welded to the Common Terminal. However, because of the tension applied by the “Compression Spring”, the “Common Contact Bar” will always be held against one of the two contacts. When the actuator is moved, energy is stored in the compression spring until the “Actuator” passes its mid-position. At that time, the “Common Contact Bar” is forced in the opposite direction by the “Compression Spring”, acting via the “Actuator Bar”, thus breaking the connection with the existing contact and making connection with the other contact.
The “Common Contact Bar” is made of copper, with an inlay of harder contact metal on the underside. While it is free to move the required distance lengthwise, it is constrained from moving sideways by the construction of the moulded plastic “Body”.
The Screw terminals are hollow and allow up to three wires, twisted together, to be inserted to a depth of up to 10mm and secured with a “set screw”. The contact terminal set screws are installed at a slight angle to allow easier screwdriver access when the switch is installed in a face-plate. Also shown is a “Loop” connection terminal. This terminal plays no part in the action of the switch but, because there is space to do so, it is provided as an insulated terminal for joining other wires, if required. (e.g. The Neutral wires.)
A switch of basically the same design is also used on Australian power outlet wall-plates, and it is now extremely rare to find any other type of switch in Australian homes, although the Australian Wiring Standard AS 3112 does not forbid other types. While there are many variations of Australian designs and cover plates available, some designers and renovators may import UK designed switches when they are aiming for a particular finish. However, while standard Australian wall mounting plates are of the same dimensions as those used in North America they are not of the same dimensions as those used in the UK.
Where lighting circuits must not be accidentally switched off (for example, school corridors), a vandal resistant switch may be installed. These require a key to operate and so discourage casual or accidental operation of the switch.
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.
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 were equipped with a neon lamp, making them glow faintly when they were "off" to aid 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.
A dimmer switch, properly called a dimmer control, contains a solid-state circuit to allow changing the brightness by reducing the average voltage applied to the lamp.
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.
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. Multiway switching is done using special switches that have additional contacts.
Earlier switches were made of porcelain in the surface-mounted version and operated as a rotary switch with a rotary mechanism. Due to the impact sensitivity of china, light switches made out of pressed fiberboard (Bakelite) were used later on instead. Today they are made of modern plastics. In some cases, especially in hospitals and other public facilities, light switches made of antimicrobial materials such as copper are used for infection prevention.
|Wikimedia Commons has media related to Light switches.|
- Electric Light Years 1878-1899 - England's North East
- "Flush Switch" (PDF). Google Patents. Jul 17, 1917. Retrieved 2011-11-11.
- "What's New for Your Home". Popular Mechanics: 131. March 1955. Retrieved 12 February 2013.