Light switch

The direction which represents "on" also varies by country. In the US, Canada, European countries, Russia, etc., it is usual for the "on" position of a toggle switch to be "up", whereas in many other countries such as the UK, India, Australia, New Zealand (Commonwealth countries), Ireland and China, it is "down". (In multiway switching, the correspondence between a single switch's state and whether lights are on or off depends on the state of the other switch[es] in the circuit.)

The contacts of a switch are under their greatest stress while opening or closing. As the switch is closed, the resistance between the contacts changes from almost infinite to almost 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. However, while the contacts change state, there is a brief instant of partial contact when resistance is neither zero nor infinite, and electrical power is converted into heat. If the heating is excessive, the contacts may be damaged, or may even weld themselves closed.

A switch should be designed to make its transition as swiftly as possible. This is achieved by the initial operation of the switch lever mechanism storing potential energy, usually as mechanical stress in a spring. When sufficient mechanical energy is stored, the mechanism in the switch "breaks over", and quickly drives the contacts through the transition from open to closed, or closed to open, without further action by the switch operator. This quick-break action of the switch is essential to a long life for the switch contacts, as described in Holmes' 1884 patent.

While the contacts are separating, any electrical energy stored in the inductance of the circuit being disconnected 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 limitation.

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

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 replaceable 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 a non-conductive oxide layer builds up without an arc to disperse it.

Two kinds of sparks may occur 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 by friction to incandescence, melted, and thrown off. On opening, a bluish arc may occur, with a detectable "electrical" (ozone) odor. Subsequently, 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 may 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 operator manipulates the actuator, the contacts always close or open quickly.

In the construction of many small switches, the spring that stores the mechanical energy necessary for the snap action of the switch mechanism 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.

Vintage push-button light switches

The push-button light switch has two buttons: one that closes the contacts and one that opens the contacts. Pushing the raised button opens or closes the contacts and pops out the previously depressed button so the process can be reversed. In the U.S., the buttons were commonly black; the "on" button typically had a white mother-of-pearl (real or simulated) inlay to indicate its function. By convention, the switch was customarily installed with the "on" button on top. Push-button switch reproductions are available on the market today for vintage or authentic styling.

A toggle light switch

Internal components of a toggle switch

The toggle mechanism provides "snap-action" through the use of an "over-center" geometry. The design was patented in 1916 by William J. Newton and Morris Goldberg.[2] The switch actuator does not control the contacts directly, but through an intermediate arrangement of springs and levers. Turning the actuator 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 actuator gradually stretches the spring. When the mechanism passes over the center point, the spring energy is released and the spring, rather than the actuator, 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.

A UK illuminated rocker switch in the off position

Illuminated switches incorporate a small neon lamp or LED, allowing the user easily to locate the switch in the dark. Household illuminated switches were introduced in the mid-1950s.[4]

Single-pole illuminated switches derive the power to energize their in-built illuminating source (usually, a "neon" lamp) from the current passing through the lamp(s) which they control. Such switches work satisfactorily with incandescent lamps, halogen lighting, and non-electronic fluorescent fixtures, because the small current required for the switch's illuminating source is too small to produce any visible light from such devices controlled by the illuminated switch. However, if they control only compact fluorescent lamps (CFLs) and/or LED lamps, the small amount of current required to energize the lighting source within switch also slowly charges the internal input capacitor in the electronic ballast of the CFL or LED until the voltage across it rises to the point where it produces a brief discharge through the CFL. This cycle may repeat indefinitely, resulting in repetitive brief flashing of the lamp(s) (and the light inside the switch) while the illuminated switch is in the "off" position.

A rocker switch

A European style rocker switch

An alternative design to the toggle switch is the rocker switch. Large switches of this design are commonly known in the United States as "decorator" or "Decora" style.[5][6] "Decora" is a trademark of Leviton, and the term in recent years has become a genericized trademark in the United States for any rocker light switch regardless of brand. Switches of this design sit almost flush with the wall plate, and are activated by "rocking" a flat, broad lever, rather than pushing a short protruding actuator up or down.

In Europe, Hong Kong, Malaysia, Singapore, and India this type is near-universal, and toggle style switches would be considered old-fashioned.

3 Australian light switches and a "neon" indicator—in a 4-gang wall-plate. The lower switch is in the "on" or "down" position.

Two Australian rocker switches (disassembled), together with a cutaway view, and a view of the switch connections

An Australian architrave-mounted 2-gang switch plate and switches.

In Australia and New Zealand, a small rocker switch is almost universally used, in the form of a 16 mm (0.63 in) switch mechanism, which is mounted from behind into a wall-plate—attaching via mounting lugs, as shown in the photo on the left. A slightly larger "cover plate", supplied with the wall-plate, or additional to it, then clips over the assembly, as an additional insulating barrier covering the deep set wall-plate mounting screws - which are "deep set" to prevent inadvertent human contact. The "cover plate" can be removed without the use of tools, such as when wall painting is required.

While larger "decorator" style switches are readily available in Australia, the advantage of the smaller mechanisms is that wall-plates are available 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 could be mounted. Since the mechanisms are small, they can also be mounted into "architrave" plates, for mounting in positions where it is not possible to mount a "standard" sized wall-plate. An example is shown in the picture below on the right. All of the switch mechanisms have no exposed metal parts requiring grounding (earthing). While switches, wall-plates, and cover plates from different manufacturers tend not to be interchangeable, switch mechanisms of this type have been available in Australia since 1951.[7]

The keystone module system for extra-low voltage electrical jacks (patented in 1975) is somewhat similar in appearance to these modules, but the design of the keystone mountings are different, and keystone modules can be removed without a tool. (A similar system, but with bigger switches, is used in Italy.)

As shown in the disassembly photo, the switch 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 pressure applied by the compression spring, the common contact bar will always be held against one of the two contacts.

When the actuator is moved, mechanical 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 molded plastic body.

The screw terminals are hollow and allow up to at least three 1 mm (CSA) wires, twisted together, to be inserted to a depth of up to 10 millimetres (0.39 in) and secured with a set screw. The contact terminal set screws are installed at a slight angle to allow easier screwdriver access after the switch mechanism has been installed into a wall-plate - before fixing the wall-plate to the wall. Also shown is a "loop" connection terminal. This terminal plays no part in the action of the switch but, because there is available space, it is provided as an insulated terminal for joining other wires, if required (such as the neutral wires). Each Australian rocker switch mechanism is actually a single-pole, double-throw (SPDT) Switch, also known as a "two-way switch", and has three terminals.

A switch of basically the same design is also used on Australian power outlet wall-plates. 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 many variations of Australian designs and cover plates are available, some designers and renovators may import UK- and European-designed switches when they desire a particular finish. However, while standard Australian wall mounting plates have the same dimensions as those used in North America, they have different dimensions from those used in the UK or Europe.

Switches (and other mechanisms) of this Australian design series are currently available in the UK (and other countries), together with wall-plates appropriate to the mounting standards of the countries concerned.[8]

For a short time, Australian rocker switches were exported to the United States in the 1960s. Although the switches had adequate ratings for usage on 120 V circuits and had advantages of compactness and distinctive appearance, they failed to establish themselves in the American market.

Tamper resistant switch[9]

Where lighting circuits must not be accidentally switched off, for example, corridor and restroom lighting controls in public buildings such as schools, a tamper-resistant switch may be installed. These require a key to operate and so discourage casual or accidental operation of the switch.[10]

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.

Mercury light switch from General Electric c. 1960, 120 V 15 A

Around the middle of the 20th century, "quiet" switches were introduced that used mercury to make the connection, avoiding the loud snap made by the spring-loaded toggle switches that were then the norm. One form of mercury light switch uses a sealed glass vial that is tipped by the toggle lever, causing a drop of liquid metallic mercury to roll from one end to the other, bridging a pair of contacts to complete the circuit. Another style of mercury light switch uses a metal wheel with an insulator separating the two halves that has a hole which allows the mercury to connect either side of the wheel in only one position.[11]

Some of these switches were also equipped with a neon lamp connected across the contacts, and thus in series with the electrical load. This caused the indicator to glow faintly when the switch was off, as an aid to finding the switch in a dark room.

By the 1970s, other quiet switch designs that did not require mercury (and were thus simpler and cheaper to produce) came into widespread use, gradually replacing both conventional "snap" toggle switches and mercury-based quiet switches.

An example of a British corded pull switch

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, UK wiring regulations required that all bathroom switches were operated by pull cords.

This type of switch is mounted on the power cord, both in middle or at the end. They are light weight, and they are used mainly for low power devices, like table lamps, table fans, radios and many other low power devices. Usually they are single pole single throw, but can also double pole single throw. They can be mounted either in the middle of the plug and consumer or they may be mounted at the end of the cable. There are also more sophisticated cord mounted switches, like multiple switching (for lamps with more than one bulb, or fans with more speeds, in this case they have one input and 2 or more outputs), variators and dimmers (light dimmers, electric pillows and matresses or some fans with more speeds). Some older heaters may also use such switches, but with higher ratings. Usually their current rating is between 0.5 A, mainly limited to low powered lamps at mains voltage , up to 6 or even 10 A (like heaters) . Higher current ratings may also allow usage in high current but low voltage (6-24 V) applications.

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.

Dimmer with RF-based remote control[12]

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. As of 1986 these remain specialty items. Electronic switches provide flexibility in terms of different interfaces for their operations, such as touch plates, soft-touch controls, pressure or light sensor based control, interactive touch-screens (which are widely used in aircraft for lighting control), and others.

Public buildings such as hospitals frequently save energy by using motion detector switches, also known as occupancy sensors. The occupancy sensors can also be used in residential applications such as in bathrooms, garages, and hallways.[13]

A wireless light switch provides remote control of lighting using a hand-held transmitter. While the controlling device may be a unit dedicated to this purpose, increasingly such switches may be controlled by the technology (such as Bluetooth or Wi-Fi) now found in smartphones.

Wired remote control of lighting switches is also possible using, for example, X10 signaling over the power wires.