In
electrical engineering
Electrical engineering is an engineering discipline concerned with the study, design, and application of equipment, devices, and systems which use electricity, electronics, and electromagnetism. It emerged as an identifiable occupation in the l ...
, a switch is an
electrical component
An electronic component is any basic discrete device or physical entity in an electronic system used to affect electrons or their associated fields. Electronic components are mostly industrial products, available in a singular form and are not ...
that can disconnect or connect the conducting path in an
electrical circuit
An electrical network is an interconnection of electrical components (e.g., batteries, resistors, inductors, capacitors, switches, transistors) or a model of such an interconnection, consisting of electrical elements (e.g., voltage sources, ...
, interrupting the
electric current
An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The moving pa ...
or diverting it from one conductor to another. The most common type of switch is an electromechanical device consisting of one or more sets of movable
electrical contact
An electrical contact is an electrical circuit component found in electrical switches, relays, connectors and circuit breakers. Each contact is a piece of electrically conductive material, typically metal. When a pair of contacts touch, they ca ...
s connected to external circuits. When a pair of contacts is touching current can pass between them, while when the contacts are separated no current can flow.
Switches are made in many different configurations; they may have multiple sets of contacts controlled by the same knob or actuator, and the contacts may operate simultaneously, sequentially, or alternately. A switch may be operated manually, for example, a
light switch
In electrical 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 may have a light switch mounted on the socket, base, or i ...
or a keyboard button, or may function as a
sensing element
A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon.
In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...
to sense the position of a machine part, liquid level, pressure, or temperature, such as a
thermostat
A thermostat is a regulating device component which senses the temperature of a physical system and performs actions so that the system's temperature is maintained near a desired setpoint.
Thermostats are used in any device or system tha ...
. Many specialized forms exist, such as the
toggle switch
In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type of ...
,
rotary switch
A rotary switch is a switch operated by rotation. These are often chosen when more than 2 positions are needed, such as a three-speed fan or a CB radio with multiple frequencies of reception or "channels".
A rotary switch consists of a spindl ...
,
mercury switch
A mercury switch is an electrical switch that opens and closes a circuit when a small amount of the liquid metal mercury connects metal electrodes to close the circuit. There are several different basic designs (tilt, displacement, radial, etc ...
,
push-button switch,
reversing switch
In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type of ...
,
relay
A relay
Electromechanical relay schematic showing a control coil, four pairs of normally open and one pair of normally closed contacts
An automotive-style miniature relay with the dust cover taken off
A relay is an electrically operated switch ...
, and
circuit breaker
A circuit breaker is an electrical safety device designed to protect an electrical circuit from damage caused by an overcurrent or short circuit. Its basic function is to interrupt current flow to protect equipment and to prevent the risk ...
. A common use is control of lighting, where multiple switches may be wired into one circuit to allow convenient control of light fixtures. Switches in high-powered circuits must have special construction to prevent destructive arcing when they are opened.
Description
![switches-electrical](https://upload.wikimedia.org/wikipedia/commons/3/3a/Switches-electrical.agr.jpg)
The most familiar form of switch is a manually operated electromechanical device with one or more sets of
electrical contacts
An electrical contact is an electrical circuit component found in electrical switches, relays, connectors and circuit breakers. Each contact is a piece of electrically conductive material, typically metal. When a pair of contacts touch, they ca ...
, which are connected to external circuits. Each set of contacts can be in one of two states: either "closed" meaning the contacts are touching and electricity can flow between them, or "open", meaning the contacts are separated and the switch is nonconducting. The mechanism actuating the transition between these two states (open or closed) is usually (there are other types of actions) either an "''alternate action''" (flip the switch for continuous "on" or "off") or "''momentary''" (push for "on" and release for "off") type.
A switch may be directly manipulated by a human as a control signal to a system, such as a computer keyboard button, or to control power flow in a circuit, such as a
light switch
In electrical 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 may have a light switch mounted on the socket, base, or i ...
. Automatically operated switches can be used to control the motions of machines, for example, to indicate that a garage door has reached its full open position or that a machine tool is in a position to accept another workpiece. Switches may be operated by process variables such as pressure, temperature, flow, current, voltage, and force, acting as
sensor
A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon.
In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...
s in a process and used to automatically control a system. For example, a
thermostat
A thermostat is a regulating device component which senses the temperature of a physical system and performs actions so that the system's temperature is maintained near a desired setpoint.
Thermostats are used in any device or system tha ...
is a temperature-operated switch used to control a heating process. A switch that is operated by another electrical circuit is called a
relay
A relay
Electromechanical relay schematic showing a control coil, four pairs of normally open and one pair of normally closed contacts
An automotive-style miniature relay with the dust cover taken off
A relay is an electrically operated switch ...
. Large switches may be remotely operated by a motor drive mechanism. Some switches are used to isolate electric power from a system, providing a visible point of isolation that can be padlocked if necessary to prevent accidental operation of a machine during maintenance, or to prevent electric shock.
An ideal switch would have no voltage drop when closed, and would have no limits on voltage or current rating. It would have zero
rise time In electronics, when describing a voltage or current step function, rise time is the time taken by a signal to change from a specified low value to a specified high value. These values may be expressed as ratiosSee for example , and . or, equivale ...
and
fall time
In electronics, fall time (pulse decay time) t_f is the time taken for the amplitude of a pulse to decrease (fall) from a specified value (usually 90% of the peak value exclusive of overshoot or undershoot) to another specified value (usually 10 ...
during state changes, and would change state without "bouncing" between on and off positions.
Practical switches fall short of this ideal; as the result of roughness and oxide films, they exhibit
contact resistance
The term contact resistance refers to the contribution to the total resistance of a system which can be attributed to the contacting interfaces of electrical leads and connections as opposed to the intrinsic resistance. This effect is describe ...
, limits on the current and voltage they can handle, finite switching time, etc. The ideal switch is often used in circuit analysis as it greatly simplifies the system of equations to be solved, but this can lead to a less accurate solution. Theoretical treatment of the effects of non-ideal properties is required in the design of large networks of switches, as for example used in telephone exchanges.
Contacts
![On-Off Switch](https://upload.wikimedia.org/wikipedia/commons/8/80/On-Off_Switch.jpg)
In the simplest case, a switch has two conductive pieces, often
metal
A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...
, called ''contacts'', connected to an external circuit, that touch to complete (make) the circuit, and separate to open (break) the circuit. The contact material is chosen for its resistance to corrosion, because most metals form
insulating oxides that would prevent the switch from working. Contact materials are also chosen on the basis of
electrical conductivity
Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current. A low resistivity indicates a material that readily allow ...
, hardness (resistance to abrasive wear), mechanical strength, low cost and low toxicity. The formation of oxide layers at contact surface, as well as surface roughness and contact pressure, determine the
contact resistance
The term contact resistance refers to the contribution to the total resistance of a system which can be attributed to the contacting interfaces of electrical leads and connections as opposed to the intrinsic resistance. This effect is describe ...
, and
wetting current
In electrical and electronics engineering, wetting current is the minimum electric current needing to flow through a contact to break through the surface film resistance at a contact. It is typically far below the contact's nominal maximum curr ...
of a mechanical switch. Sometimes the contacts are
plated
Plating is a surface covering in which a metal is deposited on a conductive surface. Plating has been done for hundreds of years; it is also critical for modern technology. Plating is used to decorate objects, for corrosion inhibition, to improv ...
with
noble metal
A noble metal is ordinarily regarded as a metallic chemical element that is generally resistant to corrosion and is usually found in nature in its raw form. Gold, platinum, and the other platinum group metals (ruthenium, rhodium, palladium, o ...
s, for their excellent conductivity and resistance to corrosion. They may be designed to wipe against each other to clean off any contamination. Nonmetallic conductors, such as conductive plastic, are sometimes used. To prevent the formation of insulating oxides, a minimum
wetting current
In electrical and electronics engineering, wetting current is the minimum electric current needing to flow through a contact to break through the surface film resistance at a contact. It is typically far below the contact's nominal maximum curr ...
may be specified for a given switch design.
Contact terminology
![Tpst](https://upload.wikimedia.org/wikipedia/commons/d/da/Tpst.jpg)
In electronics, switches are classified according to the arrangement of their contacts. A pair of contacts is said to be "''closed''" when current can flow from one to the other. When the contacts are separated by an
insulating air gap, they are said to be "''open''", and no current can flow between them at normal voltages. The terms "''make''" for closure of contacts and "''break''" for opening of contacts are also widely used.
The terms pole and throw are also used to describe switch contact variations. The number of "''poles''" is the number of electrically separate switches which are controlled by a single physical actuator. For example, a "''2-pole''" switch has two separate, parallel sets of contacts that open and close in unison via the same mechanism. The number of "''throws''" is the number of separate wiring path choices other than "open" that the switch can adopt for each pole. A single-throw switch has one pair of contacts that can either be closed or open. A double-throw switch has a contact that can be connected to either of two other contacts, a triple-throw has a contact which can be connected to one of three other contacts, etc.
In a switch where the contacts remain in one state unless actuated, such as a
push-button switch, the contacts can either be normally open (abbreviated "n.o." or "no") until closed by operation of the switch, or normally closed ("n.c." or "nc") and opened by the switch action. A switch with both types of contact is called a ''changeover switch'' or ''double-throw switch''. These may be "make-before-break" ("MBB" or shorting) which momentarily connects both circuits, or may be "break-before-make" ("BBM" or non-shorting) which interrupts one circuit before closing the other.
These terms have given rise to abbreviations for the types of switch which are used in the
electronics
The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons using electronic devices. Electronics uses active devices to control electron flow by amplification ...
industry such as "''single-pole, single-throw''" (SPST) (the simplest type, "on or off") or "''single-pole, double-throw''" (SPDT), connecting either of two terminals to the common terminal. In
electrical power
Electric power is the rate at which electrical energy is transferred by an electric circuit. The SI unit of power is the watt, one joule per second. Standard prefixes apply to watts as with other SI units: thousands, millions and billions o ...
wiring (i.e., house and building wiring by
electrician
An electrician is a tradesperson specializing in electrical wiring of buildings, transmission lines, stationary machines, and related equipment. Electricians may be employed in the installation of new electrical components or the maintenance ...
s), names generally involve the suffix ''"-way"''; however, these terms differ between
British English
British English (BrE, en-GB, or BE) is, according to Lexico, Oxford Dictionaries, "English language, English as used in Great Britain, as distinct from that used elsewhere". More narrowly, it can refer specifically to the English language in ...
and
American English
American English, sometimes called United States English or U.S. English, is the set of variety (linguistics), varieties of the English language native to the United States. English is the Languages of the United States, most widely spoken lan ...
(i.e., the terms ''two way'' and ''three way'' are used with different meanings).
Switches with larger numbers of poles or throws can be described by replacing the "S" or "D" with a number (e.g. 3PST, SP4T, etc.) or in some cases the letter "T" (for "triple") or "Q" (for "quadruple"). In the rest of this article the terms ''SPST'', ''SPDT'' and ''intermediate'' will be used to avoid the ambiguity.
Contact bounce
Bounce
![Bouncy Switch](https://upload.wikimedia.org/wikipedia/commons/a/ac/Bouncy_Switch.png)
Contact bounce (also called ''chatter'') is a common problem with mechanical switches and
relay
A relay
Electromechanical relay schematic showing a control coil, four pairs of normally open and one pair of normally closed contacts
An automotive-style miniature relay with the dust cover taken off
A relay is an electrically operated switch ...
s, which arises as the result of electrical
contact resistance
The term contact resistance refers to the contribution to the total resistance of a system which can be attributed to the contacting interfaces of electrical leads and connections as opposed to the intrinsic resistance. This effect is describe ...
(ECR) phenomena at interfaces. Switch and relay contacts are usually made of springy metals. When the contacts strike together, their momentum and elasticity act together to cause them to bounce apart one or more times before making steady contact. The result is a rapidly pulsed electric current instead of a clean transition from zero to full current. The effect is usually unimportant in power circuits, but causes problems in some
analogue and
logic circuit
A logic gate is an idealized or physical device implementing a Boolean function, a logical operation performed on one or more binary inputs that produces a single binary output. Depending on the context, the term may refer to an ideal logic gate, ...
s that respond fast enough to misinterpret the on‑off pulses as a data stream.
[Walker, PMB, ''Chambers Science and Technology Dictionary'', Edinburgh, 1988, ] In the design of micro-contacts, controlling surface structure (
surface roughness) and minimizing the formation of
passivated layers on metallic surfaces are instrumental in inhibiting chatter.
In the
Hammond organ
The Hammond organ is an electric organ invented by Laurens Hammond and John M. Hanert and first manufactured in 1935. Multiple models have been produced, most of which use sliding drawbars to vary sounds. Until 1975, Hammond organs generated s ...
, multiple wires are pressed together under the piano keys of the manuals. Their bouncing and non-synchronous closing of the switches is known as ''Hammond Click'' and compositions exist that use and emphasize this feature. Some
electronic organ
An electric organ, also known as electronic organ, is an electronic keyboard instrument which was derived from the harmonium, pipe organ and theatre organ. Originally designed to imitate their sound, or orchestral sounds, it has since developed ...
s have a switchable replica of this sound effect.
Debouncing
The effects of contact bounce can be eliminated by:
* use of
mercury-wetted contacts, but these are now infrequently used because of the hazards of mercury.
* Alternatively, contact circuit voltages can be
low-pass filter
A low-pass filter is a filter that passes signals with a frequency lower than a selected cutoff frequency and attenuates signals with frequencies higher than the cutoff frequency. The exact frequency response of the filter depends on the filter des ...
ed to reduce or eliminate multiple pulses from appearing.
* In digital systems, multiple samples of the contact state can be taken at a low rate and examined for a steady sequence, so that contacts can settle before the contact level is considered reliable and acted upon.
See also
Keyboard technology #Debouncing.
* Bounce in
SPDT ("single-pole, double-throw") switch contacts signals can be filtered out using an
SR flip-flop (latch) or
Schmitt trigger
In electronics, a Schmitt trigger is a comparator circuit with hysteresis implemented by applying positive feedback to the noninverting input of a comparator or differential amplifier. It is an active circuit which converts an analog input ...
.
All of these methods are referred to as 'debouncing'.
Arcs and quenching
When the power being switched is sufficiently large, the electron flow across opening switch contacts is sufficient to
ionize
Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule ...
the air molecules across the tiny gap between the contacts as the switch is opened, forming a
gas plasma
Plasma ()[πλάσμα](_blank)
, Henry George Liddell, R ...
, also known as an
electric arc
An electric arc, or arc discharge, is an electrical breakdown of a gas that produces a prolonged electrical discharge. The electric current, current through a normally Electrical conductance, nonconductive medium such as air produces a plasma (p ...
. The plasma is of low resistance and is able to sustain power flow, even with the separation distance between the switch contacts steadily increasing. The plasma is also very hot and is capable of eroding the metal surfaces of the switch contacts (the same true for vacuum switches). Electric current arcing causes significant
degradation of the contacts and also significant
electromagnetic interference
Electromagnetic interference (EMI), also called radio-frequency interference (RFI) when in the radio frequency spectrum, is a disturbance generated by an external source that affects an electrical circuit by electromagnetic induction, electros ...
(EMI), requiring the use of
arc suppression methods.
Where the voltage is sufficiently high, an arc can also form as the switch is closed and the contacts approach. If the voltage potential is sufficient to exceed the
breakdown voltage
The breakdown voltage of an insulator is the minimum voltage that causes a portion of an insulator to experience electrical breakdown and become electrically conductive.
For diodes, the breakdown voltage is the minimum reverse voltage that mak ...
of the air separating the contacts, an arc forms which is sustained until the switch closes completely and the switch surfaces make contact.
In either case, the standard method for minimizing arc formation and preventing contact damage is to use a fast-moving switch mechanism, typically using a spring-operated
tipping-point mechanism to assure quick motion of switch contacts, regardless of the speed at which the switch control is operated by the user. Movement of the switch control lever applies tension to a spring until a tipping point is reached, and the contacts suddenly snap open or closed as the spring tension is released.
As the power being switched increases, other methods are used to minimize or prevent arc formation. A plasma is hot and will rise due to
convection
Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convec ...
air currents. The arc can be quenched with a series of non-conductive blades spanning the distance between switch contacts, and as the arc rises, its length increases as it forms ridges rising into the spaces between the blades, until the arc is too long to stay sustained and is extinguished. A ''puffer'' may be used to blow a sudden high velocity burst of gas across the switch contacts, which rapidly extends the length of the arc to extinguish it quickly.
Extremely large switches often have switch contacts surrounded by something other than air to more rapidly extinguish the arc. For example, the switch contacts may operate in a vacuum, immersed in
mineral oil
Mineral oil is any of various colorless, odorless, light mixtures of higher alkanes from a mineral source, particularly a distillate of petroleum, as distinct from usually edible vegetable oils.
The name 'mineral oil' by itself is imprecise, ...
, or in
sulfur hexafluoride
Sulfur hexafluoride or sulphur hexafluoride (British spelling) is an inorganic compound with the formula SF6. It is a colorless, odorless, non- flammable, and non-toxic gas. has an octahedral geometry, consisting of six fluorine atoms attached ...
.
In AC power service, the current periodically passes through zero; this effect makes it harder to sustain an arc on opening. Manufacturers may rate switches with lower voltage or current rating when used in DC circuits.
Power switching
When a switch is designed to switch significant power, the transitional state of the switch as well as the ability to withstand continuous operating currents must be considered. When a switch is in the on state, its resistance is near zero and very little power is dropped in the contacts; when a switch is in the off state, its resistance is extremely high and even less power is dropped in the contacts. However, when the switch is flicked, the resistance must pass through a state where a quarter of the load's rated power (or worse if the load is not purely resistive) is briefly dropped in the switch.
For this reason, power switches intended to interrupt a load current have spring mechanisms to make sure the transition between on and off is as short as possible regardless of the speed at which the user moves the rocker.
Power switches usually come in two types. A momentary on‑off switch (such as on a
laser pointer
A laser pointer or laser pen is a small handheld device with a power source (usually a battery) and a laser diode emitting a very narrow coherent low-powered laser beam of visible light, intended to be used to highlight something of interest by ...
) usually takes the form of a button and only closes the circuit when the button is depressed. A regular on‑off switch (such as on a
flashlight
A flashlight ( US, Canada) or torch ( UK, Australia) is a portable hand-held electric lamp. Formerly, the light source typically was a miniature incandescent light bulb, but these have been displaced by light-emitting diodes (LEDs) since the ...
) has a constant on-off feature. Dual-action switches incorporate both of these features.
Inductive loads
When a strongly
inductive load such as an
electric motor
An electric motor is an Electric machine, electrical machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a Electromagneti ...
is switched off, the current cannot drop instantaneously to zero; a
spark
Spark commonly refers to:
* Spark (fire), a small glowing particle or ember
* Electric spark, a form of electrical discharge
Spark may also refer to:
Places
* Spark Point, a rocky point in the South Shetland Islands
People
* Spark (surname)
* ...
will jump across the opening contacts. Switches for
inductive loads must be rated to handle these cases. The spark will cause
electromagnetic interference
Electromagnetic interference (EMI), also called radio-frequency interference (RFI) when in the radio frequency spectrum, is a disturbance generated by an external source that affects an electrical circuit by electromagnetic induction, electros ...
if not suppressed; a
snubber
A snubber is a device used to suppress ("snub") a phenomenon such as voltage transients in electrical systems, pressure transients in fluid systems (caused by for example water hammer) or excess force or rapid movement in mechanical systems.
Ele ...
network of a
resistor
A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active el ...
and
capacitor
A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals.
The effect of ...
in series will quell the spark.
Incandescent loads
![General-Electric-silent-T-rated-light-switch](https://upload.wikimedia.org/wikipedia/commons/c/cd/General-Electric-silent-T-rated-light-switch.jpg)
When turned on, an
incandescent lamp
An incandescent light bulb, incandescent lamp or incandescent light globe is an electric light with a wire filament heated until it glows. The filament is enclosed in a glass bulb with a vacuum or inert gas to protect the filament from oxid ...
draws a large
inrush current
Inrush current, input surge current, or switch-on surge is the maximal instantaneous input current drawn by an electrical device when first turned on. Alternating-current electric motors and transformers may draw several times their normal full-lo ...
of about ten times the steady-state current; as the filament heats up, its resistance rises and the current decreases to a steady-state value. A switch designed for an incandescent lamp load can withstand this inrush current.
Wetting current
''
Wetting current
In electrical and electronics engineering, wetting current is the minimum electric current needing to flow through a contact to break through the surface film resistance at a contact. It is typically far below the contact's nominal maximum curr ...
'' is the minimum current needing to flow through a mechanical switch while it is operated to break through any film of
oxidation
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...
that may have been deposited on the switch contacts. The film of oxidation occurs often in areas with high
humidity
Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present.
Humidity depe ...
. Providing a sufficient amount of wetting current is a crucial step in designing
systems
A system is a group of interacting or interrelated elements that act according to a set of rules to form a unified whole. A system, surrounded and influenced by its environment, is described by its boundaries, structure and purpose and express ...
that use delicate switches with small contact pressure as sensor inputs. Failing to do this might result in switches remaining electrically "open" due to contact oxidation.
Actuator
The moving part that applies the operating force to the contacts is called the ''
actuator
An actuator is a component of a machine that is responsible for moving and controlling a mechanism or system, for example by opening a valve. In simple terms, it is a "mover".
An actuator requires a control device (controlled by control signal) a ...
'', and may be a toggle or ''dolly'', a rocker, a push-button or any type of mechanical linkage ''(see photo).''
Biased switches
A switch normally maintains its set position once operated. A biased switch contains a mechanism that springs it into another position when released by an operator. The momentary
push-button switch is a type of biased switch. The most common type is a "push-to-make" (or normally-open or NO) switch, which makes contact when the button is pressed and breaks when the button is released. Each key of a computer keyboard, for example, is a normally-open "push-to-make" switch. A "push-to-break" (or normally-closed or NC) switch, on the other hand, breaks contact when the button is pressed and makes contact when it is released. An example of a push-to-break switch is a button used to release a door held closed by an
electromagnet
An electromagnet is a type of magnet in which the magnetic field is produced by an electric current. Electromagnets usually consist of wire wound into a coil. A current through the wire creates a magnetic field which is concentrated in the ...
. The interior lamp of a household
refrigerator is controlled by a switch that is held open when the door is closed.
Rotary switch
![Rotaryswitch](https://upload.wikimedia.org/wikipedia/commons/b/b3/Rotaryswitch.gif)
A rotary switch operates with a twisting motion of the operating handle with at least two positions. One or more positions of the switch may be momentary (biased with a spring), requiring the operator to hold the switch in the position. Other positions may have a detent to hold the position when released. A rotary switch may have multiple levels or "decks" in order to allow it to control multiple circuits.
One form of rotary switch consists of a spindle or "rotor" that has a contact arm or "spoke" which projects from its surface like a cam. It has an array of terminals, arranged in a circle around the rotor, each of which serves as a contact for the "spoke" through which any one of a number of different electrical circuits can be connected to the rotor. The switch is layered to allow the use of multiple poles, each layer is equivalent to one pole. Usually such a switch has a detent mechanism so it "clicks" from one active position to another rather than stalls in an intermediate position. Thus a rotary switch provides greater pole and throw capabilities than simpler switches do.
Other types use a cam mechanism to operate multiple independent sets of contacts.
Rotary switches were used as channel selectors on television receivers until the early 1970s, as range selectors on electrical metering equipment, as band selectors on multi-band radios and other similar purposes. In industry, rotary switches are used for control of measuring instruments,
switchgear
In an electric power system, a switchgear is composed of electrical disconnect switches, fuses or circuit breakers used to control, protect and isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be ...
, or in control circuits. For example, a
radio control
Radio control (often abbreviated to RC) is the use of control signals transmitted by radio to remotely control a device. Examples of simple radio control systems are garage door openers and keyless entry systems for vehicles, in which a small ...
led overhead crane may have a large multi-circuit rotary switch to transfer hard-wired control signals from the local manual controls in the cab to the outputs of the remote control receiver.
Toggle switch
![Toggle switch logic](https://upload.wikimedia.org/wikipedia/commons/8/8f/Toggle_switch_logic.jpg)
A toggle switch or tumbler switch is a class of electrical switches that are manually actuated by a mechanical
lever
A lever is a simple machine consisting of a beam or rigid rod pivoted at a fixed hinge, or ''fulcrum''. A lever is a rigid body capable of rotating on a point on itself. On the basis of the locations of fulcrum, load and effort, the lever is div ...
, handle, or rocking mechanism.
Toggle switches are available in many different styles and sizes, and are used in numerous applications. Many are designed to provide the simultaneous actuation of multiple sets of
electrical contact
An electrical contact is an electrical circuit component found in electrical switches, relays, connectors and circuit breakers. Each contact is a piece of electrically conductive material, typically metal. When a pair of contacts touch, they ca ...
s, or the control of large amounts of
electric current
An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The moving pa ...
or
mains voltages.
The word "toggle" is a reference to a kind of mechanism or joint consisting of two arms, which are almost in line with each other, connected with an elbow-like pivot. However, the phrase "toggle switch" is applied to a switch with a short handle and a positive snap-action, whether it actually contains a toggle mechanism or not. Similarly, a switch where a definitive click is heard, is called a "positive on-off switch". A very common use of this type of switch is to switch lights or other electrical equipment on or off. Multiple toggle switches may be mechanically interlocked to prevent forbidden combinations.
In some contexts, particularly
computing
Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes, and development of both hardware and software. Computing has scientific, e ...
, a toggle switch, or the action of toggling, is understood in the different sense of a mechanical or software switch that alternates between two states each time it is activated, regardless of mechanical construction. For example, the
caps lock key on a computer causes all letters to be generated in capitals after it is pressed once; pressing it again reverts to lower-case letters.
Special types
![Float switch on](https://upload.wikimedia.org/wikipedia/commons/3/3f/Float_switch_on.jpg)
Switches can be designed to respond to any type of mechanical stimulus: for example, vibration (the trembler switch), tilt, air pressure, fluid level (a
float switch A float switch is a type of level sensor, a device used to detect the level of liquid within a tank. The switch may be used to control a pump, as an indicator, an alarm, or to control other devices.
One type of float switch uses a mercury switch ...
), the turning of a key (
key switch
A key switch (sometimes called a keyswitch or lock switch) is a key-operated switch. Key switches are used in situations where access needs to be restricted to the switch's functions.
Key switches are available as components with solder connecti ...
), linear or rotary movement (a
limit switch In electrical engineering, a limit switch is a switch operated by the motion of a machine part or the presence of an object. A limit switch can be used for controlling machinery as part of a control system, as a safety interlock, or as a counter en ...
or
microswitch
A miniature snap-action switch, also trademarked and frequently known as a micro switch, is an electric switch that is actuated by very little physical force, through the use of a tipping-point mechanism, sometimes called an "over-center" mechan ...
), or presence of a magnetic field (the
reed switch). Many switches are operated automatically by changes in some environmental condition or by motion of machinery. A
limit switch In electrical engineering, a limit switch is a switch operated by the motion of a machine part or the presence of an object. A limit switch can be used for controlling machinery as part of a control system, as a safety interlock, or as a counter en ...
is used, for example, in machine tools to interlock operation with the proper position of tools. In heating or cooling systems a
sail switch ensures that air flow is adequate in a duct.
Pressure switch
A pressure switch is a form of switch that operates an electrical contact when a certain set fluid pressure has been reached on its input. The switch may be designed to make contact either on pressure rise or on pressure fall. Pressure switches ar ...
es respond to fluid pressure.
Mercury tilt switch
The mercury switch consists of a drop of
mercury
Mercury commonly refers to:
* Mercury (planet), the nearest planet to the Sun
* Mercury (element), a metallic chemical element with the symbol Hg
* Mercury (mythology), a Roman god
Mercury or The Mercury may also refer to:
Companies
* Merc ...
inside a
glass
Glass is a non-crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling (quenching) of ...
bulb with two or more contacts. The two contacts pass through the glass, and are connected by the mercury when the bulb is tilted to make the mercury roll on to them.
This type of switch performs much better than the ball tilt switch, as the
liquid metal
A liquid metal is a metal or a metal alloy which is liquid at or near room temperature.
The only stable liquid elemental metal at room temperature is Mercury (element), mercury (Hg), which is molten above −38.8 °C (234.3 K, −37.9&nbs ...
connection is unaffected by dirt, debris and oxidation, it wets the contacts ensuring a very low resistance bounce-free connection, and movement and vibration do not produce a poor contact. These types can be used for precision works.
It can also be used where arcing is dangerous (such as in the presence of explosive vapour) as the entire unit is sealed.
Knife switch
![Sectionneur](https://upload.wikimedia.org/wikipedia/commons/1/1f/Sectionneur.JPG)
Knife switches consist of a flat metal blade, hinged at one end, with an insulating handle for operation, and a fixed contact. When the switch is closed, current flows through the hinged pivot and blade and through the fixed contact. Such switches are usually not enclosed. The knife and contacts are typically formed of
copper
Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...
,
steel
Steel is an alloy made up of iron with added carbon to improve its strength and fracture resistance compared to other forms of iron. Many other elements may be present or added. Stainless steels that are corrosion- and oxidation-resistant ty ...
, or
brass
Brass is an alloy of copper (Cu) and zinc (Zn), in proportions which can be varied to achieve different mechanical, electrical, and chemical properties. It is a substitutional alloy: atoms of the two constituents may replace each other with ...
, depending on the application. Fixed contacts may be backed up with a spring. Several parallel blades can be operated at the same time by one handle. The parts may be mounted on an insulating base with terminals for wiring, or may be directly bolted to an insulated switch board in a large assembly. Since the electrical contacts are exposed, the switch is used only where people cannot accidentally come in contact with the switch or where the voltage is so low as to not present a hazard.
Knife switches are made in many sizes from miniature switches to large devices used to carry thousands of amperes. In electrical transmission and distribution, gang-operated switches are used in circuits up to the highest voltages.
The disadvantages of the knife switch are the slow opening speed and the proximity of the operator to exposed live parts. Metal-enclosed safety
disconnect switches are used for isolation of circuits in industrial power distribution. Sometimes spring-loaded auxiliary blades are fitted which momentarily carry the full current during opening, then quickly part to rapidly extinguish the arc.
Footswitch
A footswitch is a rugged switch which is operated by foot pressure. An example of use is in the control of a machine tool, allowing the operator to have both hands free to manipulate the workpiece. The foot controls of an electric guitarist's
effects pedals
An effects unit or effects pedal is an electronic device that alters the sound of a musical instrument or other audio source through audio signal processing.
Common effects include distortion/overdrive, often used with electric guitar in ele ...
and
amp #REDIRECT Amp
{{Redirect category shell, {{R from other capitalisation{{R from ambiguous page ...
are also footswitches.
Reversing switch
A DPDT switch has six connections, but since polarity reversal is a very common usage of DPDT switches, some variations of the DPDT switch are internally wired specifically for polarity reversal. These crossover switches only have four terminals rather than six. Two of the terminals are inputs and two are outputs. When connected to a battery or other DC source, the 4-way switch selects from either normal or reversed polarity. Such switches can also be used as intermediate switches in a
multiway switching
In building wiring, multiway switching is the interconnection of two or more electrical switches to control an electrical load from more than one location. A common application is in lighting, where it allows the control of lamps from multiple ...
system for control of lamps by more than two switches.
Light switches
In building wiring, light switches are installed at convenient locations to control lighting and occasionally other circuits. By use of multiple-pole switches,
multiway switching
In building wiring, multiway switching is the interconnection of two or more electrical switches to control an electrical load from more than one location. A common application is in lighting, where it allows the control of lamps from multiple ...
control of a lamp can be obtained from two or more places, such as the ends of a corridor or stairwell. A
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. There are different ways to communicate between the switch and the fixt ...
allows remote control of lamps for convenience; some lamps include a
touch switch A touch switch is a type of switch that only has to be touched by an object to operate. It is used in many lamps and wall switches that have a metal exterior as well as on public computer terminals. A touchscreen includes an array of touch switches ...
which electronically controls the lamp if touched anywhere. In public buildings several types of
vandal resistant switch
Vandal-resistant switches (also referred to as vandal-proof switches) are electrical switches designed to be installed in a location (or device) and application where they may be subject to abuse and attempts to damage them, as in the case of pe ...
es are used to prevent unauthorized use.
Slide switches
Slide switches are mechanical switches using a slider that moves (slides) from the open (off) position to the closed (on) position.
Electronic switches
![Tactile switches](https://upload.wikimedia.org/wikipedia/commons/1/11/Tactile_switches.jpg)
A
relay
A relay
Electromechanical relay schematic showing a control coil, four pairs of normally open and one pair of normally closed contacts
An automotive-style miniature relay with the dust cover taken off
A relay is an electrically operated switch ...
is an electrically operated switch. Many relays use an electromagnet to operate a switching mechanism mechanically, but other operating principles are also used.
Solid-state relays
Solid state contactor
PCB mount solid-state DIL relay
A solid state relay (SSR) is an electronic switching device that switches on or off when an external voltage (AC or DC) is applied across its control terminals. They serve the same function ...
control power circuits with no moving parts, instead using a semiconductor device to perform switching—often a
silicon-controlled rectifier
A silicon controlled rectifier or semiconductor controlled rectifier is a four-layer solid-state current-controlling device. The name "silicon controlled rectifier" is General Electric's trade name for a type of thyristor. The principle of four ...
or
triac
A TRIAC (triode for alternating current; also bidirectional triode thyristor or bilateral triode thyristor) is a three terminal electronic component that conducts current in either direction when triggered. The term TRIAC is a genericised tradem ...
.
The
analogue switch
The analogue (or PETR) switch, also called the bilateral switch, is an electronic component that behaves in a similar way to a relay, but has no moving parts. The switching element is normally a pair of MOSFET transistors, one an N-channel device ...
uses two
MOSFET
The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...
transistors in a
transmission gate
A transmission gate (TG) is an analog gate similar to a relay that can conduct in both directions or block by a control signal with almost any voltage potential. It is a CMOS-based switch, in which PMOS passes a strong 1 but poor 0, and NMOS passes ...
arrangement as a switch that works much like a relay, with some advantages and several limitations compared to an electromechanical relay.
The
power transistor(s) in a
switching voltage regulator, such as a
power supply unit
A power supply is an electrical device that supplies electric power to an electrical load. The main purpose of a power supply is to convert electric current from a source to the correct voltage, current, and frequency to power the load. As a res ...
, are used like a switch to alternately let power flow and block power from flowing.
Many people use
metonymy
Metonymy () is a figure of speech in which a concept is referred to by the name of something closely associated with that thing or concept.
Etymology
The words ''metonymy'' and ''metonym'' come from grc, μετωνυμία, 'a change of name' ...
to call a variety of devices "switches" that conceptually connect or disconnect signals and communication paths between electrical devices, analogous to the way mechanical switches connect and disconnect paths for electrons to flow between two conductors. Early telephone systems used an automatically operated
Strowger switch
The Strowger switch is the first commercially successful electromechanical stepping switch telephone exchange system. It was developed by the Strowger Automatic Telephone Exchange Company founded in 1891 by Almon Brown Strowger. Because of it ...
to connect telephone callers; telephone exchanges contain one or more
crossbar switch
In electronics and telecommunications, a crossbar switch (cross-point switch, matrix switch) is a collection of switches arranged in a matrix configuration. A crossbar switch has multiple input and output lines that form a crossed pattern of int ...
es today.
Since the advent of
digital logic
A logic gate is an idealized or physical device implementing a Boolean function, a logical operation performed on one or more Binary number, binary inputs that produces a single binary output. Depending on the context, the term may refer to an id ...
in the 1950s, the term ''switch'' has spread to a variety of digital
active device
Passivity is a property of engineering systems, most commonly encountered in analog electronics and control systems. Typically, analog designers use ''passivity'' to refer to incrementally passive components and systems, which are incapable of p ...
s such as
transistor
upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink).
A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch e ...
s and
logic gate
A logic gate is an idealized or physical device implementing a Boolean function, a logical operation performed on one or more binary inputs that produces a single binary output. Depending on the context, the term may refer to an ideal logic gate, ...
s whose function is to change their output state between two
logic level
In digital circuits, a logic level is one of a finite number of states that a digital signal can inhabit. Logic levels are usually represented by the voltage difference between the signal and ground, although other standards exist. The range ...
s or connect different
signal
In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The '' IEEE Transactions on Signal Processing' ...
lines, and even computers,
network switch
A network switch (also called switching hub, bridging hub, and, by the IEEE, MAC bridge) is networking hardware that connects devices on a computer network by using packet switching to receive and forward data to the destination device.
A netw ...
es, whose function is to provide connections between different
ports
A port is a maritime facility comprising one or more wharves or loading areas, where ships load and discharge cargo and passengers. Although usually situated on a sea coast or estuary, ports can also be found far inland, such as H ...
in a
computer network
A computer network is a set of computers sharing resources located on or provided by network nodes. The computers use common communication protocols over digital interconnections to communicate with each other. These interconnections are ...
. The most widely used
electronic switch
In electronics, an electronic switch is an electronic component or device that can switch an electrical circuit, interrupting the current or diverting it from one conductor to another. Electronic switches are considered binary devices because the ...
in
digital circuits
Digital electronics is a field of electronics involving the study of digital signals and the engineering of devices that use or produce them. This is in contrast to analog electronics and analog signals.
Digital electronic circuits are usually ...
is the
metal–oxide–semiconductor field-effect transistor
The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...
(MOSFET).
The term 'switched' is also applied to
telecommunications network
A telecommunications network is a group of nodes interconnected by telecommunications links that are used to exchange messages between the nodes. The links may use a variety of technologies based on the methodologies of circuit switching, message ...
s, and signifies a network that is
circuit switched
Circuit switching is a method of implementing a telecommunications network in which two network nodes establish a dedicated communications channel ( circuit) through the network before the nodes may communicate. The circuit guarantees the full ...
, providing dedicated circuits for communication between end nodes, such as the
public switched telephone network
The public switched telephone network (PSTN) provides Communications infrastructure, infrastructure and services for public Telecommunications, telecommunication. The PSTN is the aggregate of the world's circuit-switched telephone networks that ...
. The common feature of all these usages is they refer to devices that control a
binary
Binary may refer to:
Science and technology Mathematics
* Binary number, a representation of numbers using only two digits (0 and 1)
* Binary function, a function that takes two arguments
* Binary operation, a mathematical operation that ta ...
state
State may refer to:
Arts, entertainment, and media Literature
* ''State Magazine'', a monthly magazine published by the U.S. Department of State
* ''The State'' (newspaper), a daily newspaper in Columbia, South Carolina, United States
* ''Our S ...
: they are either ''on'' or ''off'', ''closed'' or ''open'', ''connected'' or ''not connected''.
Other switches
*
Centrifugal switch
A centrifugal switch is an electric switch that operates using the centrifugal force created from a rotating shaft, most commonly that of an electric motor or gasoline engine. The switch is designed to activate or de-activate as a function of th ...
*
Company switch
A company switch is a basic electrical switch design. They are used for power distribution
Electric power distribution is the final stage in the delivery of electric power; it carries electricity from the transmission system to individual con ...
*
Dead man's switch
A dead man's switch (see alternative names) is a switch that is designed to be activated or deactivated if the human operator becomes incapacitated, such as through death, loss of consciousness, or being bodily removed from control. Originally a ...
*
Fireman's switch
*
Hall-effect switch
A Hall effect sensor (or simply Hall sensor) is a type of sensor which detects the presence and magnitude of a magnetic field using the Hall effect. The output voltage of a Hall sensor is directly proportional to the strength of the field. ...
*
Inertial switch
An inertial switch is a switch, firmly mounted upon a vehicle or other mobile device, that triggers in the event of shock or vibration. It is a part of electrical circuits that may either enable or disable some function.
Disconnect
The switch sh ...
*
Isolator switch
In electrical engineering, a disconnector, disconnect switch or isolator switch is used to ensure that an electrical circuit is completely de-energized for service or maintenance. They are often found in electrical distribution and industrial a ...
*
Key switch
A key switch (sometimes called a keyswitch or lock switch) is a key-operated switch. Key switches are used in situations where access needs to be restricted to the switch's functions.
Key switches are available as components with solder connecti ...
*
Kill switch
A kill switch, also known as an emergency stop (E-stop), emergency off (EMO) and as an emergency power off (EPO), is a safety mechanism used to shut off machinery in an emergency, when it cannot be shut down in the usual manner. Unlike a normal ...
*
Latching switch
A latching switch is a switch that maintains its state after being activated. A push-to-make, push-to-break switch would therefore be a latching switch – each time you actuate it, whichever state the switch is left in will persist until the switc ...
*
Light switch
In electrical 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 may have a light switch mounted on the socket, base, or i ...
*
Load control switch
Demand response is a change in the power consumption of an electric utility customer to better match the demand for power with the supply. Until the 21st century decrease in the cost of pumped storage and batteries electric energy could not be ...
*
Membrane switch
A membrane switch is a custom switch assembly that can open or close the conducting path in an electrical circuit and requires at least one contact made of or attached to a flexible substrate. Its assembly differs from traditional mechanical switc ...
*
MEMS switch
A radio-frequency microelectromechanical system (RF MEMS) is a microelectromechanical systems, microelectromechanical system with electronic components comprising moving sub-millimeter-sized parts that provide radio-frequency (RF) functionality. ...
*
Optical switch An optical transistor, also known as an optical switch or a light valve, is a device that switches or amplifies optical signals. Light occurring on an optical transistor's input changes the intensity of light emitted from the transistor's output ...
*
Piezo switch
*
Pull switch
A pull switch, also known as pull-cord switch, or light pull, or pull chain is a switch that is actuated by means of a chain or string.
An electric pull switch is attached to a toggle type switch: one pull to switch on and next pull to switch of ...
*
Push switch
A push switch (button) is a momentary or non-latching switch which causes a temporary change in the state of an electrical circuit only while the switch is physically actuated. An automatic mechanism (i.e. a spring) returns the switch to it ...
*
Railroad switch
A railroad switch (), turnout, or ''set ofpoints () is a mechanical installation enabling railway trains to be guided from one track to another, such as at a railway junction or where a spur or siding branches off.
The most common ty ...
*
Sense switch
A sense switch, or program switch, is a switch on the front panel of a computer whose state can be tested by conditional branch instructions in software.Rudolf F. Graf ''Modern dictionary of electronics seventh edition'', Newnes, 1999 Most early ...
*
Slotted optical switch
The slotted optical switch, sometimes known as opto switch or optical switch but not to be confused with the optical component, is a device comprising a photoemitter (e.g. LED) and a photodetector (e.g. photodiode) mounted in a single package ...
*
Stepping switch
In electrical control engineering, a stepping switch or stepping relay, also known as a uniselector, is an electromechanical device that switches an input signal path to one of several possible output paths, directed by a train of electrical pulse ...
*
Thermal switch
*
Time switch
A time switch (also called a timer switch, or simply timer) is a timer that operates an electric switch controlled by the timing mechanism.
Intermatic introduced its first time switch in 1945, which was used for "electric signs, store window lig ...
*
Touch switch A touch switch is a type of switch that only has to be touched by an object to operate. It is used in many lamps and wall switches that have a metal exterior as well as on public computer terminals. A touchscreen includes an array of touch switches ...
*
Transfer switch
A transfer switch is an electrical switch that switches a load between two sources. Some transfer switches are manual, in that an operator effects the transfer by throwing a switch, while others are automatic and trigger when they sense one of t ...
*
Wire switches
*
Zero speed switch
See also
*
Circuit breaker
A circuit breaker is an electrical safety device designed to protect an electrical circuit from damage caused by an overcurrent or short circuit. Its basic function is to interrupt current flow to protect equipment and to prevent the risk ...
*
Commutator (electric)
A commutator is a rotary electrical switch in certain types of electric motors and electrical generators that periodically reverses the current direction between the rotor and the external circuit. It consists of a cylinder composed of multiple ...
*
Contact resistance
The term contact resistance refers to the contribution to the total resistance of a system which can be attributed to the contacting interfaces of electrical leads and connections as opposed to the intrinsic resistance. This effect is describe ...
*
DIN rail
A DIN rail is a metal rail of a standard type widely used for mounting circuit breakers and industrial control equipment inside equipment racks. These products are typically made from cold rolled carbon steel sheet with a zinc-plated or chroma ...
*
Electric switchboard
An electric switchboard is a device that distributes electricity from one or more sources of supply to several smaller load circuits. It is an assembly of one or more panels, each of which contains switching devices for the protection and contro ...
*
Fuse cutout
In electrical distribution, a fuse cutout or cut-out fuse (often referred to as a cutout) is a combination of a fuse and a switch, used in primary overhead feeder lines and taps to protect distribution transformers from current surges and overl ...
*
RF switch matrix
An RF switch matrix is an array of RF switches arranged to route radio frequency (RF) signals between multiple inputs and multiple outputs. Applications requiring RF matrices include ground systems, test equipment, and communication systems.
An ...
*
Switch access
Switch access is the use of one or more switches to operate computers and other devices, and is primarily used by people with severe physical or cognitive impairment. A switch is an assistive technology device that replaces the need to use a comp ...
*
Switchgear
In an electric power system, a switchgear is composed of electrical disconnect switches, fuses or circuit breakers used to control, protect and isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be ...
References
External links
*
{{Authority control
Electrical components
Human–machine interaction