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108-442: Some power strips include a master switch to turn all devices on and off. This can be used with simple devices, such as lights, but not with most computers, which must use shutdown commands from the software first. Computers may have open files, which may be damaged if the power is simply turned off. Some power strips have individually switched outlets. " Master/slave " strips can detect one "master" device being turned off (such as

216-412: A ceramic sintering process, which exhibited a voltage-current function similar to that of a pair of back-to-back Zener diodes . This type of device became the preferred method for protecting circuits from power surges and other destructive electric disturbances, and became known generally as the metal-oxide varistor (MOV). The randomness of orientation of ZnO grains in the bulk of this material provided

324-574: A circuit breaker integrated to prevent overload. In the UK, power strips are required to be protected by the fuse in the BS 1363 plug. Some also feature a 13A BS1362 fuse in the socket end. Power strips are generally considered a safer alternative to “double adapters”, “two-way plugs”, “three-way plugs”, or “cube taps” which plug directly into the socket with no lead for multiple appliances. These low-cost adapters are generally not fused (although more modern ones in

432-407: A laser pointer ) 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 ) has a constant on-off feature. Dual-action switches incorporate both of these features. When a strongly inductive load such as an electric motor is switched off, the current cannot drop instantaneously to zero; a spark will jump across

540-460: A multiway switching system for control of lamps by more than two 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 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 allows remote control of lamps for convenience; some lamps include

648-500: A radio controlled 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. A toggle switch or tumbler switch is a class of electrical switches that are manually actuated by a mechanical lever , 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

756-417: A sail switch ensures that air flow is adequate in a duct. Pressure switches respond to fluid pressure. The mercury switch consists of a drop of mercury inside a glass 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

864-446: A touch switch which electronically controls the lamp if touched anywhere. In public buildings several types of vandal resistant switches are used to prevent unauthorized use. Varistor A varistor (a.k.a. voltage-dependent resistor (VDR) ) is a surge protecting electronic component with an electrical resistance that varies with the applied voltage. It has a nonlinear , non- ohmic current–voltage characteristic that

972-444: A MOV device. A varistor provides no equipment protection from inrush current surges (during equipment startup), from overcurrent (created by a short circuit), or from voltage sags ( brownouts ); it neither senses nor affects such events. Susceptibility of electronic equipment to these other electric power disturbances is defined by other aspects of the system design, either inside the equipment itself or externally by means such as

1080-526: A UPS, a voltage regulator or a surge protector with built-in overvoltage protection (which typically consists of a voltage-sensing circuit and a relay for disconnecting the AC input when the voltage reaches a danger threshold). Another method for suppressing voltage spikes is the transient-voltage-suppression diode (TVS). Although diodes do not have as much capacity to conduct large surges as MOVs, diodes are not degraded by smaller surges and can be implemented with

1188-468: 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

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1296-431: 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

1404-421: A fire hazard. While a MOV is designed to conduct significant power for very short durations (about 8 to 20 microseconds), such as caused by lightning strikes, it typically does not have the capacity to conduct sustained energy. Under normal utility voltage conditions, this is not a problem. However, certain types of faults on the utility power grid can result in sustained over-voltage conditions. Examples include

1512-445: 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 , steel , or brass , depending on the application. Fixed contacts may be backed up with a spring. Several parallel blades can be operated at

1620-453: A fuse rated at not more than 13 A. Hence, in the UK and in other countries using BS 1363 plugs , this fused plug provides overload protection for any power strip. The fuse must be replaced if the power strip is overloaded, causing the fuse to operate. In Australia and New Zealand the rating for a standard socket outlet is 10 Amperes but these outlets are provided on circuits usually protected by circuit breakers of 16 or 20 A capacity. Also, it

1728-562: A given switch design. 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 "

1836-504: 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 . 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 sensors in

1944-402: A large " wall wart " transformer at its end may cover up multiple sockets. Various designs address this problem, some by simply increasing the spacing between outlets. Other designs include receptacles which rotate in their housing, or multiple short receptacle cords feeding from a central hub. A simple DIY method for adapting problematic power strips arrangements to large "wall warts" is to use

2052-507: A live (hot) wire, a neutral wire, and a ground wire. Low-cost power strips often come with only one MOV mounted between the live and neutral wires. More complete (and desirable) power strips will have three MOVs, connected between each possible pair of wires. Since MOVs degrade somewhat each time they are triggered, power strips using them have a limited, and unpredictable, protective life. Some power strips have "protection status" lights which are designed to turn off if protective MOVs connected to

2160-484: A loss of a neutral conductor or shorted lines on the high voltage system. Application of sustained over-voltage to a MOV can cause high dissipation, potentially resulting in the MOV device catching fire. The National Fire Protection Association (NFPA) has documented many cases of catastrophic fires that have been caused by MOV devices in surge suppressors, and has issued bulletins on the issue. A series connected thermal fuse

2268-413: A lower "clamping voltage". MOVs degrade from repeated exposure to surges and generally have a higher "clamping voltage" so that leakage does not degrade the MOV. Both types are available over a wide range of voltages. MOVs tend to be more suitable for higher voltages, because they can conduct the higher associated energies at less cost. Another type of transient suppressor is the gas-tube suppressor. This

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2376-449: 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 and fall time 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 , limits on

2484-447: 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

2592-418: A microscopic scale. This type is commonly known as the metal-oxide varistor ( MOV ). Varistors are used as control or compensation elements in circuits either to provide optimal operating conditions or to protect against excessive transient voltages . When used as protection devices, they shunt the current created by the excessive voltage away from sensitive components when triggered. The name varistor

2700-409: A neighbor forms a diode junction, which allows current to flow in only one direction. The accumulation of randomly oriented grains is electrically equivalent to a network of back-to-back diode pairs, each pair in parallel with many other pairs. When a small voltage is applied across the electrodes, only a tiny current flows, caused by reverse leakage through the diode junctions. When a large voltage

2808-467: A process and used to automatically control a system. For example, a thermostat is a temperature-operated switch used to control a heating process. A switch that is operated by another electrical circuit is called a relay . 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

2916-513: 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 , or in control circuits. For example,

3024-535: 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

3132-408: A shorted circuit condition as the energy discharges create a conductive channel through the oxides. The main parameter affecting varistor life expectancy is its energy (Joule) rating. Increasing the energy rating raises the number of (defined maximum size) transient pulses that it can accommodate exponentially as well as the cumulative sum of energy from clamping lesser pulses. As these pulses occur,

3240-411: A single power strip can have two or more kinds of sockets. Socket arrangement varies considerably, but for physical access reasons there are rarely more than two rows. In Europe, power strips without surge suppression are normally single row, but models with surge suppression are supplied both in single and double row configurations. If sockets on a power strip are grouped closely together, a cable with

3348-652: A slave socket on such a strip as it can damage them if they are switched off incorrectly (for example the inkjet printer may not have capped the print head in time, and consequently the ink will dry and clog the print head.) Within Europe, power strips with energy-saving features are within the scope of the Low Voltage Directive 2006/95/EC and the EMC Directive 2004/108/EC and require a CE mark . In some countries where multiple socket types are in use,

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3456-461: A stack and connecting the terminal ends and the center in an anti-parallel configuration, as shown in the photo of a Western Electric Type 3B varistor of June 1952 (below). The Western Electric type 500 telephone set of 1949 introduced a dynamic loop equalization circuit using varistors that shunted relatively high levels of loop current on short central office loops to adjust the transmission and receiving signal levels automatically. On long loops,

3564-438: 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

3672-451: A strike may melt, burn, or even vaporize the varistor. This thermal runaway is due to a lack of conformity in individual grain-boundary junctions, which leads to the failure of dominant current paths under thermal stress when the energy in a transient pulse (normally measured in joules ) is too high (i.e. significantly exceeds the manufacture's "Absolute Maximum Ratings"). The probability of catastrophic failure can be reduced by increasing

3780-545: A sufficient amount of wetting current is a crucial step in designing systems 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. The moving part that applies the operating force to the contacts is called the actuator , and may be a toggle or dolly , a rocker , a push-button or any type of mechanical linkage (see photo). A switch normally maintains its set position once operated. A biased switch contains

3888-470: A surge-protected power strip to a UPS, but instead to rely solely on surge protection provided by the UPS itself. More-elaborate power strips may use inductor-capacitor networks to achieve a similar effect of protecting equipment from high voltage spikes on the mains circuit. These more-expensive arrangements are much less prone to silent degradation than MOVs, and often have monitoring lights that indicate whether

3996-617: A switch has two conductive pieces, often metal , 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 , 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

4104-529: A three-way socket adapter to extend the socket above its neighbors, providing the required clearance. Many power strips have built-in surge protectors or EMI/RFI filters: these are sometimes described as surge suppressors or electrical line conditioners . Some also provide surge suppression for phone lines, TV cable coax, or network cable. Unprotected power strips are often mistakenly called "surge suppressors" or "surge protectors" even though they may have no ability to suppress surges . Surge suppression

4212-404: 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 , a toggle switch, or the action of toggling,

4320-419: A varying resistance in dependence on the polarity and magnitude of applied voltage. It was constructed from a small copper disk, on one side of which, a layer of cuprous oxide was formed. This arrangement provides low resistance to current flowing from the semiconducting oxide to the copper side, but a high resistance to current in the opposite direction, with the instantaneous resistance varying continuously with

4428-469: A very fast, <1 ns rise-time transient, response times for the MOV are in the 40–60 ns range. A typical surge protector power strip is built using MOVs. Low-cost versions may use only one varistor, from the hot (live, active) to the neutral conductor. A better protector contains at least three varistors; one across each of the three pairs of conductors. Some standards mandate a triple varistor scheme so that catastrophic MOV failure does not create

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4536-427: Is a portmanteau of varying resistor . The term is only used for non-ohmic varying resistors. Variable resistors , such as the potentiometer and the rheostat , have ohmic characteristics. The development of the varistor, in form of a new type of rectifier based on a cuprous oxide (Cu 2 O) layer on copper, originated in the work by L.O. Grondahl and P.H. Geiger in 1927. The copper-oxide varistor exhibited

4644-400: Is a common problem with mechanical switches, relays and battery contacts , which arises as the result of electrical contact resistance (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

4752-487: 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 circuits that respond fast enough to misinterpret the on‑off pulses as a data stream. 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

4860-419: 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 (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

4968-408: Is applied, the diode junction breaks down due to a combination of thermionic emission and electron tunneling , resulting in a large current flow. The result of this behavior is a nonlinear current-voltage characteristic, in which the MOV has a high resistance at low voltages and a low resistance at high voltages. A varistor remains non-conductive as a shunt -mode device during normal operation when

5076-622: Is conducted by a MOV, it may fail catastrophically to an open circuit condition, keeping the load connected but now without any surge protection. A user may have no indication that the surge suppressor has failed. Under the right conditions of over-voltage and line impedance, it may be possible to cause the MOV to burst into flames, the root cause of many fires which is the main reason for NFPA's concern resulting in UL1449 in 1986 and subsequent revisions in 1998 and 2009. Properly designed TVSS devices must not fail catastrophically, instead resulting in

5184-488: 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. The most familiar form of switch is a manually operated electromechanical device with one or more sets of electrical contacts , which are connected to external circuits. Each set of contacts can be in one of two states: either "closed" meaning

5292-601: Is intended only for indoor use as a temporary extension of a grounding alternating-current branch circuit for general use. In Europe, plugs and sockets without additional control or surge protection circuits are outside the scope of the Low Voltage Directive 2006/95/EC and controlled by National regulations, and therefore must not be CE marked. In the UK the legal requirements for plugs and sockets are listed in Statutory Instrument 1994 No. 1768, The Plugs and Sockets etc. (Safety) Regulations 1994. This regulation lists

5400-608: Is often defined using standardized transients such as 8/20 microseconds or 10/1000 microseconds, where 8 microseconds is the transient's front time and 20 microseconds is the time to half value. Typical capacitance for consumer-sized (7–20 mm diameter) varistors are in the range of 100–2,500 pF. Smaller, lower-capacitance varistors are available with capacitance of ~1 pF for microelectronic protection, such as in cellular phones. These low-capacitance varistors are, however, unable to withstand large surge currents simply due to their compact PCB-mount size. The response time of

5508-462: Is on. Better-quality surge-protected strips have additional lights to indicate the status of the surge protection system, however these are not always reliable as an indicator. Some power strips have energy-saving features, which switch off the strip if appliances go into standby mode. Using a sensor circuit, they detect if the level of power through the socket is in standby mode (less than 30 watts), and if so they will turn off that socket. This reduces

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5616-447: Is one solution to catastrophic MOV failure. Varistors with internal thermal protection are also available. There are several issues to be noted regarding behavior of transient voltage surge suppressors (TVSS) incorporating MOVs under over-voltage conditions. Depending on the level of conducted current, dissipated heat may be insufficient to cause failure, but may degrade the MOV device and reduce its life expectancy. If excessive current

5724-482: Is possible to insert an Australian/NZ 10 A plug into socket outlets rated at up to 32 A. Hence, all power strips sold in Australia and New Zealand with three or more 10A socket outlets are required to have overload protection so that if the total current drawn exceeds 10 A the inbuilt circuit breaker will operate and disconnect all connected devices. These power strips have a reset button for the circuit breaker, which

5832-525: Is similar to that of a diode . Unlike a diode however, it has the same characteristic for both directions of traversing current. Traditionally, varistors were indeed constructed by connecting two rectifiers , such as the copper-oxide or germanium-oxide rectifier in antiparallel configuration. At low voltage the varistor has a high electrical resistance which decreases as the voltage is raised. Modern varistors are primarily based on sintered ceramic metal-oxide materials which exhibit directional behavior only on

5940-468: 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

6048-513: 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. 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 ),

6156-520: Is used to return the strip to service after an overload has caused it to trip. On some power strips, the red, lighted rocker switch actually controls a thermal circuit breaker which flips to off when tripped. Resetting is done by flipping the switch back on. Electrical overloading can be a problem with any sort of power distribution adapter. This is especially likely if multiple high-power appliances are used, such as those with heating elements, like room heaters or electric frying pans. Power strips may have

6264-505: Is usually provided by one or more metal-oxide varistors (MOVs), which are inexpensive two-terminal semiconductors . These act as very high speed switches, momentarily limiting the peak voltage across their terminals. By design, MOV surge limiters are selected to trigger at a voltage somewhat above the local mains supply voltage, so that they do not clip normal voltage peaks, but clip abnormal higher voltages. Mains electrical power circuits are generally grounded (earthed), so there will be

6372-485: The Hammond organ , 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 organs have a switchable replica of this sound effect. The effects of contact bounce can be eliminated by: All of these methods are referred to as 'debouncing'. When

6480-436: The contact resistance , and wetting current of a mechanical switch. Sometimes the contacts are plated with noble metals , 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 may be specified for

6588-420: The electronics 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 wiring (i.e., house and building wiring by electricians ), names generally involve the suffix "-way" ; however, these terms differ between British English and American English (i.e.,

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6696-405: The liquid metal 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 switches consist of

6804-417: The "clamping voltage" it provides during each event decreases, and a varistor is typically deemed to be functionally degraded when its "clamping voltage" has changed by 10%. Manufacturer's life-expectancy charts relate current , severity, and number of transients to make failure predictions based on the total energy dissipated over the life of the part. In consumer electronics, particularly surge protectors ,

6912-527: The LVD by complying with the requirements of EN 61643-21:2001. Connecting MOV-protected power strips in a " daisy chain " (in a series, with each power strip plugged into a previous one in the chain) does not necessarily increase the protection they provide. Connecting them in this manner effectively connects their surge protection components in parallel, in theory spreading any potential surge across each surge protector. However, due to manufacturing variations between

7020-421: The MOV is not standardized. The sub-nanosecond MOV response claim is based on the material's intrinsic response time, but will be slowed down by other factors such as the inductance of component leads and the mounting method. That response time is also qualified as insignificant when compared to a transient having an 8 µs rise-time, thereby allowing ample time for the device to slowly turn-on. When subjected to

7128-489: The MOV varistor size employed is small enough that eventually failure is expected. Other applications, such as power transmission, use VDRs of different construction in multiple configurations engineered for long life span. MOVs are specified according to the voltage range that they can tolerate without damage. Other important parameters are the varistor's energy rating in joules, operating voltage, response time, maximum current, and breakdown (clamping) voltage. Energy rating

7236-638: The MOVs, the surge energy will not be spread evenly, and will typically go through the one that triggers first. Daisy chaining of power strips (known in building and electric codes as multi-plug adapters or relocatable power taps), whether surge protected or not, is specifically against most codes. As an example, the International Code Council's International Fire Code 2009 Edition in 605.4.2 states, "Relocatable power taps shall be directly connected to permanently installed receptacles." Where

7344-522: The PC itself in a computer setup, or a TV in a home theatre) and turn everything else on or off accordingly. Remote control strips are used in data centers , to allow computer systems or other devices to be remotely restarted, often over the Internet (although this leaves them vulnerable to outside attacks). Many power strips have a neon or LED indicator light or one per output socket to show when power

7452-508: The UK and Ireland are). Therefore, in many cases the only protection against overload is the branch circuit fuse which may well have a rating higher than the adapter. The weight of the plugs pulling on the adapter (and often pulling it part way out of the socket) can also be a problem if adapters are stacked or if they are used with brick-style power supplies. Such adapters, while still available, have largely fallen out of use in some countries (although two- and three-way adapters are still common in

7560-560: The US, UK, and Ireland). When plugging a device into a power strip, a buildup of carbon or dust can cause sparking to occur. This generally doesn't pose much of a risk in a non-explosive atmosphere, but explosive atmospheres (for example, near a gasoline refueling station or a solvent cleaning facility) require specialized explosion-proof sealed electrical equipment. 1.1 These requirements cover cord-connected, relocatable power taps rated 250 V AC or less and 20 A AC or less. A relocatable power tap

7668-551: The arc. 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

7776-461: The arc. For example, the switch contacts may operate in a vacuum, immersed in mineral oil , or in sulfur hexafluoride . 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. When a switch is designed to switch significant power,

7884-524: The conducting path in an electrical circuit , interrupting the electric current 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 contacts 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

7992-488: The consumption of standby power used by computer peripherals and other equipment when not in use, saving money and energy Some more-sophisticated power strips have a master and slave socket arrangement, and when the "master" socket detects standby mode in the attached appliance's current it turns off the whole strip. However, there can be problems detecting standby power in appliances that use more power in standby mode (such as plasma televisions) as they will always appear to

8100-451: 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

8208-415: 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. In the simplest case,

8316-409: The current decreases to a steady-state value. A switch designed for an incandescent lamp load can withstand this inrush current. Wetting current is the minimum current needing to flow through a mechanical switch while it is operated to break through any film of oxidation that may have been deposited on the switch contacts. The film of oxidation occurs often in areas with high humidity . Providing

8424-401: The current rating of the socket outlet, plug and lead of the power strip is equal to the rating of the circuit breaker supplying the circuit concerned, additional overload protection for the power strip is unnecessary, since the existing circuit breaker will provide the required protection. However, where the rating of a socket outlet (and, hence, the plug and lead of the power strip) is less than

8532-482: The early 1920s. However, these were usually not in a long "strip" like in modern devices. Examples of power strips exist in the U.S. patent system dating back as far as 1929, starting with the creation of Carl M. Peterson's "Table Tap". Another early example was created by Allied Electric Products in 1950. Perhaps the first modern designs for the power strip were created by the U.S. firm Fedtro , which filed two patents in 1970 for designs that were close to designs used in

8640-407: 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

8748-401: The live wire have failed, but such simple circuits cannot detect all failure modes (such as failure of a MOV connected between neutral and ground). The surge-induced triggering of MOVs can cause damage to an upstream device, such as an uninterruptible power supply (UPS), which typically sees an overload condition while the surge is being suppressed. Therefore, it is recommended not to connect

8856-456: The modern day. One early iteration, called a "power board", was invented in 1972 by Australian electrical engineer Peter Talbot working under Frank Bannigan, managing director of Australian company Kambrook . The product was hugely successful, however, it was not patented and market share was eventually lost to other manufacturers. Switch In electrical engineering , a switch is an electrical component that can disconnect or connect

8964-413: The opening contacts. Switches for inductive loads must be rated to handle these cases. The spark will cause electromagnetic interference if not suppressed; a snubber network of a resistor and capacitor in series will quell the spark. When turned on, an incandescent lamp draws a large inrush current of about ten times the steady-state current; as the filament heats up, its resistance rises and

9072-444: The opening of a thermal fuse or something equivalent that only disconnects MOV devices. A MOV inside a transient voltage surge suppressor (TVSS) does not provide complete protection for electrical equipment. In particular, it provides no protection from sustained over-voltages that may result in damage to that equipment as well as to the protector device. Other sustained and harmful over-voltages may be lower and therefore ignored by

9180-448: 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 . The interior lamp of a household refrigerator is controlled by a switch that is held open when the door is closed. A rotary switch operates with a twisting motion of the operating handle with at least two positions. One or more positions of

9288-424: The power being switched is sufficiently large, the electron flow across opening switch contacts is sufficient to ionize the air molecules across the tiny gap between the contacts as the switch is opened, forming a gas plasma , also known as an electric arc . 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

9396-400: The power strip to be switched on. When using a master–slave power strip, one way to avoid such problems is to plug an appliance with a lower standby wattage (such as a DVD player) into the master socket, using it as the master control instead. A different power strip design intended to save energy uses a passive infrared (PIR) or ultrasonic sound detector to determine if a person is nearby. If

9504-501: The protective circuitry is still connected. In the European Union, power strips with surge suppression circuits can demonstrate compliance with the (LVD) Low Voltage Directive 2006/95/EC by complying with the requirements of EN 61643-11:2002+A1. The standard covers both the performance of the surge suppression circuit and their safety. Likewise, power strips with telecoms surge suppression circuits can demonstrate compliance with

9612-436: The rating of the circuit breaker supplying the circuit concerned, overload protection for the power strip and its supply cable is necessary. In the UK, standard BS 1363 plugs and sockets are rated at 13 A but are provided on circuits protected by circuit breakers of up to 32 A. However, UK Consumer Protection legislation requires that plug-in domestic electrical goods must be provided with plugs to BS 1363, which include

9720-413: The rating, or using specially selected MOVs in parallel. Cumulative degradation occurs as more surges happen. For historical reasons, many MOVs have been incorrectly specified allowing frequent swells to also degrade capacity. In this condition the varistor is not visibly damaged and outwardly appears functional (no catastrophic failure), but it no longer offers protection. Eventually, it proceeds into

9828-484: The requirements for all domestic plugs and sockets; including socket outlet units (power strips), see Electrical Equipment - Requirements for Plugs & Sockets etc. - Guidance notes on the UK Plugs & Sockets etc. (Safety) Regulations 1994 (S.I. 1994/1768). The regulation requires all socket outlet units to comply with the requirements of BS 1363-2 Specification for 13A switched and unswitched socket-outlets and with

9936-404: The requirements of BS 5733 Specification for General requirements for electrical accessories. Sockets and socket outlets do not require independent approval under the regulations. Any plug fitted to the socket outlet unit must comply with the requirements of BS 1363-1 Specification for rewirable and non-rewirable 13A fused plugs. Plugs must also be independently approved and marked in accordance with

10044-404: The requirements of the regulation. If a socket outlet unit contains additional control circuits or surge protection circuits they will fall within the scope of the Low Voltage Directive 2006/95/EC and must be CE marked. Socket outlet units with control circuits also fall within the scope of the EMC Directive 2004/108/EC. Outlet multipliers with the same purpose as power strips go back at least to

10152-513: 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 or a keyboard button, or may function as a sensing element to sense the position of a machine part, liquid level, pressure, or temperature, such as a thermostat . Many specialized forms exist, such as the toggle switch , rotary switch , mercury switch , push-button switch, reversing switch , relay , and circuit breaker . A common use

10260-581: 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

10368-435: The same voltage-current characteristics for both directions of current flow. The most common modern type of varistor is the metal-oxide varistor (MOV). This type contains a ceramic mass of zinc oxide (ZnO) grains, in a matrix of other metal oxides, such as small amounts of bismuth, cobalt, manganese oxides, sandwiched between two metal plates, which constitute the electrodes of the device. The boundary between each grain and

10476-439: The sensors don't detect any motion for a preset period of time, the strip shuts off several outlets, while leaving one outlet on for devices that should not be powered off. These so-called "smart power strips" are intended to be installed in offices, to shut down equipment when the office is unoccupied. It is recommended that appliances that need a controlled shutdown sequence (such as many ink-jet printers) not be plugged into

10584-449: The simultaneous actuation of multiple sets of electrical contacts , or the control of large amounts of electric current 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

10692-437: 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 air currents. The arc can be quenched with

10800-413: 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

10908-415: 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

11016-451: The terms two way and three way are used with different meanings). Form A 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 (also called chatter )

11124-420: 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

11232-407: The turning of a key ( key switch ), linear or rotary movement (a limit switch or microswitch ), 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 is used, for example, in machine tools to interlock operation with the proper position of tools. In heating or cooling systems

11340-415: The varistors maintained a relatively high resistance and did not alter the signals significantly. Another type of varistor was made from silicon carbide (SiC) by R. O. Grisdale in the early 1930s. It was used to guard telephone lines from lightning. In the early 1970s, Japanese researchers recognized the semiconducting electronic properties of zinc oxide (ZnO) as being useful as a new varistor type in

11448-434: The voltage across it remains well below its "clamping voltage", thus varistors are typically used for suppressing line voltage surges. Varistors can fail for either of two reasons. A catastrophic failure occurs from not successfully limiting a very large surge from an event like a lightning strike, where the energy involved is many orders of magnitude greater than the varistor can handle. Follow-through current resulting from

11556-643: The voltage applied. In the 1930s, small multiple-varistor assemblies of a maximum dimension of less than one inch and apparently indefinite useful lifetime found application in replacing bulky electron tube circuits as modulators and demodulators in carrier current systems for telephonic transmission. Other applications for varistors in the telephone plant included protection of circuits from voltage spikes and noise, as well as click suppression on receiver ( ear-piece ) elements to protect users' ears from popping noises when switching circuits. These varistors were constructed by layering an even number of rectifier disks in

11664-466: The voltage potential is sufficient to exceed the breakdown voltage 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

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