A power strip with two USB power ports that includes a built in surge protector
A power strip[lower-alpha 1] is a block of electrical sockets that attaches to the end of a flexible cable (typically with a mains plug on the other end), allowing multiple electrical devices to be powered from a single electrical socket. Power strips are often used when many electrical devices are in proximity, such as for audio, video, computer systems, appliances, power tools, and lighting. Power strips often include a circuit breaker to interrupt the electric current in case of an overload or a short circuit. Some power strips provide protection against electrical power surges. Typical housing styles include strip, rack-mount, under-monitor and direct plug-in.[1]
Illuminated power switch on a power stripGerman master-slave remote controlled power strip
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 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).[citation needed]
Indication
Many power strips have a neon or LED indicator light or one per output socket to show when power 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.[2]
Energy-saving features and standby power
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.[3][4] This reduces the consumption of standby power used by computer peripherals and other equipment when not in use, saving money and energy [5] 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 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 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 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.
Socket arrangement
Italian power strip with two different types of socket
In some countries where multiple socket types are in use, 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 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 a three-way socket adapter to extend the socket above its neighbors, providing the required clearance.[6]
Schuko surge protected power strip (this model is unlikely to incorporate an inductor-capacitor network, due to lack of internal physical space)
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 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. In the US, this is (nominally) 120VAC. It should be borne in mind that this voltage specification is RMS, not peak, and also that it is only a nominal (approximate) value.
Mains electrical power circuits are generally grounded (earthed), so there will be 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 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 a surge-protected power strip to a UPS,[7] 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 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 [8] 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 the LVD by complying with the requirements of EN 61643-21:2001.
Daisy chaining and surge protection
North American surge suppressor power strip with separate protection status indicator lights
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.[9] 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 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."
Overload protection
Where 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 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 BS1363, which include 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.
Australian/NZ 10 A, 6 Outlet Power Board, with resettable overload protection
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 is possible to insert an Australian/NZ 10 A plug into socket outlets rated at up to 32 A.[10] 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 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.[11][12]
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 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 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 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.
US regulations
US power strip with switch
Underwriters Laboratories standard 1363 contains requirements for relocatable power taps. Included in the scope section of the standard is the statement:
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 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.[13] This regulation lists the requirements for all domestic plugs and sockets; including socket outlet units (power strips), see[14] 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 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 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.
History
Australian / New Zealand power board with individual switches
Outlet multipliers with the same purpose as power strips go back at least to the early 20s. 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".[15] Another early example was created by Allied Electric Products in 1950.[16]
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 the modern day.[17][18]
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.[19]
↑ Also known as an extension block, extension box, power board, power bar, plug board, pivot plug, trailing gang, trailing socket, plug bar, trailer lead, multiple socket, multi-socket, multiplug, multigang, multi-box, multibox, socket board, super plug, multiple outlet, polysocket and by many other variations
Related Research Articles
Mains electricity or utility power, power grid, domestic power, and wall power, or, in some parts of Canada, hydro, is a general-purpose alternating-current (AC) electric power supply. It is the form of electrical power that is delivered to homes and businesses through the electrical grid in many parts of the world. People use this electricity to power everyday items by plugging them into a wall outlet.
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 result, power supplies are sometimes referred to as electric power converters. Some power supplies are separate standalone pieces of equipment, while others are built into the load appliances that they power. Examples of the latter include power supplies found in desktop computers and consumer electronics devices. Other functions that power supplies may perform include limiting the current drawn by the load to safe levels, shutting off the current in the event of an electrical fault, power conditioning to prevent electronic noise or voltage surges on the input from reaching the load, power-factor correction, and storing energy so it can continue to power the load in the event of a temporary interruption in the source power.
A circuit breaker is an electrical safety device designed to protect an electrical circuit from damage caused by overcurrent. Its basic function is to interrupt current flow to protect equipment and to prevent the risk of fire. Unlike a fuse, which operates once and then must be replaced, a circuit breaker can be reset to resume normal operation.
IEC 60320 Appliance couplers for household and similar general purposes is a set of standards from the International Electrotechnical Commission (IEC) specifying non-locking connectors for connecting power supply cords to electrical appliances of voltage not exceeding 250 V (a.c.) and rated current not exceeding 16 A. Different types of connector are specified for different combinations of current, temperature and earthing requirements. Unlike IEC 60309 connectors, they are not coded for voltage; users must ensure that the voltage rating of the equipment is compatible with the mains supply. The standard uses the term coupler to encompass connectors on power cords and power inlets and outlets built into appliances.
A surge protector (or spike suppressor, surge suppressor, surge diverter, surge protection device or transient voltage surge suppressor is an appliance or device intended to protect electrical devices in alternating current circuits from voltage spikes, which can arise from a variety of causes including lightning strikes in the vicinity and have a very short duration measured in microseconds.
A power cord, line cord, or mains cable is an electrical cable that temporarily connects an appliance to the mains electricity supply via a wall socket or extension cord. The terms are generally used for cables using a power plug to connect to a single-phase alternating current power source at the local line voltage. The terms power cable, mains lead, flex or kettle lead are also used. A lamp cord is a light-weight, ungrounded, single-insulated two-wire cord used for small loads such as a table or floor lamp.
Schuko is a plug/socket system used in much of Europe and in many countries east of there. It is a registered trademark referring to a system of AC power plugs and sockets that is defined as "CEE 7/3" (sockets) and "CEE 7/4" (plugs). A Schuko plug features two round pins of 4.8 mm diameter for the line and neutral contacts, plus two flat contact areas on the top and bottom side of the plug for protective earth (ground). The socket has a predominantly circular recess which is 17.5 mm deep with two symmetrical round apertures and two earthing clips on the sides of the socket positioned to ensure that the earth is always engaged before live pin contact is made. Schuko plugs and sockets are symmetric AC connectors. They can be mated in two ways, therefore line can be connected to either pin of the appliance plug. As with most types of European sockets, Schuko sockets can accept Europlugs. Schuko plugs are considered a very safe design when used with Schuko sockets, but they can also mate with other sockets to give an unsafe result.
A residual-current device (RCD), residual-current circuit breaker (RCCB) or ground fault circuit interrupter (GFCI) is an electrical safety device that quickly breaks an electrical circuit with leakage current to ground. It is to protect equipment and to reduce the risk of serious harm from an ongoing electric shock. Injury may still occur in some cases, for example if a human receives a brief shock before the electrical circuit is isolated, falls after receiving a shock, or if the person touches both conductors at the same time.
In electronics and electrical engineering, a fuse is an electrical safety device that operates to provide overcurrent protection of an electrical circuit. Its essential component is a metal wire or strip that melts when too much current flows through it, thereby stopping or interrupting the current. It is a sacrificial device; once a fuse has operated, it is an open circuit, and must be replaced or rewired, depending on its type.
A distribution board is a component of an electricity supply system that divides an electrical power feed into subsidiary circuits while providing a protective fuse or circuit breaker for each circuit in a common enclosure. Normally, a main switch, and in recent boards, one or more residual-current devices (RCDs) or residual current breakers with overcurrent protection (RCBOs) are also incorporated.
AC power plugs and sockets connect devices to mains electricity to supply them with electrical power. A plug is the connector attached to an electrically-operated device, often via a cable. A socket is fixed in place, often on the internal walls of buildings, and is connected to an AC electrical circuit. Inserting the plug into the socket allows the device to draw power from this circuit.
Appliance classes specify measures to prevent dangerous contact voltages on unenergized parts, such as the metallic casing, of an electronic device. In the electrical appliance manufacturing industry, the following appliance classes are defined in IEC 61140 and used to differentiate between the protective-earth connection requirements of devices.
An arc-fault circuit interrupter (AFCI) or arc-fault detection device (AFDD) is a circuit breaker that breaks the circuit when it detects the electric arcs that are a signature of loose connections in home wiring. Loose connections, which can develop over time, can sometimes become hot enough to ignite house fires. An AFCI selectively distinguishes between a harmless arc, and a potentially dangerous arc.
In electricity supply design, a ring circuit is an electrical wiring technique in which sockets and the distribution point are connected in a ring. It is contrasted with the usual radial circuit, in which sockets and the distribution point are connected in a line with the distribution point at one end.
Electrical wiring in the United Kingdom is commonly understood to be an electrical installation for operation by end users within domestic, commercial, industrial, and other buildings, and also in special installations and locations, such as marinas or caravan parks. It does not normally cover the transmission or distribution of electricity to them.
The Europlug is a flat, non-rewirable two-pole, round-pin domestic AC power plug, rated for voltages up to 250 V and currents up to 2.5 A. It is a compromise design intended to connect low-power Class II appliances safely to the many different forms of round-pin domestic power socket used across Europe. However, it is not compatible with the rectangular-pin BS 1363 sockets found in Cyprus, Gibraltar, Singapore, Malaysia, United Arab Emirates, Hong Kong, Ireland, Malta and the United Kingdom. By the standard, Europlugs must be non-rewirable and must be supplied attached to a power cord, anything else is non-compliant.
AS/NZS 3112 is the harmonised Australian and New Zealand standard for AC power plugs (male) and sockets (female). The standard is used in Australia, Argentina, New Zealand, Fiji, Tonga, Solomon Islands, Papua New Guinea and several other Pacific island countries. The International Electrotechnical Commission (IEC) "world plugs" Web site calls this plug Type I.
NEMA connectors are power plugs and receptacles used for AC mains electricity in North America and other countries that use the standards set by the US National Electrical Manufacturers Association. NEMA wiring devices are made in current ratings from 15 to 60 amperes (A), with voltage ratings from 125 to 600 volts (V). Different combinations of contact blade widths, shapes, orientations, and dimensions create non-interchangeable connectors that are unique for each combination of voltage, electric current carrying capacity, and grounding system.
In electrical safety testing, portable appliance testing is a process in the United Kingdom, Ireland, New Zealand and Australia by which electrical appliances are routinely checked for safety. The formal term for the process is "in-service inspection & testing of electrical equipment". Testing involves a visual inspection of the equipment and that any flexible power cables are in good condition, and also where required, verification of earthing (grounding) continuity, and a test of the soundness of insulation between the current carrying parts, and any exposed metal that may be touched. The formal limits for pass/fail of these electrical tests vary somewhat depending on the category of equipment being tested.
Plugs and sockets for electrical appliances not hardwired to mains electricity originated in the United Kingdom in the 1870s and were initially two-pin designs. These were usually sold as a mating pair, but gradually de facto and then official standards arose to enable the interchange of compatible devices. British standards have proliferated throughout large parts of the former British Empire.
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