A power strip (also known as an extension block, power board, plug board and by many other variations) 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 and computer systems. Power strips often include a circuit breaker to safely limit the electric current flowing through them.
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Power strips can include a switch to turn all devices on and off. Some have outlets which are individually switched. "Master/slave" strips can detect one "master" device being turned on or 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 allow a group of devices to be switched remotely, usually over the Internet. These types of power strips are usually used in a data centre.
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 cannot always be replied upon.[1]
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 current flowing through the socket is in standby mode (less than 30 watts), and if so they will turn off that socket. This reduces 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 (e.g. plasma televisions) as they will always appear to be switched on to the power board. 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.
It is recommended that appliances that need a controlled shutdown sequence (e.g. 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.
In some countries where multiple socket types are in use, a single power strip can have two or more kinds of socket.
Socket arrangement varies considerably, but for 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.[2]
Many power strips have built-in surge protectors and/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) 120 VAC. It should be borne in mind that this voltage specification is RMS, not peak, and also that it is only a nominal value.
In most of the developed world, mains electrical circuits are (supposed to be) grounded (earthed), so there will be a live wire, a neutral wire, and a ground wire. 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, mounted 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.
This arrangement 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. Consequently it is recommended not to connect a surge-protected power board to a UPS.[3]
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 less prone to silent degradation than MOVs.
Within the EU power strips with surge suppression circuits can demonstrate compliance with the (LVD) Low Voltage Directive 2006/95/EC [4] by complying with the requirements of EN 61643-11:2002+A1. The standard covers both the performance of the surge suppression circuit and the 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.
Connecting Power Strips in a daisy chain does not necessarily increase the protection they provide.[5] Connecting them in this manner connects their surge protection components in parallel, in theory spreading any potential surge across each surge protector. However, due to manufacturing tolerances of the MOVs, the surge will not be spread evenly and will typically go through the one that trips first.
Overload protection is different from surge protection. Some power strips only have overload protection, which does not provide protection from electricity spikes (surges); it only means that the device will trip (switch off) when the devices plugged into it are drawing more power than the strip is rated to supply. For example, the standard rating for overload protected powerboards is 2400W in Australia, so exceeding that amount of power will make the board trip.
Power boards offering only overload protection generally do not have a power light, but they do have a reset switch, which is used to return the board to service after an overload has caused it to trip.
Overloading can be a problem with any sort of power distribution adaptor. This is especially likely if multiple high-power appliances are used, such as those with heating elements, like room heaters or electric frying pans.
In the US and some other countries, power strips generally 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. While this is not much use if they are being fed with a 13A plug it can be very helpful for providing safe protection to adaptor leads from a higher current plug type.
Power strips are generally considered a safer alternative to "double adaptors", "two-way plugs", "three-way plugs" or "cube taps" which plug directly into the socket with no lead for multiple appliances. These low-cost adaptors are generally not fused (although more modern adaptors in the UK and Ireland are). Therefore in many cases the only protection against overload is the circuit fuse which may well have a rating higher than the adaptor. The weight of the plugs pulling on the adapter (and often pulling it part way out of the socket) can also be an issue if adaptors are stacked or if they are used with brick-style power supplies. Such adaptors, while still available, have largely fallen out of use in some countries (although two and three-way adaptors are still common in the UK and Ireland).
When plugging a device into a power strip, a build up of carbon can cause sparking to occur. This generally doesn't pose much of a risk, but it can bother some people.
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.[1] This regulation lists the requirements for all domestic plugs and sockets; including socket outlet units (power strips), see [2] 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 unit with control circuits also fall within the scope of the EMC Directive 2004/108/EC.
In 1972, the electrical "power-board" was invented 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.[6]