Power strip
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A power strip (also known as a power bar, gangplug or multibox) is a strip of sockets that attaches to the end of a flex and allows multiple devices to be plugged in. As such it can be considered a type of trailing socket though that term is more often used for single and double cable mounted sockets. The term is also used to refer to the complete assembly with the power strip on one end and a plug on the other. Power strips are often used when many electrical devices are in close proximity, especially with audio/video and computer systems.
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Control
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Power strips can include a switch to turn all devices on and off. In a few cases they may even have all outlets individually switched. Some very expensive strips exist that can detect one device being turned on (say the PC itself in a computer setup) and turn everything else on. Remote control strips also exist to allow a group of devices to be switched remotely.
Surge Protection and Filtering
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Many power strips have built in surge protectors and/or EMI/RFI filters: these are sometimes described as electrical line conditioners. Some also provide surge suppression for phone lines, TV cable coax, or network cable.
Surge suppression is usually provided by one or more metal-oxide varistors (MOVs), which are inexpensive two-terminal semiconductors. They act as very high speed switches, closing at any of several designed voltages. The most commonly used are built to close at a voltage somewhat above the local mains supply. In the US, this is (nominally) 115 VAC. however it should be borne in mind that this voltage is RMS not peak and also that it is only a nominal value.
In most of the civilised 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.
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. Such arrangements are more expensive, but less prone to silent degradation than MOVs.
Indication
Many power strips have a neon light for power indication. Surge protected strips are likely to have more lights to indicate the status of the surge protection system.
Socket arrangement
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Socket arrangement varies considerably, but for access reasons there are rarely more than two rows. In the UK units featuring four sockets in a square arrangement are fairly common although these are not strip-shaped and hence not normally referred to as power strips.
The bulkiness of European plugs and sockets tends to mean that European power strips have somewhat fewer outlets than their American counterparts and usually only have one row.
In some countries where multiple socket types are in use, a single power strip can have two or more kinds of sockets.
Safety
Overloading can be a problem with any sort of power distribution adaptor. This is especially likely if multiple appliances with heating elements, such as electric room heaters or benchtop cooking appliances like electric frying pans are plugged into a power strip or similar device by an ignorant user.
In the US and some other countries, power strips generally have a circuit breaker integrated to prevent overload. In the UK power strips are usually protected by the fuse in the BS 1363 plug.
Power strips are generally considered a safer alternative to using "double adaptors" or "two-way plugs" / "three-way plugs" which plug directly into the socket with no lead for multiple appliances. Two-way and three-way plugs are generally not fused (although later three-way adaptors in the UK 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 wall warts. Such adaptors, while still available, have largely fallen out of use.