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__TOC__ THE THREE CONTACTS In most countries, household power is Single-phase Electric Power , in which a single live conductor brings Alternating Current into a house, and a neutral returns it to the power supply. Many plugs and sockets include a third contact used for a protective earth ground, which only carries current in case of a fault in the connected equipment. Live or Phase The live contact (also known as '''phase''', '''hot''' or '''active'''), along with the '''neutral''' contact, carries power from the power source to the equipment. The voltage between them varies by country, as set by national standards. In some installations, there may be two live conductors, either being two phases from a Three-phase system or being both phases from a Split Phase system. Some plug/socket combinations are designed in a way that a plug can be inserted only one possible way — this is referred to as a ''polarized'' plug (not to be confused with positive and negative polarity). Others allow the plug to be inserted with live and neutral either way round — this is referred to as an ''unpolarized'' plug. Furthermore even if live and neutral can only connect one way, in some countries it is common to wire them without regard for which is which. This will be hazardous with equipment that connects the neutral to the case. Neutral The neutral contact, along with a '''live''' contact, completes the circuit between the power source and equipment. It is chosen as the zero voltage reference point, with the live contact's voltage measured with respect to it. In many cases local electrical regulations require the neutral to be connected to earth ground. In such systems even though the neutral conductor is at a very low voltage with respect to ground, it is insulated for the full supply voltage in case of a fault such as a break in the wiring between netural and source. Another possibility is that the live and neutral may be reversed or crossed by improper installation. Neutral And Earth (ground) are closely related and are usually connected at some point. However, extra connections between the neutral and the earth should be avoided unless the relevant jurisdiction's regulations allow it. Connecting neutral and earth at more than one point can sometimes create a dangerous Ground Loop in the system. Earth/Ground The earth contact (known as '''ground''' in American English ) is only intended to carry Electric Current when connected to equipment that has developed an insulation fault (except for EMI/RFI filters which do cause a small current down the earth). The earth connection was added to modern plugs because, if a live wire or other component in a device touches the metal casing, anybody touching the device may receive a dangerous Electric Shock . In many countries devices with metal cases must have the case connected to the earth contact. This reduces but does not eliminate the possibility of the case developing a high voltage relative to the earth and grounded metalwork. It is a common misconception that the purpose of the earth connection is to take fault currents safely to earth. The primary purpose of the earthing system is to cause a fuse to blow or a breaker or a Residual-current Device (RCD) to trip to automatically disconnect the power supply to any device or cable which develops a wiring fault. The secondary purpose is to hold all touchable metal in a house to the same voltage to prevent electrical shocks when touching two metal objects at the same time. In addition, some equipment such as Surge Protector s require an earth connection to function properly because they operate by shorting the excess current to the earth. There are two main approaches to the problem of how to disconnect power when a live wire comes into contact with metalwork attached to the earthing system. One way is to get the resistance through the fault path and back to the supply very low by having a metallic connection from the earth back to the supply transformer (a TN system). Then when a fault happens a very high current will flow rapidly blowing a fuse (or tripping a Circuit Breaker ). Where such a direct connection is not used (a TT system) the resistance of the fault path back to the supply is almost invariably far higher and as a result the fault current is generally too low to reliably blow fuses (or trip Circuit Breaker s). Therefore an RCD must normally be used to disconnect the fault. The neutral core could in theory be used as a ground, but this would be dangerous if the core broke, so this is not normally used in building wiring or portable appliances. It is, however, used in some other situations with special precautions. For instance, in Switzerland, sockets in houses with the old two wire installation have the ground and neutral contacts connected together, probably supposing that the professionally maintained house installation is much more reliable than plugged-in device. Also using the neutral as a ground prevents the use of RCDs. Differences in terminology There are significant differences between American English and British English in talking about power plugs and sockets. In the United States , the live contact may be called ''live'' or ''hot''. The neutral contact may be called ''cold'', ''neutral'', ''return'', ''the grounded conductor'', or (in the National Electrical Code ), the ''identified conductor''. The earth contact is called ''ground'' or ''the ground'''ing''' conductor''. In the United Kingdom the word "line" is occasionally used to denote the live terminal or wire. This terminology derives from its being at the line voltage relative to neutral and ground, as distinct from the "phase" voltage, between lines on different phases of the supply. Live conductors are called ''phases'' when there is more than a single phase in use. Pins are also known as ''prongs'', ''contacts'' or ''terminals''. In Australia , the live contact is called ''active''. HISTORY OF PLUGS AND SOCKETS When Electricity was first introduced into the household, it was primarily used for lighting. At that time, many electricity companies operated a split-tariff system where the cost of electricity for lighting was lower than that for other purposes. This led to low-power appliances (such as vacuum cleaners and hair driers) being connected to the light fitting. The picture to the right shows a 1909 electric Toaster with a Light Bulb socket plug. However, as electricity became a common method of heating houses and operating labour-saving appliances, a means of connection to the electric system other than using a light socket was needed. The original two prong electrical plug and socket were invented by Harvey Hubbell and Patent ed in 1904. ;French type E France , Belgium , Poland , Czech Republic , Slovakia (after 1 July 2008 also Denmark) and some other countries have standardised on a socket which is not compatible with the CEE 7/4 socket (type F) that is standard in Germany and other continental European countries. The reason for incompatibility is that earthing in the E socket is done by a round male pin permanently mounted in the socket. Sockets are installed with the earth pin upwards and wired with left as live and right as neutral. The plug itself is round with two round pins measuring 4.8 × 19 mm, spaced 19 mm apart and a hole for the socket's earth pin. It will accept Europlug and CEE 7/17 plugs. As with the German plug below this plug will fit some other types of socket either easily or with force. However, there is no earth connection with such sockets! Also in some cases if the plug is forced in, the socket may be damaged when the plug is removed. Type F (German 2-pin, side clip earth) ;CEE 7/4 (German "Schuko" 16 A/250 V earthed) > > The type F plug, defined in CEE 7/4 and commonly called a " Schuko plug", is like type E except that it has two earthing clips on the sides of the plug instead of a female earth contact. The Schuko connection system is symmetrical and allows live and neutral to be reversed. The socket also accepts Europlugs and CEE 7/17 plugs. It supplies up to 16 amperes. Above that, equipment must either be wired permanently to the mains or connected via another higher power connector such as the IEC 309 system. "Schuko" is an abbreviation for the German word ''Schutzkontakt'', which means "Protective (that is, earthed) contact". ;Gost 7396 (Russian 10 A/250 V earthed) The countries of the CIS use a standard plug and socket similar to the Schuko standard, defined in Russia n Standard Gost 7396. The contacts are also 19 mm apart, but the diameter of the pins is 4.0 mm instead of 4.8 mm. And hence the connectors are rated at 10 rather than 16 amps. It is possible to insert Russian plugs into Schuko outlets, but Russian sockets will not accept type E or F plugs because the holes are too small. This socket also accepts Europlugs, but does not accept CEE 7/17 plugs because they use the larger pin size. Many official standards in Eastern Europe are virtually identical to the Schuko standard. One of the protocols governing the reunification of Germany required that the DIN and VDE standards would prevail without exception, so the former East Germany had to conform to the Schuko standard. Most other Eastern European countries use the Schuko standard internally but, prior to its collapse, they exported large volumes of appliances to the Soviet Union with the Soviet standard plug installed. Because of that, many of the Russian plugs found their way into other Eastern European countries. Type E and F hybrid ;CEE 7/7 (French/German 16 A/250 V earthed) > In order to bridge the differences between sockets E and F, the CEE 7/7 plug was developed. It has earthing clips on both sides to connect with the CEE 7/4 socket and a female contact to accept the earthing pin of the type E socket. Nowadays, when appliances are sold with type E/F plugs attached, the plugs are CEE 7/7 and non-rewirable. This means that the ''plugs'' are now identical between countries like France and Germany; only the ''sockets'' are different. Type E and F plugs that are not compatible with both types of socket are only found if a cheap replacement plug has been attached to a cord that originally had another plug. Better-quality replacements are standard CEE 7/7 and are compatible with both Schuko and French standard sockets. Note that the CEE 7/7 plug is polarized to prevent the live and neutral connections from being reversed when used with a type E outlet, but allows polarity reversal when inserted into a type F socket. The plug is rated at 16 A. Above that, equipment must either be wired permanently to the mains or connected via another higher power connector such as the IEC 309 system. Type G (Irish/British 3-pin) ;BS 1363 (Irish/British 13 A/250 V 50 Hz earthed and fused) The BS 1363 plug,BS 1363 (1995), ''13 A plugs, socket-outlets and adaptors. Specification for rewirable and non-rewirable 13 A fused plugs'', British Standards Institute commonly known as a "13-amp plug", is a large plug that has three rectangular prongs forming a triangle. Live and neutral are 4 × 6 × 18 mm spaced 22 mm apart. 9 mm of insulation over the base of the pins prevents people from touching a bare connector while the plug is partly inserted. Earth is 4 × 8 × 23 mm. The plug is unusual in that it has a fuse inside, for protection, in addition to a circuit breaker in the distribution panel. The fuse is required to protect the cord, as British wiring standards allow very high-current circuits to the socket. Accepted practice is to choose the smallest standard fuse (3 A, 5 A, or 13 A) that will allow the appliance to function. Using a 13 A fuse on an appliance with thin cord is considered bad practice. The fuse is 1 Inch long, conforming to standard BS 1362. UK wiring regulations ( BS 7671 ) require sockets in homes to have shutters over the live and neutral connections to prevent children from inserting metal objects into them, and this is incorporated into all BS 1363 sockets. It also helps prevent the use of plugs made to other standards. On plugs for Class II appliances that do not require an earth, the pin is often Plastic . It is sometimes possible to open the shutters with a screwdriver to insert other plug types but this should be avoided as such plugs will not have a fuse. BS 1363 plugs and sockets started appearing in 1946 and BS 1363 was first published in 1947. By the end of the 1950s, it had replaced the earlier standard (type D) ( BS 546 ) in new installations, and by the end of the 1960s, most earlier type D installations had been rewired to BS 1363 standards. Socket-outlets usually include switches on them for convenience and safety. BS 1363 is considered a very safe system. Type H (Israeli 3-pin) ;SI 32 (Israeli 16 A/250 V earthed) This plug, defined in SI 32 (IS16A-R), is unique to Israel and is incompatible with all other sockets. It has three flat pins to form a Y-shape. "Live" and "Neutral" are spaced 19 mm apart. The Type H plug is rated at 16 A but in practice the thin flat pins cause the plug to overheat when connecting large appliances. In 1989, the SI 32 was revised to use three round 4 mm pins in the same locations as the older standard. Sockets made since 1989 accept both flat and round pins in order to be compatible with both old and new plugs. This also allows the Type H socket to accommodate type C plugs which are used in Israel for non-grounded appliances. Older sockets, from about the 1970s, have both flat and round holes for "Live" and "Neutral" in order to accept both Type C and Type H plugs. As of 2007, "pure" Type H sockets (which accept only old standard Type H plugs) are very rare in Israel. This plug is also used in the areas controlled by the Palestinian National Authority in the West Bank and all of the Gaza Strip Type I (Australian/New Zealand & Chinese/Argentine 2/3-pin) ;AS 3112 (Australian 10 A/240 V) This plug, used in Australia , New Zealand , Fiji and Papua New Guinea , has an earthing pin, and two flat pins forming an upside down V-shape. AS/NZS 3112:2004, ''Approval and test specification - Plugs and socket-outlets'', Standards Australia and Standards New Zealand The flat blades measure 6.5 × 1.6 mm and are set at 30° to the vertical at a nominal pitch of 13.7 mm. Australasian wall sockets almost always have switches on them for extra safety, as in the UK. An unearthed version of this plug with two angled power pins but no earthing pin is used with small double-insulated appliances, but the power (wall) outlets always have three pins, including an earth pin. There are several AS/NZS 3112 plug variants,2 including one with a wider earth pin used for devices drawing up to 15 A; sockets supporting this pin will also accept 10 A plugs. There is also a 20 A variant, with all three pins oversized, and 25 and 32 A variants, with the 20 A larger pins and the earthing pin forming an inverted "L" for the 25 A and a horizontal "U" for the 32 A (the 5 variants {10; 15; 20; 25 & 32 ampere sockets} will accommodate all the plugs that are equal or of a lesser current carrying capacity, but not a higher value; i.e. a 10 A plug will be accommodated by all sockets but a 20 A plug will fit only 20, 25 and 32 A outlets). Australia's standard plug/socket system was originally codified as standard C112 (floated provisionally in 1937, and adopted as a formal standard in 1938), which was superseded by AS 3112 in 1990. As of 2005, the latest major update is AS/NZS 3112:2004, which mandated insulated pins3 by 2005. However, equipment and cords made before 2003 can still be used. ;CPCS-CCC (Chinese 10 A/250 V) Although the pins on the Chinese plug are 1 mm longer, the Australasian plug can be used with Mainland Chinese socket. The standard for Chinese plugs and sockets is set out in GB 2099.1–1996 and GB 1002–1996. As part of China's commitment for entry into the WTO , the new CPCS (Compulsory Product Certification System) has been introduced, and compliant Chinese plugs have been awarded the CCC (China Compulsory Certification) Mark by this system. The plug is three wire, grounded, rated at 10 A, 250 V and used for Class 1 applications. In China, the sockets are installed upside down relative to the Australian one shown in the picture. China also uses American/Japanese "Type A" sockets and plugs for Class-II appliances. However, the voltage across the pins of a Chinese socket will always be 220, no matter what the plug type. ;IRAM 2073 (Argentine 10 A/250 V) The Argentine plug is a three-wire earthed plug rated at 10 A, 250 V defined by IRAM and used in Class 1 applications in Argentina and Uruguay . This plug is similar in appearance to the Australasian and Chinese plugs. The pin length is same as the Chinese version. The most important difference from the Australasian plug is that the Argentine plug is wired with the live and neutral contacts reversed. Type J (Swiss 3-pin) ;SEV 1011 (Swiss 10 A/250 V) Switzerland has its own standard which is described in SEV 1011. (ASE1011/1959 SW10A-R) This plug is similar to the type C europlug (CEE 7/16), except that it has an earth pin off to one side. Swiss sockets can take Swiss plugs or europlugs (CEE 7/16). This connector system is rated for up to 10 amperes. There is also a less common variant with 3 square pins rated for 16 A. Above 16 A, equipment must either be wired permanently to the electrical supply system with appropriate branch circuit protection, or connected to the mains with an appropriate high power industrial connector. Switzerland also has a two-pin plug, with the same pin shape, size and spacing as the SEV 1011's live and neutral pins, but with a more flattened hexagonal form. It fits into both Swiss sockets (round and hexagonal) and CEE 7/16 sockets, and is rated for up to 10 A. ;IEC 60906-1 (Brazilian 16 A/250 V) In 1986, the International Electrotechnical Commission published IEC 60906-1 , the specification for a plug that looks similar but is not identical to the Swiss plug. This plug was intended to become one day the common standard for all of Europe and other regions with 230 V mains but the effort to adopt it as a European Union standard was put on hold in the mid 1990s.4 Brazil — which uses a mix of Europlug and NEMA plugs — later adopted it as national standard NBR 14136 in 2001NBR 14136:2002 - Plugues e tomadas para uso doméstico e análogo – Padronização (Plugs and socket-outlets for household use and similar purposes - Specification) and it will be the only plug permitted to be sold with domestic appliances in Brazil from 2009. Type K (Danish 3-pin) ;DS Afsnit 107-2-D1 (Danish 10 A/250 V) The 7 July 2005 23:46 (UTC)--> The current rating on both plugs is 10 A. Adapter plugs exist to allow connection of CEE 7/7 prongs to non-computer outlets. These usually are not sold at the local supermarket so visitors wishing to be safe should contact an electrician. Since the early 1990s grounded outlets have been required in all new electric installations in Denmark. After 1 July 2008 Type E (French 2-pin, female earth) will be permitted for installations in Denmark. Type L (Italian 3-pin) The Italian earthed plug/socket standard, CEI 23-16/VII, includes two models rated at 10 A and 16 A that differ in contact diameter and spacing. Both are symmetrical, allowing the live and neutral contacts to be inserted in either direction. CEE 7/16 (type C) unearthed Europlugs are also in common use, and standardized in Italy as CEI 23-5. Appliances with CEE 7/7 Schuko-French plugs are often sold in Italy, but not every socket will accept them. Adapters are cheap and commonly used to connect CEE 7/7 plugs to CEI 23-16/VII sockets. ;CEI 23-16/VII (Italian 10 A/250 V) The 10 ampere style extends CEE 7/16 by adding a central earthing pin. Thus, CEI 23-16-VII 10 A sockets can accept CEE 7/16 Europlugs. This is the plug shown in the illustrations. Outside of Italy, this plug is found in Syria , Libya , Ethiopia , Chile , Argentina , Uruguay , various countries in North Africa , and occasionally in older buildings in Spain . ;CEI 23-16/VII (Italian 16 A/250 V) The 16 ampere style looks like a bigger version of the 10 A style. The pins are a couple of Millimetre s further apart, and all three are slightly thicker. The packaging on these plugs in Italy may claim they are a "North European" type. They were also referred to as ''industriale'' ("industrial") although this is not a correct definition. ;Variations in sockets Two types of sockets are in common use in modern installations in Italy. One type has a central round hole and two 8-shaped holes above and below. This design allows the connection of both styles of type L plugs (CEI 23-16/VII 10 A and 16 A) and the type C CEE 7/16 Europlug. The advantage of this socket type is its small footprint. The other type looks like a type F socket, but adds a central grounding hole. This design accepts CEE 7/7 (type E/F) plugs, in addition to type C and type L 10 A plugs; its disadvantage is that it is twice as large as a normal type L socket. Some of these sockets also have extra holes to accept type L 16 A plugs. Older installations often have sockets that are limited to either the 10 A or the 16 A style plug, requiring the use of an adapter if the other style needs to be connected. Type M (see D) ;BS 546 (South African 15 A/250 V) Type M is sometimes used to describe the 15 A version of the old British type D, used in South Africa and elsewhere. See Type D for details. ''' outlet supporting American and European 2-pin plugs, and also earthed American plugs]] Multi standard sockets Sockets that take a variety of incompatible plug types are often seen in developing countries where electrical standards are either lacking or unenforced. These sockets may accept both 120 V and 240 V plugs raising a significant risk of devices being damaged by the wrong voltage. Sometimes they have one or more earth holes to allow 3-pin plugs, but there is a good chance that the ground contact may not actually be connected to earth and the ground contact certainly will not mate with Schuko or French plugs. Great care should be taken to avoid incompatible voltage and grounding connections when using such outlets. Multi-standard devices designed to auto-adapt to different voltage and frequency standards, and devices which do not require a ground contact are best used with these sockets. SAFETY NOTES Connecting a plug or socket may seem simple, but if done improperly, can result in a working but highly dangerous installation. Portable electrical devices are designed and approved to work with a particular electrical system, so interchanging devices between systems requires knowledge of the basis of approval. The main issues are: |
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