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 portable appliances (such as vacuum cleaners, electric fans, and hair driers) being connected to the light fitting.
U.S. Patent 774,250. The first electric power plug and receptacle.However, as electricity became a common method of lighting houses and operating labour-saving appliances, a safe means of connection to the electric system other than using a light socket was needed. The original two pin electrical plug and socket were invented by Harvey Hubbell and patented in 1904. Other manufacturers adopted the Hubbell pattern and by 1915 they were widespread, although in the 1920s and even later, household and light commercial equipment was still powered through cables connected with Edison screw-base adapters to lampholders.
The three-prong plug was invented by Philip F. Labre, while he was attending the Milwaukee School of Engineering (MSOE).[contradiction] It is said that his landlady had a cat which would knock over her fan when it came in the window. When she plugged the fan back in, she would get an electric shock. Labre figured out that if the plug were grounded, the electricity would go to earth through the plug rather than through his landlady. A german patent for the grounded design called the schuko was issued two years earlier in 1926, and is still in use today in over 40 countries. He applied for and was issued a US patent for grounding receptacle and plug on June 5, 1928. As the need for safer installations became apparent, earthed three-contact systems were made mandatory in most industrial countries.
 Proliferation of standards
During the first fifty years of commercial use of electric power, standards developed rapidly based on growing experience. Technical, safety, and economic factors influenced the development of all wiring devices and numerous varieties were invented. Gradually the desire for trade eliminated some standards that had been used only in a few countries. Former colonies may retain the standards of the colonising country, occasionally"as with the UK and a number of its former colonies"after the colonising country has changed its standard. Sometimes offshore industrial plants or overseas military bases use the wiring practices of their controlling country instead of the surrounding region. Hotels and airports may maintain receptacles of foreign standards for the convenience of travellers. Some countries have multiple voltages, frequencies and plug designs in use, which can create inconvenience and safety hazards.
 Design for safety
Design features and aspects of plugs and sockets have evolved to reduce the risk of electric shock and equipment damage. Depending on the plug/socket system, safety measures may include pin and slot configuration to permit only the correct insertion of plug into socket, earth pins longer than power pins so the device becomes earthed before power is connected, insulated pin shanks to reduce or eliminate live-contact exposure when a plug is partially inserted in a socket, socket slot shutters that open only for the correct plug, as well as inbuilt fuses and switches.
 Consolidation of standards
In recent years many countries have settled on one of a few de facto standards, which became formalised as official national standards, although there remain older installations of obsolete wiring in most countries. Some buildings have wiring that has been in use for almost a century and which pre-dates all modern standards.
There has been some movement towards consolidation of standards for international interoperability. For example, the CEE 7/7 plug (see below) has been adopted in several European countries and is compatible with both Type E and Type F sockets, while the unpolarised Europlug is compatible with an even greater proportion of European and other socket types. IEC 60906-1 has been proposed as a common standard for all 230-V plugs and sockets worldwide but has only been adopted in Brazil to date.
IEC power cord with CEE 7/7 plug at left end.Many manufacturers of electrical devices like personal computers have adopted the practice of putting a single world-standard IEC connector on the device, and supplying for each country a power cord equipped with a standard IEC connector on one end and a national power plug at the other. The device itself is designed to adapt to a wide range of voltage and frequency standards. This has the practical benefit of reducing the amount of testing required for approval, and reduces the number of different product variations that must be produced to serve world markets.
 World maps
See also: Mains power around the world
Plugs.There are two basic standards for voltage and frequency in the world. One is the North American standard of 110"120 volts at a frequency of 60 Hz, which uses plugs A and B, and the other is the European standard of 220"240 volts at 50 Hz, which uses plugs C to M. The differences arose for historical reasons as discussed in the article Mains electricity.
Countries on other continents have adopted one of these two voltage standards, although some countries use variations or a mixture of standards. The outline maps show the different plug types, voltages and frequencies used around the world, color-coded for easy reference.
 Types in present use
Electrical plugs and their sockets differ by country in shape, size and type of connectors. The type used in each country is set by national standards legislation. In this article each type is designated by a letter designation from a U.S. government publication , plus a short comment in parentheses giving its country of origin and number of contacts. Subsections then detail the subtypes of each type as used in different parts of the world.
IEC Classes are assigned to electrical devices depending on whether or not they are earthed and the degree of insulation they incorporate. Class I, for example, refers to earthed equipment, while class II refers to unearthed equipment protected by double insulation.
Special purpose sockets may be found in residential, industrial, commercial or institutional buildings. These may be merely labelled or coloured, or may have different arrangements of pins or keying provisions. Some special-purpose systems are incompatible with general-purpose lighting and appliances. Examples of systems using special purpose sockets include:
"clean" ground for use with computer systems,
emergency power supply,
uninterruptible power supply, for critical or life-support equipment,
isolated power for medical instruments,
"balanced" or "technical" power used in audio and video production studios,
outlets for electric clothes dryers, electric ovens, and air conditioners with higher current rating.
Depending on the nature of the system, special-purpose sockets may just identify a reserved use of a system (for example, computer power) or may be physically incompatible with utility sockets to prevent use of unintended equipment which could create electrical noise or other problems for the intended equipment on the line.
 Type A
Main article: NEMA 1
Unpolarized type A plugNEMA 1"15 (North American 15 A/125 V ungrounded)
Unusual American 5-receptacle Type A outlet, ca. 1928This plug and socket, with two flat parallel non-coplanar blades and slots, is used in most of North America and on the east coast of South America on devices not requiring a ground connection, such as lamps and "double-insulated" small appliances. It has been adopted by 38 countries outside North America[vague], and is standardized in the U.S. by the National Electrical Manufacturers Association. NEMA 1"15 sockets have been prohibited in new construction in the United States and Canada since 1962, but remain in many older homes and are still sold for replacement. Type A plugs are still very common because they are compatible with type B (three-prong) sockets.
Initially, the plug's prongs and the socket's slots were the same height, so the plug could be inserted into the socket either way around. Most sockets and plugs manufactured from the 1950s onward are polarized by means of a neutral blade/slot wider than the live blade/slot, so the plug can be inserted only the right way. Polarized type A plugs will not fit in unpolarized type A sockets, because both slots are narrow, but both unpolarized and polarized type A plugs will fit in polarized type A sockets and in type B (three-prong) sockets. Some devices that do not distinguish between neutral and live, such as sealed electronic power supplies, are still produced with unpolarized type A pins (both narrow).
Japanese outlet with earthing post, for a washing machine.JIS C 8303, Class II (Japanese 15 A/100 V ungrounded)
The Japanese plug and socket are identical to NEMA 1"15. However, the Japanese system incorporates stricter dimensional requirements for the plug housing, different marking requirements, and mandatory testing and approval by MITI or JIS.
Many Japanese outlets and multi-plug adapters are unpolarized"the slots in the sockets are the same size"and will accept only unpolarized plugs. Japanese plugs generally fit into most North American outlets without trouble, but polarized North American plugs may require adapters or replacement non-polarized plugs to connect to older Japanese outlets. However, in Japan voltage is supplied at only 100 volts and the frequency in eastern Japan is 50 rather than 60 Hz, so North American devices which can be plugged into Japanese sockets may not function properly though devices with rectified power supplies may work without problems.
 Type B
Main article: NEMA 5
NEMA 5"15 plug, left. Decorative-style duplex outlet, center. Ordinary duplex outlet, right.NEMA 5"15 (North American 15 A/125 V grounded)
The type B plug has two flat parallel blades like type A, but has a round or U-shaped grounding prong (American standard NEMA 5-15/CSA 22.2, No.42). It is rated for 15 amperes at 125 volts. The ground pin is longer than the live and neutral blades, so the device is grounded before the power is connected. Nearly always both current blades on type B plugs are narrow since the ground pin enforces polarity. This has the unfortunate effect that when a misguided individual cuts off the ground pin to fit it to a type A outlet or extension cord, the live/neutral polarity is lost. Type A plugs are also compatible with type B sockets. In this case, the socket retains polarity enforcement. Adapters that allow a type B plug to be fitted to a type A outlet are readily available. Proper grounding is dependent on the outlet being an ordinary duplex receptacle with a grounded center screw, and the grounding tab of the adapter being connected to that screw.
The 5"15 socket is standard in all of North America (Canada, the United States and Mexico). It is also used in Central America, the Caribbean, northern South America (Colombia, Ecuador, Venezuela and part of Brazil), Japan, Taiwan and Saudi Arabia. Looking directly at a type B outlet with the ground at the bottom, the neutral slot is on the left, and the live slot is on the right. They may also be installed with the ground at the top or on either side.
In some parts of the United States and all of Canada, tamper-resistant outlets are now required in new construction. These prevent contact by objects like keys or paper clips inserted into the receptacle.
5"20R T-slot receptacle mounted with the ground hole up. The neutral connection is the wider T-shaped slot on the lower right.In the theater, this connector is sometimes known as PBG for "Parallel Blade with Ground", Edison or Hubbell, the name of a common manufacturer.
NEMA 5"20 (North American 20 A/125 V grounded)
This is a 20-amp receptacle with a T-slot for the neutral blade, and allows either 15-ampere parallel-blade plugs or 20-ampere plugs to be used.
JIS C 8303, Class I (Japanese 15 A/100 V grounded)
Japan also uses a Type B plug similar to the North American one. However it is less common than its Type A equivalent.
 Type C
CEE 7/16 plug and socket(Not to be confused with the 3-blade C13 and C14 IEC connectors)
CEE 7/16 (Europlug 2.5 A/250 V ungrounded)
For more details on this topic, see Europlug.
This two-prong plug is popularly known as the Europlug. The plug is ungrounded and has two round 4 mm (0.157 in) pins, which usually converge slightly towards their free ends. It is described in CEE 7/16 and is also defined in Italian standard CEI 23-5 and Russian standard GOST 7396. This plug is intended for use with devices that require 2.5 amps or less. Because it is unpolarised, it can be inserted in either direction into the socket, so live and neutral are connected at random. The separation and length of the pins allow its safe insertion in most CEE 7/17, Type E (French), type F (CEE 7/4 "Schuko"), Type H (Israeli), CEE 7/7, Type J (Swiss), Type K (Danish) and Type L (Italian) outlets, as well as BS 4573 UK shaver sockets. It can be forced into type D (5 amp) and G sockets, it is a common misconception that this is unsafe; however, there is, technically, no reason why it would create any more of a hazard than if it were plugged into a normal Type C socket. The caveat is that plugs going into type G should be fused appropriately as a faulty device can pull up to 32 A from the ring circuit.
The Europlug (plug only, not socket from the picture) is used in Class II applications throughout continental Europe (Austria, Belgium, Bosnia and Herzegovina, Bulgaria, Czech Republic, Croatia, Denmark, Estonia, Finland, France, Germany, Greece, Greenland, Hungary, Iceland, Italy, Latvia, Lithuania, Luxembourg, Macedonia, the Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and Ukraine). It is also used in the Middle East, most African nations, South America (Brazil, Chile, Argentina, Uruguay, Peru and Bolivia), Asia (Bangladesh, Indonesia and Pakistan) as well as Russia and the former Soviet republics, such as Armenia, Georgia, and many developing nations. It is also used alongside the BS 1363 in many nations, particularly former British colonies.
CEE 7/17 plugCEE 7/17 (German/French 16 A/250 V ungrounded)
This plug also has two round pins but the pins are 4.8 mm (0.189 in) in diameter like types E and F and the plug has a round plastic or rubber base that stops it being inserted into small sockets intended for the Europlug. Instead, it fits only into large round sockets intended for types E and F. The base has holes in it to accommodate both side contacts and socket earth pins. It is used for large appliances, and in South Korea for all domestic non-earthed appliances. It is also defined in Italian standard CEI 23-5. Can also be safely inserted in to Israeli type H sockets, although with some difficulty.
BS 4573 socketBS 4573 (UK shaver)
In the United Kingdom and Ireland, what appears to be a larger version of the type C plug exists for use with shavers (electric razors) in bath or shower rooms. In fact it was not derived from the type C plug at all, but was a legacy from the obsolete 2 pin 5 amp plug used in Britain in the 1920s and 1930s but still prevalent, especially in bathrooms, as late as the 1960s. It has 0.2 in (5.08 mm) diameter pins 5⁄8 in (15.88 mm) apart, and the sockets for this plug are often designed to accept unearthed CEE 7/16, US or Australian plugs as well. Sockets are often able to supply either 230V or 115V. In wet zones, they must contain an isolation transformer compliant with BS 3535.
Soviet plug (6 A/250 V ungrounded)
The Soviet plug, still widely used in modern Russia, has pin dimensions