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Domestic AC power plugs and sockets are devices that connect the home appliances and portable light fixtures commonly used in homes to the commercial power supply so that AC electric power can flow to them. more...

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Power plugs are male electrical connectors that fit into female electrical sockets. They have contacts that are pins or blades which connect mechanically and electrically to holes or slots in the socket. Plugs usually have a live or hot contact, a neutral contact, and an optional earth or ground contact. Many plugs make no distinction between the live and neutral contacts, and in some cases they have two live contacts. The contacts may be brass, tin or nickel plated.

Power sockets are female electrical connectors that have slots or holes which accept the pins or blades of power plugs inserted into them and deliver electricity to the plugs. Sockets are usually designed to reject any plug which is not built to the same electrical standard. Some sockets have one or more pins that connect to holes in the plug.

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) carries alternating current from the power source to the equipment. The voltage 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 can be hazardous with some equipment in which the neutral is connected directly to the chassis.

Neutral

The neutral contact returns current from the equipment back to the power source or distribution panel. It is in most (but not all) cases referenced to the earth. Except under fault conditions it does not pose a danger because the voltage between the neutral contact and the earth is close to zero, but is nevertheless treated as live in most installation practices because it can develop a high voltage under fault conditions.

Read more at Wikipedia.org