The polarization has nothing to do with the polarity of the circuit, as in DC circuits, but it has everything to do with safety. Your home (assuming you live in North America, other locations have equivalents) is connected to a 240 VAC two-phase mains supply, with two hot circuits and a neutral return. The two hot circuits are 120 VAC each and operate 180 degrees out of phase with one another. One half of the electrical panel is fed with one phase and the other half with the other phase. This provides your home with two 120 VAC single phase circuits and the ability to power 240 VAC appliances. Each household circuit uses three wires: hot, neutral, and ground. The ground wire is physically tied to earth somewhere in your home, often to a buried copper rod or to the water supply main. The neutral wire in the circuit connects back to the mains neutral and is tied to ground inside the electrical panel. The hot circuit connects to one of the two 120 VAC circuits. The plugs and outlets are polarized to ensure that the wiring pattern is consistent throughout the home. Some electrical devices do not require polarization and these may come with an unpolarized two-pin plug. This is done to protect the user from electrical shocks caused by short and open circuits. If the hot lead touches the neutral or ground, the circuit breaker will trip. If the wiring in your house is done properly, you can touch the ground and neutral wires without receiving a shock. Circuit breakers, fuses, and power switches are always placed in the hot circuit. This ensures that the power to an electrical device can be disconnected. For example, if the power switch for a lamp is mistakenly placed in the neutral circuit, you will be able to turn the lamp off and on, but the lamp will always be 'live'. You don't want to put your finger in a live socket and complete the circuit to ground, do you? Another example: a damaged appliance cord touching the sheet metal case. This will cause the circuit breaker to trip, if the house and appliance are wired properly. If it is not, or if the plug is not polarized, the case could become live and present a serious shock hazard. A three-pin 120 VAC plug has a wide blade, a narrow blade, and a round pin. The wide blade is neutral, the narrow blade is hot, and the pin is ground. In-wall wiring is colour-coded with white for neutral, black for hot, and bare for ground. Power leads usually use a green wire for ground, as all three wires are insulated. All homes built in the past 40 years or so have three-wire circuits. You can check the wiring at each electrical outlet using a plug-in circuit tester that costs about $10. You should always check the wiring in any home you plan to buy to ensure the house is wired properly. It is not uncommon to find outlets wired improperly in older homes, with the hot and neutral leads swapped or a disconnected ground.