YES! Though the term “polarity”, as it is commonly used, is a mis-nomer when referring to AC electricity (the kind that powers our homes, etc). AC electricity doesn't have polarity in the sense that DC electricity (the kind produced by batteries) does. The term AC stands for Alternating Current and refers to the fact that the direction of current flow changes or alternates, in North America usually at a frequency of 60Hz. DC goes one direction only, AC goes back and forth. Put batteries in something the wrong way around and that device may not work, or may even be damaged. Connect the wires to a light, switch or receptacle in your house the wrong way around and it works fine, it's just not safe!

If you look into a light socket you will see metal at the sides, either the whole way around or as a tab on the side, as well as a small metal tab in the centre at the back of the socket. The screw or wire to the tab at the back of the socket must be connected to switchlag, the wire that is live when the switch is on. The screw or wire to the side of the socket must be connected to neutral. This way when you forget to turn the switch off while changing a dead bulb, you would have to poke your finger all the way to the back of the socket to touch a live part. It is much easier to accidentally touch the metal at the side of the light socket, and if the light were connected incorrectly that side part could be live rather than the tab at the back of the socket!

Perhaps even more dangerous is a switch wired incorrectly. Switches must break the circuit in the hot wire! Too often I've found wiring where a do-it-yourself-er broke the circuit in the neutral wire. When this happens the light is always live, even when the switch and light are off. The light turns off because the circuit is broken, but it is still energized and a person touching it, to change a bulb for instance, can still get a dangerous shock.

“Polarity” is also important when connecting receptacles. Modern receptacles are three-prong, with distinct hot, neutral and ground connections. Equipment and appliances are manufactured to take advantage of this receptacle design for safe operation and to reduce the risks if something goes wrong in the equipment or appliance. On each half of the face of a household variety receptacle, where a plug would be inserted, are two slots and a semi-oval hole. The smaller of the two slots is the hot side, the larger of the slots is the neutral and the semi-oval hole is the ground. Typically these correspond on the back of the receptacle to a brass coloured screw, or screws, for hot, silver for neutral and green for ground. When receptacles are connected wrong (hot and neutral wires reversed for example), or mismatched receptacles and plugs are used (such as breaking off the ground pin of a plug to plug into an old two-prong receptacle), the safety built in to appliances and equipment can be defeated. A dangerous condition is created even though the appliance may operate at the time.

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