You've probably noticed that your 120-volt appliances, power tools and electronic equipment don't all have the same types of plugs; some have two prongs that are the same size, some have two prongs of different sizes and some have three prongs. Plugs with two prongs of the same size are unpolarized, while the other two types of plugs are polarized. Polarized and unpolarized plugs aren't interchangeable.
What's the Difference?
In the early days of electricity, 120-volt appliances, lamps and other electrical equipment all came supplied with unpolarized two-prong cords, and all receptacles were similarly unpolarized. These plugs and outlets made no distinction between the hot leg of the electrical circuit and the neutral leg, and shocks were frequent. In a polarized appliance, the hot and neutral legs are distinguished from one another, so when you switch the appliance off, you can be sure no voltage is being supplied to any part of it.
Polarization was introduced to North American circuitry in 1928 by Philip F. Labre, who is responsible for the addition of the grounding pin to 120-volt plugs. The grounding pin connects to a third wire in the circuit with the sole purpose of conducting stray current to earth. Because the plug has three pins, it can only be inserted into the outlet in one direction. A modern polarized plug doesn't always have the grounding pin, and if not, one pin is larger than the other. The smaller of the two pins is the hot leg of the circuit.
The Case of the Light Socket
In a screw-in light bulb socket, the metal casing around the inside of the socket is neutral, and the pin on the bottom of the socket is hot. As long as the socket is connected with a polarized plug, you can't get a shock when replacing the bulb -- even if the switch is on -- because by the time the bulb is screwed in and the pin contacts the bottom of the bulb, the threads are inaccessible. If the plug weren't polarized, though, the socket could be hot, and you could be shocked.
Identifying the Wires
When you're replacing a plug on a polarized light fixture or appliance, you must maintain the polarization by connecting terminals of the new plug to the same wires as the old one. That means you need to be able to distinguish the hot wire from the neutral one. Electrical manufacturers use one of three methods to make this distinction:
- Ribbing: When the rubber or plastic insulation on a pair of wires is fused into a single cord, one side of the cord is ribbed or textured. The wire with the ribbing is neutral.
- White stripe: When the insulation of a pair of wire is fused into a single cord, the insulation of one of the wires may have a white stripe. The wire with the white stripe is neutral.
- Different colors: When each wire in a bundled pair has its own insulation, the wire with white insulation is neutral and the one with black or red insulation is hot.
Some appliances and power tools have non-polarized two-prong plugs. The reason is that the casing of the device is designed in such a way that the hot wire can never come in contact with the casing, or the casing is plastic and won't conduct electricity. These appliances and tools are double-insulated, and that fact is prominently displayed on a label.