Why do the plugs on some appliances have two prongs and others have three?

by | Sep 25, 2008 08:00 PM ET

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Marshall Brain

Hi. I'm Marshall Brain with today's question. Why do the plugs on some appliances have two prongs and others have three prongs?

Let's start with what the holes in an outlet do. When you look at a normal 120-volt outlet in the United States, there are two vertical slots and then a round hole centered below them. The left slot is slightly larger than the right. The left slot is called neutral, the right slot is called hot, and the hole below them is called ground. The prongs on the plug fit into these slots in the outlet.

In a house outlet, power flows from hot to neutral. Let's say you plug a light bulb into the outlet. The power will flow from the hot prong through the filament and back to the neutral prong, creating light in the process. The ground hole and the neutral slot of an outlet are identical. That is, if you go back to the fuse box, you'll find that the neutral and ground wires from all the outlets go to the same place. They all connect to ground. Since they both go to the same place, why do you need both?

If you look around your house, what you'll find is that just about every appliance with a metal case has a three-prong plug. This may also include some things, like your computer, that have a metal encased power supply inside, even if the device itself comes in a plastic case. The idea behind grounding is to protect the people who use metal encased appliances from electric shock. The casing is connected directly to the ground prong.

Let's say that a wire comes lose inside an ungrounded metal case and the lose wire touches the metal case. If the lose wire is hot, then the metal case is now hot, and anyone who touches the metal case will get a potentially fatal shock. With the case grounded, the electricity from the hot wire flows straight to ground and this trips the fuse in the fuse box. Now the appliance won't work, but it won't kill you either.

What happens if you cut off the ground prong or you use a cheater plug so you can plug a three-prong appliance into a two-prong outlet? Nothing really happens. The appliance will still operate. What you've done, however, is you've disabled an important safety feature that protects you from an electric shock if a wire comes lose.

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