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

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. Take a look at this HowStuffWorks article to learn why appliance cords are shaped this way.

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Episode Transcript

Speaker 1 (00:00):
Welcome to brain Stuff from house stuff works dot com,
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(00:22):
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 hundred twenty 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,

(00:46):
the right slot is called hot, and the whole 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 let. The power will flow
from the hot prong through the filament and back to
the neutral prong, creating light in the process. The groundhole

(01:08):
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 pronged plug. This may also

(01:32):
include some things like your computer, that have a metal
in case 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 and cased appliances
from electric shock. The casing is connected directly to the
ground prong. Let's say that a wire comes loose inside

(01:54):
an ungrounded metal case and the loose wire touches the
metal case. If the loose 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.

(02:16):
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 pronged appliance into a two pronged 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

(02:36):
an electric shock if a wire comes loose. Do you
have any ideas or suggestions for this podcast? If so,
please send me an email at podcast at how stuff
works dot com. For more on this and thousands of
other topics, go to how stuff works dot com