Q: What’s that third hole in electrical outlets for?

Physicist: Ground.

You can put a paperclip practically anywhere.

You can put a paperclip practically anywhere. All the same, don't try it.

The zero volt, large-slit wire is called the “return” or “neutral” line.  If it seems strange that the power company would supply you with a wire that has no voltage, keep in mind that what you really need is a voltage difference.  That’s why everything that uses electricity has at least two wires connecting it to its power supply.

Green=ground, Black=power, White=neutral

Green=ground, Black=120v, White=neutral. Notice that you can easily remove the green lines on the right without actually affecting anything.

The difference between the neutral line and the ground line is that the neutral line carries current (all the current that enters through the 120 volt line has to go back out), while the ground line is literally connected to the ground (usually through your plumbing) and carries no current.  The ground and neutral lines are spliced together at the fuse box (and generally again out by the local transformer).  You’d expect that since the lines are connected there’s no difference between them, and that’s almost entirely true.  The ground line, having no current and a shorter path to the ground, is always at exactly 0 volts, which is important information for many electronic devices to have access to.

In your house it’s important for most electronics to have a wire to dump energy out of in case of a short.  This can be the ground or neutral lines, since they’re practically identical.  If a device needs to know the difference between the 120v and neutral lines, then it will have one big tine and one small one at the end of its cord.  So when you’re wiring up a house it’s important to keep track of which line is the 120v line and which is the neutral line.  A good way to keep track is to wire up outlets with a temporary connection between the ground and neutral lines during installation, so that  if the electrician accidentally switches the 120v and neutral lines there will be a flash, a pop, a puff of smoke, and no lights.  So that’s one use for the nearly useless ground line.

It’s a bad idea in general to connect the neutral and ground lines.  Nothing bad will happen (if the wiring is up to code), but you will be creating a loop (check the figure above and connect the white and green lines at an outlet).  Loops are bad because they turn changing magnetic fields into changing electric current (and vice versa).  There are plenty of random magnetic fields out there, so you’d be introducing an unpredictable source of current to your electrical system.  Still, you’d probably never notice if the loop is relatively small (say, smaller than an entire house).

So the reason that ground lines run to outlets is that every now and then it’s nice to have access to a 0 volt, 0 current wire.  But it’s not really that important, which is why so many outlets don’t have a third hole.

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3 Responses to Q: What’s that third hole in electrical outlets for?

  1. Chris says:

    “So when you’re wiring up a house ” bad idea…leave to a pro ” it’s important to keep track of which line is the 120v line and which is the neutral line. A good way to keep track is to wire up outlets with a temporary connection between the ground and neutral lines during installation” very bad idea ” so that if the electrician accidentally switches the 120v and neutral lines” unlikely ” there will be a flash, a pop, a puff of smoke, and no lights. So that’s one use for the nearly useless ground line.”

    The ground line is like an insurance policy. If everything is perfect and no bare wires come in contact with the frame it does nothing. However, wires chafe, parts wear out and generally things don’t stay perfect forever. If an electrical fault does occur, having a ground line can save your life. The Neutral is NOT connected to the frame of the appliance and therefore will not carry any fault current. Ground is the ONLY thing that will protect you if the frame becomes energized. I don’t think it is accurate to characterize the ground line as “nearly useless.”

    As for connecting G and N together in a receptacle, I agree with your later statement that it is a bad idea, but for different reasons. First, if you tie the G and N terminals together on a GFCI outlet, by design, it will not work. The same holds true for a circuit protected by a GFCI Circuit breaker. Second, if H and N were reversed and you used a small enough wire to connect what would then be H to G, you may not pull enough current trip the breaker, but you could start a fire. There are certainly better ways to test if a circuit is wired properly and in general, it is best to leave the wiring to a licensed, professional electrician.

  2. The Physicist The Physicist says:

    Absolutely. To all of our non-electrician readers out there, this post is more of a thumb nail sketch and in no way a guide. For safety’s sake, pretend you never read it, and when there’s a problem hire professional electricians as often as you can afford. Electricity is nasty stuff.

  3. Michael Crofford says:

    The difference between the neutral and ground is this. The neutral is connected to the system neutral that comes into the house usually a bare wire. This is part of the system and goes all the way back to the source and is common with your neighbors house. The ground is exactly what it says it goes to ground like being connected to copper plumbing. If you have plastic plumbing you need ground roods in the earth 6′ to 8′ in length. This is also bridged to the system neutral. This will protect you if a metal appliance is energized by a short or in some cases protect your home from a lightning strike.

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