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I recently bought a townhouse built in 1956 that has mostly 2 prong outlets. I am mostly clueless about electrical stuff. The electrical system was partially updated in 1990 to add GFCI outlets in the kitchen and bathroom, and a circuit breaker box was added.

A few 3 prong outlets have been installed, but no ground wire was connected. I need to add some 3 prong outlets and am wondering about the ground. I'm thinking outlets for the stereo system, TV, etc. need to be grounded. Is that right? What good is adding a 3 prong outlet if it is not grounded other than so that it will accept a 3 prong plug? I think I need to determine if my boxes are grounded, but I don't know how? Then, can I just attach a ground wire from the green screw in the receptacle to a hole in the back of the metal box if they are grounded? What if they are not grounded?

I did a search on this board for this but didn't come up with an answer. If I missed it, feel free to direct me there.

Thanks in advance.

Greg
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As a devout do-it-yourselfer, I say: call an electrician.

Yes, things that have three-prong plugs ins do so because they need a ground wire. Not to mention a surge protector, but that's not the question.

Determine exactly which plugs you want converted. Make sure you know what will be plugged into them (so the electrician will know the load). Then call one.

Oh, and don't plug anything you value into those non-grounded three-prong plugs.

The longest part of the whole thing will be the electrician figuring out where to shove new cable through the wall....we're just gonna use a convenient telephone wire hole for ours. Once that's done, it's a piece of cake.

impolite
who has Cousin (electrician) coming over tonight to re-wire a plug or three in her new (old) home
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This is a link to a prior thread which may be of some help. I had bookmarked it because I needed to understand a bit better also.

http://boards.fool.com/Message.asp?mid=18132202&sort=whole

Bob
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i would agree that it would be best to call an electrician about this if you aren't positive.

however, we just had an electrician ground one of our 3 prong outlets(after completing an unrelated job) and then he went over the process with us so we are doing the rest ourselves. our apt. was built in the 50's too and the metal boxes are grounded, so we we are just attaching the ground wire to the metal box then to the green ground screw on the 3-prong plug.




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The third wire is an electrical safety ground...see links others have posted that probably explain this.

Just some notes on why you need 2 or 3 prongs.

Electrically or safety-wise the 3rd prong is not needed for things like TVs and stereos, nor are they usually needed for better picture quality or sound quality. On older appliances, for example, the third wire would be connected to the outer case when it was metal. This is so you don't kill yourself if the appliance malfunctions (internal short) and you are grounded, like standing in a puddle of water at the same time. Most appliances, like drills, sanders, kitchen stuff like blenders, etc usually have plastic cases, can't have this kind of problem, by design and only have two prongs. The same goes for most TVs and stereos, but not all.

Almost all PCs have 3 prongs because they have metal cases and metal cases on the internal power supplies. Laptops have only 2 prongs because they have a small (brick-like) transformer in a plastic case and have only DC power into them.

If you have something that comes with a 3-pronged plug you need to ground the third prong to insure safety.

Some cords have only two prongs but one blade is wider. This insures that you plug the hot side (smaller) into the correct side.

Mike
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the metal boxes are grounded, so we we are just attaching the ground wire to the metal box then to the green ground screw on the 3-prong plug

It technically meets code to do it that way, but there can be problems with it. I assume you have BX "armored" cable for your wiring. The cable "armor" often does not carry the ground well, and often results in a high resistance ground. It acts fine until you need it in a safety situation - and then the ground opens up because it can't handle the current.
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