CAUTION: The color codes for wires referred to in this article are for USA only - I haven't a clue as to what the wire color codes for house wiring are in any other countries. Consult your local electrician for information.
Circuit breakers and fuses are there to protect a circuit from becoming overloaded and failing. They do nothing to protect a person form getting a shock. GFIs, on the other hand, can protect a person from getting shocked, or electrocuted, but they give a false sense of security since one can still be electrocuted even though a circuit has a GFI protecting it.
A bit of technical talk here - not too complicated, but enough to allow you to get a picture of what a GFI can do for you. Electricity needs a complete circuit in order to do its thing, to allow electrons to flow through a circuit to an appliance and perform some sort of function. Heating the elements of a toaster, lighting a bulb and making sweet music come from a CD are all examples of electrons flowing in a circuit.
When you look at a plug on an appliance you will see a minimum of two blades sticking out of the plug. Sometimes there are three, the third being a rounded pin. On most plugs one of the blades is fatter than the other and is designed that way so one can insert the plug into a wall socket only one way, sometimes a little annoying, but as you will see for your own protection.
All electrical circuits in your house are wired with three wires (we're talking about the standard 110 volt stuff into which you plug your toaster). One is called the "hot" wire and is normally color coded with black insulation. The second wire is called the "common" or "ground" wire and is color coded with white insulation. The third is truly a ground wire and is not meant to carry any current under normal operation. It is either color coded with green insulation or it is just a bare wire buried inside the insulation containing the other two wires. For more detail see the article on house wiring.
Under normal conditions the current to operate a device such as a toaster or a TV travels in a circuit starting at the circuit breaker box and through the black wire into the appliance where it "does its work" then out of the appliance and back to the power plant via the white or "common" wire. Like I said, the green ground wire does not carry any of the functional current. So why is it there?
The "ground wire" is attached to the metal case of things like your microwave oven. It is there in the unlikely event that the hot wire inside the appliance comes in contact with the case of, for example, your microwave oven. If that happens, the current is carried back to earth ground and the circuit breaker trips (shuts off). Why? Well one thing I forgot to tell you is that the white "common" wire also goes back to good old mother earth so if the microwave oven failed as described above, and there were no green wire attached to the microwave oven, and you were somehow in contact with earth ground (like touching a kitchen faucet) your bod would complete the circuit back to earth ground and soup would pop out of your ears!! Not a good thing - could really ruin your weekend.
OK, so what's this GFI got to do with all of this? Remember that ALL of the current going into an appliance like your microwave oven, comes out through the white wire. So, if there were a "Ground Fault", i.e. the ground wire was not connected to the microwave oven frame, annnnd your microwave oven failed like I mentioned above so that the hot wire was touching the frame of the microwave oven, annnnnd you were touching the kitchen faucet at the same time you were touching the microwave oven, the GFI (remember it's Ground FAULT Interrupter) would save your young hide from getting fried and no soup would come pournig out of your ears. How does it do that??
Inside the GFI are two sensing coils which measure the current going out of the black wire and coming back through the white wire. If some of that current going out through the black wire doesn't come back through the white wire the circuit inside the GFI box senses that you are about to get fried (via the unbalance of the outgoing and returning current) and shuts the circuit off within about 10 milliseconds! You might just begin to feel a tingle if you are very sensitive to electricity, but it will not persist long enough to do any harm!!
That is the good news. Now here is the bad news. You can still get killed by a circuit that has a GFI protecting it. How? Just become a part of the circuit that normally heats the toaster element. Grab the black wire with your left hand, and the white wire with your right hand. All of the current going out of the GFI through the black wire will go right through your heart and return through the white wire and you will never get a chance to eat the toast that comes out of your ears! There is one way to save yourself. Make sure that you are standing in a grounded puddle of water so that some of the current that travels to your left hand via the black wire goes through your body and into the puddle. That way there will be less current going back through the white wire and the GFI will shut the circuit off and your friends will have to get their bread toasted some other way! IT WORKS ON PAPER BUT I WOULDN'T TRUST ANY GFI TO SAVE MY LIFE IN A SILLY SITUATION LIKE I JUST DESCRIBED!
Oh yes, I forgot to tell you why the blades on most plugs are different sizes so that you can plug it in only one way. That is because manufacturers want their devices to be "cold" when the switch on the appliance is turned off. To guarantee that, they make sure that the "hot" wire is always the "hot" wire inside the appliance and they attach the switch to that wire. So when your toaster isn't toasting bread there is no voltage anywhere inside the toaster except for a very small piece of wire that is connected to the hot side of the switch. It is the same reason that the three pronged "polarized" plugs of a three wire device are made that way. Unless some stupid idiot cuts the ground pin from the plug, there is no way to plug the device in backwards. Of course YOU would never think of doing a stupid thing like that, would you??
One final comment. Several years ago manufacturers decided to save some money by making the housings of things like electric drills and circular saws out of plastic. One benefit of that is that there is no metal housing for you to touch to possibly get electrocuted. This invention is called "double insulation". Of course there is no longer any need for a three conductor wire for these devices since there is no metal to attach the green wire to inside the device - another cost savings for the manufacturer.
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