A few years back, a friend hired me to check out her new house-to-be. It was a grand old house, about 80-years-old, but with plenty of new stuff, including an upgraded electrical system.
As part of the electrical upgrade, the homeowner had installed ground-fault circuit interrupters (GFCIs for short). GFCIs are good things. In modern houses, they shock-proof the receptacles in wet or potentially wet areas, such as kitchens, bathrooms, bars, garages, crawl spaces, decks, and porches. A GFCI can save a person from getting electrocuted.
For instance, if you settle into a nice bubble bath, then some recently-escaped psycho killer jumps out from behind the bathroom door and throws a plugged-in radio into the tub with you, the GFCI will cut the power before the electricity can stop your heart. Swell, huh? Better yet, GFCIs are cheap-about eight bucks apiece anywhere in America. People in underdeveloped countries probably have to pay twice that much.
Anyhow, the big problem with the electrical upgrade at my friend's house was that the handy homeowner had wired all the GFCIs ass-backwards. This would've left everybody in the house vulnerable to radio-wielding killers.
Y'see, GFCI receptacles come with in-holes and out-holes, not unlike the amplifier on your stereo system. There's the input side, labeled "line," and there's the output side, labeled "load." In the box with every GFCI, there's an instruction sheet with microscopic type that explains all this. However, as you might suspect, a whole lot of people don't read the instructions. They just start stripping wires and plugging 'em in randomly, until the lamp plugged into the receptacle comes on. With the use of lax wiring standards, the functioning lamp is the signal that the wiring job is done and that it's time to bust open a case of beer.
Once a GFCI is wired up, some safety-minded folks might go so far as to push in the little "test monthly" button. When you push that button, the "reset" button should pop out, and it usually does. Anybody with decent common sense would figure that if the "reset" button pops out, the receptacle is dead. After all, if it weren't dead, why would you need to push the "reset" button back in?
Well, not so fast, bubba. These $8 safety devices aren't that smart. Here's what happens when somebody wires a GFCI receptacle with the load and line wires reversed: The GFCI will work, in the sense that you can plug in a hair dryer and the hair dryer will blow hot air. But when you push the little "test monthly" button, and the "reset" button pops out, the receptacle stays live. That means the thing looks like it's working, and it acts like it's working, but it's not really working. If the load and line wiring gets messed up, a ground fault (radio in the tub) won't trip the GFCI. There is no protection; there's only the appearance of protection. The GFCI is a booby trap.
I'm sorry to say, we find this very problem about once a week. Sometimes it's in a brand-new house, freshly approved by the municipal codes inspector and guaranteed for one whole year by the friendly builder. Sometimes it's in an upgraded older house. From what we've seen, real enough electricians are about as likely to screw up the job as handymen and homeowners.
As safety hazards go, this one is pretty easy to find and pretty easy to fix. Here's what you do: Go to any well-stocked hardware store and buy a little three-light tester, the kind that plugs right into a receptacle. They only cost about three bucks. Every homeowner ought to have one. The tester comes with a legend, printed right on it, that will show you which lights mean what. These testers are easy to understand: A 12-year-old kid who's had to repeat a few grades can figure out how to work one.
Plug the tester into the GFCI, then push the "test" button on the front of the GFCI. This will make the "reset" button pop out. If the lights on the tester stay on, the GFCI is still live, and that means it's wired wrong. Get an electrician to fix it.
Now, you might be thinking, "I could've done that test with a lamp. I don't need any $3, three-light tester." Well, I say you do need it, because it's also useful for telling you if a receptacle is grounded, and if the right wires are hooked up to the right holes. You ought to stick that tester in every receptacle in your house. Likely as not, you'll find out that some of the wiring in your house is wrong.
If it's wrong, call an electrician and get your wiring fixed. Then, when the electrician tells you everything's fixed, break out your little three-light tester and check his work.