What does a ground fault interrupter do?

Dear Cecil: What does a house ground fault interrupter look like, as opposed to the 3x5 inch box on an individual electrical outlet, and what does it do? J. Smith, Chicago

Cecil replies:

I suppose you’re talking about a GFI circuit breaker, which looks like an ordinary circuit breaker except that it’s got a white test button on it. (Mine does, anyway.) Works the same as a GFI outlet. And how is that, you ask? Conventional fuses and circuit breakers work on the assumption that if something is going to short out, it’s going to short big–the flow of current to ground will be more massive than any possible legitimate load, melting the fuse or tripping the breaker. However, modern science has discovered that sometimes a relatively small amount of juice can put you in some serious hurt, even though it’s not enough to trip a conventional protective device. Only 100 milliamps is enough to cause ventricular fibrillation. If you were drying your hair while lounging in the bathtub (a bathtub filled with water, I suppose I should clarify), and your hand were somehow to contact a live wire, the fuse might not blow because the relatively high resistance of the water would keep down the current flow. But you might still fry. The ground fault interrupter prevents this by comparing the amount of juice in the hot wire versus the neutral wire–the black wire versus the white wire, to put it in practical terms. If volts in exceeds volts out by more than a certain percentage, that means some of the current is taking a detour, possibly by way of your trembling body. So the GFCI outlet trips. Unless it’s malfunctioning, which Cecil can attest, it often is. Go test that thing now. (The white button. See, you forgot already.) We print folk have lost enough readers to the electronic age as it is.

Cecil Adams

Send questions to Cecil via cecil@straightdope.com.