I love brilliant inventions - especially the ones that are super simple yet perfectly designed - rare as they are.
A GFI is one such invention. You know, the AC plug near your bathroom sink with the two little red buttons you never think about? Ever wonder what it does and how it works?
A GFI outlet makes sure you don't get shocked. It works in the cleverest of fashions.
If you stick your finger in an AC outlet you get shocked because you look like a wire to the power and it travels right through you to ground. Stand in a puddle of water and you look like an even better connection to ground. So if you're in the tub and an electrical wire or appliance falls into the tub the GFI will make sure to cut the power and save you. How does it do that? By using you as the wire.
That's right, the very principal that kills you saves you with a GFI.
Here's how it works. There's a tiny coil of wire connected in such a way as to get power through it only if a circuit is completed between hot and ground. If you happen to become that circuit the little coil is energized which activates a relay and disconnects the power.
The idea is to use the very thing that might kill you to save you. Now that's clever.