Function

• Electric current is like water through a water wheel: the amount of water entering the wheel should be the same as the amount of water leaving the wheel. Current enters a receptacle through the "hot" wire and the same amount of current should leave it through the "neutral" wire. A GFCI receptacle compares these two currents. If they differ beyond a relatively tiny margin, the receptacle "trips" and disconnects or "interrupts" both of the wires. This is important because a difference in current indicates that some of the current is getting to ground through an unintended pathway--possibly you. Electrical code often requires the installation of GFCI receptacles in bathrooms and kitchens.
  
  

Components

• Both hot and neutral currents flow through a tiny transformer in opposite directions inside the GFCI receptacle. The transformer detects differences in the currents and passes the information to a "comparator" microchip. That chip is what ultimately detects the tiniest of differences in currents. If there is a difference, it directs the electrical contacts that connect the hot and neutral wires to the receptacle to open, thereby removing all power from the receptacle.
  
  

Current Limits

• If you look inside your circuit breaker box, you'll notice the most common breakers are 15-amp and 20-amp. Unfortunately for your body, you can be fatally electrocuted at one-tenth of 1 amp. The comparator in a GFCI receptacle trips at five-one thousandths of an amp.