How Does a Ground Fault Circuit Breaker Work?

What Does a GFCI Do?

A GFCI, or ground fault circuit interrupter, is a device designed to prevent accidental electrocutions, particularly in high-risk areas with flowing water such as bathrooms or kitchens. Normal circuit breakers detect when the current flowing through a circuit gets too high for the house wiring to handle, but they are not sensitive enough to detect an electrocution before it is too late. GFCIs are designed specifically to detect when electricity flows somewhere it shouldn't and are much more sensitive than circuit breakers.

How Electric Wiring Works

Electricity normally flows in through the skinny slot in a wall plug (called the "hot wire") and back out through the fatter slot (called the "neutral wire"). Many outlets also have a small hole for "ground." The ground wire is attached to the plumbing or something else, which goes to the ground and disperses stray electricity. Only a very small bit of electricity should ever go to ground. When an electrocution happens, a lot of electricity flows to ground. For example, if you dropped a dryer in the tub, the electricity would flow through the water into the plumbing and, if you happened to be in the tub, through you. The GFCI detects this large burst of ground electricity and quickly shuts off the circuit.

How the GFCI Works

The GFCI does not directly monitor the ground wire. Instead, it compares the amount of electricity coming into the hot line with the amount going out through the neutral line. Usually, these two should be exactly the same. If some of the electricity is going to ground, less will come back out of the neutral wire than went in. The GFCI will detect this difference. If the difference exceeds a certain threshold--usually about 5 milliamps--the GFCI will open the circuit. The GFCI can turn off power very quickly. If you were in a tub when the hair dryer fell in, you would receive a painful jolt, but the electricity would turn off quickly enough to save you from serious electrocution.