How Does a GFCI Circuit Breaker Work?

Description

A ground fault circuit interrupter (GFCI) is a safety device designed to help prevent electrocution. They are typically used wherever water and electricity come in close proximity. In a home, GFCI receptacles are required in kitchens and bathrooms within a given distance of a sink or bathtub. Some appliances, such as hair dryers, that are typically used near water sources have GFCI's as part of the plug. GFCI circuit breakers can protect an entire circuit as opposed to a single receptacle.

Differential Current

AC current passes between the hot and neutral wires. If it is flowing properly, the difference between the current in the hot wire and in the neutral wire will be near zero. If there is an improper ground situation, such as a hair dryer falling into a bath tub, the current is siphoned off through the new ground situation and the difference between the hot and neutral increases.

Operation

A GFCI circuit breaker continuously measures the differential current in the circuit branch. Any time there is an improper ground, or ground fault, and the differential reaches a preset threshold, the breaker trips, cutting off all current to that circuit branch. Sometimes, a pulse in the current can create a differential large enough to trip the breaker. There is a lag of several milliseconds between the time the fault occurs and the time it is detected and trips the device. Since a circuit breaker is located farther away than a receptacle, the delay will be increased by the time it take the current to reach the breaker.

Testing

GFCI's all have test buttons that creates a simulated ground fault and a way to reset the device if it trips. There are also separate devices that are designed to create a true ground fault to test the device. These devices are considered more accurate and more reliable than the GFCI "test" button. For GFCI circuit breakers, it is best to test the device using one of these instruments at each receptacle on the branch controlled by the breaker. This can be time consuming for larger branches, but regular testing can ensure reliable operation.