How GFCI Receptacles Sense 5 Milliamps and Protect Other Outlets

A ground fault circuit interrupter shuts off power when it sees a difference of just 5 milliamps between the current going out to a device and the current coming back. That tiny imbalance tells it current is going somewhere it shouldn't, such as water or a metal pipe. This Old House master electrician Heath Eastman walked host Kevin O'Connor through how the devices work and how to tell whether an outlet is protected.

What a ground fault is

A ground fault happens when the hot conductor makes contact with a non-current-carrying metal part, such as a metal box or a pipe. Near water, a failing cord or appliance chassis can let current leak, and a person can become part of that path. The GFCI is there for personal protection. Eastman says the devices have been around since roughly the early 60s and were required in some areas of houses in the 70s.

Where they are required

Kitchens and bathrooms are the key locations. Eastman also names outdoors, damp basements and garages, plus other areas where something might be plugged in.

Spotting a protected outlet

The easiest indicator is the test and reset buttons in the middle of the receptacle. The one shown also has a green light. A plug-in tester lights up when the outlet works. Pressing the test button leaves power to the device but nothing comes out, and the reset button restores it.

Line and load

One GFCI can protect other receptacles. Power from the panel comes into the line terminal and goes out on the load terminal to the next receptacles, so everything downstream is protected from that point forward. Eastman demonstrates this by pressing the button and showing the downstream outlet go off too. A plug-in tester can show whether an outlet is tied to a GFCI.

When it won't reset

When someone replaces a GFCI, it is easy to confuse line and load. With the wires backwards, the device will not reset, and the outlets downstream never get power. Eastman describes this as failing safe.

A breaker instead

If an outlet shuts off and no GFCI receptacle is visible, there may be a GFCI breaker in the panel instead. It does the same thing, but it is rare and more expensive.