No, it’s not the name of an insurance company or a European sports car—GFCI is an abbreviation/acronym for Ground Fault Circuit Interrupter. They’ve been required in many localities for electrical outlets located near sinks or the outside of residences since the early 1970s.

The two types of GFCIs you’ll encounter are either built into the power outlet itself (Figure 1, left) or inside the circuit breaker at the power panel (Figure 1, right). Both do exactly the same thing: they watch for electricity that’s going someplace it shouldn’t in an electrical Circuit by way of a Fault to Ground and then Interrupt the flow by tripping the circuit breaker. Rearrange the letters and we get GFCI. That’s how the name is derived.

Why Do We Need A GFCI?

Human heart muscle is very sensitive to electrical shock. While it takes around 8/10ths of an amp (800 milliamperes, or 800 mA) of current to power a 100-watt light bulb, it takes less than 1 percent of that same current (10 to 20 mA) to send a heart into fibrillation, which can cause death by electrocution. That’s why the NEC (National Electrical Code) now requires a special type of circuit breaker for damp locations that can tell the difference between the normal currents feeding an electrical appliance and the currents accidentally flowing through you to ground.

And while a GFCI sometimes trips unexpectedly, it’s really there to save human lives, as well as the lives of appliances and other electrical components. The most dangerous situation we typically find on a live stages is a performer getting between a backline guitar amplifier with a broken ground connection and a properly grounded microphone.

How Does A GFCI Work?

It’s a very ingenious system that uses a small current transformer to detect an imbalanced current flow, so let’s use a water pump analogy to review the typical current path in a standard electrical circuit. In Figure 2, let’s imagine the pump at the top is pushing 7 gallons per minute (GPM) of water current around in a circle that the little turbine at the bottom is happily using to spin and do some work.

I’ve added flow meters at the bottom left and right of the illustration so we can keep track of these currents. Now since our pipes have no leaks, the current going out of the pump from the black pipe will exactly equal the return current coming back in the white pipe. This will be an exact balance since no water is lost in this closed loop. That is, if 7.000 GPM of water is flowing out of the black pipe, then 7.000 GPM will be returning to the pump via the white pipe. There are no water losses in this perfect system.

Keeping In Balance

Let’s add an extra meter in this system so we can keep track of the water flow a little easier. Notice there’s now a center meter that will show the difference in flow between the other two meters. If the left and right meters show exactly the same water flow, the center meter will indicate 0.000 (zero) GPM of flow by centering its needle.

This is exactly what should happen in an electrical circuit that’s working properly. So, for example, if a light bulb has exactly 1 amp of current flowing out from the black (hot) wire, then exactly 1 amp of current should be flowing back in the white (neutral) wire. And an electric griddle that has 10 amps of current flowing out the black wire should have exactly 10 amps of current flowing back in the white wire.

If there’s nothing wrong in the light bulb or griddle circuit, this electrical current balance will be pretty close to perfect, out to at least 3 decimal places, in other words, 10.000 amps of current flow going out will equal 10.000 amps of current flow coming back in.