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Sound Advice: Playing Safe With AC Power

The what and how of Ground Fault Circuit Interrupters (GFCIs) and why they really matter.

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.

Out Of Balance

In Figure 3, I’ve added a leak in the black outgoing pipe via the red pipe sticking out to the left. You can see from the red pipe’s meter that 5 GPM of water is flowing out onto the ground. And since only 7 GPM of water is coming out of the black pipe on the pump, there can be only 2 GPM of water returning into the white pipe on the right.


Those 5 GPM of imbalance show up in the center balance meter, which alerts us to the fact that there’s a leak somewhere in the system.

Now, we really would like to know about small leaks as well, so that center meter will tell us about an imbalance down to very small drips, say less than 1/1000 of a GPM.

The same is true of an electrical circuit where we’re interested in currents in the 1/1000 of an ampere range (1 mA). That’s because just 10 to 20 mA of misdirected current flow is close to the danger level for stopping your heart.

Teeter Totter

In an electrical system, a similar type of detector is used at the center of the circuit that is acting like a balance beam. So if 7 amps of current shows up on both sides of the balance, then the beam will be exactly level.

However, put 7 amps of current on the left side and 2 amps of current on the right side, and that 5 amps of imbalance will tip the scales, just like the teeter totter ride you took as a child (Figure 4).


In a GFCI circuit, this is a much more sensitive balance beam that only needs 5 mA (or 0.005 amps) of current imbalance to tip over, rather than the 5 GPM we’ve shown in the water pump illustration.

The reason for needing this much sensitivity is that our hearts can go into fibrillation from just 5 mA of AC current flow, so we would like to detect and stop that flow before it gets to our heart.

The Bottom Line

So here’s where it all comes together. Notice that our guy in Figure 5 is unwisely touching a hot wire with a hand while his foot is in contact with the earth. And while the electrical outlet might have been supplying 7.000 amps of outgoing current to an appliance with exactly 7.000 amps of return current, there are now 7.005 amps going out and only 7.000 amps coming back.


Those extra 0.005 amps of current (5 mA) are taking a side trip from his hand to his foot via the heart. Meanwhile the current balance circuit inside the GFCI is sensitive enough to recognize that imbalance and trip the circuit open with as little as 5 or 6 mA of current flowing someplace it shouldn’t be going.

The click that’s heard when a GFCI trips is its spring-loaded contact opening up and interrupting the current flow in the circuit before it causes electrocution. That’s the entire reason for the existence of GFCI – to save us from electrocution and keep our electrical systems safe from damage. Pretty cool, eh?

Also note that the GFCI doesn’t really need a direct ground-bond connection via the ground wire to do its job. Yes, one is required to properly “earth” the entire circuit, but the current balancing act is only between the black and white wires going to the outlet.

If the current flow in the white wire exactly matches the current flow in the black wire to within 5 mA, the circuit stays activated. If the current flow is unmatched by any more than 5 mA, say by someone touching a live wire and the earth at the same time, then the trigger circuit inside trips a little switch and the current flow is stopped. It’s that simple.

All of this means we should install GFCI breakers where required, and don’t remove or bypass them on stage if there’s false “nuisance” tripping. That so-called false tripping hints there’s something else wrong in the sound system or backline that’s leaking current to some place it doesn’t belong. And fixing that electrical leak is important since getting a body in the middle of the current leak leads to shock or even electrocution.

Let’s play safe out there…

Note: This article is provided as an educational assist in your gigs and is not intended to have you circumvent a licensed electrician or technician. If you even remotely believe you have a dangerous electrical condition on stage or on a project site, make sure to contact a qualified, licensed electrician to address it.

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