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Sound Advice: Playing Safe With Power

The what and how of GFCIs, and why they really matter.

By Mike Sokol August 27, 2018

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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About Mike

Mike Sokol
Mike Sokol

System Designer & Audio Educator
Mike Sokol does sound system design and training for JMS Productions, his consulting company in Western Maryland. Visit www.livesoundadvice.com for his educational articles and videos, and email him at [email protected]advice.com with comments and suggestions.
http://www.livesoundadvice.com

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usg 3d says

Right here is the right web site ffor anybody who wants to find out about tbis topic.

You understand so mujch its almost touhgh
to argue with you (not that I really will need to…HaHa).
You definitely put a fresh spin on a subject that has been discussed for many years.
Wonderful stuff, just wonderful!

DAVID L TIPTON says

So, what do you do when the bass players amp (class AB+B) has enough asymmetrical inrush current to trip the GFCI? He refuses to use a different amp.

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