GFCI breaker tripping at floor box

Shujinko

Senior Member
I have a project in a classroom where, at a teacher's podium/lectern, there is a floorbox with receptacles. The branch circuit for these is protected by a 20A/1P GFCI circuit breaker. The podium/lectern has some sound system equipment in it that plugs into the floor box. The GFCI breaker keeps tripping for some reason. I have inspected the installation of the floor box, panelboard, and GFCI circuit breaker with the contractor and have not found anything out of the ordinary.

Therefore my theory is that the electronics of the sound system equipment is creating enough harmonic current to trip the GFCI circuit. I am assuming that the 5mA sensor on the GFCI breaker is sensing that the current difference between hot and the neutral is greater than 5mA and is tripping the breaker.

Would this be a good analysis or am I missing something else I should be looking at? Has anyone else run into this situation?
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
You need to be clearer as to whether only a certain load trips the GFCI. If so, try that load on another GFCI device.

Yes, a GFCI device compares the currents of the line conductors, and assumes any difference is causing a shock.
 

Shujinko

Senior Member
It’s an audio/visual multifunction digital media electronic headend that is tripping the breaker and it’s tripped a couple of different GFCI Circuit breakers.
 

Another C10

Electrical Contractor 1987 - present
Location
Southern Cal
Occupation
Electrician NEC 2020
and it’s tripped a couple of different GFCI Circuit breakers
Maybe its a fault within the equipment being used being that different GFI systems are effected as well, does the GFI have issues when no audio is being used or utilizing the protected power source.
 

Shujinko

Senior Member
You need to be clearer as to whether only a certain load trips the GFCI. If so, try that load on another GFCI device.

Yes, a GFCI device compares the currents of the line conductors, and assumes any difference is causing a shock.
Then you know it's the device. Why doesn't matter to you (in the business sense), so just tell them it needs servicing.
Because the only reason there is even GFCI protection for the floor box is that the electrician kept saying it was required by code because anything installed in a concrete slab is considered a wet location. And also I just wanted to pick everyone's brain if my analysis about the harmonics was correct. And also I want to give my client good service as I am the electrical engineer who designed the project.
Maybe its a fault within the equipment being used being that different GFI systems are effected as well, does the GFI have issues when no audio is being used or utilizing the protected power source.
The GFCI is only tripping when the A/V headend is plugged in. So it's definitely the equipment. However, I think it's the harmonic currents that the A/V equipment produces and maybe not an internal fault in the A/V equipment. Have you had harmonics issues for electronics at the branch circuit level with breakers tripping whether they are GFCI protected or not?
 

mpoulton

Senior Member
Location
Phoenix, AZ, USA
"Harmonics" shouldn't trip a modern GFCI. Harmonics do not cause a current imbalance between hot and neutral - only an actual ground fault can really do that (Kirchhoff's First Law). Harmonics do cause non-sinusoidal current, which used to confuse some GFCI's and trick them into falsely sensing an imbalance where none exists. Modern GFCI's handle high-harmonic loads (anything electronic) all the time without problems.

What does trip GFCI's, though, are faulty surge suppressors. Your AV equipment almost certainly has built-in surge suppressors (MOV's). Surge suppressors often fail by starting to leak excessive current. Since the suppressors are connected between each conductor and ground, this leakage is a real ground fault. 5mA isn't much leakage, and that's all it takes.

I assume the AV equipment has a grounding path. If it's all 2-prong with no grounding path for anything then I'm way off-base here - but I'll bet it's grounded either through the plug or through shielded AV cables. If you want to prove the source of the problem, try measuring the ground leakage from that equipment. I'll bet it's a few milliamps - it should be a tiny fraction of a milliamp. The proper test for this requires using a variac to raise the line voltage a bit higher than usual to see if the leakage increases dramatically (typical for failing MOV's).
 
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Shujinko

Senior Member
Wow this is a really good point. The A/V system could have surge suppression problems. THe A/V headend has a 3 prong plug. Also the podium/lectern that the A/V headend is located in actually also has an 8 outlet rack with integral surge suppression. The A/V headend plugs into the outlet rack, the outlet rack plugs into the floor box receptacle, and the floor box receptacle is protected by a 20A/1P GFCI circuit breaker which is tripping. So this 8 outlet rack with surge suppression could definitely be a source of the the GFCI tripping. Thanks for the info. I appreciate it.

Now as far as Harmonics goes. I'd say harmonics do cause current imbalance as the harmonic frequencies cause higher current in the neutral. This is why you see oversized neutrals on some systems because of the increased currents on the neutrals due to harmonics and the heat that the additional harmonic currents create on the neutral. I will definitely look at the GFCI breaker cutsheet though because I want to see the info about it being able to handle high harmonics.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Engineer
Now as far as Harmonics goes. I'd say harmonics do cause current imbalance as the harmonic frequencies cause higher current in the neutral. This is why you see oversized neutrals on some systems because of the increased currents on the neutrals due to harmonics and the heat that the additional harmonic currents create on the neutral.
Harmonics do not create current when none exists before. The increased neutral current, in a circuit such as 208Y/120 3Ph 4-wire has to do with the harmonics causing a phase shift in the current so they do not cancel in the neutral conductor. Harmonic addition is not an issue with a simple 120V 2-wire circuit.

IMHO harmonics were a big deal several decades ago, but their impact has all but disappeared with the advent of switched mode power supplies. They are blamed for many more issues than they actually cause.

Go with the failing or 'cheap' surge arrestors angle.
 

hbiss

EC, Westchester, New York NEC: 2014
Location
Hawthorne, New York NEC: 2014
Occupation
EC
It's not harmonics. It's the surge suppressor(s). We've talked about this here before, you can't use surge suppressors on a GFCI circuit or with a GFCI receptacle. It's going to cause random tripping. Nothing to do with cheap or failing surge suppressors either, except maybe the cheap ones that don't do much will work.

I would suggest getting rid of the GFCI breaker, it's not needed anyway.

-Hal
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Agreed that harmonics will not directly cause a current imbalance that would trip the GFCI.

Because harmonics are higher in frequency, they could cause increased conductor across the surge suppression system. This would mean increased leakage through the MOVs to ground. The GFCI would then trip on the current leakage.

-Jon
 

kwired

Electron manager
Location
NE Nebraska
It's not harmonics. It's the surge suppressor(s). We've talked about this here before, you can't use surge suppressors on a GFCI circuit or with a GFCI receptacle. It's going to cause random tripping. Nothing to do with cheap or failing surge suppressors either, except maybe the cheap ones that don't do much will work.

I would suggest getting rid of the GFCI breaker, it's not needed anyway.

-Hal
A lot of electronic equipment has some surge protection built into it - even if only very basic MOV's across input leads and EGC.

Still going to trip the GFCI if an event occurs where more than 4-6 mA gets diverted to EGC.

Whole house protector at the service probably lessens how often this may happen.
 

kwired

Electron manager
Location
NE Nebraska
Wow this is a really good point. The A/V system could have surge suppression problems. THe A/V headend has a 3 prong plug. Also the podium/lectern that the A/V headend is located in actually also has an 8 outlet rack with integral surge suppression. The A/V headend plugs into the outlet rack, the outlet rack plugs into the floor box receptacle, and the floor box receptacle is protected by a 20A/1P GFCI circuit breaker which is tripping. So this 8 outlet rack with surge suppression could definitely be a source of the the GFCI tripping. Thanks for the info. I appreciate it.

Now as far as Harmonics goes. I'd say harmonics do cause current imbalance as the harmonic frequencies cause higher current in the neutral. This is why you see oversized neutrals on some systems because of the increased currents on the neutrals due to harmonics and the heat that the additional harmonic currents create on the neutral. I will definitely look at the GFCI breaker cutsheet though because I want to see the info about it being able to handle high harmonics.
Harmonics do not create current when none exists before. The increased neutral current, in a circuit such as 208Y/120 3Ph 4-wire has to do with the harmonics causing a phase shift in the current so they do not cancel in the neutral conductor. Harmonic addition is not an issue with a simple 120V 2-wire circuit.

IMHO harmonics were a big deal several decades ago, but their impact has all but disappeared with the advent of switched mode power supplies. They are blamed for many more issues than they actually cause.

Go with the failing or 'cheap' surge arrestors angle.
To add to what Jim said, current is same in both conductors of a two wire circuit, multiwire circuits is where harmonic currents can have additive effects on the neutral, and is worst on wye systems with harmonics adding on the neutral from all three phases of the system.
 

hbiss

EC, Westchester, New York NEC: 2014
Location
Hawthorne, New York NEC: 2014
Occupation
EC
GFCI's have MOVs inside of them.
Correct. They are connected from each line conductor to ground. When a surge or spike comes along that's greater than their breakdown rating they shunt it to ground. If they trip a GFCI it shows they are just doing their job.

So now the elephant in the room needs to be addressed. With the expansion of GFCIs into more and more areas, how does anybody know if what is going to be connected to them is going to have internal surge suppression? What do you tell your customer when their new appliance trips the GFCI? How much trouble is this going to cause us?

-Hal
 

kwired

Electron manager
Location
NE Nebraska
Correct. They are connected from each line conductor to ground. When a surge or spike comes along that's greater than their breakdown rating they shunt it to ground. If they trip a GFCI it shows they are just doing their job.

So now the elephant in the room needs to be addressed. With the expansion of GFCIs into more and more areas, how does anybody know if what is going to be connected to them is going to have internal surge suppression? What do you tell your customer when their new appliance trips the GFCI? How much trouble is this going to cause us?

-Hal
And 2020 NEC requires surge protection at the service or main feeder, this should help transients coming from utility side, direct or near direct lightning strikes, anything can happen.
 
Harmonic addition is not an issue with a simple 120V 2-wire circuit.
Jim is right. In a two wire circuit, how could one wire (say the neutral) have more current running through it than the other wire (the "hot" wire)?

See Kirchoff's and Ohm's laws.

Harmonics adding together is a three phase or multiple wire circuit thing. You have to have different sources of harmonics for them to add. ;)
 

Todd0x1

Senior Member
Location
CA
I have experienced this quite a bit with A/V equipment. Sometimes its leakage from filter capacitors in the device's power supply, theyll put capacitors L-N, L-G, N-G. One device may work, but add a few and the leakage adds up enough to trip a GFCI. What sort of shape are the circuit conductors in and how long are they? Might be a little leakage there. Maybe there is some gunk in the receptacle making a little more leakage. Could also be a pinched wire in a device -had that happen too.

Plug the devices in one at a time. See if there is one particular device that trips the GFCI or if its some combination of devices.
 
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