GFCI and Overload Protection. Why does it trip when I overload the receptacle? But not the main breaker??

[Will Prowse]

Just took down my video on outlet boxes. For years, I thought GFCI receptacles had their own overload and short circuit protection. Because every time I overload one, it trips (like with power tools in my garage). My breaker in the panel does not trip when I overload these circuits. The GFCI does. It even happened a week ago.

And I looked this up ages ago, and everywhere I read, it stated it does:

"The main purpose of a circuit breaker is to prevent an overload or short circuit from damaging wiring. GFCIs are designed to protect against electrocution and other hazards. In addition to protecting against overloads and shorts, GFCIs also detect small imbalances in electrical current. This issue can be caused by faulty wiring or damaged appliances."

But I think they are referring to the circuit breakers with built-in GFCI. Not just a receptacle.

I used my experience and verified it with the Internet, but I did not know that the receptacle version do not have this feature.

Now my question is, why does my GFCI trip when I overload it with tools? There is no ground fault at all. We use too many power tools on a single circuit, and then it trips. Why is it doing this? We are using proper gauge conductors and new equipment. What is going on here?

@Quattrohead caught this mistake. Thank you

 

[Will Prowse]

I have a GFCI circuit running a mini split, and it runs 247 for months. If I run a power tool on that circuit at the same time, it will overload and trip the GFCI. Not the main panel breaker.

So I walk over and turn off the mini split. Then there is no more overload, and the GFCI does not trip. Then I run multiple power tools off this circuit all day long. No issues at all. Then at the end of the day, I walk back over to the mini split and turn it on. No problem.

There is zero fault to ground either way. But when I overload the circuit, or one of the construction crews do, I have to run over to the garage to reset it. Not the main breaker panel.

What kind of magic is going on here?!

I bet @timselectric can correct me in two seconds. I am completely lost on this one.

 

[Hedges]

I've installed AFCI breakers and combination AFCI/GFCI breakers. Even the AFCI only tripped when I thought the issue was leakage current.

GFCI is a magnetic core with two wires (L and N) wrapped around it. Plus one more winding, which gets an induced current based on imbalance between the other two.

Could be the balance in magnetic coupling isn't perfect, so higher current amplifies the imbalance.

Could be the magnetic flux leakage (into the air rather than coupled in the core) reaches out and tickles the sense coil.
This leakage is something I work with regarding power transformers - they function by coupling with the core, but flux leakage to the air affects our sensitive instrument so I work to reduce it.

And finally, the GFCI could sense voltage drop or current and trip to protect itself.


How about doing a study, various sorts of overload and map out what trips each device?
Ideally you would capture voltage waveform and ground current waveform.

By the way, GFCI can get tripped by capacitively coupled current from EMI filters.
Split-phase and 3-phase filters are symmetric, near zero injected current.
Single phase filters typically inject about 0.5 mA at 60 Hz. Around 10 of those could trip GFCI.

If your overload induces voltage harmonics, pulling down line voltage, the higher frequency harmonics will be higher current through EMI filter capacitors.

 

[Will Prowse]

I've installed AFCI breakers and combination AFCI/GFCI breakers. Even the AFCI only tripped when I thought the issue was leakage current.

GFCI is a magnetic core with two wires (L and N) wrapped around it. Plus one more winding, which gets an induced current based on imbalance between the other two.

Could be the balance in magnetic coupling isn't perfect, so higher current amplifies the imbalance.

Could be the magnetic flux leakage (into the air rather than coupled in the core) reaches out and tickles the sense coil.
This leakage is something I work with regarding power transformers - they function by coupling with the core, but flux leakage to the air affects our sensitive instrument so I work to reduce it.

And finally, the GFCI could sense voltage drop or current and trip to protect itself.


How about doing a study, various sorts of overload and map out what trips each device?
Ideally you would capture voltage waveform and ground current waveform.

By the way, GFCI can get tripped by capacitively coupled current from EMI filters.
Split-phase and 3-phase filters are symmetric, near zero injected current.
Single phase filters typically inject about 0.5 mA at 60 Hz. Around 10 of those could trip GFCI.

If your overload induces voltage harmonics, pulling down line voltage, the higher frequency harmonics will be higher current through EMI filter capacitors.

Great points!! Very true. And yes, doing actual testing and measurements would be the best way. I never would have considered these factors at all.

 

[ULR]

Now my question is, why does my GFCI trip when I overload it with tools? There is no ground fault at all. We use too many power tools on a single circuit, and then it trips. Why is it doing this? We are using proper gauge conductors and new equipment. What is going on here?

@Quattrohead caught this mistake. Thank you

I've noticed that the GFCIs in my garage tripped when power first came after installation, and also on power restoration after a line outage. Perhaps they're (improperly) sensitive to sudden voltage changes.

Edit: Some that tripped didn't have any loads on them - either directly or slave outlets. Just the turn-on transient did it.

Another possibility is leakage (e.g. capacitive leakage) from the windings to the cores in the motor, and thus to the green-wire ground. Enough hand tools might add up to something the GFCI doesn't like.

==============

Offtopic: Don't backfeed through a GFCI. It can make (at least some models) self-destruct. In a ground fault they expect to use power from the line to activate the mechanism for the disconnect. If the line side is what's down, as the disconnect starts to open, the power to it disappears, so it doesn't complete the disconnection, falls back to the connected position, and repeat. "Buzzzzzzz Fry!" Dies within about a second.

(Lost a $75 30A panel breaker for my travel trailer at-home outlet that way when I fed a 600W portable generator into the townhouse during a long outage when I tried to power my critical loads (all on the same side of the split-phase as the trailer). I opened the main breaker and all but the trailer and critical load breakers, hooked the generator up with a "widow-maker" double-male extension cord, and fired it up. Worked fine. Until I hit the GFCI test button. (The special-order replacement breaker was now about $130 B-b ) )

Saw the same thing backfeeding a GFCI outlet on the side of the house, too - with a real ground fault from the rain. But much cheaper.

(Fortunately, the townhouse is on rotating outage block 50 - i.e. don't cut it. Probably because I'm three blocks from city hall and the police station, and about six from the west-coast air traffic control center, and get the benefit of their priorities. So I don't get many outages and when I do they're usually fixed very quickly. But that's no help when a goose flies across the line-pole primary wires and the resulting current melts and drops a primary wire.)

 

[jharrell]

There is zero fault to ground either way.

I wouldn't count on that, we are talking only a 5-30ma of imbalance between hot and neutral, maybe both are leaking a little and together they add up to the trip amount, this is in contrast to the overload protection which is not going to instant trip a breaker until 10x the rated current or maybe 2x the rated over 5 minutes.

GFCI's are very sensitive devices but only to imbalance not total current. Could just be an old GFCI to getting too sensitive or having other issues, these are not supposed to be OCP devices.

Breakers protect equipment, GFCI's protect people.

 

[JoeHam]

I have limited knowledge here but I will note that I have a Cotek inverter which has a GFCI outlet on it that I use exclusively for the AC output.

It seems to trip when the temperature in my seacan ham shack gets warm. If I put a fan on the inverter it does not seem to trip with the exact same loads.

Just empirical observation, I’ve never tried to figure out anything further since the fan solves my immediate needs.

This works despite adding on the (small) load of the fan in addition to the loads that tripped it in the first place .

 

[Quattrohead]

GFCI and AFCI are their own madness, there's no point in trying to understand them.

 

[SparkyJJO]

GFCI and AFCI are their own madness, there's no point in trying to understand them.

Black magic.

 

[Supervstech]

Think about it like this...

What does a megohm meter detect? It detects winding insulation leakage to ground...

EVERY SINGLE PIECE OF WIRE leaks to atmosphere, through surfaces etc.

Motors have a LOT of wire in them.
More motors, more leakage.

I find it very difficult to gfci protect minisplit wiring.

The inverters, and control wiring all add up, and as humid as our area is, a minisplit will always trip a gfci.

Having a minisplit, and power tools on the same gfci protected circuit even in arizona... yeah, its going to trip.

 

[Hedges]

Black magic.

Pretty simple, actually.
Line and neutral carry current in opposite directions and cancel each other's magnetic field.
Any difference is transformer-coupled into a secondary wire and measured.
> 5 mA trips human safety GFCI in the US, or 30 mA for equipment protection.
In Europe, 30 mA for whole house.

https://i.ytimg.com/vi/CL1zIVWvr9g/maxresdefault.jpg

I find it very difficult to gfci protect minisplit wiring.

I suspect VFD input current waveform has something to do with that.
Or output pulses into capacitance, high frequencies in edges causing higher currents.

 

[WorldwideDave]

GFCI and AFCI are their own madness, there's no point in trying to understand them.

Some cool videos from their manufacturers do exist. Think Schneider has some cool science fair like demo rigs. Pretty cool.

 

[WorldwideDave]

Some cool videos from their manufacturers do exist. Think Schneider has some cool science fair like demo rigs. Pretty cool.

Found it.

 

[windsurfer]

Just try running a vfd on a gfci protected circuit.

I had to add a dedicated circuit without gfci in my shop to run the lathe. Otherwise every time I turned the speed down or shut down the drive the gfci trips.

 

[jharrell]

The common mode noise from the VFD is leakage current the GFCI sees. Add a VFD from a brushless power tool and you get a trip.

These things aren't magic, electricity can just do surprising things and 5 milliamps is not a lot of current out of say 10 amps its only .05% of the current to cause a trip.

Here is a good explanation of why a VFD trips a GFCI: https://acim.nidec.com/drives/kbele...updated-using-vfds-on-gfci-devices.ashx?la=en

 

[going_commando]

Pretty simple, actually.
Line and neutral carry current in opposite directions and cancel each other's magnetic field.
Any difference is transformer-coupled into a secondary wire and measured.
> 5 mA trips human safety GFCI in the US, or 30 mA for equipment protection.
In Europe, 30 mA for whole house.

View attachment 280468

https://i.ytimg.com/vi/CL1zIVWvr9g/maxresdefault.jpg

I suspect VFD input current waveform has something to do with that.
Or output pulses into capacitance, high frequencies in edges causing higher currents.

With VFDs, it could also be the massive amounts of harmonics that are discharged to ground through shielding. When we install a VFD we always put in a line reactor (basically a 1:1 isolation transfomer), and in many cases a load reactor, and use either shielded VFD cable or ferrous metal conduit for the run. Also, appliances and such are built way crappier than they use to be which is why the NEC made dishwashers require GFCI protection. The manufacturers can't make one that is safe, so we have to add extra protection. Bosch refrigerators have so much leakage they can't even be installed on GFCI protected circuits (at least the last Bosch french door fridge model I dealt with), because the variable speed compressor has too much noise and leakage to ground.

Hey Will, I never really thought about it with GFCIs acting as overload protection, but they definitely do. I wonder if they have inherent protection on the CTs or something in the device so the electronics don't get damaged by an overload, but they don't want to advertise it as overcurrent protection and have to deal with all the UL testing and expense to say they have it on a $10 component.

 

[jharrell]

Hey Will, I never really thought about it with GFCIs acting as overload protection, but they definitely do. I wonder if they have inherent protection on the CTs or something in the device so the electronics don't get damaged by an overload, but they don't want to advertise it as overcurrent protection and have to deal with all the UL testing and expense to say they have it on a $10 component.

No they definitely don't unless they are a combo breaker. The CT's are over both hot and neutral so they get nothing without an imbalance, they can't even see how much balanced current exists. Try putting a clamp meter around hot and neutral and reading the total current, all you will see is the leakage, now your a GFCI.

Here is a teardown and detailed explanation of operation: https://www.powerelectronictips.com/teardown-leviton-ground-fault-circuit-interrupter/

GFCI outlets are not OCPD's the panel breaker is.

 

[Hedges]

When we install a VFD we always put in a line reactor (basically a 1:1 isolation transfomer), and in many cases a load reactor, and use either shielded VFD cable or ferrous metal conduit for the run.

Not a transformer, which would require big iron to transfer 100% of the power.

It is a choke, to reduce edge rates I think. It seems to be wired as a common-mode choke, which might be of some use given the 60 Hz common mode that appears on capacitors (relative to ground) from rectifying 3-phase input. Maybe also provides some differential impedance, which is what I would want to reduce PWM edge rates.

I did install one between VFD and pump, thinking it could prevent damage to winding insulation due to inductive voltage spikes.
I've since read another issue is current flowing through bearings. Contacts to ground shaft are one solution.

You could take one of those reactors and use it as an isolation transformer, but around 2V to 10V max. To drive a choke into saturation so I can generate BH curve and determine its parameters, I use a Variac and then step down output voltage with a transformer, use that to drive choke. Chokes operate with a voltage across windings that is a fraction of line voltage. 3% choke I think drops about 3% of line voltage at full current.

Three Phase Dry Type Reactor

Supplies 3 phase reactor with case or not, Shuenn Liang’s three phase reactor line are required quality certifications and toxic tested.

www.suenn.com

 

[Quattrohead]

The theory of operation is quite simple, apparently manufacturing them to be long-term reliable is difficult sometimes.
I put all the required GFCI outlets into a house that I refurbished and in less than 2 years every single one of them had failed, apparently a bad batch from Home Depot / Leviton.

 

[Supervstech]

The theory of operation is quite simple, apparently manufacturing them to be long-term reliable is difficult sometimes.
I put all the required GFCI outlets into a house that I refurbished and in less than 2 years every single one of them had failed, apparently a bad batch from Home Depot / Leviton.

Yup, they are one of the more common repairs i have to make on calls.

 

[Hedges]

I think GFCI breakers are more reliable than outlets.
Only killed one, that was a dead short.

I've since discovered "WR" GFCI outlets for wet locations. Maybe that makes a difference.
I think I have replaced several GFCI in wet locations, but all the ones I installed in kitchen and even bath 20 years ago are fine. I think those were Leviton.

GFCI | Outdoor / Weather-Resistant Outlets | Leviton

Get electrical shock protection, outdoors with Leviton's Weather-Resistant GFCI Outlet/Receptacles.

leviton.com

 

[WorldwideDave]

Yes, I like the heavy duty, weather resistant, non-tamper resistant ones. Not always easy to find, and sometimes twice the price.

 

[haywire]

Now my question is, why does my GFCI trip when I overload it with tools? There is no ground fault at all.

@Quattrohead caught this mistake. Thank you

This is probably too basic to be the issue in your circuit but I had this recently and discovered that the RCD protecting against ground faults was installed after a neutral earth bond rather than before it(after as in mains -> switchboard(bond(N+E bbar) -> RCD -> breakers)-> appliances. It was only noticed after trips started to occur under high loads.

The issue was resolved once the N was exclusively first run through RCD then N+E bond was made.

EDIT: Never mind - This was reversed in my head. After reading the comments that follow and checking again - the bond is indeed at the start of the panel.

 

[pone]

My understanding is that when you overpower a circuit you can get conditions - like voltage drops, overheating, or minor insulation breakdown - that may allow a small amount of current to leak to ground. It only takes about 5 milliamps of leakage to trip the GFCI.

The other side of this is that I have had GFCI receptacles give false positives, and they can get very quirky.

 

[Hedges]

This is probably too basic to be the issue in your circuit but I had this recently and discovered that the RCD protecting against ground faults was installed after a neutral earth bond rather than before it(after as in mains -> switchboard(bond(N+E bbar) -> RCD -> breakers)-> appliances. It was only noticed after trips started to occur under high loads.

The issue was resolved once the N was exclusively first run through RCD then N+E bond was made.

That may be how you do things Down Under. And perhaps in Europe, where 30 mA whole-house RCD are also used.

In the US, we have N-G bond at first disconnect (main breaker by utility meter.)
Downstream of that are many branch circuits. Some, like an electric stove, don't have GFCI (what we call RCD Up Over).
We do use GFCI breakers for kitchen, bath, outdoor 20A convenience outlets. Or, to retrofit an older house that didn't have them originally, we use GFI outlets.

Having N-G (or N-E as you call it) bond after RCD may not provide the protection I want.
If I touch an appliance having ground fault and touch a pipe or even stand on wet ground, I would think current could flow through me, back up the G (or E) connection, complete the circuit without causing imbalance in RCD.
I want any current flowing through me to get back to the grid bypassing RCD, so RCD trips.

It could be you have N-E bond and your house, and neighbor has another N-E bond, so some current goes through his bond causing your RCD to trip. But then, might require much greater than 30 mA through the victim before RCD trips.

Our human-safety GFCI trip at 5 mA because 30 mA is enough to prevent a person from letting go, causes their muscles to tense up.