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Author Topic: Tripping GFI  (Read 16874 times)

John Roberts {JR}

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Re: Tripping GFI
« Reply #10 on: July 08, 2016, 05:16:48 PM »

I put a gfci in my kitchen outlet, because safety grounds were not a concept when my house was built. Since then I jury rigged a ground wire just for this one outlet.

To get on point, this GFCI has tripped maybe 2x in the last year for no known reason. In fact I would often just notice that some appliance was not getting power and discover the GFCI was tripped.


JR
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Mike Sokol

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Re: Tripping GFI
« Reply #11 on: July 10, 2016, 09:06:46 AM »

To get on point, this GFCI has tripped maybe 2x in the last year for no known reason. In fact I would often just notice that some appliance was not getting power and discover the GFCI was tripped.

I've been able to randomly trip a few GFCI's when powering on gear, but once it was on it didn't trip again. My hypothesis at the time was there must have been sufficient inrush current to increase the normal hot-to-chassis leakage currents inside the gear to above the 6 mA trip point. This is because normal gear doesn't have zero-crossover turn-on circuitry. When you flip the power switch "ON", it could be close to the zero point of the 60-Hz sine wave, or halfway up around 80 volts, or near the peak of 170 volts. Each of these different timing conditions will change how much inrush current occurs, which can induce a fault current pulse to the chassis. If that pulse is large enough and long enough it should possible for it to "randomly" trip the GFCI.

It would be interesting to set up a test with power amplifiers to measure their Chassis-to-EGC leakage current under different music loads. Will a big kick drum pulse create more EGC leakage? Who knows unless we try?

I'm of an opinion that there are very few truly random events. Just ones that you can't observe the trigger mechanism.
« Last Edit: July 10, 2016, 09:10:33 AM by Mike Sokol »
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Jay Barracato

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Re: Tripping GFI
« Reply #12 on: July 10, 2016, 09:52:01 AM »

I've been able to randomly trip a few GFCI's when powering on gear, but once it was on it didn't trip again. My hypothesis at the time was there must have been sufficient inrush current to increase the normal hot-to-chassis leakage currents inside the gear to above the 6 mA trip point. This is because normal gear doesn't have zero-crossover turn-on circuitry. When you flip the power switch "ON", it could be close to the zero point of the 60-Hz sine wave, or halfway up around 80 volts, or near the peak of 170 volts. Each of these different timing conditions will change how much inrush current occurs, which can induce a fault current pulse to the chassis. If that pulse is large enough and long enough it should possible for it to "randomly" trip the GFCI.

It would be interesting to set up a test with power amplifiers to measure their Chassis-to-EGC leakage current under different music loads. Will a big kick drum pulse create more EGC leakage? Who knows unless we try?

I'm of an opinion that there are very few truly random events. Just ones that you can't observe the trigger mechanism.

In my lab, it is not uncommon for a device ( think hot plate/stirrer that has both a resistive element and a motor) that has been sitting to pop the gfci on the initial plug in, but then be okay after resetting. I wonder if some devices just build up charge while sitting and then dump it to ground on the initial plug in.

Sent from my XT1254 using Tapatalk

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Mike Sokol

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Re: Tripping GFI
« Reply #13 on: July 10, 2016, 10:07:08 AM »

In my lab, it is not uncommon for a device ( think hot plate/stirrer that has both a resistive element and a motor) that has been sitting to pop the gfci on the initial plug in, but then be okay after resetting. I wonder if some devices just build up charge while sitting and then dump it to ground on the initial plug in.

Sent from my XT1254 using Tapatalk

Not by building up a charge.... I think this gear is being "randomly" turned on at the top of the 60 Hz sine wave and drawing a current pulse with a fast enough rise time to capacitively couple back though the chassis to the EGC wire. And if you plug in a piece of gear with its power switch already on, you'll create a lot of arcs and sparks with high-frequency content. I think that's whats causing the GFCI's to trip....

John Roberts {JR}

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Re: Tripping GFI
« Reply #14 on: July 10, 2016, 11:04:05 AM »

Yes there may be some stray transient current leakage that only occurs at turn-on.

Some power amps with switching PS connected directly to the mains may have some leakage issues. If there are some marginal amps out there I bet the manufacturers have heard about them by now.

My GFCI trips appear random to me, because I was not even in the room and did not see them occur, only later discovered that power was off to my answering machine. (perhaps lightning came in through the phone line, not ring voltage related because typical phone calls don't trip it AFAIK).

JR
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Frank Koenig

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Re: Tripping GFI
« Reply #15 on: July 10, 2016, 12:20:13 PM »

My hypothesis at the time was there must have been sufficient inrush current to increase the normal hot-to-chassis leakage currents inside the gear to above the 6 mA trip point. This is because normal gear doesn't have zero-crossover turn-on circuitry. When you flip the power switch "ON", it could be close to the zero point of the 60-Hz sine wave, or halfway up around 80 volts, or near the peak of 170 volts. Each of these different timing conditions will change how much inrush current occurs, which can induce a fault current pulse to the chassis. If that pulse is large enough and long enough it should possible for it to "randomly" trip the GFCI.

Funny beasts GFCIs. Sometimes, maybe 5% of the time, when I turn OFF one of my welders, which is an older 60Hz transformer type, it trips the GFCIs in my kitchen. The welder is in a separate building with a subpanel but on the same service. All the building wiring is standard as I did it myself in the early '90s. I'm guessing the voltage spike that results from opening the circuit to the transformer primary near the instantaneous peak current in the cycle plays a role.

And, as I complained about here last fall, I had a gig where I had horrendous problems with the venue's GFCIs tripping. I could not reproduce the problem at home using the exact same configuration of gear, nor was the measured leakage > 5 mA. I suspect high frequency hash from the switch-mode power supplies my have confused some older, out of spec, GFCIs, but I really don't know.

Let's hope that over time GFCIs will improve with respect to nuisance tripping while continuing to provide shock protection.

Best,

--Frank
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John Roberts {JR}

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Re: Tripping GFI
« Reply #16 on: July 10, 2016, 01:29:51 PM »

Funny beasts GFCIs. Sometimes, maybe 5% of the time, when I turn OFF one of my welders, which is an older 60Hz transformer type, it trips the GFCIs in my kitchen. The welder is in a separate building with a subpanel but on the same service. All the building wiring is standard as I did it myself in the early '90s. I'm guessing the voltage spike that results from opening the circuit to the transformer primary near the instantaneous peak current in the cycle plays a role.

And, as I complained about here last fall, I had a gig where I had horrendous problems with the venue's GFCIs tripping. I could not reproduce the problem at home using the exact same configuration of gear, nor was the measured leakage > 5 mA. I suspect high frequency hash from the switch-mode power supplies my have confused some older, out of spec, GFCIs, but I really don't know.

Let's hope that over time GFCIs will improve with respect to nuisance tripping while continuing to provide shock protection.

Best,

--Frank
Large transformers can draw massive surge current at turn-on. After turned on and stable, there is no net DC across the transformer winding, but when first turned on if the mains waveform is not coincidentally near 0V, there will be a short term DC component. This DC creates a bias away from the magnetic circuits design center zero.

Various techniques are used to manage this. Inrush current limiters help, also zero-cross coordinated switches that only turn on power near zero. Many designs just slightly oversize the magnetic circuit (copper and iron) to handle the extra transient stress.

Large surge current can induce voltage/current into nearby mains wiring. I suspect big welders do not incorporate expensive surge suppression. 

JR

PS no welders in my kitchen.
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Mike Sokol

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Re: Tripping GFI
« Reply #17 on: July 10, 2016, 03:54:29 PM »

Large surge current can induce voltage/current into nearby mains wiring. I suspect big welders do not incorporate expensive surge suppression. 

I suspect that since many switching power supplies include line-to-chassis RF caps to reduce their own high frequency trash, that it's possible for those same capacitors to act as a shunt for an external transient spike from something like a welding transformer magnetic field collapsing. It could be anywhere on the same pole transformer and still get into your power distro system, and back into your GFCI. Is that what's working its way back into these GFCI sensing circuits and causing them to trip "randomly"?

Guy Holt

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Re: Tripping GFI
« Reply #18 on: July 10, 2016, 07:55:51 PM »

GFCis are pretty sensitive-they only take .006 amps to trip-so it could have just been moisture from the rain and damp.  It certainly wouldn't have to be laying in a puddle to trip.

Donít underestimate the effect of humidity on ground faults.  The climatic rooftop scene of the Ryan Reynolds/Jeff Bridges movie RIPD was shot inside an old shipyard building on Quincy Bay outside Boston. 


The roof of the massive building (see picture of set below) leaked badly and every other pane of glass was missing from the elephant doors on the bayside of the building.


Film style 100A Shock Blocks, like that pictured below, were deployed because the riggers were receiving small shocks. The Shock Blocks have a series LED to indicate the level of leakage current.


What opened my eyes to the effect of humidity on ground leakage is that every evening when the sun set, a moist breeze would come in off the bay and blow through the building.


With every gust of wind through the building the series LED indicators on the Shock Blocks would run up into the red zone indicating an increase in leakage current and then run down as the wind subsided.

Guy Holt, Gaffer
ScreenLight & Grip
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John Roberts {JR}

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Re: Tripping GFI
« Reply #19 on: July 10, 2016, 08:06:32 PM »

Lucky for me there is no humidity in Mississippi.  :o

To reinforce Guy's point, there are lots of reports about nuisance trips with RCDs in EU where they are on every branch circuit. When it rains housing with marginal wiring (or leaky roofs ) trip the RCDs.

I guess if there is dangerous leakage the trip is not really a nuisance just evidence of a problem that needs fixing. 

JR
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Re: Tripping GFI
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