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Old 11-06-2008, 07:15 PM   #16
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GFCI outlets or breakers?


This is getting fun :D

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Old 11-06-2008, 07:19 PM   #17
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GFCI outlets or breakers?


Quote:
Originally Posted by chris75 View Post
So you still wanna explain the nuisance trips with gfci's and fridges?


Why the heck would you even bring up AFCI's? Your talk is something, but your electrical knowledge is on a different level.
OK.
GFIs have a 4 mA "budget" that gets used up by resistive leakage or capacitive reactance rather than valid leakage due to someone getting shocked.

One of these days I'll have to measure the capacitance of a motor shell to ground, not that that will help much.

I agree with you that what I know is on a different level.
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Old 11-06-2008, 07:24 PM   #18
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GFCI outlets or breakers?


If the fridge or motor is tripping the gfci, then it is time to replace the fridge!
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Old 11-06-2008, 07:35 PM   #19
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GFCI outlets or breakers?


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Originally Posted by jbfan View Post
If the fridge or motor is tripping the gfci, then it is time to replace the fridge!
I have to disagree, on this particular point. I have seen several types of motor loads trip a GFCI. It doesn't happen often, that's why it's called a nuisance trip. A fridge, a dentist's chair, and a large venthood that was accidentally wired on a GFCI. I have only witnessed it with induction motors, never universal type motors.

These devices were not defective but still tripped the GFCI.
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Old 11-06-2008, 08:24 PM   #20
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GFCI outlets or breakers?


Quote:
Originally Posted by InPhase277 View Post
I have to disagree, on this particular point. I have seen several types of motor loads trip a GFCI. It doesn't happen often, that's why it's called a nuisance trip. A fridge, a dentist's chair, and a large venthood that was accidentally wired on a GFCI. I have only witnessed it with induction motors, never universal type motors.

These devices were not defective but still tripped the GFCI.

Did you megger them?


Look, if a appliance is tripping a GFCI, than that device is WAY beyond the allowed current leakage by UL, and is a hazard.

Last edited by chris75; 11-06-2008 at 08:28 PM.
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Old 11-06-2008, 08:25 PM   #21
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GFCI outlets or breakers?


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Originally Posted by jbfan View Post
If the fridge or motor is tripping the gfci, then it is time to replace the fridge!
I agree 100%
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Old 11-06-2008, 08:49 PM   #22
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GFCI outlets or breakers?


Quote:
Originally Posted by chris75 View Post
Did you megger them?


Look, if a appliance is tripping a GFCI, than that device is WAY beyond the allowed current leakage by UL, and is a hazard.
Megger? No. But we know that current can only flow when a potential difference is present. With the dental chair I did disconnect the ground and put a meter on it to see if there was voltage. The chair tripped the GFCI every few starts, but never was a voltage present on the ground of the chair.

As far as leakage, I agree that if there is leakage, the device needs to be repaired or replaced. But, I have read how rapid fluctuations or high frequencies can cause nuisance tripping of GFCIs due to capacitive coupling. An actual fault need not exist. But newer GFCIs are smart enough that this doesn't happen as much, if at all anymore. This isn't just me making this up. I have witnessed it, true, but the manufacturers have too, and have worked to fix it.
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Old 11-06-2008, 11:18 PM   #23
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GFCI outlets or breakers?


Quote:
Originally Posted by InPhase277 View Post
But, I have read how rapid fluctuations or high frequencies can cause nuisance tripping of GFCIs due to capacitive coupling. An actual fault need not exist. But newer GFCIs are smart enough that this doesn't happen as much, if at all anymore. This isn't just me making this up. I have witnessed it, true, but the manufacturers have too, and have worked to fix it.
You will have to prove this theory to me for me to believe it.
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Old 11-07-2008, 02:04 PM   #24
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GFCI outlets or breakers?


Motor winding to motor shell.

4 mA at 120v is 30 kiloohm resistance or reactance.
At 60 Hz this is .09 uF.
900' of romex could have this much capacitance, wire to wire or wire to ground.

Some washing machines only trip the GFI on the agitator part of the wash cycle, not the draining part. The agitating part suddenly pulls more current as the clutch engages.

This link
http://www.freepatentsonline.com/7088206.html
may or may not show the improvements you can expect for this gadgets.

BTW, according to a formula in UL1436, 4 mA should take a maximum of 10 seconds to trip. From the same formula, 1.1 mA could take one minute to trip, but there might be built-in circuitry to prevent the thing from responding to current differences this small.

The formula is on page 8 in the middle.
http://www.idealindustries.com/media...uctions_v4.pdf

Last edited by Yoyizit; 11-07-2008 at 02:14 PM.
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Old 11-07-2008, 03:41 PM   #25
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GFCI outlets or breakers?


Quote:
Originally Posted by Yoyizit View Post



Some washing machines only trip the GFI on the agitator part of the wash cycle, not the draining part. The agitating part suddenly pulls more current as the clutch engages.
So what if the motor all of a sudden pulls more current? The gfci will never know the difference. its still measuring what comes in, goes out. remember?
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Old 11-07-2008, 08:54 PM   #26
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So what if the motor all of a sudden pulls more current? The gfci will never know the difference. its still measuring what comes in, goes out. remember?
Ideally, "yes".

If you look at the schematic on page 6 of the link below, it seems straightforward enough. The heart is the toroid transformer.
http://www.national.com/ds/LM/LM1851.pdf

But,
practically,
in some cases,
from what I've heard on this forum and others,
I believe the answer is
"no".

Once I worked with a guy who built a device that didn't work as planned. When I told him about it, he indignantly said, "I don't design things to not work properly." I said, "Nobody does, but somehow it happens."

I don't think most people start out saying, "I am now going to build or design a worthless piece of excrement that will cause nothing but trouble for whoever is dumb enough to buy it".
But, has it ever happened?

I have certainly bought some of these things.

These little gadgets are not perfect. And there are probably patent applications out there right now to overcome all these little quirks.
Meanwhile, the manus are working to make their stuff compatible with as much as they can of what is already out there, to the extent that is economically feasible.

I should also point out that the transient performance of circuits can be way different from the steady state performance.
E.g., if you have a coil in a circuit and you interrupt the current in that coil you will get a huge voltage spike of a polarity opposite to what was applied.
This voltage is equal to -L dI/dT, with dI being the change in current and dT being the time it takes for the current to change.
The coil doesn't "know" if there was 12v applied or 120v or 1200v. It only "knows" the change in current.
With one henry and a one amp change within 100 uS (the duration of the arc you get when you open the switch) you momentarily get 10kV (in the absence of parasitic capacitance, etc., etc.) across the opened contacts.
This is why, back when there were breaker points you could get 200v across the points when they open, and this was in a 12v or 6v auto system.

That's the problem with Reality.
I take no pleasure in telling you, Mr. 75, that reality relentlessly forces you to give up your ideas, and ideals, about how the world works or "should" work. And the older you get, the more of these notions you have to give up.

Some equipment problems I saw didn't seem possible. When they were finally solved it always turned out that I was assuming something that wasn't true.
Finding your own faulty assumptions is the real work in troubleshooting.

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