Dryer trips washer GFCI

[winnie]

If that grounded metal (EGC) is in fact carrying current (because the dryer was on a 3-wire cord and its chassis was touching the washer chassis, which current would be increased if the cord neutral connection was compromised), can that current induce enough current on the neutral to cause a GFCI to trip?

Or putting it another way, say I have a 12/3 cable which is carrying the hot and neutral of a GFCI protected circuit, and the nuetral only of some other circuit, whose hot is routed through another cable some distance away (obviously a bad design, although for non-metallic wiring methods, allowed by the NEC). Is there a risk of the GFCI tripping due to the unbalanced current carried in the same cable as the two GFCI protected conductors?

Cheers, Wayne

Think of the 12/3 cable as a loosely coupled transformer. Current on the EGC could induce current on the neutral, but that current would still have to flow in a complete circuit that includes the hot.

Perhaps current on the EGC could cause high frequency common mode currents that are tripping the GFCI; a couple of years back there was a thread about refrigerators tripping GFCIs...

 

[wwhitney]

the GFCI saw a current on the conduit/EGC/neutral with nothing on the phase and tripped.

That's not how GFCIs directly work. They monitor the circuit conductors and detect any imbalance on those. If those are balanced, and the EGC is carrying current from another circuit, that EGC current does not get directly detected.

The question in my last post was whether that current could indirectly affect the circuit conductors and cause an imbalance that would trip the GFCI.

Cheers, Wayne

 

[wwhitney]

Think of the 12/3 cable as a loosely coupled transformer. Current on the EGC could induce current on the neutral, but that current would still have to flow in a complete circuit that includes the hot.

That sounds reasonable (my transformer knowledge is not quite strong enough to convince myself with complete certainty).

How does capacitive coupling work to trip a GFCI? GFCI instructions often have a warning to limit the length of circuit conductors protected by the GFCI, and my understanding is that it is due to capacitive coupling of the two circuit conductors to each other. Would the capacitive coupling from a 3rd unbalanced conductor to one or both of those circuit conductors act similarly, and over a much shorter length?

Cheers, Wayne

 

[wwhitney]

GFCI instructions often have a warning to limit the length of circuit conductors protected by the GFCI, and my understanding is that it is due to capacitive coupling of the two circuit conductors to each other.

Or is it due to capacitive coupling of the circuit conductors to the EGC? In which case a 2-wire circuit without EGC or any nearby metal would not be subject to the GFCI circuit length limit.

Cheers, Wayne

 

[Elect117]

That's not how GFCIs directly work. They monitor the circuit conductors and detect any imbalance on those. If those are balanced, and the EGC is carrying current from another circuit, that EGC current does not get directly detected.

The question in my last post was whether that current could indirectly affect the circuit conductors and cause an imbalance that would trip the GFCI.

Cheers, Wayne

"The point of the second toroid is to sense current flowing from ground to neutral, rather than a difference in the line and neutral current. One point of confusion among some online commenters is how current can flow in the ground lead without causing a difference in the hot/neutral current that the first toroid would sense. The answer: One can envision a scenario in which current flows from hot to load, then to neutral, and then to ground. There would potentially be no difference in the line and neutral current. So the GFCI circuit must sense this fault current with the operating assumption that the hot and neutral currents are the same."

Teardown: Leviton ground fault circuit interrupter - Power Electronic Tips

www.powerelectronictips.com

The UL standard has a ground - neutral trip test.


IF anyone has a GFCI and some wire it would be pretty easy to test. Swap the EGC terminal wire with the neutral of another circuit (L1) and have the neutral still connected from load 2. Have a small but low load on the normal hot of load 2. A coin operated washer should "always" be on right? Just the electronics would be energized to monitor the coin system.

Then put something on the load 1 and see if the GFCI trips.

 

[kwired]

I thought about the neutral to ground bond on the dryer, but that should only trip the gfi if the dryer is on one, shouldn’t affect the washer.

Or if older wiring that had no EGC who knows what kind of bootlegging may have been done and you could get some unbalancing current to flow over the EGC between appliances and trip a GFCI.

Other possibility is variable speed driven washer or dryer that doesn't play well with GFCI because of high frequency leakage. That sort of thing can sometimes give problems with GFCI's that are not even on same branch circuit.

 

[Elect117]

P.S. I am not trying to be combative. I am genuinely curious if the ground- neutral trip test would cover that scenario.

Maybe I am misunderstanding it.

 

[winnie]

GFCIs are supposed to have separate circuitry to detect ground to neutral faults.

But I don't think it works the way the article you linked describes. I think the second coil is supposed to inject current that would cause unbalanced current if there is a ground to neutral fault.

I need to understand the circuit more to see if this additional capacity makes the GFCI susceptible to tripping when current is imposed on the EGC. I may need to try it out on the bench

-Jonathan

 

[wwhitney]

The UL standard has a ground - neutral trip test.

Yes, and I didn't follow the description of how that test works in the link you provided. But I believe the text and diagram indicate that the method is based on detecting the loop formed by the neutral/EGC due to the line side neutral/EGC connection at the MBJ/SBJ, and the load side neutral/EGC fault.

If we just inject current from another source onto the EGC on the load side of the GFCI, that does not create such a loop. From the teardown, it looks like that EGC current would all stay on the yoke and that the circuit board doesn't directly reference the EGC. [GFCIs function without a line side EGC.]

Cheers, Wayne

 

[Flicker Index]

Thanks for the tips everyone I went back and here is the update,
Short piece of wire touching washer and dryer triped GFCI.
checked the cords, washer cord ohmed out ok no ground faults replaced it anyway, gfci still tripped when washer touched dryer.
Checked dryer cord it was very stuck in the receptacle, plastic around neutral pin slightly melted.
Replaced that with a 4-wire cord/ and recept and the problem is solved.
So a weak neutral to the dryer, but whats weird is dryer was working, and the problem was only with a GFCI.
Thanks again for all the tips.

Is there anything connected downstream of the GFCI?

I don't see how current flowing through washer chassis and back to ground would trip the GFCI. Even if you were to jumper the washer case to a neutral at an outlet that is upstream of the GFCI, it shouldn't notice it. EGC current is not monitored in GFCI. It's only monitoring for hot and neutral downstream of the GFCI. So, if the hot at the GFCI outlet or downstream of it was to return via anything but the paired neutral, it would trip. If current was to return to neutral downstream of GFCI from another hot, it will trip.

 

[Flicker Index]

GFCIs are supposed to have separate circuitry to detect ground to neutral faults.

But I don't think it works the way the article you linked describes. I think the second coil is supposed to inject current that would cause unbalanced current if there is a ground to neutral fault.

I need to understand the circuit more to see if this additional capacity makes the GFCI susceptible to tripping when current is imposed on the EGC. I may need to try it out on the bench

-Jonathan

Wrap around a lamp cord with a clamp meter that reads into mA level. No matter how many amps pass through the lamp cord, the clamp meter will read 0 mA as long as the current only passes between these pairs. However, if you attach a 6W light bulb between the neutral past the coil, and a different power source, you'll have 50mA in neutral that's not cancelled out by opposite current, so the coil will pickup the 50mA and trip.

 

[CoolWill]

I think the leak is going the other way, from washer to dryer. A leaky motor winding could be energizing the frame and finding a path through the dryer. The EGC of the washer circuit could be almost non-existent, just enough to light the neon lamp in a plug tester but not pass enough current to trip the GFCI.

 

[ptonsparky]

I think the leak is going the other way, from washer to dryer. A leaky motor winding could be energizing the frame and finding a path through the dryer. The EGC of the washer circuit could be almost non-existent, just enough to light the neon lamp in a plug tester but not pass enough current to trip the GFCI.

Voltage between the washer & dryer should be measurable. I've been on more than one service call that the owner did detect it. Painfully.


pre GFCI

 

[CoolWill]

Voltage between the washer & dryer should be measurable. I've been on more than one service call that the owner did detect it. Painfully.


pre GFCI

A leaky winding wouldn't have to be on the high side of the winding, it could be closer to the neutral side and not be much voltage to trip the GCFI. And the washer goes through several cycles. With different components being energized at different times. There could be a fault in any one of those components leading to what seems to be a random tripping of the GFCI.

To me the only thing that makes sense would be current from the washer to the dryer frame and not the other direction.

 

[Little Bill]

A leaky winding wouldn't have to be on the high side of the winding, it could be closer to the neutral side and not be much voltage to trip the GCFI. And the washer goes through several cycles. With different components being energized at different times. There could be a fault in any one of those components leading to what seems to be a random tripping of the GFCI.

To me the only thing that makes sense would be current from the washer to the dryer frame and not the other direction.

I might have agreed with you, but the OP said this....

Checked dryer cord it was very stuck in the receptacle, plastic around neutral pin slightly melted.
Replaced that with a 4-wire cord/ and recept and the problem is solved.

I can't explain this, but that's what he said.

 

[kwired]

GFCIs are supposed to have separate circuitry to detect ground to neutral faults.

But I don't think it works the way the article you linked describes. I think the second coil is supposed to inject current that would cause unbalanced current if there is a ground to neutral fault.

I need to understand the circuit more to see if this additional capacity makes the GFCI susceptible to tripping when current is imposed on the EGC. I may need to try it out on the bench

-Jonathan

A voltage is injected that will cause imbalance of the normal sensing coil if there is a neutral to ground fault. Would also cause similar imbalance if neutral contacted another neutral or even if the ungrounded conductor contacted another ungrounded conductor. Of course if another "phase conductor" you will trip breaker on short circuit current, but if you happen to contact the ungrounded with another ungrounded of same potential it would trip on a low current imbalance. This low current circuit path can/will pass though the source if that is only path to allow it to make a circuit to do it's thing. I believe it is a low DC voltage that is injected, not sure how many volts.