Crazy gfci

[Nightglow]

Im having issues with a kitchen countertop gfci at a friends house. It all started when they decited to upgrade their appliances. There was a 20a deticated circuit ran above the range for a microwave. When the new microwave was pluged in it tripped a countertop gfci... Here is the odd things and what i have tried.


The microwave has a deticated circuit, 20a straight from the outlet to the panel but when the microwave is pluged in it trips a countertop gfi that is on a seperate kitchen circuit. I have changed the gfi, ran a new circuit to microwave, pluged the microwave into other areas in the house, excanged the microwave for another new microwave, ran a new circuit to the gfci, and everytime the microwave is pluged in... That same gfi on a different circuit trips every time. Here is the kicker, you dont even have to plug in the microwave all the way into the outlet, all that has to touch is the grounding prong from the microwave to to the grounding inlet of any receptical in the house. Nothing else seemes to trip the gfi, if i plug in a gfi tester to the microwave outlet and push test it doesnt trip the countertop gfi all shows its wired correctly just like it should but the microwave will do it every time. I have changed the breaker for the microwave to the other phase in the panel and also made sure that none of the netural or grounding wires are together (under same screw pretaining to these circuits). I have tightened all connections and still have the same problem. I found out one more thing before i left it alone for the day, if i unhook the load side from the gfi everything works fine, so my next attack i will diagnose the gfi circuit and break it in the center etc. to try to find why it trips. Even if that does cure the problem and lets say i "fix it", the question still how is it possable to trip a gfi on a seperate circuit from the one you are using and why does it only trip that gfi and not any of the others in the house? Also i dont know if i have to say this but if you didnt allready guess, if i put a 3 prong to 2 prong adaptor on the microwave everything works as it should but we all know thats not the correct fix to the problem.


Thank you in advance for any help or sugestions.
- Chris

 

[Strathead]

Im having issues with a kitchen countertop gfci at a friends house. It all started when they decited to upgrade their appliances. There was a 20a deticated circuit ran above the range for a microwave. When the new microwave was pluged in it tripped a countertop gfci... Here is the odd things and what i have tried.


The microwave has a deticated circuit, 20a straight from the outlet to the panel but when the microwave is pluged in it trips a countertop gfi that is on a seperate kitchen circuit. I have changed the gfi, ran a new circuit to microwave, pluged the microwave into other areas in the house, excanged the microwave for another new microwave, ran a new circuit to the gfci, and everytime the microwave is pluged in... That same gfi on a different circuit trips every time. Here is the kicker, you dont even have to plug in the microwave all the way into the outlet, all that has to touch is the grounding prong from the microwave to to the grounding inlet of any receptical in the house. Nothing else seemes to trip the gfi, if i plug in a gfi tester to the microwave outlet and push test it doesnt trip the countertop gfi all shows its wired correctly just like it should but the microwave will do it every time. I have changed the breaker for the microwave to the other phase in the panel and also made sure that none of the netural or grounding wires are together (under same screw pretaining to these circuits). I have tightened all connections and still have the same problem. I found out one more thing before i left it alone for the day, if i unhook the load side from the gfi everything works fine, so my next attack i will diagnose the gfi circuit and break it in the center etc. to try to find why it trips. Even if that does cure the problem and lets say i "fix it", the question still how is it possable to trip a gfi on a seperate circuit from the one you are using and why does it only trip that gfi and not any of the others in the house? Also i dont know if i have to say this but if you didnt allready guess, if i put a 3 prong to 2 prong adaptor on the microwave everything works as it should but we all know thats not the correct fix to the problem.


Thank you in advance for any help or sugestions.
- Chris

Draw it out. Bottom line is that the electrons on the load side wiring of the GFI are finding a path (either hot or neutral) through the ground prong of the microwave back to the panel. The only quick thing I can think of is, that the mounting hardware for the microwave is pinching or piercing the wiring of the GFI.

 

[kwired]

Make sure there is no bootlegged equipment ground from a neutral somewhere, and happens to be load side of this GFCI?

 

[PetrosA]

Is it a ducted microwave? If so, maybe there's a short between the duct and a neutral, hot or ground from the countertop GFI somewhere down the line.

 

[Nightglow]

Very good ideas. I am going back on thursday to check all the outlets on the load side. I will def. check out the duct and mounting brackets. If they are in contact i should be able to turn the circuits off and check continuity from bracket or duct to gfi load ground/neutral! Thanks, hopefully its this simple *fingers crossed* ill update if fixed

-chris

 

[Strathead]

Very good ideas. I am going back on thursday to check all the outlets on the load side. I will def. check out the duct and mounting brackets. If they are in contact i should be able to turn the circuits off and check continuity from bracket or duct to gfi load ground/neutral! Thanks, hopefully its this simple *fingers crossed* ill update if fixed

-chris

What you should be able to do is disconnect the leads from the load side of the GFI. Check hot to ground with a meter for continuity. Check neutral to ground for continuity. There should be infinity given your description Then plug in the microwave. Check voltage to ground on hot and neut for safety. Then check hot and neut to ground for continuity actually any resistance change from infinity. There should be a resistance other than infinity on one of the leads. Now you have something to look for.

 

[growler]

if i unhook the load side from the gfi everything works fine, so my next attack i will diagnose the gfi circuit and break it in the center etc. to try to find why it trips.

The only quick thing I can think of is, that the mounting hardware for the microwave is pinching or piercing the wiring of the GFI.

I think strathead nailed it. The microwave frame and mounting hardware would not be grounded until the microwave is plugged in and then it would provide a different path to ground.

With the load side of the GFCI disconnected you should be able to use a meter and see which of the conductors (load side ) is picking up a ground when the microwave is plugged in (no need for the power to be on).

Could easily be one of those mounting lag bolts.

 

[charlie b]

What happens if you plug a different 3-prong appliance (e.g., a food processor) or some other tool (e.g., leaf blower) into the microwave's outlet?

Also, you say the trip happens when you plug the (new?) microwave into any outlet in the house. How do you do that? Is the microwave still installed in its place, and do you use an extension cord to reach the other outlets, in order to do the test?

One more thought. When any new symptom arises, the first question to ask should be, "what recently changed?" In this case, what changed is that a new microwave was purchased, and it is the type that sits over the stove. I replaced our over-stove microwave a couple years ago, and I helped someone else replace theirs as well. This process often involves installing new mounting hardware that is unique to the make and model. My two installs required us to pull a metal mounting plate off the wall and install a new one. For the install you are dealing with, I would wonder whether some wire internal to the wall might have gotten nicked, when new screws or other devices were used to mount the plate onto the wall. This is one possible explanation for how plugging the microwave into one outlet can trip a GFCI for an entirely separate circuit.

 

[charlie b]

OK, OK. I started typing, then got distracted, and came back to finish my thought. In the meantime, someone else posted the same idea. But I had the idea first. Really! :happyyes:

 

[kwired]

I think strathead nailed it. The microwave frame and mounting hardware would not be grounded until the microwave is plugged in and then it would provide a different path to ground.

With the load side of the GFCI disconnected you should be able to use a meter and see which of the conductors (load side ) is picking up a ground when the microwave is plugged in (no need for the power to be on).

Could easily be one of those mounting lag bolts.

But the strange part of this deal is he said he can plug it in most anywhere in the house and it still trips the same GFCI. I can think of a few things that could be issues, until this problem is mentioned.

I guess one should make sure the GFCI is properly connected - like line load reversal or line side hot and neutral side load on the line terminals, older GFCI's did not respond to as many miswiring scenarios as newer ones do. Maybe there is grounding and bonding issues with the premises wiring and plugging this microwave in somehow makes up some path that puts fault current through the GFCI.

He did not say how he was plugging into other outlets, if microwave is in place but running extension cords to plug it in, maybe it is a mounting screw threaded into GFCI protected wiring but the offending screw is isolated until equipment ground conductor is connected. In fact this is only thing mentioned so far that really makes much sense.

 

[growler]

But the strange part of this deal is he said he can plug it in most anywhere in the house and it still trips the same GFCI. I can think of a few things that could be issues, until this problem is mentioned.

I'm assumming this is an undercounter type microwave and not something that would be carried to different outlets and plugged in. If they use an extension cord to plug the microwave to diffent outlet around the house it wouldn't make any difference. The thing is that the frame of the microwave is not grounded until it is plugged in and then it picks up a ground through the receptacle.

If a lag bolt is used to mount the microwave (furnished by manufacturer ) and it were to pierce the load side conductor of the tripping GFCI that would explain everything. It wouldn't matter where the microwave is plugged in it would still ground the frame and the load side conductor.

 

[growler]

I will def. check out the duct and mounting brackets. If they are in contact i should be able to turn the circuits off and check continuity from bracket or duct to gfi load ground/neutral! Thanks, hopefully its this simple *fingers crossed* ill update if fixed

-chris

Could even be the hot GFCI load side conductor that is making contact through the mounting hardware or duct of the microwave.

 

[kwired]

Could even be the hot GFCI load side conductor that is making contact through the mounting hardware or duct of the microwave.

Could be, but you are more likely to have some fireworks also in this case. If the neutral is all that is faulted the GFCI just trips with no bonus fireworks show.

 

[mjf]

I'm assumming this is an undercounter type microwave and not something that would be carried to different outlets and plugged in. If they use an extension cord to plug the microwave to diffent outlet around the house it wouldn't make any difference. The thing is that the frame of the microwave is not grounded until it is plugged in and then it picks up a ground through the receptacle.

If a lag bolt is used to mount the microwave (furnished by manufacturer ) and it were to pierce the load side conductor of the tripping GFCI that would explain everything. It wouldn't matter where the microwave is plugged in it would still ground the frame and the load side conductor.

cabinet

 

[PetrosA]

I like the pierced wire idea in addition to the shorted duct. It wouldn't be my first guess, but maybe that's because I rarely run wiring where I know upper cabinets or a hood/microwave would get hung. That's just asking for problems

 

[kwired]

I like the pierced wire idea in addition to the shorted duct. It wouldn't be my first guess, but maybe that's because I rarely run wiring where I know upper cabinets or a hood/microwave would get hung. That's just asking for problems

The one circuit that is necessary in the cabinet area is the one the OP is having trouble with

 

[PetrosA]

The one circuit that is necessary in the cabinet area is the one the OP is having trouble with

True, but it wouldn't be necessary for that wire to run through the studs to the next outlet at upper cabinet height...

 

[Strathead]

The one circuit that is necessary in the cabinet area is the one the OP is having trouble with

Not from my thinking, or I am confused on your assumption. The load side wiring of the GFI that is tripping is one circuit. The microwave is on an independent dedicated home run. Since the OP indicated that he personally installed this microwave circuit, it can't be tied to the load side of the GFI is any "intentional way". If the microwave being plugged in is causing the the GFI to trip then there has to be an unintentional connection between the micro circuit and the GFI wiring. The ONLY plausible path I can see for GFI electrons back to ground is from the GFI through an unintentional connection, through the metal frame of the the microwave, then stopped until the microwave is plugged in. Now along the ground prong through building wire to ground. There may be other ways, but that, to me is Colonel Mustard in the Library with the knife.

And I must include, a perfect example of why I tell my guys that it pays to think. Draw things out, understand how stuff works, and use logic to find solutions. Don't just check a bunch of things willy-nilly without collating the results.

 

[kwired]

Not from my thinking, or I am confused on your assumption. The load side wiring of the GFI that is tripping is one circuit. The microwave is on an independent dedicated home run. Since the OP indicated that he personally installed this microwave circuit, it can't be tied to the load side of the GFI is any "intentional way". If the microwave being plugged in is causing the the GFI to trip then there has to be an unintentional connection between the micro circuit and the GFI wiring. The ONLY plausible path I can see for GFI electrons back to ground is from the GFI through an unintentional connection, through the metal frame of the the microwave, then stopped until the microwave is plugged in. Now along the ground prong through building wire to ground. There may be other ways, but that, to me is Colonel Mustard in the Library with the knife.

And I must include, a perfect example of why I tell my guys that it pays to think. Draw things out, understand how stuff works, and use logic to find solutions. Don't just check a bunch of things willy-nilly without collating the results.

I agree that it is not a good idea to run wiring through studs in area where cabinets are to be hung. My point was this is is a little unavoidable for the wiring supplying an outlet placed in a cabinet. Had nothing to do with exactly which line or load wire was where.

You are correct though - if the OP has a dedicated circuit running straight to the panel for the microwave. My guess is this is either not the case or something fairly abnormal is going on. For the alleged separate circuit to trip as soon as a ground pin in an appliance cord makes contact in what sounds like is any receptacle in the house to me means there is probably some kind of issue with neutrals and grounds in this house and by connecting this appliance he is somehow bridging something to make the GFCI trip.

But at same time if the load side of the GFCI is isolated from everything else it should not matter, so there is likely more than one issue contributing to the problem here. Load side of GFCI is likely faulted to something, but that something is floating until the microwave is plugged in then it is (grounded not likely the right word) connected to an alternate path of some type.

 

[cowboyjwc]

Someone may have already pointed it out, but sounds to me like it might have less to do with the wiring and more to do with the microwave.