Crazy gfci

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cowboyjwc said:
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.
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You would think that, but OP said it trips this countertop GFCI outlet even when plugged into outlets in other parts of the house, which shouldn't matter unless some interconnection between the load side of the problem GFCI is happening somehow.
 
Strathead said:
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.
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kwired said:
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.
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What do we actually know about this problem? GFCI (small appliance circuit) is tripping when the microwave is grounded. The OP stated that only the ground prong (microwave plug) was necessary for the GFCI to tip.

This means that if you were to ground the frame of the microwave by any means the SA GFCI would trip if the load side conductors were connected.

This circuit was working OK before the microwave was installed so what happened.

Installation hardware for a microwave consist of a mounting bracket, 2 lag bolts and 2 toggle bolts.

What is the most "plausible" way for the load side conductors to have been damaged during installation that would result in a ground path being established when the microwave is plugged-in (ground prong only)?
 
growler said:
What do we actually know about this problem? GFCI (small appliance circuit) is tripping when the microwave is grounded. The OP stated that only the ground prong (microwave plug) was necessary for the GFCI to tip.

This means that if you were to ground the frame of the microwave by any means the SA GFCI would trip if the load side conductors were connected.

This circuit was working OK before the microwave was installed so what happened.

Installation hardware for a microwave consist of a mounting bracket, 2 lag bolts and 2 toggle bolts.

What is the most "plausible" way for the load side conductors to have been damaged during installation that would result in a ground path being established when the microwave is plugged-in (ground prong only)?
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I think I made similar suggestion but got shot down, but at same time we have not clarified if OP removed microwave when plugging it into other receptacles throughout the house or if he just ran an extension cord to those other outlets. Running an extension cord doesn't change the possibility of mounting hardware being in contact with GFCI protected circuit when connecting to another circuit. This hardware then becomes grounded when plugged in to any receptacle with good equipment grounding.
 
kwired said:
You would think that, but OP said it trips this countertop GFCI outlet even when plugged into outlets in other parts of the house, which shouldn't matter unless some interconnection between the load side of the problem GFCI is happening somehow.
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Ok, but couldn't the grounding and grounded conductors in the microwave be interconnected somehow?

But then why is it only tripping the counter top GFCI and not any other ones?

Because I believe he said he ran a whole new circuit, meaning that the only place they could be interconnected is at the panel and again wouldn't that affect other GFCI's also?

Unless, as Growler has suggested, the counter GFCI wiring has been comprimised.
 
cowboyjwc said:
Unless, as Growler has suggested, the counter GFCI wiring has been comprimised.
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Folks Strathead solved this problem and not me, I was just agreeing with him.


I have installed several microwaves without any problems but once I read his explanation of what could easily have happened it made me think and it was a very logical explanation.

Even if you locate the problem quicky this could end up with an hour or two of extra time to correct the problem. I don't feel bad about charging as much as I do to install a microwave because you really do need to be carefull.


I think this would make a great test question for instructors, it's simple enough but does require some thought.
 
growler said:
Folks Strathead solved this problem and not me, I was just agreeing with him.


I have installed several microwaves without any problems but once I read his explanation of what could easily have happened it made me think and it was a very logical explanation.

Even if you locate the problem quicky this could end up with an hour or two of extra time to correct the problem. I don't feel bad about charging as much as I do to install a microwave because you really do need to be carefull.


I think this would make a great test question for instructors, it's simple enough but does require some thought.
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So have we ever found out what the problem was or are we still waiting for an answer?
 
growler said:
Folks Strathead solved this problem and not me, I was just agreeing with him.
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He was first to mention most logical thing that seems to fit the description of problems here. Problem is OP has not replied to confirm a few assumptions. OP has not told us just exactly how this microwave was plugged into other outlets in the house. I don't think we even know for certain that it is one fastened to a wall or cabinet, but have kind of assumed it was. If it is fastened to wall and a fastener has compromised the GFCI in question, and plugging in to other outlets was done by just running an extension cord to those outlets but leaving the microwave in place, then it all makes perfect sense. If the assumptions are wrong it is still puzzling.
 
kwired said:
He was first to mention most logical thing that seems to fit the description of problems here. Problem is OP has not replied to confirm a few assumptions. OP has not told us just exactly how this microwave was plugged into other outlets in the house. I don't think we even know for certain that it is one fastened to a wall or cabinet, but have kind of assumed it was. If it is fastened to wall and a fastener has compromised the GFCI in question, and plugging in to other outlets was done by just running an extension cord to those outlets but leaving the microwave in place, then it all makes perfect sense. If the assumptions are wrong it is still puzzling.
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Thank you, and I prefer the way kwired put it. "first to mention most logical thing" It is kind of nice to feel I contributed something minor, because frankly, some of you here are pretty intimidating, but that is why I am smart enough to come here. I would hope that the OP and other Journeyman types take a little time to think through problems like this and come up with exactly the most logical thing or things to look at. As someone else mentioned this is a perfect example for teaching, even if this isn't ultimately the solution. I am with Cowboy, in wondering if the OP will get back to us with a follow up.

Once again I will post the "clues" that my assumption is based on:
  1. Newly pulled dedicated circuit to the microwave.
  2. Existing GFCI immediately trips when the new microwave is plugged in
  3. If the Microwave plug is equipped with a 2 prong adapter, the GFCI doesn't trip.
  4. Existing GFCI with load side wiring all existing undisturbed (intentionally anyway)
  5. Existing GFCI operated 100% OK until the microwave was changed
  6. Disconnecting the load side wires from the GFCI eliminates the tripping problem
  7. So, in order for the GFCI to trip it is necessary for the current to be unbalanced on the load side wires of the GFCI.
  8. An unbalance can only (reasonably) occur when one leg "bleeds" current to ground or some outside circuit injects current in to one leg. (the electrons have to go somewhere else, or come from somewhere else period) This is why I told the OP to DRAW it out. When you see it on paper it is easier.
  9. Clue 3 indicates that the grounded parts of the microwave ARE integral to the tripping.
  10. So, most likely the (unbalanced) electrons must flow back to the source along the ground for the microwave.
Obviously, if any one of the things we have been told is not accurate, then everything above is invalid.
 
Hey sounds good to me.

I had a house once where they were doing a kitchen remodel and someone else had done the wiring. Now they're doing this and trying to keep the kitchen in working order, which is important to the story. They call me over and have a dead short. Start going through stuff and find something, fix it, still a short. Find something else, fix it still a short. Messed with it for a couple of hours and told the guy I would have to come back. Layed in bed that night and tossed and turned and then about two in the morning sat bolt up right and say "got it". Scared the heck out of my wife. Went back first thing in the morning and disconnected the dishwasher and fixed!. The guy had said they had a short and that the diswasher had quit working, well the reason the dishwasher quit working was because he fried the motor with one of the original problems. I was so concerned about how bad the wiring was that I forgot to look at the stuff that was plugged into it.
 
I have gotten a bit lost within all the discussions above. So the following statement might be a repetition of what someone else has already said. Let me just clarify my earlier suggestion to say that installation of the new microwave's hardware may have damaged the wires downstream of the "problem GFCI." So the fact that you replaced the wiring serving the new microwave will not have resolved that problem.
 
charlie b said:
I have gotten a bit lost within all the discussions above. So the following statement might be a repetition of what someone else has already said. Let me just clarify my earlier suggestion to say that installation of the new microwave's hardware may have damaged the wires downstream of the "problem GFCI." So the fact that you replaced the wiring serving the new microwave will not have resolved that problem.
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That is our most logical conclusion at this point. Op has not replied to confirm some assumptions.
 
Nightglow said:
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!
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kwired said:
He was first to mention most logical thing that seems to fit the description of problems here. Problem is OP has not replied to confirm a few assumptions. OP has not told us just exactly how this microwave was plugged into other outlets in the house. I don't think we even know for certain that it is one fastened to a wall or cabinet, but have kind of assumed it was. If it is fastened to wall and a fastener has compromised the GFCI in question, and plugging in to other outlets was done by just running an extension cord to those outlets but leaving the microwave in place, then it all makes perfect sense. If the assumptions are wrong it is still puzzling.
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I think we can be pretty sure this is a built-in microwave and is fastened in place with mounting hardware.

I would rather make the assumption that an extension cord was used than make the assumption that anyone was dumb enough to carry a big old built-in microwave around to different outlets just to test it.

The fact that the explanation given (by strathead) makes perfect sense is why I said he solved the problem.
 
growler said:
I would rather make the assumption that an extension cord was used than make the assumption that anyone was dumb enough to carry a big old built-in microwave around to different outlets just to test it.
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cowboyjwc said:
I know. I mean we took the time to figure it for him, the least he could do is post and tell us we were right. :roll:
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Maybe the OP is in the hospital having back surgery because he did carry a big old built-in micro wave around to every receptacle in the house to test it. :slaphead:
 
growler said:
Maybe the OP is in the hospital having back surgery because he did carry a big old built-in micro wave around to every receptacle in the house to test it. :slaphead:
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Maybe it is like Garage Sale signs. You put them up all over the neighborhood, and then when the sale is over you don't bother to go around and take them down. :)
 
I would change the GFCI first to make sure it isn't bad.

Its much faster than trying to trying to trouble shoot the microwave and all the house wiring.

If that doesn't fix the problem, then you might start planning to stay a while.
 
Solved!

Solved!

Sorry it took so long to post a reply, my wife recently had a baby so iv been pretty busy.


So in conclusion, problem is solved! It ended up after reading a couple posts and asking the homeowner a few more questions i found the problem... When the unit was installed the homeowner used long 3"-4" toggle bolts to secure the microwaves mounting bracket. I turned off both circuits, disconnected the countertop outlet to the right and left of the microwave and checked continuity from any grounding (metal) point of the microwave (unplugged) to the wires that ran behind the microwave for the countertop outlets. I found that i did indeed have continuity from the microwave to the neutral wire on the countertop. My best guess without cutting into the wall was, the toggle bolt when installed caught the countertop circuit with the wing, and when he tightened it down it pinched the neutral wire just enough to break into the jacket and scratch the neutral but not enough to cause a short. Thus resulting in every time the microwaves cord made connection to its outlet at the grounding pin it completed the ground to neutral connection to trip the gfci on the countertop.


I ran a new wire from the right to the left of the microwave for the countertop outlets and abandoned the old wire to test my theory... Everything worked as expected and the homeowners were very happy.


Thank you everyone for your replies and time to post it helped me allot and hopefully the people reading this will save some diagnostic time in the future.

-Chris
 
Congrats on the baby! I am tickled pink that we solved the problem with theory and deduction. I try to get my guys to understand how important theory is.
 
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