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Old 05-08-2009, 01:02 AM   #1
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Microwave ground prong sparks/melts upon plugging it in


We just moved into an old (1950) house that has some obvious DIY wiring from the previous owner, but everything appears to be in working order.

We purchased a microwave/hood combo (Whirlpool Gold GMH3174 - 1000w hood combo).
The contractor that fixed up our kitchen sheet rock & cabinets moved an outlet up from a lower position on the wall for this microwave/hood. When I hung the unit and attempted to plug it in, sparks flew and the ground prong blackened/melted a bit on the tip. At this time, one circuit breaker flipped, but not the one (assumed) that the microwave is hooked up to, but a 15 amp breaker that handles the living room and bedroom lights and outlets. Only the ground prong of the 3-prong plug actually made it into the receptacle when this happened.

Here is where it gets weird.

We have used our regular counter top microwave plugged into this outlet without incident for 4 weeks. I started asking around to trouble shoot the problem. I tried plugging the new unit into a known, grounded, home run outlet with its own 15 amp with nothing else on it breaker. Same thing: sparks fly, ground prong melted a bit and the breaker for the living room flipped, but not the one I was plugged into.

Blame the microwave, plug in the old microwave an use it for two weeks without incident, take the new microwave back and order another new one.
Now it gets weirder.

New microwave arrives, so I plug it in FIRST to the known, grounded by-itself home run outlet. No problem, no sparks; must be fine. Hang it, bolt it, plug it into the other outlet: SPARKS, MELTED PRONG, OTHER BREAKER FLIPS...just like before. Huh. Immediately I plug the microwave back into the by-itself outlet, and the same sparks and other breaker tripped. Huh.
No extension cords are used, and the outlets are fairly new as well as the wiring. That I can see inside the box.

I can assume that the microwave got screwed up somehow, or at least the cord since it worked in the by-itself outlet first. The microwave/hood unit is "plug & play" , all sealed up and there are no switches to change the voltage. I checked the breaker box an there does not seem to be any thing run 220 or improperly connected, but I can honestly say I have no idea what is going on in the wall or attic, nor can I access either in that area.


What really gets me is that my other microwave (5 year old panasonic 1000w) was plugged into the other outlet and used for weeks, even between incidents.

More weird:

I had my voltage detector out; the suspect outled does not set it off, but the new unit, unplugged and just hanging there, seems to make it beep as if it is electrically live. The only breaker that went off was the 15 amp living room lights breaker. I tried th short the suspect receptacle with a curve of wire and it also sparked, melted and shot firey wads of molten copper towards my face, but did not flip a breaker.

I am a amateur DIY-er, and appreciate any advice that only ends in "call a pro."

The GF's dad is pretty sharp, and has helped on some other issued, but he is baffled by the scenario.

thanks

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Old 05-08-2009, 01:25 AM   #2
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Microwave ground prong sparks/melts upon plugging it in


The ground pin could be hot via the living room circuit somehow, and the living room circuit is on the opposite phase, resulting in 240 volts between hot and ground. That would probably fry any electronics. If the living room is a 15A breaker, and the kitchen is a 20A breaker, the 15A would trip first.

That's just a guess though. Another possibility is that you have a the hot and neutral reversed somewhere. Have you checked anything with a meter? What happens?

Whatever it is, it's not correct, and not safe. Don't use any of the kitchen outlets where this occurred for anything without fixing it.

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Old 05-08-2009, 01:30 AM   #3
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Microwave ground prong sparks/melts upon plugging it in


No, I just realized, you've probably pierced the insulation of the living room hot conductor with a screw while mounting the bracket, which makes the bracket, and the case of the microwave hot. When you plug it in, the living room circuit shorts to ground via the microwave cord.

This could be confirmed if your voltage tester doesn't read voltage on the microwave chassis if you turn off the living room circuit.

EDIT: Therefore, it's very dangerous right now. Anybody who touches the microwave and something else metal could get electrocuted. This is the very reason that appliances are grounded--so if the chassis becomes energized somehow, a breaker will trip before someone gets electrocuted. Of course, it's not plugged in, so there is no short to ground that would trip the breaker, so that sucker's live. (And now we come to the reason GFCIs were invented, but I digress...)

Bottom line: Before you touch the microwave again, kill the living room breaker!

Last edited by thegonagle; 05-08-2009 at 02:48 AM.
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Old 05-08-2009, 01:35 AM   #4
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Microwave ground prong sparks/melts upon plugging it in


My first guess is you put a screw through the wire that runs the living room circuit. That's the only thing that really seems to make sense with what you have going on here, un-mount the microwave and try it again, I bet it'll work fine (provided the electronics aren't already fried).
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Old 05-08-2009, 01:35 AM   #5
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Microwave ground prong sparks/melts upon plugging it in


Call the contractor back tell him you have a problem. He probably had one of his guys move the outlet instead of an electrician, no telling what they did.
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Old 05-08-2009, 02:39 AM   #6
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Microwave ground prong sparks/melts upon plugging it in


Quote:
Originally Posted by microwaveman View Post
The contractor that fixed up our kitchen sheet rock & cabinets moved an outlet up from a lower position on the wall for this microwave/hood. When I hung the unit and attempted to plug it in, sparks flew and the ground prong blackened/melted a bit on the tip. At this time, one circuit breaker flipped, but not the one (assumed) that the microwave is hooked up to, but a 15 amp breaker that handles the living room and bedroom lights and outlets. Only the ground prong of the 3-prong plug actually made it into the receptacle when this happened.
Now what I underlined it that is a serious issue there the the microwave hood unit must be on it own circuit.

And you will have to get a hold of your contractor to come back and address this issue but get a licensed electrician!! that is the only way to slove the issue you got there now.

I know many pas legit contractors will do this in very cheap way and not realized the codes are there.

RenoDon have right words there.

Do not use that circuit at all until electrician check out the system very carefully.

Merci,Marc
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Old 05-08-2009, 06:22 AM   #7
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Microwave ground prong sparks/melts upon plugging it in


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I tried th short the suspect receptacle with a curve of wire and it also sparked, melted and shot firey wads of molten copper towards my face, but did not flip a breaker.
That will win you a Darwin Award!

Gary
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Old 05-08-2009, 08:25 AM   #8
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Microwave ground prong sparks/melts upon plugging it in


I appreciate the input. I will grab my trusty curved wire, pliers an a meter and see what happens. I will respond as soon as the issue is reassessed.
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Old 05-08-2009, 10:27 AM   #9
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Microwave ground prong sparks/melts upon plugging it in


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When I hung the unit and attempted to plug it in, sparks flew and the ground prong blackened/melted a bit on the tip.

High current in the ground.

one circuit breaker flipped, but not the one (assumed) that the microwave is hooked up to

Dunno'

Only the ground prong of the 3-prong plug actually made it into the receptacle when this happened.

Microwave shell is energized, or the shell is grounded and the ground is energized.

I tried plugging the new unit into a known, grounded, home run outlet with its own 15 amp with nothing else on it breaker. Same thing: sparks fly, ground prong melted a bit and the breaker for the living room flipped, but not the one I was plugged into.

see above

Blame the microwave, plug in the old microwave an use it for two weeks without incident

Dunno'

New microwave arrives, so I plug it in FIRST to the known, grounded by-itself home run outlet. No problem, no sparks; must be fine. Hang it, bolt it, plug it into the other outlet: SPARKS, MELTED PRONG, OTHER BREAKER FLIPS...just like before. Huh. Immediately I plug the microwave back into the by-itself outlet, and the same sparks and other breaker tripped. Huh.
Can you summarize what all combinations with what outlets you tried or didn't try? You seem to have a known good microwave and at least one known good outlet. It seems like your bolting process contacted a live wire.
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Old 05-08-2009, 10:46 AM   #10
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Microwave ground prong sparks/melts upon plugging it in


You could just build an isolation booth around the microwave. Put a raised wooded floor down, covered with a rubber mat. Build the booth big enough so it is impossible for someone touching the microwave to also reach a grounded surface. As a back up, hang some lineman's gloves by the entrance for use when operating the MW. Then just plug it in with a grounding adapter...
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Old 05-08-2009, 10:48 AM   #11
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Microwave ground prong sparks/melts upon plugging it in


two outlets: microwave over the range plug and the kitchen island plug. The island plug is known to be home run and on its own 15 amp breaker. It has the GD and DW hooked up to it.

Summary:

Mounted the new microwave, plugged into over range outlet= sparks
Then, unmounted microwave and plugged it into island outlet= sparks (possibly because microwave was already fried? )

2nd new microwave, unmounted, plugged into island plug first- okay
mounted it, plugged it into over range outlet- sparks. I did not try to plug it back into the island plug on account of the ground prong is now 20% melted off.

The idea of a screw to a wire is looking plausible. I have removed the mounting plate and bolts and I will be cutting sheetrock to look into this sometime this weekend.

The over range outlet is supposed to be its own homerun line, but I will have to wait to confirm that this weekend.

If the mircowave is "fried" could that cause it to spark even plugged into other outlets?

Last edited by microwaveman; 05-08-2009 at 10:51 AM.
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Old 05-08-2009, 11:08 AM   #12
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Microwave ground prong sparks/melts upon plugging it in


Quick Update:

I cleaned up the ground prong of the MW and plugged it into the island outlet. It comes on, beeps blows and starts cooking. I did not run it too long, but it seems to be functioning fine.

I grabbed an extension cord to plug it into the over range outlet. Seems to function fine there too. No sparks, no muss, no mounting plate screws.

Looks like the mounting bracket screw through wire is gaining speed here. I am going to cut the sheetrock away where the mount plate was screwed in to check for any damage. I am hoping this is the only culprit.

Thank you again for all your input. I will be back.
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Old 05-08-2009, 12:22 PM   #13
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Microwave ground prong sparks/melts upon plugging it in


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Looks like the mounting bracket screw through wire is gaining speed here.
Check it. Run a bulb or voltmeter from the screw to a known good ground.
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Old 05-08-2009, 02:17 PM   #14
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Microwave ground prong sparks/melts upon plugging it in


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Originally Posted by InPhase277 View Post
You could just build an isolation booth around the microwave. Put a raised wooded floor down, covered with a rubber mat. Build the booth big enough so it is impossible for someone touching the microwave to also reach a grounded surface. As a back up, hang some lineman's gloves by the entrance for use when operating the MW. Then just plug it in with a grounding adapter...
I would add to build a faraday cage around the unit to protect from the microwaves themselves.
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Old 05-08-2009, 03:33 PM   #15
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Microwave ground prong sparks/melts upon plugging it in


...I will not even begin to speculate where the conundrum in the wiring is. I'll leave that to the great analyzers on this forum. One thing, though, is certain. This is a classical case that proves certain things in life are best left to professionals. Now, not to discourage any future amateurs/do-it-yourselfers. If someone has sufficient knowledge and is mechanically inclined, they could, under the supervision of a professional electrician (after having read and understood the step-by-step instructions of a project book) perform the wiring of an apartment. There are so many things that could go wrong, and the lights may even work initially. But life-threatening hazards may exist in the wiring, which one day might cost life or property!!!

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