Microwave blowout mystery.

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  #1  
Old 05-22-14, 09:14 AM
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Microwave blowout mystery.

I've been trying to find info on my issue, but it's so specific, I need to just post here.

We just moved into a condo built in the mid-1990s. Small kitchen with an electric range/oven with a separate microwave w/venting system and lights mounted above. 4-5 grounded outlets above the counters, plus one outlet in the cabinet above the microwave for its power.

The microwave is a cheapo Magic Chef model MCO165UW.

It was working fine when we moved in. But the other night, we were using maybe 2 of the burners on the range + the oven. We weren't using the microwave to cook, but we did have the vent fan going and the lights on. We then plugged in our Waring Toaster Oven (which we only use occasionally) into an outlet on the opposite side of the kitchen to toast some bread. Suddenly, the microwave's vent fan and lights shut off and the LCD display went haywire. And it just stayed like that until we finished with the toaster and unplugged it, at which point the microwave immediately came back to life. We just assumed the toaster was drawing too much power on that circuit and didn't think anything of it.

Then the next day we went to use the microwave...and it wouldn't heat up. It works fine otherwise (LCD, vent, lights, turntable all normal), it just doesn't cook.

So, it seems obvious that the magnetron or some other internal component has died. Which is fine...it's a cheap microwave. But we don't understand how or why. I've had circuits trip because of too much load before, but nothing like this. None of the circuits tripped when this happened. Not only that, but when we tried to determine which of the breakers controls the microwave's outlet...we couldn't! We've switched them all off in turn and the microwave never loses power. But if we unplug it, it does go out...so there's no battery backup or anything. Furthermore, one of the breakers DOES control the outlet that the toaster was plugged into when the microwave went berzerk that night.

So, a couple questions...

1. Is it possible that the outlet for the microwave doesn't have a breaker?
2. Regardless of #1, it appears that the microwave and the toaster weren't sharing a circuit when this happened. So why did the toaster have any effect on it at all?
3. If the toaster somehow was the culprit, is it possible that drawing power away from the microwave caused the magnetron (or whatever) to blow? I could understand a power surge killing it...but a power drop?

Our landlord is fine with us just replacing the microwave. But we need to make sure we don't blow the next one.

We'll call an electrician out if we need to. But we're wondering if this scenario seems obvious to anyone. Any ideas on what's going on here?

Thanks!
 
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  #2  
Old 05-22-14, 05:54 PM
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Most likely a problem developed in a neutral wire. This caused the voltage to seesaw with one circuit going well over 120 volts and another circuit going well under 120 volts. The overvoltage almost certainly burned out one or more appliances including your microwave oven.

Some appliances can also be damaged by significant undervoltage.

Unplug all electronics (includes microwave ovens) from the affected or suspected circuits. You can conduct tests using incandescent lights and hair dryers (and toasters), measuring the voltage at the receptacles with different combinations of equipment turned on. Do not use electronic devices for these tests.
 
  #3  
Old 05-22-14, 06:45 PM
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Welcome to the forums.

As Allan mentioned it sounds like a multi wire branch circuit with a neutral problem.

This kind of circuit use two hot wires on two separate breakers but a common neutral wire. Those two separate breakers need to be right next to each other in the panel. Each wire carries one leg of 120v each. If the neutral opens or or is not solid.... the circuit voltage can go way over 120vac.

The first check you can make is see if those two separate circuits/breakers are next to each other.
 
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Old 05-22-14, 07:11 PM
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Our landlord is fine with us just replacing the microwave
Sounds like a landlord problem to me, I'd call him and let him deal with it. Isn't that part of why you are paying the landlord rent?
 
  #5  
Old 05-23-14, 06:12 AM
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Sounds like a landlord problem to me, I'd call him and let him deal with it. Isn't that part of why you are paying the landlord rent?

Read more: http://www.doityourself.com/forum/el...#ixzz32XmcW7ul
Ha...touché. Our landlords live in Philly and we have a really great relationship with them. They're happy to take care of anything we need, but we enjoy fixing things and are just generally curious about this type of stuff. What I meant was that they've already agreed to just replace the unit outright, but we want to continue using the toaster oven and don't want to destroy another microwave.
 
  #6  
Old 05-23-14, 06:26 AM
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Thanks for all the replies. All very helpful. I wasn't aware that undervoltage could damage something like a microwave. Interesting indeed.

Regarding the neutral wire issue. Is this is "problem", or just a disadvantage of the particular wiring setup of our condo? Is this type of issue indicative of a shoddy job by the contractor?

Thanks!
 
  #7  
Old 05-23-14, 08:28 AM
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This kind of neutral wire issue is a problem, not a disadvantage. But some proper wiring methods (two hots sharing one neutral aka multiwire branch circuit) are more susceptible to this problem than other methods. This problem can exist in the main feed and affect the entire house, also. This issue does not point to a shoddy job by a contractor but a shoddy job can increase the chances of this neutral issue occurring.
 
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Old 05-23-14, 08:47 AM
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Ah, okay. That makes sense. I'm always curious how systems without moving parts like this develop issues in the first place. How does it happen? And if we opt to get it taken care of, how will an electrician typically address it?
 
  #9  
Old 05-23-14, 02:00 PM
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Every time you plug something in or unplug something, or if a heavy truck rumbles by on the street outside, there may be a tiny movement of the receptacle (or switch) and the wires.

Back stab connections, where you push the wire end into a hole and it supposedly stays in place without the need to tighten any screws, are more susceptible to developing a loose contact compared with screw terminals or screw tightened clamp terminals.

Unfortunately, some electricians will say you are paranoic if you tell them to redo all the connections to switches and receptacles using the screw terminals.
 
  #10  
Old 05-23-14, 02:03 PM
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Man I'm glad I posted here. This is all great stuff. I feel alot better about talking to my LL now and explaining why they should have an electrician come over.

Much appreciated.
 
  #11  
Old 05-23-14, 02:57 PM
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In the diagram below say your toaster oven was plugged into circuit 1 and the microwave was plugged into circuit 2. The toaster oven is upwards of up to 1500 watts. The microwave is maybe 20 watts at standby(not operating). Comparing the toaster oven to the microwave... the toaster oven would be considered an almost dead short. That means that you toaster oven saw a few volts of AC but your microwave saw almost 240v of AC.

Name:  MWBCD.jpg
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The neutral connection in a MWBC is critical.
 
  #12  
Old 05-23-14, 03:23 PM
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This diagram is extremely helpful. It's really making sense to me now. So perhaps it wasn't necessarily an undervolt that killed the microwave, but rather the toaster was pulling too many volts through the microwave?

EDIT: Also, why don't the breakers do their thing in this scenario?
 
  #13  
Old 05-23-14, 03:32 PM
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The breakers only trip with a short circuit..... there was no short circuit.

but rather the toaster was pulling too many volts through the microwave?
Technically yes..... something that is not supposed to happen.
 
  #14  
Old 05-24-14, 08:48 AM
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This is not easily explained to civilians . :-(

But as you have been told , most likely a bad / loose connection .

May be a lost neutral , as has been described . This can throw wild voltages to appliances . In theory , from 0 to 240 VAC . In practice , not that extreme , but bad . The device on one hot will experience greater than normal 120 VAC . The device on the other hot , less than 120 VAC .

Either condition can mess up what ever is being feed by those hots .

With a lost neutral , if one appliance would normally be pulling twice as much current as the appliance on the other hot , one appliance could experience twice the voltage as the other . And since the 2 appliances would effectively be connected in series , I do not think either would be drawing the proper current ( that they are suppose to ) .

In school , we set up a demonstration with a 60 watt lamp on one hot and an electric drill motor on the other hot . With the drill off , nothing worked . When the trigger of the variable speed drill was slowly depressed , the lamp gradually brightened to " normal " .

As the trigger of the drill was continued to be further depressed , the lamp got brighter . Carried to an extreme , one or both could be burned out . Probably , the lamp first .

God bless
Wyr
 
  #15  
Old 06-02-14, 09:19 AM
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Update for those of you interested...

Electrician just left. As it turns out, the microwave outlet in the upper cabinet was tapped off of the 15 amp circuit for the living room lighting and outlets. Which is why we couldn't find the breaker...we didn't think to test those not labeled as kitchen-related.

Anyway, our above-the-range microwave is fairly new. The owner had it installed about a year ago. Before that, there was just a hood vent and tenants used a countertop microwave. The contractor must have just hoped to use the same power source (which was fine for a simple hood vent), so he just put an outlet on there.

So, part of the mystery solved. And they're coming back over to install a dedicated 20 amp circuit. But when I told the electrician about how the toaster caused the microwave to go screwy that night, he said nah. They don't even share a neutral. Just a coincidence. But it's not. I was there and I know for certain it had something to do with it. But he did say that when they take the breaker panel off, they might find something that could cause what all of you were describing, regardless of the issue above.

Now that we know what circuit it was on, does this mean that all my electronics in the living room are at risk, too? They're on surge protectors, of course. But they never tripped. Scary to think I could've lost thousands of dollars that night.
 
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