Microwave Mystery, fuse blows repeatedly

[dinofx]

When I moved into my house, the microwave was not working, and GE "repaired" it. About 2 years later, it stopped working again [tripped the breaker and blew fuse]. I replaced the 20 amp fuse in it (probably the original "repair" as well), two times over a few months. Someone told me the circuit breaker would eventually fail and I should fix the problem, which I assumed was the microwave. The 3rd time it just tripped the breaker, then the 4th time days later the fuse and breaker both blew/tripped again. Bought a new microwave, but had the same problem on the FIRST day.

What could be causing the microwave's 20 amp fast-blow fuse to break repeatedly, and trip the breaker? I can't imagine I have two defective microwaves. Is there a short somewhere in the wiring to my outlet? If the voltage sagged somehow (bad breaker?), would the microwave try to compensate by drawing too much current?

The outlet measures 122V, with the "live" wire being on the right side. The microwave is on a dedicated outlet, which is the only place the problem has occurred, and pretty much the only place we've used the microwave.

Thanks for your suggestions.

 

[dknguyen]

Wait, is it your microwave fuse that is blowing? Or your main breaker? I would think a short in your lines would cause a fire or something and not destroy your microwave...but then again, it's AC and not DC so I could be wrong.

If your voltage sags, it should cause the oven to draw less current. Unless some inductances are coming to work.

Are you sure your kitchen outlets (which should be wired differently than regular outlets around the house to handle higher current of small appliances like toasters) is actually wired properly? And not wired like the regular outlets?

 

[Nigel Goodwin]

I would suggest faulty house wiring?, spikes down the mains will blow the fuse in a microwave quite easily.

 

[oldtimer]

I would determine where the fault lies in the oven or in the wiring by hooking the oven up to another socket in the house or a genny if you have one.

 

[dinofx]

Did I mention I have two identical ovens? Both have blown their internal fuse. Unless it is a design flaw, it is unlikely that both are defective.

I have the original oven with a new fuse plugged into a separate outlet and circuit. My guess is it will work perfectly at this new location, but I really need to put it back inside the cabinet where it belongs. It is a tabletop microwave, but with the trim kit to mount inside a cabinet, which has the dedicated breaker and outlet inside it.

 

[dinofx]

If your voltage sags, it should cause the oven to draw less current.

If the microwave operates off DC, or has a variable step-up transformer in it, couldn't it draw more current?

I'll remove the outlet and check for voltage sag under the load of a 300W halogen lamp.

 

[moody07747]

I would suggest faulty house wiring?, spikes down the mains will blow the fuse in a microwave quite easily.

im also going to go with the faulty house wiring, go to the orange store (home depot) or lowes and get yourself one of those plug in outlet testers with the 3 lights on it.

see if your outlet is wired right, if not...call an electrician in to find and fix the problem.

this may be something in the breaker panel thats causing a problem.

my friend from welding had a sub panel put in his garage for a welder and a few other things. A dedicated 20A circuit was run to an outlet for a 120V 15A chop saw. Every time he used the saw his breaker would trip. He tested it with me here and it all checked out. I swapped out for a new breaker and the problem went away. The old breaker was about 3 months old but must have been made wrong.

also, try plugging the microwave into another kitchen outlet...most kitchen outlets should be 20A, its in the NEC...if the microwave works in the new location its the circuit with a problem, if the microwave tripps the breaker in the panel again get a different microwave.

you didnt say weather its the fuse in the microwave thats blowing or the one at the main panel where the power comes into the house.

 

[Nigel Goodwin]

MCB's are just too sensitive, we had to replace one in the shop at work with an old style fusewire holder - as it permanently tripped when we turned the power on - due to the surge of the display TV's as they powered up. An identical value fusewire has never failed.

 

[RODALCO]

Try to run it from an other powerpoint first and see if it re occurs.

You could have a neutral fault, and if your supply is 2 phase there may be times that the voltage is a lot higher than 120 Volts if the load over the house is unbalanced.

I'm not sure if you are an electrician, but if so, isolate the supply to the circuit concerned and do an insulation test on the cable from the main braker board to the socket. Should read 20 Mega ohms or better, as per local regulations.

Continuity test of cable to confirm that all is continiuous with a loop impedance tester, will show up if connections are crooked.

Get connections checked at the sockets and braker panel.

 

[Nigel Goodwin]

Get connections checked at the sockets and braker panel.

Yes, loose screws in plugs and sockets can cause mains spikes, these often blow the fuses in microwave ovens - basically because there's a mains filter immediately after the fuse - the filter absorbs the spike, causing a high current pulse to pass through (and often blow) the fuse.

 

[dinofx]

Usually, both the fuse AND the circuit breaker would both "trip" at the same time. This is what seems really strange to me. I would expect either one or the other to fail, but not both.

I'm going to replace the outlet first, as that is probably the cheapest made component.

 

[stevez]

I'd add my vote for "faulty wiring" that would include loose screws and poor wire to wire connections. I would go to every possible connection point from the breaker panel to the receptacle to verify that good contact is made. I'd remove the breaker to inspect the contact points to the main panel. Given the problems you've had one would expect to see some evidence of heating.

 

[dinofx]

OK, this is getting ridiculous. Two identical microwaves had the same problem on the same outlet (and thus breaker). I returned the second microwave, since it didn't solve the problem.

The original microwave just blew its fuse again (perhaps the 5th time), on a completely different outlet and circuit breaker in my home! So it wasn't the outlet, it wasn't the breaker, and apparently it wasn't the microwave either. So, what does that leave?

On what might be a related note, my old battery backup power supply (APC UPS 650VA) occasionally emits a single long buzzer noise. It can be while the computer is on or off. Could this noise indicate some kind of voltage spike? I assumed it just meant lead acid battery probably needed replacing, or it had decided to cycle the battery or something.

 

[RODALCO]

Have you done a Voltage check on the cct in question over a longer period of time ?
Ideally do it with a DMM and a Analogue Voltmeter at the same time as well, it may show you the variations in voltage a bit better than jumping digits on the DMM.
The Analogue Voltmeter imposes a bigger burden 20kΩ / Volts instead of MΩ / volts for a DMM.

Other option to try is, leave ppt with no load on it and put a plug in surge detector on it only. If that trips the braker by itself there may be a voltage problem.

 

[moody07747]

MCB's are just too sensitive, we had to replace one in the shop at work with an old style fusewire holder - as it permanently tripped when we turned the power on - due to the surge of the display TV's as they powered up. An identical value fusewire has never failed.

you could have tried a slow trip type breaker.

i know square D has them but im not sure about others.

 

[Hero999]

If your voltage sags, it should cause the oven to draw less current.

This isn't always the case, some loads like AC motors actually draw more current at lower voltages!

 

[Nigel Goodwin]

The original microwave just blew its fuse again (perhaps the 5th time), on a completely different outlet and circuit breaker in my home! So it wasn't the outlet, it wasn't the breaker, and apparently it wasn't the microwave either. So, what does that leave?

As we've been saying all along, it's a fault with the mains supply to the oven - either in your house somewhere (most likely) or external to it. You're almost certainly getting large mains spikes, the filter in the oven absorbs the spike, and in doing so draws enough current through the fuse to blow it. As previously suggested, this may be as simple as a loose screw on an outlet 'somewhere'.

Don't know about the USA?, but in the UK you can ask the electicity company to come and install a data recorder for a while to monitor the voltage.

 

[Shiin]

If you're experienceing trouble with Electric stuff in your house, in USA all you gotta do is call the eleectric company and tell them your house may be getting either insufficient or excessive amount of voltage\amps\current and your appliances are being minorly damaged by the issue.

Usualy its' free for them to come out and inspect your sources of problems; it only usualy costs something when they "repair" the issue.

We got ours inspect for free, we thought we had 160+ volts coming into the house, but turned out it was a faulty Meter we used - they did the test to make sure of course, didnt cost us a dime.



Microwaves, including TV's and other appliances that have a 'spin up' or 'warm up' thing before they reach operating standards draw higher current than when during normal operation.

I used to have that problem with my Bandsaw ... after using it for a while, if turned off and let it fully stop, it would 50% of the time blow the breaker when you turned it back on (from the current load it induced when 1st coming on) - other times it just made the lights dim, noticeably, for about a second till uit hit normal operating speed. then it could run all day long, constantly on, without a problem.

 

[HiTech]

Is the AC circuit breaker rated for 20 amps as well or is it the more common 15 amp?

 

[RODALCO]

As Nigel sais that in the UK they fit a datarecorder for a week or so. Our POCO's do the same in New Zealand,
I work for Siemens in the distribution side of the electricity and sometimes have to fit dataloggers in transformers and at customers premises when the guy who normally does it is sick or on leave.
I see is I can attach a graph later, have to get one from work first.

They have a power quality section which deals with customer complaints re Voltage, Current, Powerfactor , Harmonics etc.
You can have a plug in Voltage datalogger installed for a week usually for domestic sites, for free, which records Voltage sags, peaks, and spikes.

A very accurate graph can be drawn from it which can be expanded out to a very good resolution, to analyse the data.
This type of loggers are the "Osprey" Voltage recorders, think they are UK made and very good.