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Why does it trip the breaker every time I start it up?

Hi all, my microwave would trip the breaker every time I try to start it. Here's what I've done so far:

I've been using this microwave for around a year without any issues. This started happening in the past few days. The breaker is a 20A breaker, which I think is shared between the fridge, stove, and microwave. Again, this worked for about a year without issues. The microwave alone pulls 12A.

I used my multimeter's continuity test, and confirmed that both thermostats and all 3 door switches are working. The only thing that didn't pass the continuity test is the lamp unit (Light Bulb). The lamp unit has 66 ohms of resistance. However, the light turns on every time I open the door. The internal fuse also has continuity, here's a picture of it.

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While keeping the fridge and stove operating, I also plugged a kettle into the same outlet, which didn't trip the breaker. The kettle draws around 9A.

I tried finding parts diagrams or wiring schematics, but can't find any. If anyone got any insight on this, that'd be greatly appreciated!

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I tried plugging the microwave in another outlet in another room. Immediately tripped another breaker.

I suppose that's a clear sign of the house wiring in good shape, microwave not working.

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Hi @jacksonmuir

Just verifying that the primary and secondary interlock door switches (both N.O. types) test open circuit at normal (door open) and short circuit when operated (door closed) and that the monitor switch (N.C. type) tests short circuit at normal (door open) and open circuit when operated by the door latches (door closed)?

Were the fridge and stove both operating when you started the microwave and it tripped the breaker?

Were they both operating when you tested the breaker supply circuit with a kettle and it didn't trip?

If they were both operating with the oven and with the kettle then it could be the breaker or breaker supply circuit. There may be something else being supplied by the breaker and the oven draws enough to overload and trip it but the kettle doesn't or it could be a faulty breaker.

If they both weren't both operating with either oven or kettle connected, just wondering if there is a problem in the microwave, that could cause the breaker to trip, why it doesn't blow the supply fuse in the oven itself. Most microwave ovens have an internal 20A fuse on the input, mounted near the power cord connections in the oven.

Is the breaker a GFCI type or a normal type of breaker?

If the breaker is a GFCI type perhaps it could be a problem with the earth to the oven or something is shorting to earth (wire framing to chassis?) in the HV section of the oven and when the oven is started it trips the breaker.

Just some thoughts.

Update (01/13/26)

@jacksonmuir

Very strange that it doesn't blow the fuse in the microwave though.

Did you check for a short between the L1 (active) and the chassis with the door closed?

Here's a link that will help to find out what's wrong. You may have to substitute "fuse" for breaker in this instance as it's very unusual for it not to blow if the current is sufficient to trip the house breaker unless the breakers are the GFCI type

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Hey @jayeff,

All three switches work as you described.

The fridge and stove were both operating when I started the microwave. This tripped the breaker. And they were also both operating when I turned on the kettle, which didn't trip the breaker. To be clear, the fridge and stove never operated with BOTH the microwave AND the kettle. I just SUBSTITUTED the microwave with the kettle, which combined with the fridge & stove didn't trip the breaker.

The microwave's internal fuse passed the continuity test.

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@jacksonmuir

If the breaker trips when the fridge, stove and microwave were operating, but not when the fridge, stove and kettle were operating to me it seems like the breaker is being overloaded.

It may be functioning at the top end of its rating and the microwave is just enough to go over the rating.

Does the breaker trip if you turn on the microwave with the fridge operating but not the stove?

You may have to check what else is on the circuit that the breaker feeds.

If there is an internal problem in the microwave then the microwave internal fuse should have blown. Did you check its rating when you tested it (usually stamped on the end caps)? If it's >20A maybe that is why it didn't blow before the breaker tripped.

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Could it be the breaker getting old? Because for a year straight, I used the microwave while both the fridge and stove are running (Like when cooking), and there was never an issue.

I checked the internal fuse, and I can't make out any amperage stamping on it. But there's this chart next to it with multiple amperages, and the 20A box is checked. I guess that shows it's a 20A fuse, and it hasn't blown since it has continuity.

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@jacksonmuir

Maybe check how many outlets are connected to the breaker.

There may be something else has been connected to it that is also on and you've forgotten about it.

Easiest way is to use a hair dryer and then with the breaker turned off is to plug the hair dryer into all the outlets and check which ones work and which don't.

Also check any other electrical equipment that may not be working e.g. pumps, air con etc etc when the breaker is tripped as they may be permanently wired in

This of course assumes that there's more than 1 breaker for the power outlets in the premises.

Where I am the first breaker is only allowed to supply 11 outlets and the 2nd, 14 outlets for a standard 3-4 bedroom house. This is when it is first built of course. If more outlets are needed later on then more breakers have to be installed.

Also stoves are usually wired in and require separate dedicated breakers. (unless they're single phase working which is more and more rare and not two phase work which is more usual)

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Generally, when a MW trips the breaker it is the monitor switch. However, you said it trips when you try to start it. That would not be any switches. A bad monitor switch or worn mechanism affecting the monitor switch will trip the breaker when you close the door. In your case, I would check the next most common failure item - the HV diode. If it is shorted, that might do it. The capacitor might also be shorted, but those rarely fail. Other than those parts, it could be the transformer or Mag. Those items rarely fail also, but if they do, they are likely uneconomical to fix.

Obviously, anything that is across the AC line, could be suspect, like the fan or maybe the turntable motor. Since it is likely a short, it should be easy to check with your MM once you get access to the part.

If it is tripping when you close the door, you can test the monitor switch by unplugging it and putting your MM on the plug prongs and open/closing the door. If you get a short, it is the mechanism because you said you tested the switches.

It seems like a 20A breaker is quicker than a fuse with a dead short.

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Thanks for the insights @bill!

How can I test the HV diode, the fan, turntable motor, etc.? Should I keep using the continuity test?

Closing the microwave's door doesn't trip the breaker. Only attempting to start it trips the breaker, so the mechanism is most likely ok. But how can I confirm that? How can I tell there's a "short" when opening/closing the door?

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