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I have a brand new Bosch Microwave (model: hmd8451uc) installed as the only device on the entire circuit. Anytime I use the Microwave on this circuit, it triggers the breaker's ARC Fault. Breaker (Murray MP-GAT2 20AMP).

I bought one of these guys (https://www.amazon.com/dp/B07PMDRHXB) and pushed it into the socket where the microwave is plugged in, and everything passed. I was also able to test the ARC Fault mechanism, which seemed to work just fine.

After some research, I found the following Q&A Why does my microwave trip my AFCI only when it's mostly empty? So I tried the method that was described adding a full glass of water, it improves the problem slightly, (the microwave will last longer on), but sometimes it'll still trigger the breaker.

Seems like some people in that Q&A above had luck switching out the breaker to a different model or brand. Everyone in that thread had Siemens + Panasonic, I have Murray + Bosch so I'm wondering if my issue would be fixed by switching out the breaker.

Seem like the afci breaker you have right now affected by the microwave frequency ,try to replace the afci breaker.

But before I spend more money on switching the breaker, I want to understand how a certain "Microwave Frequency" can trigger an Arc Fault and if there is any safety issues with ignoring this issue and just switching a breaker to a different brand? Also, it's unclear from the previous Q&A, is this specific breaker faulty or is the breaker brand not meshing with my microwave.

I've been eying the Leviton Smart Load Center, to just replace my entire panel, which I don't mind doing, but want to be sure it'll fix this issue in the process.


Edit: image of breaker:

Breaker Image

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    Which lights on the breaker are turning on? Also, if you put a big microwave-safe container of water in the microwave and run it, do you still get a trip, and if so, how long does it take to trip? – ThreePhaseEel Feb 7 at 21:20
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    @ThreePhaseEel I put a 32 Oz bowl of water in the microwave, tripped after 4 seconds when setting it to 3 minutes. – Arian F Feb 7 at 22:40
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    Microwaves (the traditional type with a transformer and a voltage doubler before the magnetron) are impressively bad as an AC load. In a sense, they really imitate an arc. The current is asymmetric, full of harmonics and the peaks may be well outside the 20A breaker tolerance. It is even worse if one tries to start it empty. – fraxinus Feb 7 at 23:24
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    You said that "I have a brand new Bosch Microwave (model: hmd8451uc) installed as the only device on the entire circuit." BUT can you confirm that the circuit is an uninterrupted single run from the panel directly to the microwave outlet? If there are any junctions at all along the way then you will need to ensure that none of those junctions are arcing. – MonkeyZeus Feb 8 at 14:35
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    BTW, people in the thread you read having Siemens vs you having Murray is actually not a difference -- Siemens bought out Murray years back, and at this point both lines of breakers are exactly the same, just with different stickers on them. – Nate S. Feb 8 at 17:20
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Dealing with the problem now:

One of the first things to try is just swapping the breaker with an identical one in the same panel. In my experience, there's a lot of individual variation in the exact behavior of both AFCI breakers and appliances, that sometimes you can get the problem to go away just by using a different combination of individual components. It's harder to swap the microwave (typically), but you can also try exchanging the oven at the store to see if a different one (even if the same make and model) would work okay.

That said, the only real solution to this problem is to have the breaker replaced by the manufacturer. It is almost certain that you have no actual arc fault, and that the breaker is being tripped unnecessarily.

You can also report the problem here: https://www.afcisafety.org/home-owners/unwanted-tripping/

I have been struggling with the same issue, for several years now, in a newly-constructed home where AFCI breakers were required on almost every circuit due to relatively new code requirements. It has not helped that the electrician who did all the work for the construction shows little to no interest in doing anything to investigate, never mind fix the problem. But the one thing that has, for the most part, helped is to replace the breakers. It seems the manufacturer (Square D in my case) updates the AFCI logic in the breaker over time, to reduce the number of false positives.

Since the latest attempt by the electrician to deal with the problem by replacing the most problematic breakers, while they introduced a whole new problem (breakers tripping randomly even when there's not even any load on the circuit), almost all of the appliance-related tripping has been eliminated.

(The one exception is that now we have a breaker for the furnace that trips the instant the furnace blower motor turns on, but only if the utility power is out and we're running on the generator instead. As you can see, it's a bit like playing whack-a-mole.)

You or your electrician need to work with the manufacturer to report the problem, telling them the exact make and model of appliance that is causing the problem, and get updated breakers that will work. If possible, that is…this is hit or miss under the best of circumstances, and it requires that the manufacturer (and any electrician that's involved) are willing to be cooperative and support the breaker and installation.

Why does the microwave trip the breaker?

As far as how the microwave can cause the tripping, it's simply due to the switching of electricity that occurs in the appliance. Motors with brushes are among the worst (fortunately, more and more appliances are being designed with DC brushless motors), but other things like plasma screen TVs (which use tiny arcs to generate the plasma discharge that lights the pixels), higher-end toaster ovens (we have a Breville that modulates the heating element intensity in a way that can trip the AFCI), and yes, microwave ovens, can all have similar problems.

In the case of the microwave oven, it will depend on the individual design. But ovens have motors (which are a possible problem, depending on the motor), for the carousel, ventilation, and the "stirrer fan", and the switching on and off of the magnetron can also create signals that would fool the breaker.

I can't tell you exactly what component in the microwave is causing the false tripping, but it's not surprising that something is. And because of the chaotic nature of the AFCI behavior, you may or may not get the exact same behavior from a different individual breaker or oven, even if the same make and model, never mind if you tried a different manufacturer for either.

Why bother using the web form to report the problem?

Note that by reporting the problem on the form linked above, you can at least give the manufacturer a chance to investigate and update the design of their breaker for that particular scenario. It doesn't necessarily help your specific issue right now, but it could improve the situation for you and everyone else in the long run.

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  • Thank you Peter! This was super informative! I reported the issue to AFCI Safety – Arian F Feb 7 at 23:01
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    @ArianF -- good idea, a lot of mystery AFCI trips never get followed up on with the proper instrumentation – ThreePhaseEel Feb 7 at 23:19
  • Next case: arc breaker vs arc welder. – Joshua Feb 8 at 16:46
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    To my surprise, I just received an email from Siemens Customer Care Center, it looks like ACFI Safety forwarded my issue to them! Filling out the ACFI Safety form definitely seems like the way to go. – Arian F Feb 8 at 17:26
  • @ArianF: I'm actually not too surprised. Enough people mad about Arc breakers, enough people pulling them from their own houses when they can't get things working and the electrical code will fall. – Joshua Feb 8 at 18:09
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Don't ignore the possibility that this may be an actual fault in the appliance, or at least that the manufacturer may accept responsibility.

I have had a similar issue with two different appliances (dishwasher and coffee machine) and in both cases the manufacturers agreed to treat it as a warranty issue. In one case they did a repair, and in the other they did a series of replacements (and ultimately an upgrade to a model that didn't have the problem, because every identical replacement did have it).

Regardless of the cause, these manufacturers seemed to accept the notion that they can't sell equipment which is incompatible with code compliant and increasingly common arc fault protected circuits.

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Just got off the phone with both Leviton and Siemen! Thank you to Peter Duniho for having me file that AFCI Safety form, was super helpful in me getting in touch with Siemen!

I'll put everything I've learned here so everyone else could potentially benefit from this.

What can be the issue?

False Positive: Some appliances have the tendency to produce a sine wave that mimic/triggers the AFCI breaker's detection algorithms. Newer breaker are updated with newer detection algorithms.

Real Arc Faults: Danger! https://en.wikipedia.org/wiki/Arc_fault

How to test if it's an actual Arc Fault vs a False Positive

Was told by my the Siemen AFCI technician, I should plug in a hair dryer to test this. It's a good test appliance because it's portable and it doesn't internally create much noise (sine wave) going back into the breaker. So if running a hair dryer triggers the Arc Fault in the breaker, you likely have a real arc fault (as long as your hair dryer itself is working correctly). Siemen AFCI tech mentioned that you need to pull about 2 or 3 Amps before the Arc Fault detection actually turns on and starts detecting, which is why they recommend using a hair dryer as a test. They recommend keeping the hair dryer running for 3-5 minutes to generate sufficient load to be certain.

@StayOnTarget brings up a good point, this test will only test between the socket and the breaker. The ARC could still be between the socket and appliance, either in the appliance cord or appliance itself.

Plugged in my hair dryer and no AFCI fault was tripped.

To fix a false positive

If you think it could be a false positive, there are a few different approaches you can take. I'll list them below, I'll order them by easiest/cheapest first.

  • Buy a Filter (you can diy) $
  • Swap existing AFCI breakers you have installed (requires electrician or a savvy DIYer) $$
  • Replace the breaker to a newer version (requires electrician) $$
  • Upgrade the Load Center to a Smart one (requires electrician) $$$$

Install an EMI/RFI filter between your appliance and the socket

This solution is the cheapest and fastest solution, and doesn't require an electrician to come out. The only downside is you need about an inch of space between your appliance and the outlet, which may not always be possible with all appliance like washer/dryers.

It works by sending all the noise (sine waves) that the appliance generates into the ground wire. The one that Siemen Tech recommended was:

Minuteman MMS110 Surge Protector

MMS 110 MMS 110 description

I've tried this and it works well so far haven't had any breaker trips.

Swap the breaker from a different circuit (matching the AMPs)

The Siemen tech didn't recommend this in my case since all my other ACFI breakers were not tripping. But mentioned that different combinations of breakers for each circuits could magically work. As Peter mentioned, this is a lot like wack-a-mole, and not something I personally want to do... So not going to go into this solution.

Update the circuit breaker

The Siemen tech mentioned that each AFCI breaker has a specification that it needs to function by, so there is no warranty coverage applicable here. The breaker is functioning to its manufacturing specs. And they are constantly releasing updated version.

My breaker version is 2b. The versions go 2b, 3a, 3b, and 3c. Each version has been updated with new sine wave detection algorithms to work with more appliances, having less false positives.

And when you purchase from amazon you won't know version version you are getting. So you'd need to go to like Home Depot and see the breaker in person, here is how to identify which version you have (which is not documented anywhere lol):

  • 2b - No label - 2018 and Pre 2018 era
  • 3a - 2 Green Vertical Bars with Black Line between bars.  2019 era
  • 3b - White label with Black letter N and Down Arrow.  2019/2020 era
  • 3c - Black label with White letter N and Down Arrow.  2020/2021 era

Here is version 3b, see if you can identify the version with the above info:

Siemen Version 3b

I absolutely hate that this is even an expected solution for AFCI false positives. How can they expect that when ever you update an appliance, you may also need to update your breaker to be compatible with it?! How is this a sustainable solution? It'll make Siemen a lot of money, but having to call an electrician just to install a new microwave or TV sounds like a flaw in the system!

Get a Leviton Smart Load Center

This is probably the most expensive thing to do, have an electrician come out and replace your entire Load Center. But there are some long term benefits to this solution! (I'll probably do this in a couple more months)

I just spoke with a Leviton Smart Load Center engineer. I put a request in on there website to ask if their Leviton Smart Load Centers would be affected by my Microwave. And somehow my request went to a Leviton engineer, so I had a chat with him about this issue.

He stated that they didn't know about my exact microwave, but something interesting is that Leviton Smart Load Center is able to receive firmware updates to address new false positives for AFCI breakers. And that they are continuously monitoring for all false positives within all homes that have the Smart Load Centers. This allows them to continuously push firmware updated into their breakers without any intervention from the customer. So you get the latest version without having to replace the breaker! Now that sounds like a pretty neat solution to this AFCI nonsense!

After seeing the conversation between Peter and Nate in the comments for whose responsible for these issues (the appliance manufacturer or the AFCI Specs)... It's kind of refreshing that Leviton solution does work long term and is customer friendly. Siemens approach was to buy a 3rd party filter and if that doesn't work, to upgrade your breaker.. I still can't get over how they expect you to continuously buy an upgrade breaker to have the latest firmware for sine wave detection. At least with Leviton, that is done automatically since the breakers are connected to WiFi. Another benefits with Leviton is it's able to detect the difference with a series and parallel arc fault. And you can monitor your electricity for each circuit.

Leviton SLC description here Leviton Smart Load Center

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    In the how to test section... doesn't that just eliminate the wiring as a cause of the fault? How does swapping the item that is plugged in mean that the appliance was not at fault? Couldn't it have potentially dangerous arcing internally, or with its cord? – StayOnTarget Feb 8 at 21:03
  • Thanks for this follow-up...having any first-hand account like this adds a ton to the state of things from a consumer perspective. A couple of notes: 1) the device you're describing as a "filter" isn't actually a filter per se, but rather a surge protector; in my research I did find that some surge protectors include filtering capability that can prevent false-positive AFCI trips, but not all will...it's important if following this strategy to make sure the surge protector being used does. I would prefer that the industry develop a line of products specifically intended for ... – Peter Duniho Feb 8 at 21:12
  • ... filtering, so that it's easier to find the right product. 2) in the case of the Siemens/Murry breakers, is the only way to determine the revision by matching the physical features as you describe? it's hard to tell from the photo, but it looks like there's nothing on the label that describes which revision breaker it is...that seems like an especially poor approach on the manufacturer's part, considering that getting "the latest" revision is apparently a big part of their approach to "fixing" the problem. – Peter Duniho Feb 8 at 21:12
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    @StayOnTarget that's true and a good point, the arc problem could be with the appliance itself. But if you want to test the wiring from the socket to the load center, that's the method he recommended. I'll update my answer to make that clear. – Arian F Feb 8 at 21:21
  • @PeterDuniho to your first point, I've updated the answer to include a picture description that the Siemens tech provided of the product he recommended. Clearly states it's a EMI/RFI Frequency Filter Attenuation at 150KHz-100MHz – Arian F Feb 8 at 21:30

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