Preplexed by Tripping AFCI

This is my own house and not directly inspection-related but potentially educational nonetheless:

I installed a wall safe on an interior wall - 16" O.C. studs, 2014 construction. The 3" lag screws to hold the safe go from inside into the studs (3" screws for a 1.5" stud - I bet you can see where this is going). Anyway, I blow out the back of the stud right through a 12-gauge piece of romex that is unfortunately stapled about 1/2" from where I hit it (so it doesn’t move at all). The screw and/or drill bit for the pilot hole go right between the neutral and ground wires with the neutral taking the vast majority of the impact (flat wire pack with black and white on each side, ground wrapped in paper in the middle.

So, the black/load wire is completely free of any damage, the ground has a small nick and the white/neutral loses about one-third of its diameter. Presumably, the screw was shorting (or bonding for us HI geeks) the neutral and ground. So, why did the AFCI at the panel trigger? I’m glad it did or I would have had no idea what I had done (as I only cut out a hole for the safe in one stud bay) but I didn’t think neutral to ground would cause this.

For troubleshooting I first swapped the AFCI with another and confirmed the problem in both directions (problem circuit tripped the new breaker, breaker from problem circuit operated on the other circuit). So, I cut open the next stud bay over to confirm what I was 99.99% sure I’d find. Pics attached.

Anyone with knowledge of the inner-workings of AFCIs I’m curious why it tripped with neutral to ground connection. IMG_5549

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I do not have your answer, but for clarification was it a AFCI GFCI combo?

Nope, straight AFCI - master bedroom outlet circuit - had my wife not been able to watch TV I may have never known the outlets were dead :slight_smile:

I found this in a blog

When you touch neutral to ground, two things are happening.

  • The “making” of that connection may have had some initial arcing
  • You now have a neutral-ground fault

Many AFCI breakers have a design that is sensitive to ground faults, so a ground fault may trip those AFCIs. They are not rated for GFCI, however.

Also, maybe a photo of the breaker and manufacturer info could help, then we could look up the manufacturers literature.

Interesting - it sounds like it would be more likely to trigger with ground/neutral connection on a circuit being used with some live current running through it as opposed to one just sitting idle. Prior to posting, I was trying to remember if/when I’ve seen improperly bonded sub panels with AFCIs. I have to think I’ve come across some. And, of course, the neutrals and grounds are all connected at the main panel, service entry, etc.

This is what really bends my mind. How could connecting neutral/ground at my master bedroom result in the AFCI tripping when it doesn’t at the panel just 20 feet away? I was fully expecting to find the screw driven right between the black and ground wires.

The breaker and panel are Eaton brand - pics attachec. IMG_5554

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That is a brain teaser. I look forward to seeing other comments. I am knee deep in late night report writing , so I cannot go down the rabbit hole at the moment.

From Eaton, similar to Brian’s post.

If there is a bare ground wire making contact with the
neutral conductor, the breaker will trip instantly as soon as a load exceeding 40 watts is applied to the circuit.

My guess is the bit caused a spark, or the ground and neutral to make contact, and the breaker tripped, as designed to.

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Great find… thanks! As I was working on it, the AFCI continued to trip as it sat with the lag screw inserted (so after the pilot hole was drilled, screw inserted and I thought I was finished). I agree and think it probably initially tripped when I drilled the pilot and pierced the cable.

I’m wondering what would have happed if this wasn’t an AFCI circuit? Would the screw just sit embedded in the cable until a short circuit tripped the breaker?.. or caused a fire? Basically, am I only alive to type this because I have AFCI breakers? (to be fair I also have smoke detectors :slight_smile: )

Installing a wall safe seems innocent enough but this is definitely a lesson… for a guy (at the risk of sounding cocky) that is pretty educated on these things.

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Wall safes are not to be taken for granted. When I installed mine, I measured my perfect rectangle. Cut it beautifully. But, when I removed the sheetrock., there was the wire for my light switch (opposite side of the wall) running directly across my opening. So, end result. Patch, sand and paint hole number one and start again.

AFCI breaker is an advanced detection device that breaks the circuit when it detects an unsafe electric arc. It reduces the risk of electrical fire. It is capable to selectively differentiate between an innocuous arc, a harmless arc caused incidentally or through normal operation, and a potentially dangerous one.

Because the AFCI circuitry is monitoring the wiring downstream of the device not what is upstream of the device.

This reminds me of hoping that I don’t hit a cable every time I install a bracket for a wall mounted TV. Drilling 1/4"holes and installing 3" lag bolts can always be a potential problem.


With what little I know about AFCIs they seem to be a frequency sensing circuit of some sort. Carrier current has long been used to impart an intelligent signal (one that carries information) on a standard AC waveform. AM (amplitude modulation) and FM (frequency modulation) are two common types of radio transmission. There are others such as FSK (frequency shift keying) and most of these are used in various electronic devices.

With this rudimentary information it should seem obvious that an AFCI senses and trips as the result of unwanted frequencies on the AC line. These can be created just as easily on the “neutral” conductor as the hot and would include shorts to “ground” as well.

Further reading indicates that AFCI nuisance tripping was an early problem as a result of shared neutrals that were intended (multiwire circuits) and shared neutrals that were simply improperly joined in a junction box.


That is a lot of info, Bob.

Thanks for posting that & the link. :+1:

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This photo looks like a black wire with a nick. Could you clarify?

The AFCI breaker can detect this situation easily. It’s sitting at the breaker box, and compares the current going out the hot with the return current coming back via the neutral. When you shorted neutral to ground, half the return current started going down the ground wire.

Likely this circuit had some load – maybe another outlet with a desk lamp or something? Or just the TV’s vampire load (the current it uses 24/7 so the remote control works).

Your solution for next time: build your home with Knob & Tube wiring. You’ll never have this problem. The wires will be safely spread apart.

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This leads me back to wondering why they work at all since they are in the panel and neutral is always bonded (or shorted) to ground there?

Definitely a white wire once I opened it up.

Also, it’s worth mentioning that the AFCI continued to trip after everything was unplugged from the circuit. It wasn’t until I opened up the wall and pulled the wire from the screw and opened it up that the breaker would reset.

Just note: neutral and ground should not be connected at the subpanel, only at the main panel. Could you confirm how yours is run?

Can you still test your system by shorting neutral and ground at a receptacle now that the wire is fixed, and seeing if the AFCI trips just on contact or continuously?

This is not how an AFCI operates. It is, however, a basic description of a GFCI. GFCIs detect current flow, AFCIs detect spurious frequency signals such as created by an unwanted arc (spark). Early radio made use of spark generators to create radio signals. The idea is very similar.

While it may not apply in the OP’s case, recent AFCI’s include the current comparing function of a GFCI, even if they are not combination AFCI/GFCI rated.

Only bonding is at the meter base on the side of the house w/disconnect. Inside panel is +/- 20 feet away and is wired as a sub w/grounds and neutrals isolated. I was mainly referring to how I see many panels wired - neutrals/grounds bonded, AFCIs in the same panel and operating.

I’m just not seeing how/why bonding neutrals and grounds can cause an AFCI to trigger when they are always bonded somewhere on the system. Quite often literally inches from the AFCI breaker.