AFCI - Tripping at approximatly half loaded

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Missiles

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Omaha, NE
Hello,
I am having an issue with an AFCI that makes me scratch my head.
Was asked to looking at why the breaker trips when the bathroom light is turned on. House is less than a year old, circuit has worked fine until now. Verified it wasn't the switch then started checking joints and found nothing. What I did realize is that if I removed several of the lamps from the bath fixture (8 lamp "standard" vanity light), then I could turn the light on and the circuit would hold. So I verified there wasn't something on the circuit overloading it. All that was on was the bedroom fan/light and the bathroom light. When I add more load to the circuit via the closest light, tv, lamp, or another bulb in the bath light the AFCI trips.

So I am seeing the AFCI trip not when overloaded, or when any load is on but only when loaded to a certain point. (Haven't taken an amp meter to it yet to verify the exact amperage at when this happens)
Before purchasing a new $40 breaker I just removed the circuit from another AFCI breaker and attached this circuit to it. To see if it would trip that breaker (which has been working fine) and it did in fact trip that breaker. I have a brand new breaker that I am going to try but I am not convinenced that it will work.

Any thoughts on something I may be missing would be appreciated.
Thanks
 
What kind of light bulbs are in the fixtures? LEDs can be a problem. AFCIs will nuisance trip on electronic loads.

Swap out the breaker with a GFCI and see if you have a neutral to ground fault or bootleg neutral somewhere on the circuit. If the GFCI trips start opening boxes.
 
Check for loose wire at the fixture , the box , the switch.
Loose wire nut will do this.

Load that circuit up farther downstream and see what happens.
 
Lamps were incandescents, no electronics on the circuit (A tv but I unplugged that during troubleshooting).
I have checked almost every joint in the circuit (especially the switch and light).
I will try a GFI breaker and see if I can narrow it down to a N to G fault.
 
The series arc detection function of an AFCI breaker is not enabled until the circuit current crosses a set threshold, even if the breaker thinks it is seeing an arc signature. I believe that threshold is about 8 amps.

Your breaker could be constantly detecting a real or spurious arc signature waveform but not tripping on series arc detection until the current gets high enough.

Another possibility is that there is a high resistance parallel path to through ground paralleling the neutral wire somewhere in the circuit which is carrying a small fraction of the total circuit current. In that case the imbalance current will be proportional to the total circuit current and would cross the 6ma threshold at some value of the total current. This seems less likely to me, but knowing whether you are seeing an arc or a GF trip should tell you which is possible.
 
Missiles said:
House is less than a year old, circuit has worked fine until now.
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This type of statement always raises a red flag in my mind. It tells me that something happened, something was changed, something is new. I would probe the owner for information. When did the symptom first show up, and what happened earlier that day? Did they just install the fan/light, or did they just replace the one that came with the house? Did they hang a new photo on the bedroom wall (possibly hitting a wire inside the wall with a nail)? Did they move the bed to a different wall (thereby pinching the table lamp's cord that plugs into an outlet that you did not realize was on the same circuit)? These problems don't generally develop like the sand passing through a sand glass, with the symptom silently waiting for the right time to appear. They are generally caused by something that someone did. I think the worst case scenario would be if the thing that just happened was that they replaced all the light bulbs in the bathroom with higher wattage bulbs, in order to make the room brighter. That could mean that the cause of the arcing was in place from the beginning, and now there is enough current flowing to cross over the AFCI device's threshold.

Let me offer a caution to the homeowner. You might be able to make the problem stop happening (i.e., no more trips) by reducing the current (e.g., put in LED bulbs that draw less current). But that would be the wrong thing to do. If the AFCI is tripping, that means there is a real problem somewhere. That problem could start a fire, if it is not found and repaired. So your mission is not to stop the AFCI from doing its job, but rather to make the AFCI no longer have a need to do its job.
 
GoldDigger said:
The series arc detection function of an AFCI breaker is not enabled until the circuit current crosses a set threshold, even if the breaker thinks it is seeing an arc signature. I believe that threshold is about 8 amps.
Click to expand...

Some information says they can be detected "down to" 5 amps, whatever that means.

I would be looking for a series arc to start with.

I would start with the home run if that is possible and load that to the point of tripping or greater.

Charlie has a point when he suggest to ask the homeowner where they have been driving nails or cutting.
 
First intuition on this kind of situation is to look real close at a possibility of a light bulb somewhere on that circuit that may have a filament that has partially detached from its leads within the bulb or some arcing is beginning at the socket pin where the bulb tip seats. This would be a series type of arcing event that would get detected once the 5amp threashold is reached. (Seen it happen)

Then I would start asking questions.
 
Thanks for all the help, just wanted to inform all about the outcome.
Ended up fixing two houses in the same neighborhood that both had Arc Fault breakers tripping.
The first, the one I had originally posted about ended up being their cable company box. I checked all boxes/joints etc and found no issues until I started to plug everything back in. With the cable box plugged in the breaker would trip around 5 amps everytime (box on or off). Without it I ran the breaker up to 15 amps with no issues.
The second house I fixed was tripping the arc fault breaker with almost no load running. Breaker would hold as long as everything was off. I pulled all plugs and switches out to check the joints on that circuit, ensured wire nuts were tight and found nothing that looked obviously wrong. I carefully put all devices back in and the circuit worked fine. I still don't know which joint was causing the problem.
Definitely a lesson learned about how sensitive these arc fault breakers are.

Thanks again for all the help and advice.
 
Those 8 lamp vanities (like this: https://www.wayfair.com/Sunset-Ligh...3w67Am7sYgB3a7zG_ujHCeQD5GYQoGVxqVRoCsMzw_wcB) are very popular in hotels and lower end homes because they are cheap garbage fixtures.

It would not surprise me at all if there is an intermittent short in the light's internal wiring. Unwire it and try a different load at the round box wiring (temporarily install a receptacle there) and plug in a hairdryer. If it holds, the fixture is the culprit.

eta: re: the other loads tripping the AFCI, find the box where the home run comes into, check for loose wire nuts or backstab connection.
 
Use GE archys, they are a tad different , in fact our nema rep has informed us we can forgo the neutral pigtail if we'd like.....

~RJ~
 
Get some 220 or finer sandpaper. Sand the the terminal of all the light bulbs flat. In the socket, bend the spring terminal toward you. Then sand the contact point so it is clean. If that does not fix it, use a different brand of bulbs. Then, re-do all splices. If this does not solve the problem, megger all the wires.
 
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