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Intermittent Arc Fault Troubleshooting

779 Views 9 Replies 3 Participants Last post by  seharper
I live an a home in the US that was built in 2008 and has AFCI in the breaker box for the bedrooms. One of the bedrooms shares an AFCI with our bonus room and this circuit intermittently trips. I have had an electrician friend come out to replace the AFCI in the box and do a bit of troubleshooting but the problem still persists.

The faults almost always occur when something new is plugged in and turned on or when one of the switches is toggled in the bedroom. After the AFCI replacement I suspected that it was one of the switches in the bedroom so I replaced all three still to no avail.

My wife recently relocated her office to our bonus room and is not happy about losing power at inopportune times. Can I get suggestions on what I might try next in an attempt to remedy this?

Thanks in advance!
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It's exactly what it says on the tin.

You have an arc fault somewhere.

An arc fault is either a) two wires/things that are *supposed* to connect well, not connecting so well (series arc fault)... or b) two wires that are not supposed to connect occasionally arcing between them (parallel arc fault). Generally you're up against the first one, but the second is always a possibility. Usually the second will trip a GFCI.

You could actually *hear* arc faults if you could listen to the signal on the wire. If you've ever hooked up speakers or plugged in headphones with the power on, and heard sort of a crinkle-crunch sound, that's what arc faults sound like. That is what the AF breaker is listening for.

A switch can be a candidate for this sort of thing, because a hesistant throw of the switch (especially off) can cause power to arc arcoss the gap.

When they invented arc-fault breakers, the concept was to protect bedrooms from electric blanket failures, appliance cord damage from furniture being mashed into them, that kind of thing. But what actually happened was they revealed wiring faults in the walls (virtually all at wire ends):

- Top of the hit parade is the backstab connection. That's where the wire jabs into a hole, which auto-grabs it. Never use those.

- Second is weak wire-nutting. A lot of people find their wire-nut work tends to "fall apart" unless they tape the nut to the wires. That is a poor connection, and causes arc faults. They misunderstand how wire-nuts work, and their technique is quite bad, and they should fix their technique. The proof of the pudding is a pull-test: Hold the nut in one hand and pull hard on each wire in turn. Pull hard enough that you can't hold onto the nut anymore. If the wire comes out, it was a poor enough connection to cause arc faults. Tape over wirenuts is a yellow-flag, because it's how Johnny Limp-Wrist covers his bad technique. But there are legit uses for tape, especially if the nut is capping a single wire.

- Third is limp-wristed under-torquing of screw terminals on receps, switches, neutral bar or breaker.

Almost any splice method can be done wrong, except for backstabs, which make their own "wrong".

You should also evaluate socket connections. An appliance whose plug fits poorly into a socket can cause arc faults, that usually means the socket is worn out.

Obviously the original purpose of AFCIs, worn/damaged cords, is valid too.

And appliances *can* have arc faults internally, or they can falsely trip AFCIs. So if you have an appliance which "cycles", such as a dehumidifier on the circuit, that's a place to look.

Generally you're up against *series* arc faults, where things that are supposed to connect aren't doing so reliably. Here's the takeaway: series arc-faults only happen when current is flowing. So your electric blanket, which is OFF, cannot be a source of a series arc fault. If power comes from the breaker to receptacle 1 2 3 etc., and nothing is plugged into receptacles 4, 5 and 6, then a series arc fault there is highly improbable.

Oh, and there's one more wrinkle to this odyssey. Remember how I said that you can "Hear" arc faults on the wires? Well, AFCIs can "hear" arc faults *on other circuits*. So something "loudly" arc-faulting on another circuit absolutely can trip this AFCI. That generally only happens with circuits on the same pole. Most panels have poles laid out like this. There are other layouts, but you can't get AFCI breakers for those.
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Thank you so much for the extensive and humorous reply to my inquiry.

I can confirm that the backstab technique was used in the wall switches that I replaced and can bet that the wall sockets use them as well.

Since I will need to pull all of the sockets from the wall to check for loose connections, would you recommend that I 'convert' any backstabs that I find to screw terminals? If so, is there a good method to ensure that I apply enough torque to the screw to ensure proper connection?

Thanks again!!
Many of us convert every backstab we see.

As a matter of fact, the best way to assure proper torque is now a requirement in NEC 2017, a torque screwdriver. Basically a torque wrench (that'd be fine too) in a screwdriver form-factor.
I went around both rooms that hooked up to that arc breaker and sure enough all 12 outlets used backstabs. I cut them all out and attached the wires to the side terminals. I didn't use the torque screwdriver but got the screws as tight as I could.

So far we haven't had a trip yet. I hope that was the end of it.

Thanks again!
I would probably slightly loosen the clamps on the boxes too. If one ios too tight it may trip the AFCI breaker.

Hi John, thanks for the additional suggestion. Could you elaborate on what you mean be claps on the boxes or point me to a page / pic that shows what these are? :smile:
In Canada we tend to use metal boxes more then plastic, so often we find that (especially apprentices who like to use a battery driver for everything) will over tighten the metal clamps inside the box

The same situation at the panel if you are using a metal connector that has some sort of clamping arrangement such as a
or one of theses

The connector should be tight enough to secure the wire, but not so tight that it crushes it or changes it's shape.

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Thank you for the reply John. My house has plastic boxes.
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In Canada we tend to use metal boxes more then plastic, so often we find that (especially apprentices who like to use a battery driver for everything) will over tighten the metal clamps inside the box
Confiscate that. No one should be allowed near a power tool until they have mastered the hand tool.

That's a huge problem with "kids these days", they need to cut one 2x4 so off they go to the box store for a $70 circular saw, and then put the blade into their skull because they have no earthly clue how a saw blade reacts to a knot. They don't even own a $10 hand saw, that product makes no sense to their brains.
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