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Community and Q&A

AFCI Breaker Protection

KYLE WINSTON BENTLEY | Posted in Mechanicals on

Discovered this little gem while replacing a receptacle in a bedroom. Is this the type of fault an AFCI breaker is supposed to protect against, in the case of a short?

Drywallers got a little loose with the rotozip I think.

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  1. maine_tyler | | #1

    Not my area of expertise, and I'm sure Bill will chine in with all the goods, but i think it depends on the typenof fault. An exposed wore is not itself a fault. That wire could still produce any of the 3 fault types: short if it touched a neutral; ground if it touched something metal that's grounded; or arc if it comes close to any of the above but sparks instead of making solid contact.
    Acr fault protection will protect against not just exposed wires making a short, but loose connections that create arcs (sparks). Which I believe is why they can nuisance trip with certain devices that have arcing characteristics inherent to them.
    All that said, an afci would certainly add another level of protection to that situation that a normal breaker or gfci wouldn't.

    I've seen the drywalling rotozip get outta hand as well and its frustrating. Move fast, not carefully, is probably most drywallers mantra.

  2. Expert Member
    BILL WICHERS | | #2

    The short answer is "maybe", and you need to remove the nicked wire and fix it, but I'll explain:

    AFCIs protect against what I explain to customers as "sizzle sizzle" faults. Those are faults where you get little sparks THAT KEEP GOING. Those are actually pretty rare in the wiring itself, and are more likely to occur with something downstream of the hardwire system. The classic example is a loose receptacle for a vacuum cleaner, that sizzles a bit as it wiggles around in that loose receptacle as you use the vacuum. An AFCI should trip for a loose plug that is "sizzle arcing" in the contact area.

    The nicked wire you have is likely to either short to ground (note that contacting the ground wire makes a "short" just like if it contacts the neutral, a "ground fault" can be more complex, which I'll explain later), or short to neutral. The most likely scenario is that nicked area either shorts to ground as the device is pushed into the box for final installation, in which case that breaker will trip on OVERLOAD (not arcing) immediately when the circuit is first energized. The next possible scenario is that the wire will break at that weak spot when the wires bend as the device is pushed into the box for final installation. Either option is a Bad Thing. If you have some slack to pull into that box, the ideal "fix" here is to use that slack, then strip the jacket back to get fresh wire. Use the bit of damaged wire you cut off to whack your drywaller so that he's more careful next time :-)

    If you don't have any slack to work with, cut the wire at the nick so that the two ends are square, then strip the wire and put a wire nut there to "fix" the nicked spot. You do NOT want to leave a nicked wire like that, and tape alone isn't enough to fix this since you can see that the copper conductor has been nicked, and thus weakened, too. That weak spot is likely to break at some point even if it doesn't fail right away, and those time bomb problems are something you really want to avoid. I'd eyeball the other electrical boxes to see if there are any other issues. Chances are you're right, and the drywaller nicked the wire with the router, probably because the wires weren't pushed all the way back into the box prior to drywalling.

    A "ground fault" is, technically, ANYTHING that makes current flow to ground, but a direct connection is usually called a "short to ground", even though it is still technically a "ground fault". Ground faults that GFCIs see do include shorts to ground, but regular breakers will trip on shorts to ground too. What makes a GFCI special is that they they will trip on VERY SMALL amounts of current leaking to ground (typically 5 milliamps -- 5 thousandths of an amp), with the thinking being that the GFCI will now trip on "ground faults" that include small current leak paths that go through a person, such as someone using a hair dryer in a bathtub (please don't try that, even with a GFCI though!). That low current sensing of ground currents is what makes a GFCI "special".

    BTW, the way a GFCI can "fake" a ground is that a GFCI doesn't actually look at ground current at all: a GFCI makes sure the current going out on the hot is equal to the current returning back through the neutral. If those currents differ, the GFCI assumes the extra current is leaking off to ground somewhere, assumes a ground fault is present, and trips. "Grounds" faked by a GFCI on a two wire circuit can thus work for safety (although I recommend rewiring with a "real" ground), but they CANNOT function for FAULT CURRENT, so they won't work with devices like surge protectors, which need to "shunt" (transfer) current to ground to keep it from going through something else.


  3. Expert Member

    Thanks guys. It was more thought provoking than anything - it wasn't tripping any breakers, or causing any faults, I just wanted to know if that's the type of injury an AFCI would protect against, but it sounds like it's more intended for lose connections than something like that, which would short to ground using the regular old breaker systems.

    I did replace that section of wire Bill. I had enough to trim it off and put a wire nut on there.

    1. Expert Member
      BILL WICHERS | | #4

      What I've been told is that the specific issue AFCIs are intended to protect against is cords and plugs smashed behind furniture. There were some specific instances of this kind of thing, along with many daisy chained plugstrips, causing some fires in some sub-sub-sub-sub-subletted apartments. I've heard of AFCIs protecting against rat-chewed wires, and also overdriven cable staples that have smashed the wire.

      I would still try to do things safely though, and not rely on the AFCIs to cover for other problems. Don't daisy chain 20 plug strips together. Don't smash cords and plugs behind furniture (it's easy to use little pieces of scrap wood to make spacers to keep furniture from getting pushed too close to walls). I prefer the plastic cable "staples" that have two nails on the sides instead of the formed metal cable staples, since the plastic staples with nails prevent overdriven cable staple problems, and they're also easier to install in tight spaces. Rat protection is different issue, but if you have severe problems with critters chewing your cables, run MC cable or EMT conduit everywhere, either of which provides metal armor for the wire inside.

      Surprisingly enough, the only time myself or any of my electrical crews have actually seen the "sizzle sizzle" type of fault an AFCI is intended to protect against is on 277v commercial lightning circuits, and AFCIs aren't required for those (I don't think 277v AFCIs are even available). Most electricians and electrical engineers, myself included, consider AFCIs to be a solution in search of a problem, but they are currently required by code in many areas of residential structures.


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