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This is what happened a few hours back while I was installing a shelf in the bathroom.

The shelf requires three screws, but according to the stud finder, the screw locations I wanted didn't align with the studs. So I decided to use drywall anchors(a self-drilling type anchor about 1.5" long https://www.homedepot.com/p/E-Z-Ancor-Twist-N-Lock-75-lbs-Medium-Duty-Drywall-Anchors-50-Pack-25310/100040265) and pre-drill holes for the them. One of the hole locations was somewhat close to a gfci outlet, so I did AC scan using the stud finder and it didn't report anything.

So I went ahead and drilled a hole. However, when I manually screwed the anchor with a screw driver about one inch into the drywall, it felt like something was hit, so I took it out and pushed in my screw driver into the hole. To my horror, it hit something like rubber band inside wall, and I am pretty sure that it is a wire from that outlet, likely running horizontally since there was no stud.

I didn't notice anything abnormal - no sparks, no light out or flickering, no breaker tripping - and the outlet is working.

When drilling I believe I stopped as soon as the drill went through the drywall but the drill bit was likely pushed in a little further. Also, the tip of the anchor (plastic) is sharp. How likely is it that I damaged the wire? Should I worry and really open up the wall to check?

I think the chance is pretty slim, but I am being cautious and also a complete novice when it comes to diy and electric stuff, so will need some advice. Thanks in advance.

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    what style of plastic anchor did you use?
    – Jasen
    Apr 4 '20 at 8:15
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    Can you rent an inspection camera? Apr 4 '20 at 14:07
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    It was a self-drilling type anchor about 1.5" long (homedepot.com/p/…). Apr 4 '20 at 17:51
  • This is a pretty good example of "when to be worried" diy.stackexchange.com/q/97883/18078 and based on what you have shared thus far, I would not be overly worried.
    – Ecnerwal
    Apr 5 '20 at 1:15
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I have seen many damaged wires inside walls when remodeling, some with the outside jacket and both the conductors and ground exposed. Some have even damaged the wires because of visible arcs I know the breaker was tripped. Others just skinned the wires or the jacket (this doesn’t include the wires I was called to repair where they were severed in the walls). I was quite surprised on a few that they continued to work. All of this said it takes quite a bit of friction to cut through a unsupported piece of Romex. It sounds like you were careful and if you cannot see any damage and did not trip the breaker or GFCI things are probably ok. I would check the light and fan as that could have been a switched cable and the reason it did not show as hot was it was turned off. Then we can think of all the calls where the wires were cut. I would not be worried about most plastic anchors except the self drilling type. Of course the only way to know for absolute sure is to open the wall or use a bore scope to inspect but my guess with no arcs and sparks , tripped breakers or getting shocked if everything still works it’s probably fine. The last quick thing is all of these anchors are usually shorter than 1-1/4” the romex is supposed to be in holes or stapled that far back , so it was probably flopping around making it harder to cut through the insulation.

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    I've seen so many Romex cables skinned bare (all wires exposed copper) on one side for several feet & still working perfectly well (wires still sepatared by the unchewed insulation between them.) Rodents. But that did lead to a personal decision to get good at bending EMT for my own building...
    – Ecnerwal
    Apr 4 '20 at 14:43
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    @EdBeal Thanks. Looks like it is more likely the anchor rather than pre-drilling that might have damaged the wire if it ever happened? It was actually a self-drilling anchor about 1.5" long (homedepot.com/p/…) and I indeed felt resistance when I screwed it in about 1-1/4". Maybe I did 2-3 rotations against the unsupported wire before I took the anchor out. Would that puncture the insulation and likely cause any issue? Everything on that circuit works fine. Apr 4 '20 at 18:09
  • Yes those anchors are what I was talking about but I never pre drill they are designed to do it themselves. they can be backed out the hole is large enough you should be able to see if more than the outer covering was damaged could it have damaged the inner insulation , possibly but I doubt it as the wire probably was just pushed back it may have skinned the outer cover but without copper showing and no breakers tripped everything working as I said it is probably ok.
    – Ed Beal
    Apr 4 '20 at 18:40
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It seems most unlikely that a plastic anchor with moderate pressure would damage the insulation on a wire.

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