What might cause frayed insulation inside a light switch box?

Question

I opened a 2-gang light switch-box on my wall (to determine if there happened to be a neutral inside), and I found this fraying/damage to the insulation on the wire pictured below. I couldn't tell for certain, but this wire may have been larger than the others in the box. Both light switches control fixtures on the same circuit.

What could have caused this type of damage inside a switch box? The grounding tab at the top of the switch looks like it might have been touching/rubbing against the wire, but it's hard to imagine that caused all of this.

How worrying is this, and what's the right way to repair something like this? I am a novice, and will probably end up calling an electrician here. I have temporarily put some electrical tape around the grounding tab at the top.

I am assuming that the white flecks on the black wire coming out of the wire nut is paint. The home was built in 1983 (in Oregon, USA), and other than minor issues like back-stabbed switch/receptacles and a few grounds that weren't attached, the work has looked solid so far.

This also might not be a true two-gang box, but instead two 1-gang boxes that were cut together? Note how the foam cover on the right was cut to size, and the gap between the left and right boxes where they meet in the middle.

Answer 1

The frayed insulation is what happens with older, cloth insulated wiring and is quite common (though not the best). What's very unusual is that it was used in your house which you claim was built in 1983. Cloth insulation was phased out in the 1960s, so it seems incongruous to find it in a house built in the 80s. Are you certain of the build date of the house?

This image shows what the cloth insulation looks like when it's easy to view:

^{Image Source: https://www.whittinspections.com/homeowner-resources/cloth-wiring/. No relationship or recommendation implied or intended.}

To know for sure, it would help to pull the switches out of the wall to get a better look at the wiring behind. That web page (just the first search result with a good pic), of course, recommends replacing all cloth wiring and getting a licensed electrician in to do the work. It's not a bad idea to replace that wiring (especially since the insulation is fraying), and, if you're comfortable, you can DIY it, but it will take a lot of effort.

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As far as the 2-gang box, yes, that's a proper 2-gang box that's still commonly sold today. The sides remove with two screws so you can build up an x-gang box as big as you need it to be. I built a 3-gang box out of these when I was first working on my house.

I do believe that they end up being fractionally narrower than a pre-built 2-gang box, which is probably why the foam insulation gasket had to be cut down. Of course, putting on a midi or large cover plate would have probably eliminated the need to cut the gasket.

Here's one example of one of these types of boxes currently for sale:

^{Image from HomeDepot.com, no endorsement of product or vendor implied or intended.}

These are usually used in "old work" where you need to install a box in a wall that doesn't currently have one. Note the "plaster ears" (the rectangles that stick out above & below the box opening). These are to prevent the box from going too far into the wall.