THHN (Thermoplastic High Heat Nylon coated) uses a thinner layer of PVC (the Thermoplastic) insulation that other types of cable in order to make it thinner and take up less space. But the cost of that thinner insulation is an increased risk of deterioration and leakage if exposed to environmental contaminates. That's what the Nylon jacket is for (it's a common misconception that it's only there for pulling, that's a side benefit). So with the insulation exposed like that, it will not last as long as the intact ones, guaranteeing that this will be the first one to fail. How long that will take is totally dependent upon the level of contaminants, but don't be fooled by thinking there are NONE, that is never the case in the real world. Out gassing of other materials, INCLUDING PAINTS AND CLEANERS, all contribute at some level.
So like he said, re-wrapping it will help, but unless you use something as good or better than the nylon it was designed with, it's still going to be compromised. Regular splicing tape is NOT going to be as good or better. Scotch 2234 Cable Jacket Repair Tape is listed as compatible with THHN, but if the actual insulation is damaged too, you have to use 23 on that FIRST. Many heat shrink sleeves will also be as good or better, but you would have to remove the conductor from the terminal.
