I mean, the simplest answer is to lay a new cable, and that is definitely what I am going to do - that’s not my question.
But this is a long run, and it would be neat if I could salvage some of that cable. How can I discover where the cable is damaged?
One stupid solution would be to halve the cable and crimp each end, and then test each new cable. Repeat iteratively. I would end up with a few broken cables and a bunch of tested cables, but they might be short.
How do the pro’s do this? (Short of throwing the whole thing away!)
Can’t you just use a cheap non-contact voltage tester to find where the cable is damaged? Just run the tester through the cable until it suddenly stop detecting any AC voltage, which is probably where the cable is broken. But if you’re talking about ethernet cable, then I have no idea.
I just learned about these things today when researching for a device to test that there’s no voltage present before installing a ceiling light. Such a great innovation since it works even through insulation, so no risk of getting shocked.