whats the risk? was doing some crawl space replacement of K&T circuit, as it was the type k's every 8 feet on bottom of joists and beams, evidence of rat infestation and needing some heat ducts replaced, tried to correct polarity on natural k&t ins. but circuit blew when I hooked up to breaker so there is a short down stream of black to earth, or possibly combined returns in k&t system works ok as originally connected....tried arc fault it wouldnt stay on....been like that for 80 years? Is it worth the needle in a hay stack isolating of the feeds up to find the short? Chris

Might be cost effective just to replace it with current Article 300 wiring method.

having pulled out miles of K&T you are much better off just replacing the circuit than trying to repair.

my thoughts too but getting access and moving antiques to upgrade is the question of level of importance, safety? chris

Sometimes K&T had a switched neutral. You need to check before making too many changes to the system.

I agree. They were really fast and loose with keeping the pairs separate in K&T and if it is working you are usually better leaving it alone until you are ready to rip it out.
My bet is you have a borrowed neutral somewhere that is giving you your bolted fault when you "fixed" the polarity problem. That will also give you a parallel neutral path that trips AFCI/GFCIs.

switched neutral is possibly why the white wire was hot. but just reversing the polarity shouldnt trip the breaker unless there is a short to ground downstream of the panel. Is there a tool that will locate a short, Ie inspector 2 or 3 and measure leakage? this house had some remuddling done before I arrived and just added new circuits as needed. otherwise my knowledge would tell me that somewhere there has to be a grounded box with the return wire bonded to it to make a dead short when polarity was switched, or would a possible old three wire k t circuit trip a breaker if one phase was swapped with the neut.? previous owners had new K&T wires pulled to old fpe panel for new kitchen remodel, prior to 1990, and a new 200amp panel for new remodel circuits, but kept the K&T, I've bypassed the fpe but trying to ring out the old circuits has been trying.

thanks greg the borrowed neut. is possibly it, I just dont want to have floating voltage on a screw plate or water pipe but if it was bonded downstream you would only see that if you lost the neutral/return path, I am trying to talk them into ripping it out, but that may be a year away...chris

I removed a K&T circuit that had the neutral looped back to the panel so both ends were connectewd to the neutral buss.