Not sure why the ground continuity of the conduit would be affected? If the continuity was affected the welder wouldn't heat up the pipe. No thaw setting although they do make dedicated pipe thawing equipment. Not sure why but older welders can handle this with no problem, newer ones burn up. We pulled the string out with ice still attached to the string so the conduit didn't get very hot. Thawing pipes in Maine like this is a very common thing. Thawing conduits like this isn't. I've only done it a couple of times in my 25 years.
