On those little crackerbox A.C. only lincolns, that number on the dial that is circled- 75 or 80 I can't remember which- is where you are to set the machine to thaw frozen lines. That's why it was circled. It does sound bad flipping the switch to turn it on with a shorted connection, but it can take it. I've also used my tombstone lincoln AC/DC to jumpstart vehicles with dead batteries. Did it the same way- clamp the leads to the battery, set the amps to around 50, and flip the switch. Makes an arc, but it's not like you are doing it constantly. The engine fired right up and I flipped the switch back off, no worse for wear.
I built a welder out of a 4 cylinder I.H. engine and 4 chrysler alternators. Among other things, I have used it to thaw frozen water lines- since it is portable. It only puts out 12 volts D.C., but the amperage is adjustable up to over 200a. You aren't going to melt any soldered joints as long as water is in the pipe.
Pipes usually freeze in the fittings, f.y.i.
I'd put one lead on the pipe down in the water meter hole, and the other in the house, and wind it up and put the current wherever it sounded good.
Number one thing is good connections, you don't want to create an arc and burn a hole in the pipe- which could easily happen. Don't break the welding lead connections while the thing is under load. Use the on/off switch.
On edit~ years before I built mine, a friend built his chrysler alternator welder with 8 alternators and a slant 6 engine. It would produce over 400 amps (you add up the combined total output of each alt.).
Back when we had an ice plant here in town, the owner called him one subzero day and asked if he had any way to thaw a frozen 4" water line. It had been snowing, then sleeting, then below zero for a week. The plant had an exposed 4 inch line that wasn't insulated. It ran outside building for a ways before going underground and coming back up on the other side to enter the building.
My buddy took his welder over there, parked it on one end of the building and ran the leads from there to both sides of the building and clamped them to the pipe. Leads laying on top of the snow and ice. Started the welder. Set the current to W.F.O and let it cook. Can't recall how long he said it took to thaw the thing, but he said when it thawed out, his leads had melted down thru the ice cap and had frozen over again, trapping them. He had to start it up and go back and get them warmed enough he could get them unstuck.
That is the reason I built my alternator welder in the first place- seeing his and realizing how simple and powerful (and cheap) it is. 100% duty cycle and parts are basically available at any auto parts house. Power is only limited by the strength of your gasoline engine and the number of alternators you belt up to it.
I even made one with a 7.5 hp 3 phase motor once. Used it in the shop where I worked. The guys there were amazed how good it worked.