Arc welding machines have been used effectively for thawing frozen metal water pipes, either underground or through the basement walls of a building. Leads from a welding machine are attached to the pipe on each side of the frozen section and the current is turned on until the ice begins to melt and the water pressure pushes the slush ice out of the pipe. One lead may be connected to a pipe inside the basement and the other lead to a shut-off valve outside by the curb.
If an entire main is frozen, cables may be connected to fire plugs on each end of the frozen section. C-clamps can be used to connect the ends of the cables to the pipe. It is critical that the connections to the pipe are tight and that no overheating takes place. These connections should be checked frequently while the current is flowing. The welding machine and cables should also be checked and adjusted to avoid over-loading.
Copper and brass pipes have less electrical resistance than iron or steel and require higher current and longer times to thaw. Currents up to 500 amperes may be required depending on the pipe diameter. Particular care should be used with lead pipe to
prevent melting the lead or joints; current of 75 amperes may be adequate for many cases, and should not exceed 150 amperes.
Details of specific instances of pipe thawing may be obtained from manufacturers of arc welding equipment.