Our city passed an ordinance prohibiting the use of welding equipment to thaw frozen underground pipes after a fire was caused by this method. To make matters worse the fire was not in the house with the frozen water line. The plumber hooked up the welding leads to a fire hydrant and to the water line in the house. There was a nonmetallic under ground fitting in this water line. The current went from the water line to the service neutral (via the grounding electrode conductor) into the second house via the grounded conductor and to the water line in the second house and then to the fire hydrant. The water pipe grounding electrode conductor in the second house was run directly on a floor joist and the excessive current produced enough heat to ignite this joist. Fortunately this fire was discovered early and there was only a small amount of damage.
Don
