A foreman and his crew were tasked to carryout a weld repair on a new pipe line (not live). The foreman and crew went to the wrong location and started repair work without a permit to work by grinding the weld joint of a live line. On realizing it was the wrong line the crew were instructed to weld the cut area. While the welding was ongoing, the riggers came and informed the foreman that this line is live. All activities were stopped and the crew demobilized from the location.