On June 26, 2012, at 1415 Pacific daylight time, a Bell UH-1F, N4582D, was conducting long line external load operations near Concrete, Washington, when an electrical arc from an energized power line traveled down the long line and seriously injured a person on the ground. Salmon River Helicopters operated the helicopter under Title 14 Code of Federal Regulations, Part 133. The commercial pilot was not injured, and the helicopter was not damaged. Visual meteorological conditions prevailed, and a company flight plan was active. The helicopter was positioning a concrete bucket as an external load attached to a long line. At the end of the long line was a remote hook that was activated electrically from the cockpit. The helicopter maneuvered the concrete bucket attached to the long line to the work location, which placed the long line 15-20 feet from the energized power line. The energized electrical power line arced to the long line, and electrical energy was conducted down the remote hook circuit to the bucket and injured a construction worker who was ground handling the bucket. The worker's injuries involve burns and hospitalization. The operator reported that there were moderate rain showers in the vicinity. Visible moisture, rain, sleet, or snow, can facilitate an electrical arc from a hot power line.