Abstract: A physical science technician and a geologist working for the US Department of Agriculture's Geological Service were at a grade stabilization site making core samples. They were assisted by three employees of a drilling contractor. A Failing truck-mounted, hydraulic core drilling rig (Model No. 1500) with a 25-foot mast was being used to drill the cores. A 7200-volt overhead power line crossed the drilling site. The employees had been working in the same area for the past 3 days, and the rig had passed beneath the power line numerous times. The mast had not been in the fully raised position, however. A helper was directing the drilling rig operator as he was backing the rig to a new drilling location. This time, the mast was in the fully raised position; and, as the rig was moving, the mast contacted a 7200-volt overhead power line. The phase conductor of the power line burned in two, and one section of the conductor fell onto the helper. He was electrocuted.