Abstract: At 8:24 a.m. on June 18, 2003, Employee #1, a sprinkler fitter with a fire sprinkler installation company, was working at an office building undergoing renovation in El Segundo, CA. He was installing an attachment for a sprinkler water pipe to hang from the bottom of a 12 in. wide by 2 in. deep metal cable tray that was 8 ft. above the roof. The tray was designed to carry and protect electrical wires and contained a 480 volt conductor. Employee #1 discussed with his supervisor how to attach the pipe hanger. Initially, they were not sure whether to use "TEK" screws and a flange attached to the bottom of the tray, or a "Unistrut" attached to a tray support. After the discussion, Employee #1 and his supervisor went to the supervisor's truck and got the materials for both methods. The supervisor then left the site. Employee #1 returned inside and decided to use the "TEK" screw method. He was using a portable electric drill to make holes in the bottom of the tray when he drilled into the energized 480 volt line, resulting in an arc blast. Employee #1 suffered second- and third-degree burns on his hands, arms, and face that required skin grafts. He was hospitalized for between four and five weeks.