Abstract: Construction contractors were installing new underground chilled-water lines. The new lines were run in an excavation and had to pass through a concrete wall. A subcontractor was hired to bore holes through the concrete wall. Two employees working for the subcontractor were to bore two 203-millimeter-diameter holes in the 472-millimeter-thick concrete wall. The excavation for the water lines was next to the wall. A concrete slab was located in the trench, and a transformer was nearby. They were using a coring machine to bore the holes. To position the coring machine, one of the employees used a jackhammer to break off part of the concrete slab in the excavation. The jackhammer penetrated a 208-volt, three-phase underground cable embedded in the concrete and caused an electrical fault. The ensuing electric arc burned the employee who was not using the jackhammer. He was hospitalized with first-, second-, and possibly third-degree burns on both arms up to his armpits, on his left knee and thigh, and on his left waist by his stomach. The employer had not conducted a prejob survey.