Abstract: On November 12, 2004, Employees #1 through #3 and a coworker were cleaning inside breaker panels in a motor control center at a chemical manufacturing facility. The energy source was a single-circuit, three-phase, 480 volt energy source; all the panel breakers, except the main circuit breaker, were de-energized. The four workers had been instructed to clean both inside and outside, and to tighten wiring connections inside all the panels except the main. While they were working, one of the employees apparently opened the energized main panel and his hand tool contacted two of the phases. This caused an electrical explosion and an arc flash. Employees #1 through #3 sustained serious burns from the intense heat of the explosion and were hospitalized. Five weeks after the accident, Employee #3 died at the hospital while undergoing treatment for his injuries.