Abstract: A power line worker and his helper were troubleshooting a power outage. After diagnosing the problem as a burned-out fuse in a switch in an underground vault, the power line worker entered the vault to replace the fuse. He opened the cover on the switch; and, as he was replacing the fuse, an electrical fault occurred. The ensuing electric arc burned the employee. The employee used an access ladder to exit the 3.7-meter-deep vault before being transported to a hospital. He was hospitalized for 7 days with second- and third-degree burns to his face and arms. The worker had about 20 years' experience as a troubleman. He had been wearing gloves, Nomex clothing, and eye protection at the time of the accident. Another crew that had worked in the vault that same day had reversed the load-side and line-side cables.