Abstract: An electrician was troubleshooting a problem in a motor control center in a high rise building. A heating, ventilation, and air conditioning condenser water pump had tripped a circuit breaker. The employee apparently determined that the 480-volt terminal block containing the starter for the pump had shorted conductors and repaired it. As the employee tightened the lugs on the front of the terminal block in the process of reinstalling it, an electrical fault occurred. The ensuing electric arc burned him and knocked him backwards, unconscious, to the floor. It was believed that the cause of the fault was either that prongs at the rear of the block were not properly aligned and touched each other when tightened or that something behind fell onto the prongs.