At 9:20 a.m., a truck carrying fuel and lubricants in the plant's 230 kV switchyard backed into a support column for the feeder line supplying power to the Unit 1-A reserve auxiliary transformer (RAT). At the time, the 1-B RAT was de-energized for maintenance and RAT 1-A was powering both trains of emergency electrical power. The non-emergency electrical trains were being powered by back-feeding from the switchyard through the main step-up transformer to the 1-A and 1-B unit auxiliary transformers (UAT). Additionally, emergency diesel generator (EDG) 1-B was out of service for planned maintenance. After the power loss, EDG 1-A failed to start due to a protective safety trip. The resulting loss of electrical power in the plant's "vital circuits" shut down the residual heat removal (RHR) pump that was cooling the core of Unit 1 (which was nearing the end of a refueling outage) and prevented the backup RHR from activating. Even though Unit 1 was offline at the time, residual heat from the natural decay of the radioactive fuel must be removed to prevent a dangerous rise in core temperature. While the non-safety power was not interrupted, there was no physical connection between the vital and non-vital electrical trains, preventing the vital trains from receiving power from the unaffected path through the UATs.