Class 1 - DC Power from battery banks
Class 2 - AC Power from UPS
Class 3 - AC Power from Emergency DGs
Class 4 - AC Power from the Grid and the Turbo Generator of the Power Plant. In the event of a major external incident such as an earthquake or a storm, class 4 power can be interrupted. The Class 3 power is supposed to be activated by starting the emergency DGs.
Class 1E refers to the power source which is essential for the safe shutdown of the reactor, isolation of the containment and removing the residual heat from the reactor in the event of a shutdown. The Class 1E bus can draw power from the Emergency DGs as well as the Grid.