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Class of Power Supplies in Nuclear Power Stations

In a nuclear power station, the sources of power are classified on the basis of reliability into four distinct classes.  Each of these four classes is, in turn,  provided with multiple redundant system to ensure maximum reliability.
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.