This is the most stable and reliable source of electricity for a nuclear power plant and is therefore the preferred source during normal operations or when the nuclear unit is shut down. In case of its unavailability, a portion of the high voltage power generated by the plant’s main generator is fed back through unit auxiliary transformers to supply all normal house loads.15 If both systems fail, the plant is equipped with onsite emergency back-up capacities, which constitute the third level of protection of the AC electrical system. These sources, such as diesel generators, batteries or gas turbines, can be operational within a few minutes and are designed to withstand external events considered in the design bases of the plant. The main connection is the one used for export of power from the NPP generator to the main grid.