the turbine generator consists of a series of steam turbines interconnected to each other and a generator on a common shaft. there is usually a high-pressure turbine at one end, followed by an intermediate-pressure turbine, and finally one, two, or three low-pressure turbines, and the generator. as steam moves through the system and loses pressure and thermal energy, it expands in volume, requiring increasing diameter and longer blades at each succeeding stage to extract the remaining energy. the entire rotating mass may be over 200 metric tons and long. it is so heavy that it must be kept turning slowly even when shut down (at 3 rpm) so that the shaft will not bow even slightly and become unbalanced. this is so important that it is one of only six functions of blackout emergency power batteries on site. (the other five being emergency lighting, communication, station alarms, generator hydrogen seal system, and turbogenerator lube oil.)