systems could be powered by diesel generators. Each unit's transformer is therefore connected to two 6 kV main power line switchboards, A and B (e.g., 7A, 7B, 8A, 8B for generators 7 and 8), powering principal essential systems and connected to even another transformer at 4 kV, which is backed up twice (4 kV reserve busbar). The 7A, 7B, and 8B boards are also connected to the three essential power lines (for the coolant pumps), each also having its own diesel generator. In case of a coolant circuit failure with simultaneous loss of external power, the essential power can be supplied by the spinning down turbogenerators for about 45 to 50 seconds, during which time the diesel generators should start up. The generators were started automatically within 15 seconds at loss of off-site power. Turbo generators Electrical energy was generated by a pair of 2x500 MW hydrogen-cooled turbo generators per unit. These are located in the -long machine hall, adjacent to the reactor building. The turbines—the venerable five-cylinder K-500-65/3000—are supplied by the Kharkiv turbine plant; the electrical generators are the TBB-500. The turbine and the generator rotors are mounted on the same shaft; the combined weight of the rotors is almost and their speed is 3,000 revolutions per minute. The turbo generator is long and its total weight is . The coolant flow for each turbine is 82,880 t/h. The generator produces 20 kV 50 Hz AC power. The generator's stator is cooled by water while its rotor is cooled by hydrogen. The hydrogen for the generators is manufactured on-site by electrolysis. The design and reliability of the turbines earned them the State Prize of Ukraine for 1979. The Kharkiv turbine plant later developed a new version of the turbine, K-500-65/3000-2, in an attempt to reduce use of valuable metal. The Chernobyl plant was equipped with both types of turbines; block 4 had the newer ones. The newer turbines, however, turned out to be more sensitive to their operating parameters, and their bearings had frequent problems with vibrations. Reactor fleet The construction of two partially completed reactors, No. 5 and 6, were suspended immediately after the accident at reactor No. 4, and eventually cancelled in 1989. Reactors No. 1 and 3 continued to operate after the disaster. Reactor No. 2 was permanently shut down in 1991 after a fire broke out due to a faulty switch in a turbine. Reactors