PURPOSE: To separate an operation floor and the area below a generator by providing a boundary including a seal plate above a generator main circuit and a neutral point circuit insulated from the generator vibration-wise.

CONSTITUTION: A main circuit terminal box 7 and a neutral point terminal box 8 are directly supported by a generator foundation base 1 and insulated from a generator 2 vibration-wise. A generator main circuit 9 and a neutral point circuit 10 connected to them are also insulated from the generator 2 vibration-wise, and they are extended toward the area below the generator 2 respectively. An intermediate floor including a seal plate 11 provided at the vertical portion of both circuits is provided at the intermediate level between the operation floor and the downstairs of a turbine building to form a boundary 12. Outer covers of the circuits 9, 10 are formed into smooth circular arcs, and a high sealing property is obtained when the seal plate 11 is fitted and the whole periphery is welded. The boundary 12 can be surely sealed without being affected by the vibration of the generator 2, and the operation floor of the turbine building and the area below the generator 2 can be completely separated.