| 摘要 |
PURPOSE:To exactly exercise control operations by a method in which a levitation preventor is provided for the inside of the receiving frame of a cover plate, the end of the covering plate is shaped into a face matching the preventor, and the preventor is not provided for the removing side of the cover plate of the receiving frame. CONSTITUTION:A levitation preventor 2 of an inversely triangular cross section is provided for the inside of the receiving frame 1 of a covering plate 3, and the end 9 of the plate 3 is shaped into a sloped face matching the shape of the preventor 2. When a locking mechanism 6 is provided, the preventor 2 is provided on the opposite side to the side where the locking mechanism 6 is attached. Even in the case of severally divided covering plate, only one locking mechanism is sufficient enough of one manhole or inspection port, while permitting control operation to be exactly performed.
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