The Nuclear Power Plant Unit Top-level Control System databases are very complex. They are developed and modified as at the stage of design, and maintenance. At the maintenance stage the databases development is more convenient to spend out directly on the site. Exactly for these works the data preparation system was created. With the aid of this system some security and safety assurance measures are provided: unauthorized access protection, limitations of possibility for change making, obligatory verifying of made changes and support the private channel of data transfer from development to system operation.