PURPOSE: To facilitate the examination and maintenance of a sensor while a switchboard is in operation by a method wherein the sensor is provided near an examination cover.

CONSTITUTION: A sensor 3 is provided near an examination cover 4 so as to be examined from the outside of the cubicle of a switchboard 6. The examination cover 4 can be turned 180° to a direction perpendicular to the cubicle of the switchboard 6 with a hinge 5 as the center of turning. Further, a protective cover 7 is provided between the sensor 3 and the live parts of the switchboard 6 to prevent the sensor 3 from falling into the switchboard 6. As a number of holes are provided in the protective cover 7, smoke and air containing smoke produced in the board 6 can pass through the protective cover 7 and reach the sensor 3. With this constitution, the sensor 3 can be examined and maintained safely and easily.