PURPOSE: To prevent a stress concentration of ventilation parts by providing an earthed first perforated plate composed of metallic material and a movable second perforated plate being close to said first perforated plate in the connecting part of a terminal box and a ventilation box.

CONSTITUTION: First, when a damper 24 is fitted to the center of a flange in the vent hole 20a with the central phase (V phase) of a ventilation box 20 so as not to be eccentric thereto, the ventilating hole 25 of said damper 24 coincides with the ventilating hole 22 of an arc shield plate 21. Therefore, the area of said ventilating holes through said damper 24 is maximum in this case. Next, when the damper 24 is moved horizontally, the area of the ventilating holes 22 of said arc shield plate 21 is reduced by the damper 24 according to its movement. Consequently, it is possible to regulate the air flow and flow velocity of an apparatus.