Abstract: The invention relates toa circuit breaker, preferably for high or medium voltage. The movable conact of each breaker pole is connected, via mechanical means for transmitting movement, to a movable part of an electric motor (6). The movable contact and the means for transmitting movement are arranged in a gas-tight apparatus chamber (31) surrounded by a gas-tight apparatus housing (9) of insulating material such as porcelain. In accordance with the invention the motor (6) is arranged entirely in the apparatus chamber (31). The invention also relates to an electric plant provided with the breaker claimed, uses of the breaker claimed and a method of breaking electric current in which the breaker claimed is used.