A multipolar auto-isolating circuit breaker in which each pole has a feed-through insulator containing a vacuum chamber and comprising at one end thereof a first contact connected to a first terminal of the chamber so as to co-operate with a first conductor. A second end of the insulator has a second contact connected to a second terminal of the chamber so as to co-operate with a second conductor. The feed-through insulators are rigidly connected to a single metal beam (4) containing a common control shaft (38) for the chambers (14) which is actuated by a mechanism housed in a box (5) which is rigidly fixed to the beam. Said circuit breaker may be used in electrical bays and medium voltage substations.