Selective trip unit for a multipole circuit breaker

A trip unit for a multipole electrical circuit breaker comprising a selective magnetic trip unit per pole, said trip unit comprising:

• a U-shaped fixed magnetic armature located facing a movable magnetic armature with an air gap arranged between the two,
• a conductor associated to the fixed magnetic armature, through which conductor the current of the corresponding poles flows,
• an elastic part biasing the movable magnetic armature to a separated rest position corresponding to the maximum air gap,
• and time delay means cooperating with the movable magnetic armature to perform chronometric selectivity of protection,
• wherein the time delay means comprise a damping device fitted on the movable magnetic armature and designed to be in an inactive state up to an intermediate non-tripping position when the movable magnetic armature is attracted by polar parts of the fixed magnetic armature during a first travel, switching to the active state resulting from a deformation of the damping device during a second travel of the movable armature between the intermediate position and the final attraction position causing tripping of the circuit breaker.