in line circuit breakers with a combined overcurrent / short - circuit current tripping device , tripping should take place in a well defined manner in the case of an overcurrent and in the case of a short - circuit current . for this purpose , gaps need to be set precisely . if the housing is made from a cost - effective housing material such as thermosetting plastic it is subject to shrinkage . as a result , the mentioned gaps may change . an armature is mounted in such a way that it changes its rest rotary position in the event of shrinkage of the housing . a magnet yoke as part of the overcurrent / short - circuit current tripping device is mounted and shaped in such a way that the rotation is compensated for precisely , so that the mentioned gaps do not change despite the shrinkage .