The present invention is an electrical switchgear unit in a case and including at least one stationary contact and at least one movable contact supported by a contact support shaft, an opening and/or closing mechanism of the movable contact(s) fitted in a support, the mechanism including a manual operating handle of the contact(s) rotatable around a toggle pin of the case, and a high-speed closing shaft designed to keep the movable contact(s) in the open position, at the beginning of manual closing of the handle and over a certain travel, by storing energy and to release the movable contact(s) after travel of the handle so that the stored energy causes high-speed closing of the contacts. On one of its side faces, the device includes the support of the mechanism which includes an opening to accommodate at least a part of the high-speed closing shaft, which is inserted laterally at least partially into the support prior to fixing of the shaft inside the case, and the means for fixing said support inside the case is located on one side of the mechanism.