The present invention relates to an electrical switchgear unit housed in a case and comprising 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, said mechanism comprising a manual operating handle of the contact(s) fitted rotating around a toggle pin of the case, and a shaft called high-speed closing shaft designed to keep the movable contact(s) in the open position, at the beginning of the manual closing operation of the handle and over a certain travel of the latter, by storing energy and to release the movable contact(s) after this travel of the handle so that the stored energy causes high-speed closing of the contacts. This device is characterized in that, on one of its side faces, the above-mentioned support of the mechanism comprises an opening designed to accommodate at least a part of said high-speed closing shaft, the latter being thereby able to be inserted laterally at least partially into said support prior to fixing of said shaft inside the case, and that the means for fixing said support inside the case are located on one side of mechanism only with respect to the high-speed closing shaft.