Plugged fastening device with snap-action locking for control and/or signalling units

A snap-action fastening device for control and signalling units, comprising an operative unit including an elongated stem having a thread thereon and being insertable into a switchboard from an external front side thereof;

• a threaded ring nut provided at a rear side of the switchboard and threaded on said stem for fastening said operative unit to the switchboard;
• said stem including an end portion;
• and a flange unit receiving said end portion of said stem and including means for receiving an engaging part of a member containing at least one of the control and signalling units, said end portion of said stem being hollow and being provided with at least one longitudinal slot having at a side thereof a bevelled protrusion merging into a recess extending transversely to an axis of elongation of said stem, said flange unit including a flange body, an externally operated ring-shaped lever inserted in said flange body for rotation about said axis of elongation and having a central bore, and means provided in said flange body for biasing said lever in a direction of rotation, said lever including at least one tooth provided at said central bore thereof and inwardly extending therefrom and entering said longitudinal slot upon assembling of said flange unit to said stem of said operative unit, said tooth engaging said bevelled protrusion causing said lever to rotate such that said lever overcomes a force of said biasing means, said tooth being engaged in said recess as said tooth passes over said protrusion and remaining in said recess to ensure fastening of said flange unit and said operative unit, wherein said tooth becomes disengaged from said recess by externally applied rotation of said lever when desired.