Abstract: A support box (60) for supporting functional elements within the frame (28) of an electrical connection enclosure comprises at least one pair (70) of rails (72) for guiding a monitoring/control plug-in unit forming a functional element in translation, a parallelepipedal framework (62) and members for fastening the framework to the frame. The framework comprises two horizontal plates, a vertical back that is perpendicular to the horizontal plates, and two vertical walls, each supporting a rail of each pair of rails. The members for fastening the framework to the frame comprise fastening lugs that are elastically deformable and integral with the horizontal plates and elastically deformable load absorbing lugs that are integral with the vertical walls. The support box comprises at least one centring pin, vertically centring the box with respect to a post (32) of the frame, each centring pin being arranged in a positioning hole of a fastening lug.