A cabling unit for connecting electrical devices to power supply bus bars having a longitudinal cavity accessible through a longitudinal slot includes: a screw with a hammer head and a threaded shank provided with two lateral reliefs; a shaped casing configured to be received in the longitudinal cavity, in which there is identified a shaped housing configured to receive the hammer head, an elastic bottom and a through opening for the threaded shank. The elastic bottom counteracts the bottom of the longitudinal cavity and pushes the underhead surface of the hammer head against the inner wall of the longitudinal cavity opposite to the bottom and constrains the screw to the power supply bus bar when a rotation imparted to the screw arranges the hammer head transversely to the longitudinal slot. The shaped casing is provided with counteracting structure configured to cooperate with the lateral reliefs and prevent the counter-rotation of the screw when the hammer head is arranged transversely to the shaped casing and the longitudinal slot.