apart in the blocked position by the lateral sides 30a 30b of the electric relay 30 the plugging thrust pushes the body 1 downwards so that the above mentioned clearance j which appeared above the panel 20 in fig2 is to be found below the panel 20 in fig3 thus the resilient lugs in the spread apart position rest on the face of the panel by way of their shoulders 15 forming a snap in jaw and are automatically blocked in this position by the electric relay when the latter is presented in the approach position after being placed in the approach position in fig3 the electric relay 30 is secured to the device embodying the invention via the panel 20 for this purpose on the securing lugs 31 bored with holes 33 and 34 are mounted screws 35 fitted with bearing washers made captive by tightening into sleeve tubes 36 forming distance sleeves and tapped at their upper ends these sleeve tubes 36 are either mounted on or integral with the securing lugs 31 and enable a standard relay to be adapted onto the base according to the invention the screws 35 are then screwed into the internal screw threads or tapped inserts 5 or 6 and tightened to a predetermined torque so as to firmly maintain the panel 20 between the relay 30 and the device embodying the invention more precisely between an end 3 or 4 and a sleeve tube forming a.