The insulator, includes a lower bell shaped portion, an intermediate annular waist portion and an upper saddle portion for receiving and supporting an electric transmission overhead cable. An axial blind bore opens at its lower end for receiving an upstanding support pin. The saddle portion includes a transversely-curved groove, substantially normal to the axial bore, and jutting parts on each side of the cable receiving groove and laterally protruding from the waist portion. Each jutting part has an external face which is provide with a partly annular groove, generally coaxial with the bore and of greater radius than that of the waist portion. These annular grooves are adapted to positively retain the inturned flanges of a cable retaining clasp despite an upward force or a laterally upwardly directed force exerted by the cable on the clasp. The waist portion can still be used for attaching the electric wire by a tie-wire. The clasp includes male/female joints at each of its two downwardly curved arms that automatically interlock with each other when a sufficient biasing force is applied thereon by a pneumatic tool deforming the clasp arms, drawing the registering flanges of the latter toward one another to lock the insulator to the overhead electric cable.