A hammer drill, in particular a chipper comprises: a body having at least one support handle; an electric motor mounted within the body; an electric switch, capable of being switched on and off by a trigger button connected to it, to activate or deactivate respectively the electric motor, wherein the trigger button is moveable between two positions, a first position where the electric switch is off and a second position where the electric switch is on; a locking arm moveably mounted on the electric switch which, when the trigger button is located in its second position, is moveable between two positions, a first position where it is disengaged from the trigger button and a second position where it engages with the trigger button and holds the trigger button in its second position. When the locking arm is in its second position it causes the hammer to remain switch on even if the operator removes their fingers from the trigger button.