A shredder including a housing having a throat for receiving articles to be shredded, a shredder mechanism, a blocking member, and an electrically powered motor and cutter elements. A thickness detector comprises a contact member extending into the throat. The shredder includes an actuator for moving the blocking member between a retracted position and an extended position. The actuator and the thickness detector are coupled to enable the actuator to move the blocking member from the retracted position to the extended position responsive to the movement of the contact member by insertion into the throat of the at least one article above a predetermined maximum thickness threshold. The blocking member is configured such that in the extended position the blocking member prevents further insertion of the at least one article into the throat, and in the retracted position the blocking member permits further insertion thereof into the throat.