A hammer drill comprising: a motor (20); a tool holder (6) capable of holding a cutting tool (8); a drive transmission, capable of operating in at least two modes of operation (14,16,18), which, when a cutting tool (8) is held by the tool holder, is capable of converting the drive output of the motor (20) into a rotary drive for the cutting tool (8) and/or repetitive impacts which are imparted tothe cutting tool (8) depending on the mode of operation of the drive transmission; a mode change mechanism which is capable of switching the drive transmission between the at least two modes of operation (14,16,18); a switch (22) which, when activated, provides power to the motor (20); and a mechanical lock on mechanism (26) which, when activated, mechanically locks the switch (22) in its activated state to maintain the power to the motor; wherein there is further provided: a first sensing apparatus (50) which indicates the mode of operation of drive transmission; a second sensing apparatus (36) which indicates whether the lock on mechanism (26) has been activated; and a controller (40) which is capable of interfering with the operation of the motor when power is provided to it by the switch and which monitors the signals from the first and second sensing apparatuses wherein, when the controller detects that the gear transmission is in at least one certain predetermined mode of operation and the lock on mechanism has been activated, it interferes with the operation of the motor (20).