A power table saw having: a table top surface with an opening through which a blade can extend, the blade being driven by a drive motor supported by a carriage relative to the top surface, a mains switch selectively actuatable by an operator to provide power to the drive motor, a riving knife slidably supported on the carriage to selectively position the riving knife in an extended position in which the riving knife extends through the opening adjacent the blade and a retracted position in which the riving knife is retracted below the table top surface, a blade guard removably mounted to the riving knife in a position above the blade, a sensible element mounted to the riving knife; a first sensor arranged in the power table saw to detect the sensible element when the riving knife is in the retracted position, and to generate a signal when the sensible element is adjacent the first sensor; a second sensor arranged on the power table saw and configured to detect the presence of the blade guard and to generate a signal in response to the absence or improper positioning of the blade guard, characterised in that said second sensor is an optical sensor configured to optically sense the presence or absence of the blade guard, and wherein the optical sensor is configured to direct an optical beam through the opening in the table top surface to shine on the blade guard and to receive the reflected optical beam if the blade guard is detected, and a controller operable to disengage power to the drive motor when the mains switch is actuated, wherein the controller is operable to determine a state of the first sensor and a state of the second sensor and to disengage power to the drive motor when the mains switch is actuated when the state of the first sensor and the state of the second sensor are different. a table top surface with an opening through which a blade can extend, the blade being driven by a drive motor supported by a carriage relative to the top surface, a mains switch selectively actuatable by an operator to provide power to the drive motor, a riving knife slidably supported on the carriage to selectively position the riving knife in an extended position in which the riving knife extends through the opening adjacent the blade and a retracted position in which the riving knife is retracted below the table top surface, a blade guard removably mounted to the riving knife in a position above the blade, a sensible element mounted to the riving knife; a first sensor arranged in the power table saw to detect the sensible element when the riving knife is in the retracted position, and to generate a signal when the sensible element is adjacent the first sensor; a second sensor arranged on the power table saw and configured to detect the presence of the blade guard and to generate a signal in response to the absence or improper positioning of the blade guard, characterised in that said second sensor is an optical sensor configured to optically sense the presence or absence of the blade guard, and wherein the optical sensor is configured to direct an optical beam through the opening in the table top surface to shine on the blade guard and to receive the reflected optical beam if the blade guard is detected, and a controller operable to disengage power to the drive motor when the mains switch is actuated, wherein the controller is operable to determine a state of the first sensor and a state of the second sensor and to disengage power to the drive motor when the mains switch is actuated when the state of the first sensor and the state of the second sensor are different. to detect the sensible element when the riving knife is in the retracted position, and to generate a signal when the sensible element is adjacent the first sensor; a second sensor arranged on the power table saw and configured to detect the presence of the blade guard and to generate a signal in response to the absence or improper positioning of the blade guard, characterised in that said second sensor is an optical sensor configured to optically sense the presence or absence of the blade guard, and wherein the optical sensor is configured to direct an optical beam through the opening in the table top surface to shine on the blade guard and to receive the reflected optical beam if the blade guard is detected, and a controller operable to disengage power to the drive motor when the mains switch is actuated, wherein the controller is operable to determine a state of the first sensor and a state of the second sensor and to disengage power to the drive motor when the mains switch is actuated when the state of the first sensor and the state of the second sensor are different.