1. In 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 be selectively positioned in an extended position in which the riving knife extends through the opening adjacent the blade and in a retracted position in which the riving knife is retracted below the table top surface and a blade guard removably mounted to the riving knife in a position above the blade, the improvement comprising:a sensible element mounted to the riving knife;
a first sensor supported and arranged on the carriage to detect the sensible element when the riving knife is in the retracted position and configured to generate a signal when the sensible element is adjacent the sensor;
a second sensor supported and arranged on the carriage to detect the presence of the blade guard and configured to generate a signal in response thereto; and
a controller operable while the mains switch is actuated to disengage power to the drive motor upon receipt of the signal from either of said first sensor or said second sensor.