To prevent an event caused by a blocked bit by generating correct probability of blocking accidents without applying any sufficient and detailed knowledge on the physical characteristic of a power tool to generate block accidents.

A safety cut-off system for a hand held electrically powered tool with a rotatingly driven bit (8) comprises at least one sensor (56, etc.) for detecting an operating condition of the tool and generating a sensor signal representing the operating condition, and cut-off units (38, 80) for interrupting a rotary drive to the bit from a motor (10). A processing unit (42) comprises a neural network (50, etc.) which is responsive to the sensor signal or signals to generate a summed probability signal representing a probability of a blocking event and ready for a case when the summed probability signal exceeds a pre-set threshold for causing activation of the cut-off units.