be checked by operating reset device 54 to cause testing device 56 to initiate a simulated malfunction as by interrupting the supply of operating power to all transmitters if the result is positive power is again supplied to the transmitters and reset device 54 is switched to a normal operating position preferably the circuit components are interconnected so that switch 55 does not close until device 54 has been switched to this normal operating position if during operation of the machine a light beam is interrupted all movement of the operating member 2 and of the holding device 3 is stopped and restarting is possible only by operating the reset device 54 the reset device 54 is advisably so located that the operator must first step entirely away from the danger zone i or ii respectively of the machine 2 3 before the machine can be switched on again i e the operator must check out the danger zone i or ii the spatial arrangement is advisably such that the reset device is at the end of the machine opposite the operator 39 s penal the entire control system is preferably constructed using relays in such a manner that if there is a drop in voltage or a defect the machine 2 3 is.