a system is provided for automatically clamping a cable drum in the event of mechanical failure of the drive train that connects the cable drum to its prime mover . the system monitors the mechanical continuity of the drive train by comparing the number of revolutions of the prime mover with the number of revolutions made by the cable drum in the same time interval . the system actuates a caliper disc brake mounted with the drum if a discontinuity of the drive train is detected . in the electrica system , shaft encoders are used on both the cable drum axle and prime mover shaft . the cable drum encoder is selected such that the number of pulses it produces per revolution of the cable drum is approximately equal to the number of pulses produced by prime mover shaft encoder per revolution of its shaft divided by the speed reduction ratio of the drive train . the counters in the comparator are periodically reset to prevent the slight difference in the number of pulses produced by the encoders during normal operations from falsely indicating a drive train discontinuity . in the mechanical system , the prime mover shaft and cable drum axle may be connected to a mechanical phase sensing switch through respective rotation transmitting cables in order to determine if the cable drum and prime mover are rotating at the drive train speed reduction ratio .