Current through Register Vol. 54, No. 45, November 9, 2024
Section 39.251 - Control devices(a) Separately excited direct-current motors, also series motors, and motor generators and converters larger than ten kilowatts where it is possible for them to be driven at excessive speed from the direct-current end, as by a reversal of current or decrease in load, shall be provided with speed-limiting devices, unless the load and the mechanical connection to such load are of such a character as to safely limit the speed.(b) If the speed control of direct-current motors is accomplished by varying the field resistance, and the nature of the load and the range of the field rheostat are such as to make a dangerous speed attainable, and no speed limit devices are used, the field rheostats shall be arranged with novoltage releases or other devices so that the motor may not be started or continued in operation under dangerously weakened field, unless the operation of such a novoltage release might result in serious injury to service or apparatus. Motors which are designed to permit starting safely under weakened field shall not be included in the provisions of this subsection.(c) Manually controlled starters for motors shall be so designed and circuits so arranged that they return automatically to the off or starting position upon failure of the energy supply, unless the motors and their starting devices are, during operation, under supervision of qualified persons and equivalent protection is otherwise provided.(d) If speed limiting devices or remote control switches are electrically operated, the control circuits by which such devices are actuated shall be adequately guarded by conduit or otherwise, against mechanical injury.