ROTATING EQUIPMENT
§ 39.111. Speed control and stopping devices.
(a) Prime movers driving generating equipment shall be provided with automatic speed-limiting devices, in addition to their governors, if necessary, as with some types of steam turbines where harmful overspeed may otherwise occur.
(b) Separately excited direct current motors, series motors, and motor generators and converters, where possible for them to be driven at an excessive speed from the direct current end by a reversal of current or decrease of load, shall be provided with speed limiting devices, unless the load and the mechanical connections thereto are of such a character as to safely limit the speed.
(c) 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 no voltage releases or other devices so that the motor cannot be started or continued in operation under dangerously weakened field, except where a hazard to service or apparatus might result from operation if a hazard to service or apparatus might result from operation.
(d) Stopping devices, such as switches or valves which may be operated from locations convenient to machine operators, shall be provided from prime movers or motors driving generating equipment.
(e) If speed limiting or stopping devices are electrically operated the control circuits by which such devices are actuated shall be in conduit or otherwise suitably protected from mechanical injury.
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