Switch overspeed

E.3.3.2 The risk of overspeed is reduced when the driven equipment, such as a pump or fan, caimot reahstically be expected to lose function. The risk is increased when the driven equipment is a generator, since a sudden disconnection from electric power circuits will unload the HPRT. In the latter case, automatic sensing and dummy-load switching should be provided. 

This condition can be detected by a variety of devices, such as centrifugal switches, electronic tachometers, or strain-detecting devices installed on or near such components of the machine that are affected by the overspeed condition. 

The unit is fully protected with safety valves at necessary points. In addition the compressor has water temperature and pressure shutoff switches, and a suction pressure switch, which will shut it down in the event the system runs low on gas. This protects against line bursts or major leaks, or operator negligence in providing gas supply. The expander has overspeed and oil-pressure protection. The expander and compressor are linked with a unique electrical circuit, which permits them to be started separately, but which shuts down both units in case of automatic shutdown of either one. Thus any failure will close down the entire system, preventing damage to either compressor or expander. 

This could also include supervisions such as midshaft overspeed, end of wind overspeed, overwind, underwind, wrong direction of rotation and stall, broken motor-pulley/drum drive chain, rope shp (for friction hoist) and slack rope switch (for drum hoist) supervisions. 

In addition to overcurrent devices, other switching devices can reduce or eliminate electrical hazards. They include lockouts, interlocks, and thermal or overspeed switches. 

Thermal and Overspeed Cutouts The temperature of an electric motor will rise during use. If the temperature exceeds a certain value, a dangerous condition may exist. A temperature-sensitive switch with a preset temperature limit can interrupt power. Some electrical motors, equipment with resistance heaters (such as hair dryers), and other electrical equipment have thermal cutout switches. 

Overspeed switches sense when a motor or other device operates too fast. Excessive speed may create dangerous conditions or indicate failure in the equipment. If a motor reaches excessive speed, the switch interrupts power to the equipment. 

In addition to overcurrent devices, certain switching devices can reduce or eliminate electrical hazards. These include interlocks, lockouts, and thermal or overspeed switches. Interlocks are switches that prevent access to an energized or dangerous location. Often attached to access doors, panels, and gates, interlocks act to shut off power to the equipment whenever these devices are opened. Probably the most commonly used and most familiar interlock... 

Thermal and overspeed cutout devices are coimnonly used to protect electrical equipment (and thus the operator). A thermal cutout is simply a temperature-sensitive switch with a preset limit designed to interrupt power when the temperature exceeds a certain value. As its name implies, an overspeed switch operates when it senses that a motor or other device is operating at too fast a speed. Obviously, excessive speed may create dangerous conditions and indicate failure of equipment. The overspeed switch operates to shut down an overspeeding device by interrupting power to it. 

As discussed earlier the ESD system normally operates in a normally energized condition. Therefore initiating Pb, limit switch contacts, etc. shall be a normally open (NO) contact, but a few contacts such as contact from the motor control center (MOO), overspeed, reset/stop push button, etc. are normally closed (NC)-type contacts (refer Fig. VIII/4.1.4-1). 

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