Trip mechanisms

Figure 12.12 Tripping mechanism of an overcurrent-cum-single-phasing thermal relay...

Such relays are in the form of a small tube inside which is a loosely fitted rotatable shaft, held by a very thin film of this alloy. The alloy senses the motor temperature through a heater connected in series with the motor terminals and surrounding this tube. When the motor current exceeds the predetermined value, the alloy melts and enables the shaft to rotate and actuate the lever of the tripping mechanism. 

This is not quite true. Whenever we start a pump, the starting torque required to get the pump spinning requires a surge of motor amps. To avoid tripping off the motor on high amps, there is a time delay built into the trip mechanism. This delay permits the amperage load to greatly exceed the FLA point, for up to 15 to 30 s. This is too brief a period in which to overheat the motor. 

The fire protection system consists not only of a deluge system activated by the ultraviolet sensors, but also of fusible link type fire systems. The deluge system has a trip mechanism from mercury checks activated by heat-activated-devices, a manual release on the deluge valve, a pneumatic remote trip station, and an electrical push button along with the electrical trip mechanism from the U/V detectors. The remote trip stations are located by escape routes so it is possible for the operator to trip the systems as he exits the building without exposing himself to further danger. 

Steam enters a turbine through the steam chest. The steam chest typically has a strainer on the inlet side to remove solids. Inside the steam chest is a device called the governor valve. The governor valve opens and closes to admit steam into the turbine. A governor system controls the position of the governor valve. An overspeed trip mechanism is attached to the rotor and will shut off the flow of steam into the turbine when it reaches 115% of its design limit. The shutoff valve is typically located in front of the governor valve. 

During the Nazi invasion of Russia in World War II, Soviet engineers were instructed to destroy turbogenerators before retreating. First, the trip mechanism was wired so that it would stay latched. Next, the turbine was run up to maximum speed. Finally, the pump circulating lube oil to the turbine bearings was switched off. 

Steam turbine trips, 350-352 trip mechanism, 350-351 emergency lube-oil pumps, 351-352 Steam turbines, 350-352, 361 trips, 350-352... 

Bimetal Overload Relays. Traditional NEMA-style bimetal overload relays share the heater mterchangeabihty of the melting-alloy type. They may also have the ability to adjust up or down the equivalent of one heater size, or about +15 percent. lEC overloads are bimetal type, but have nonreplaceable integral heaters. To compensate for this, these units can typically be adjusted +20 to 25 percent. Bimetal devices use the heat generated to cause deflection of one or more bimetal strips. Once this deflection is great enough, the trip mechanism is operated. 

Many such overload relays are constructed with a differential trip mechanism, and are phase-loss sensitive. A phase-loss-sensitive overload relay, compensated for temperature variations, and with two poles energized at 115 percent of full-load amperage, shall trip in less than 2 h. ... 

SPECIAL CAUTIONARY NOTE AU circuit breaker manufacturers recommend that their circuit breakers be exercised annuaUy via the trip test button. Exercising the circuit breaker actuates the trip mechanism. The trip mechanism of the circuit breaker is separate from the on-off handle mechanism, and annual operation of the trip mechanism ensures that the breaker wiU trip when called on to operate. This is also a test indication if the breaker faUs to trip, it has malfunctioned and should be replaced. 

The third equipment operator then proceeded to the No. 1 AFW pump trip throttle valve. The valve had not been reset properly and he experienced great difficulty in relatching and qpening it because he had to hold the trip mechanism in the latched position and open the valve with the valve wrench. Because the trip mechanism was not reset properly, the valve shut twice before he finally opened the valve and got the pump operating. 

If the reaction does not respond to the dosage or if the trip mechanism fails the pressine will continue to rise. There is an extra-high pressine alarm which is set to warn of an impending release from the bursting disk. Site staff are trained to go to safe areas when this alarm sounds all around the plant. The result of this alarm and evac procedure is that on average (including allowance for alarm failmes) only 1 person in 10 is placed at risk when a binsting disk release occurs. 

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