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Additional resources for Aging Mgmt Guidelines for Comml Nucl Powerplants [motor ctl centers]
Molded-case circuit breakers containing only an instantaneous overcurrent trip unit are sometimes used in conjunction with thermal overload units tor motor circuit protection. This type of MCCB is often referred to as a motor circuit protector (MCP). Thermal-magnetic trip devices are simply a combination of the thermal and magnetic devices described in the preceding paragraphs; these devices provide the benefils of both types ot"elements in one unit. Thermal-magnetic breakers can be used to provide overload and short circuit protection for non-motor branch circuits.
The spring-loaded pawl is then free to move, allowing the contacts to open, which, in iurn, deenergizes the control circuit and the starter. When the alloy has cooled and subsequently hardenc,l, the unit can be manually reset. Thermal Relay Unit f 'l Ii ___ ,--, ........ To Magnet Coil Figure 3-8. Melting Alloy Thermal Overltmd Relay. 3-17 AGING MANAGEMENT GUIDELINE FOR MOTOR CONTROl. CENTERS Bimetallic Thermal Overload Relay Bimetallic relays use a bimetallic strip (as opposed to a eutectic alloy) to sense hcat as current is increased.
The bimetallic strip is designed to flex a predetermined amount at a specific temperature because of the use of two metals with differing thermal expansion coefficients. The temperature is chosen as the temperature corresponding to the point at which the motor must be disconnected fiom the line to avoid damage to the motor insulation. When this temperature is reached, the bimetal strip bends, thereby applying force to the contact mechanism, which, in turn, opens the contacts and disconnects the motor.