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The shaft of the ratchet wheel is normally secured by the solidified eutectic alloy, and the contacts remain in a closed position. Should the current drawn bv the motor become excessive for a sufficient peri_,d of time, the heat dissipated bv the heater v;ill melt the eutectic alloy, allowing the ratchet whcel to rotate. 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.

In rnost cases, plants having MCCs in harsh environment areas are older plants. The need to qualify the MCCs was frequently identified after the plant was placed in service. Accordingly, generic manufacturers' environmental qualification programs rarely exist for MCCs, and plant-specific tests and analyses were performed. ,c defined specific replacement schedules for components with organic materials. In other cases, the qualification test may have been purely for accident conditions, and no additional normal maintenance requirernerlts may have been imposed.

The barriers may be open (allowing easy access to the contacts) or closed (protecting the contacts from external effects). The terminals themselves are made of conducting material and generally use a post/nut type arrangement for fastening the wire termination to the terminal. Terminal blocks may also include other optional attachments such as protective covers and fuse holders. Another common terminal block configuration uses a sliding metal link (hence the name sliding link terminal block) that allows easy wire installation and electrical disconnection of the two posts during testing (thereby avoiding wire removal).

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Aging Mgmt Guidelines for Comml Nucl Powerplants [motor ctl centers]

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