Wilson Dam & Hydroelectric Plant, Three Phase Tri Level Circuit Breaker, Spanning Tennessee River at Wilson Dam Road (Route 133), Muscle Shoals, Colbert County, AL

Significance: In order to perform such functions as servicing, maintenance, re-configuring and fault protection, it is necessary that there be some means for disconnecting the load (electric furnaces, motors, etc.) from the generator bus lines. This is accomplished by a circuit breaker, which is basically a switch. Shown in this drawing is the version of this device installed in the switch building of Wilson Dam. As might be expected, a switch capable of interrupting hundreds of kilowatts of power must be rather large, and requires motor driven auxiliaries to move the various breaker parts. At Wilson Dam, these requirements dictated the arrangements of the circuit breakers on three separate floors, a separate breaker being needed for each of the lines from the three-phase source. This configuration also isolated each breaker in its own cubicle, an important safety feature. The principle problem to be solved in the operation of high voltage circuit breakers is that of arc suppression. Such arcs would very quickly vaporize the metal at the switch contacts. To prevent such damaging arcs, two switches are provided. On the opening cycle, the short-circuiting switch, which carries most of the load, opens first, leaving the oil-immersed switch to open next; this maintains a low voltage across the opening of the short circuiting switch, thus preventing an arc. On the closing cycle, the reverse occurs. The function of the oil in the immersed switches is to steal out any air and to insulate the gap between the contacts. A vent is provided to release oil vapors produced by the heating effects of abnormal overload currents. A practical circuit breaker must make or break contact quickly. In this design, powerful coil springs provide the needed burst of kinetic energy to push the moveable parts together or apart. In either the closed or open position, the springs are held down by a latch/toggle device that can be released or latched by a motor-actuated mechanism. After release, the springs are re-compressed. This operation, as well as that of actuating and resetting the release latches, is done by a D.C. electric motor geared for high torque. In order to prevent damage to either the motor or the mechanism itself, the motor is provided with a magnetic clutch; this clutch is activated when the spring reaches full compression. As a safety measure, a manually operated lever is provided to open and close two disconnecting switches. These switches provide a clear visual indication to workers repairing the breaker that the circuit is indeed open.
Unprocessed Field note material exists for this structure: N167
Survey number: HAER AL-47-C
National Register of Historic Places NRIS Number: 66000147