Drawout style circuit breakers are commonly used in industrial facilities in voltages ranging from 208-volts to 15 kV. Higher voltages are also used, although less frequently. These devices are frequently connected and disconnected from the bus by technicians without incident, usually by means of a manual screw-type racking mechanism as shown in Figure 1. The typical racking operation involves attaching a handle (often shaped like a speed handle) to the screw mechanism of the circuit breaker frame, rotating the handle in order to cause the circuit breaker to move in or out of the cubicle, as well as connecting or disconnecting the circuit breaker's load-carrying elements, referred to as primary disconnects or stabs. It is the connecting or disconnecting of the primary disconnects from the energized bus that creates a safety hazard. There are many situations that can cause a failure when the circuit breaker is being racked, ranging from the circuit breaker reclosing instead of discharging the springs or the circuit breaker being in the CLOSED position while being racked in or out of the cubicle. Other problems include inadequate spring tension on the primary disconnect fingers, primary disconnect fingers breaking, internal failure of the circuit breaker insulation system, and vacuum bottle or arc chute failure. There are many other reasons, but you get the idea. If anything goes wrong during the racking process and an electrical arc is created at the primary disconnect/bus connection, there are no arc chutes to extinguish the arc. It will likely be a large scale event with severe consequences as it is on the main bus, and it will do a lot of damage. Figure 2 shows a low-voltage circuit breaker primary disconnect assembly as it is about to make contact with the bus.
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