What Are Circuit Breakers?

Courtesy of Siemens

Circuit breakers are mechanical switching devices capable of making and breaking currents under either normal or specified abnormal (short circuit) conditions on the power system. 

Though circuit breakers are primarily defined by their protective capabilities and ratings under abnormal short circuit conditions, they also perform switching duties under a myriad of other system conditions, each of which has its own set of switching stresses.

Circuit breakers are rated primarily by power frequency voltage, insulation levels (BIL, switching impulse, hi-pot voltage), continuous current, short-circuit current, and interrupting time. Reference is made to IEEE C37.1001 for definitions of ratings subjects, and to IEEE C37.042 and IEEE C37.063 for values of ratings typically applied to circuit breakers.

Circuit breakers employ a variety of media for high voltage insulation and/or current interruption. The type of media employed in a specific design is often designated as a prefix in the naming of the circuit breaker, for example, vacuum circuit breaker, or sulfur hexafluoride (SF6) gas circuit breaker.

Circuit breakers are often categorized as being of either “dead tank” or “live tank” design. In the dead tank case, the interrupting contact system is enclosed in a grounded tank, typically surrounded by an insulating fluid (oil) or gas (SF6) (see Figs. 10-53 and 10-54 for examples of dead tank circuit breakers).

The electrical current enters the tank through high voltage entrance bushings, passes through the contact system, and then exits through another high voltage entrance bushing. In the live tank case, the interrupting contact system is supported by insulators at some height above ground potential, but is not contained within a grounded tank system.

There is no grounded tank or enclosure surrounding the live parts. Dead tank design allows the placement of current transformers, which are necessary for protective relaying input signals, around the high voltage entrance bushing.

Live tank design offers no location to place current transformers, and therefore must be independently placed adjacent to the circuit breaker.

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