Principles of Operation and Terminology

A low voltage power circuit breaker can be applied on any system within the interrupting rating of the circuit breaker. Low voltage power circuit breakers are ideally suited for applications where there is a requirement for the circuit breakers to be selective when faced with short-circuit conditions. In addition to our earlier discussion of selectivity, we could also say that "selective" means that the circuit breaker is capable of remaining closed for a certain period of time with a short-circuit present to allow the problem to be cleared up by a downstream device before the power circuit breakers open and the larger system is shut down (short time delay rating capacity). This is the area where short time delay ratings from 0 to 30 cycles play a key role. Obviously, it is assumed that the circuit breaker is applied properly and will not face short-circuit conditions beyond its capabilities. If it does see a condition beyond its short time rating, it will open instantaneously.

Time will be taken here to introduce several additional principles and common terms associated with low voltage power circuit breakers and their application. This material will be especially helpful from a practical standpoint. These are the types of terms and topics encountered on the job when working with low voltage power circuit breakers and their assemblies. Principles and terms presented here are certainly not all inclusive. Even after this module is completed and you return to your work location, new terms will surface that should be part of your low voltage power circuit breaker vocabulary. The intent here is to provide a solid background on which to build.

Stored Energy
Stored energy was briefly touched on earlier in this module and in Module 5. Because this is a phrase frequently heard with respect to circuit breakers, it deserves some elaboration. All low voltage power circuit breakers, whether manually or electrically operated, utilize two-step stored energy mechanisms. Stored energy mechanisms are needed to overcome inherent forces opposed to the closing process. They also make it possible to close the circuit breaker very quickly, 5 cycles or less time.

Stored energy is energy held in waiting, ready to open or close the low voltage power circuit breaker in five cycles or less. Designs are such that the energy required to open a low voltage power circuit breaker is always available.

On manually operated circuit breakers, closing springs are charged by hand. For electrically operated circuit breakers, springs are normally charged by a small electric motor, although they can also be charged manually if power is not available.

Bus
Bus refers to a conductor or conductors, usually made of copper or aluminum bars. Bus bars carry current and serve as a common connection for two or more circuits