Circuit breakers must have adequate current carrying capacity for the usual load current and also be capable of interrupting the highest fault current possible at the voltage level where the breaker is installed. They must be capable of withstanding the mechanical and thermal stresses produced by such fault currents. All of this must be provided without danger to personnel and apparatus in the vicinity of the breaker. A circuit breaker - like other devices - can be a hazard if it is subjected to currents or voltages which exceed its specified design limits.
Molded case breakers are available with voltage ratings up to 600 Volts and current ratings up to 4,000 amperes. They are available with interrupting ratings up to 200,000 amperes symmetrical at 240 Volts AC. These devices combine the convenience of a switching mechanism with the protective function of current interruption. They can be used as load disconnects and, if specifically approved for that use, as control devices. They are designed and built to conform with UL and NEMA standards. They can be built to combine an "instantaneous" trip characteristic for large overcurrents with an inverse-time characteristic that matches the damage curve for overloads.
Some molded case breakers have electronic trip circuits which allow more sophisticated control of the tripping characteristics. Some provide for ground-fault current interruption. Some are used as motor circuit protectors. These provide very rapid current interruption where currents exceed the normal inrush current for the motor. These circuit breakers have adjustable instantaneous (magnetic) trip settings, so they can be matched to a specific motor. In this application, the circuit breaker only provides protection against short-circuits occurring in the motor or its leads. It does this quickly to minimize damage. The overload relays in the starter are used to protect the circuit and the motor against overloads up to and including the locked rotor current.
The standard molded case breaker has a thermal magnetic trip. These contacts may be closed and opened many times (except, possibly, after a maximum level fault, where the number of successful reclosings may be limited). These breakers also have arc-chutes, and mechanical linkages necessary for manual operation. Some breakers can be fitted for electrical operation by signals from a remote location.
Molded case breakers are available with various voltage, load-current, and interrupting-current ratings. The frame size indicates the largest load current rating for a breaker of that size and interrupting rating, however, the actual trip-current level can be set lower than the frame size. Standard molded case breakers have the load current trip setting fixed at the time of manufacture. 
This can be an advantage when used to protect conductors of a particular size since the breaker cannot be reset to a higher trip current level which might result in damage to the conductors or protected device. However, some newer breakers have electronic trip circuits and interchangeable rating plugs which allow greater flexibility when the system load characteristics change.
Solid-state devices have made many features possible for molded case circuit breakers. Some of these features include adjustment of the long-time ampere rating, the long-time delay, short-time delay, the "instantaneous" pickup, the ground-fault pickup, and the ground-fault delay. Other features which may be ordered include push-button testing of the ground-fault detection circuit, and a ground-fault memory for intermittent faults. These devices can now be used in some situations where low-voltage power circuit breakers have previously been required.
Many circuit breakers incorporate specially designed fuses integral to the breaker assembly. These fuse elements provide greater interrupting ratings or current limitations and are called current-limiting circuit breakers. These fuses can provide interrupting ratings up to 200,000 amperes at 480 Volts within a 400 ampere frame. When any fuse opens, it releases a plunger that trips all poles of the breaker to prevent single-phasing.
Some breakers have contact linkages designed for high speed interruption and arc control to limit the current without the use of fuses. Some of these have interrupting ratings up to 200,000 amperes at 240 Volts.
Circuit Breakers For Overcurrent Protection
Circuit Breakers For Short Circuit Protection
