Circuit breakers are switching devices capable of making, carrying and breaking currents under normal circuit conditions and making, carrying for a specified time, and breaking current under specified abnormal conditions such as a short circuit. Low voltage circuit breakers have voltage ratings from 250 to 600 V AC and 250 to 700 V DC. The molded case circuit breaker is one of the two basic low voltage classes of circuit breakers. The other class is the low voltage power circuit breaker. Medium voltage circuit breakers are rated up to 72.5 kV. ANSI or IEC is normally the governing standard for medium voltage circuit power breakers. ANSI is associated with U.S. standards, and IEC is associated with international standards. For standard definitions, refer to ANSI 37.100 Definitions for Power Switchgear.
Important ratings specifications to consider for circuit breakers include rated AC voltage, rated DC voltage, continuous current rating, and rated breaking capacity (Icn). The rated AC and DC voltages are the maximum voltage for ANSI rated breakers or the Voltage Class for IEC rated breakers. The continuous current (at the rated voltage) on ANSI breakers or the normal current on IEC breakers should always be in excess of the utilization equipment rating to provide for a short-time overload capability. It is also referred to as the Frame Rating or the Frame Size. The rated breaking capacity is the rated short circuit capacity or interrupting capacity. The short circuit current is rated at the breaker's maximum voltage (ANSI) or voltage class (IEC). The number of poles and number of protected poles (if different) are also important parameters to consider.
The trip unit type for circuit breakers can be thermal overcurrent, thermal magnetic, magnetic, magnetic-hydraulic type circuit breakers, and electronic unit. In a thermal overcurrent type the tripping mechanism is comprised of a thermal actuator and a mechanical latch designed to discriminate between in-rush and temporary current surges and prolonged overloads. Applications include motors, transformers, solenoids and low voltage wiring. A thermal magnetic type of circuit breaker combines a bimetal element with a magnetic coil to provide time/current characteristics with two distinct steps. The magnetic coil activates quickly at high overcurrent values. Prolonged low value overloads cause the thermal element to activate. Applications include telecommunications, process control, and similar applications requiring precision performance. In a magnetic type circuit breaker the magnetic coil responds rapidly to high values of overcurrent. A magnetic-hydraulic a fast acting magnetic coil with hydraulically delayed switching characteristics. Electronic trip units are composed of three components, internal to the trip unit. These components are the current transformer, circuit board and flux-transfer shunt trip (for remote controlled tripping of the breaker). There are two types of electronic trip units: Analog and Digital.
Common features found on circuit breakers include molded cases, miniature construction, motor circuit protector, remote control, lock or interlock, positively trip free, snap-action actuation, trip indicator, interchangeable trip unit, undervoltage release, and auxiliary contacts. Miniature circuit breakers can have a supplementary protector, shunt trips, ground fault circuit interrupter, and duplex circuit breakers.