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 3.5.2.1 Medium and High Voltage Power Circuit Breakers (rated above 1000 VAC).
3.5.2.2 Low Voltage Circuit Breakers (rated at 1000 VAC or less).
Section 5 discusses the medium and high voltage power circuit breaker. The low voltage circuit
breaker, which includes the molded-case type, is discussed in Chapter 4, Section 6.
3.5.3 Application. Circuit breakers are used in distribution substations, switching stations,
and generating stations for protecting and switching electrical service apparatus such as
transformers, motors, generators, substation buses, and distribution feeder circuits. Circuit
breakers are normally used where less expensive fuses and switches are not adequate because
switching is frequent or available fault current is high.
3.5.4 Rating. Circuit breakers are rated in terms of voltage, continuous current, momentary
current, and interrupting capability. In the normal operation of a circuit breaker the limitations
imposed by a given breaker rating should not be exceeded; otherwise, excessive maintenance or
unsatisfactory operation may be experienced.
3.5.4.1 Voltage Rating. Circuit breakers are assigned a voltage rating that designates the
maximum system voltage at which the breaker is intended to be operated.
3.5.4.2 Continuous Current Rating. This is the maximum value of the continuous current
which the contacts and conductors are designed to carry. It should not be exceeded, except for
short periods; as in starting motors or rotary converters, or energizing cold loads. In an
emergency, circuit breakers can be overloaded provided the associated protective relays do not
trip the breaker.
3.5.4.3 Maximum Momentary Current Rating. This is the maximum instantaneous
current for which the breaker has been designed to close and latch under fault conditions.
3.5.4.4 Interrupting Rating. Circuit breakers have an interrupting rating representing their
design limitation for interrupting fault current. Associated with the interrupting rating is the
circuit breaker's capability to close into, carry, and interrupt short-circuit current. The
interrupting rating of a circuit breaker takes into account the value of current to be interrupted,
the voltage across the circuit breaker contacts at the time of current interruption, and the standard
duty cycle. The interrupting capacity is given as the maximum amperes the breaker is designed
to interrupt at normal circuit volts, as a function of the ratio of system reactance to resistance at
the point of fault.
3.5.5 Types of Circuit Breakers. Circuit breakers are divided into two broad classes: oilless
and oil circuit breakers.
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