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 4.6.4 Ratings. The ratings which apply to circuit breakers are:
(a) Voltage.
(b) Frequency.
(c) Continuous current.
(d) Interrupting current.
(e) Short-time current.
4.6.5 Trip Device Characteristics. The overcurrent trip devices considered here are integral
parts of their respective types of circuit breakers. By continually monitoring the current flowing
through the circuit breaker, they sense any abnormal current conditions, and in accordance with
their time current characteristics permit the circuit breaker operating mechanism to open its
contacts and interrupt the circuit.
4.6.5.1 Basic Characteristics. The basic overcurrent trip device characteristics used on
molded-case circuit breakers and low-voltage power circuit breakers are longtime delay and
instantaneous. The combination of these characteristics provides time delay to override transient
overloads, delayed tripping for those low-level short circuits or overloads that persist, and
instantaneous tripping for higher level short circuits.
4.6.5.2 Alternate Characteristics. Trip devices of low-voltage power circuit breakers, and
certain new molded-case circuit breakers, may provide a short-time-delay characteristic and
ground current sensing. In addition, these trip devices are equipped with long time-delay and
instantaneous characteristics. The resulting combination of long-time-delay and short-time-delay
characteristics provides delayed tripping for all levels of current up to the maximum allowable
available short-circuit current limit of the circuit breaker without instantaneous trip elements.
4.6.5.3 Trip Device Design. The basic trip device design has been of the
electromechanical type, using a displacement dashpot for the low-voltage power circuit breaker.
The thermal electromechanical type has been used for molded-case circuit breakers. In recent
years, however, the direct-acting solid-state trip device has been developed and is available for
use on both types of circuit breakers.
4.6.6 Application. The proper application of a circuit breaker, either molded-case circuit
breaker or low-voltage power circuit breaker, involves considerations that go beyond voltage,
current, and interrupting ratings. There are differences between molded-case circuit breakers
and the low-voltage power circuit breakers which affect their application.
(a) Low-voltage power circuit breakers are rated to carry 100 percent of their
continuous current rating inside enclosures at 40C ambient temperature. Molded-case circuit
breakers are rated to carry 100 percent of their rated continuous current when tested in the open
at 25C ambient temperature. Generally, molded-case circuit breakers must be derated for
4-28
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