Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 136)

4.2.2.3 Design of a circuit breaker
The parts of the circuit breaker detailed in Fig. 4.2-3 are precisely coordinated so that the
common tasks, the rapid disconnection of short-circuit currents and the dependable recognition
of overloads, can be performed optimally.
h
a
b
f
g
c
d
e
Fig. 4.2-3
The main functional elements of a circuit breaker for motor protection
a) Thermal overcurrent release
b) Electromagnetic overcurrent release
c) Main contact system
d) Auxiliary switch position
e) Switch latch
f) Arcing chamber (de-ion plates)
g) Plunger armature
h) Differential trip slide
In larger circuit breakers (> approx. 100 A), electronic trip and communication modules are
increasingly being used. These offer a high degree of flexibility with respect to the selection of
application-specific parameters and support the integration of devices in superordinated control
and management systems.
4.2.2.3.1 Thermal overcurrent releases
The thermal overcurrent releases of circuit breakers act in the same way as those of thermal
motor protection relays (bimetal overload relays) and are subject to the same standards if they
are used for motor protection. See also Section
4.2.4.1 . Tripping is normally effected via the
switch latch of the circuit breakers and results in the opening of the main contacts. Resetting is
by manual or remote actuation of the switch after the bimetals have cooled below the reset
threshold.
In the case of circuit breakers with thermally delayed overload releases and low setting currents
(ca. < 20 A), the resistance of the circuit with the heating windings of the bimetal strips and the
coil of the undelayed electromagnetic short-circuit triggers is comparatively large. It may be so
large that it damps any size of (prospective) short-circuit current to a value that the switch can
still cope with thermally and dynamically and can hence also disconnect. Such circuit breakers
are intrinsically safe against short-circuits.
LVSAM-WP001A-EN-P - April 2009 4-28