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more detailed information about the specific trip unit
used with Types DSll and DSLll circuit breakers, refer to
the specific trip unit instruction book for details.
3-7.1 ELECTRONIC TRIP UNIT
Types DSll and DSLll circuit breakers, whether manual-
ly or electrically operated, are supplied with a state-of-
the-art electronic, microprocessor-based trip units. True
RMS sensing is provided for proper correlation with
thermal characteristics of conductors and equipment.
In
addition to circuit protection, electronic trip units pro-
vide information and integral testing functions. Optional
remote communications and energy monitoring func-
tions are also available.
The electronic trip units are completely self-contained
and, when the circuit breaker is closed, no external
power is required to operate their protection systems.
They operate from current signal levels and the control
power is derived from current sensors integrally mount-
ed in the circuit breaker.
When the circuit breaker is shipped from the factory, the
trip unit's protective functions are normally set at nomi-
Trip Shaft
Lever
OTA
Actuator
Frame
Untripped
Figure 3-11 Type OSI/ Trip Actuator
Tripped
l.B.
694C694-02
nal values. For specific overload tripping characteristics
to coordinate with the load
or
system, refer to the specif-
ic trip unit instruction book.
3-7.2 SENSORS
Three current sensors are installed at the rear of the circuit
breaker on the lower studs, directly behind the main dis-
connecting contacts (Figures
3-1
and 3-2). The sensors
produce an output current proportional to the load current.
Under preselected conditions of current magnitude and
time, the sensors furnish the electronic trip unit with a sig-
nal and the energy required to trip the circuit breaker.
Since the continuous current rating for any frame size
breaker is associated with both the sensors and the trip
unit itself, a complete tabulation of available current sen-
sors is presented in the specific trip unit instruction
book, not this instruction book for the circuit breaker.
3-7.3 TRIP
ACTUATOR
The electronic trip unit provides a pulse to the trip actua-
tor, which produces a mechanical force to trip the circuit
breaker (Figure 3-11). The actuator is comprised of a
Pole Shaft Lever
----~
OTA Reset
Lever
Actuator Reset
Breaker Closed
Effective
07
/97
Courtesy of NationalSwitchgear.com
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