Effective 10/2004
Page 13
I.L. 70C1036H05
circuit breaker if the chip temperature is excessive. If the
unit trips on over-temperature, the red Long Delay Time
LED will flash and the OVER TEMP TRIP message will
appear on the display.
The Digitrip uses the Cutler-Hammer custom-designed
CHip™, an integrated circuit that includes a microcom-
puter to perform its numeric and logic functions. The
principles of operation of the trip unit are shown in Fig. 3.1.
All sensing and tripping power required to operate the
protection function is derived from the current sensor
secondary currents whenever the circuit breaker is carrying
current. These current signals develop analog voltages
across the current viewing resistors. The resulting analog
voltages are digitized by the CHip™.
The microcomputer continually digitizes these signals.
This data is used to calculate true RMS current values,
which are then continually compared with the protection
function settings and other operating data stored in the
memory. The embedded software then determines whether
to initiate protection functions, including tripping the circuit
breaker through the Trip Actuator. (TA)
3.2 Trip and Operation Indicators
The four cause of trip LEDs on the face of the trip unit,
shown in Figures 1.1 and 3.2 to 3.5, flash red to indicate
the reason for any automatic trip operation. Each LED is
strategically located in the related segment of the time-
current curve depicted on the face of the trip unit. The
reason for the trip is identified by the segment of the time-
current curve where the LED is illuminated. Following an
automatic trip operation, the backup battery shown in
Figure 3.1 continues to supply power to the LEDs. The
LED pulse circuit, also shown in Figure 3.1, is provided to
reduce battery burden and will flash the trip LED approxi-
mately every 4 seconds. Therefore, it is important to view
the unit for at least 5 seconds to detect a flashing trip
indicator. Pushing the Reset button extinguishes the LED.
NOTE:
A complete reset of the Digitrip 1150 requires the
unit to be powered (Status LED flashing) when depressing
the Reset pushbutton. Otherwise, a previous trip event
may be indicated again by the LED.
3.2.1 Status/Long Pickup LED
The green Status LED will indicate the operational status
of the protection CHip A microprocessor of the trip unit.
Even with no external power present, if the load current
through the circuit breaker exceeds approximately 12 per-
cent (3 phase power) of the current sensor rating, the LED
will flash on and off once each second indicating that the
breaker rating (
I
n). For example, 2000A:1A sensors are
used on a 2000A rated circuit breaker. There are also four
auxiliary current transformers with a ratio of 10:1 which
further step down the rated current to 100 milliamperes,
which is equivalent to 100% (
I
n) in the Digitrip 1150.
The primary current sensors produce an output propor-
tional to the load current and furnish the Digitrip 1150
family with the information and energy required to trip the
circuit breaker when functional protection settings are
exceeded.
WARNING
IF A SET OF CURRENT SENSORS WITH A DIFFERENT
RATIO ARE INSTALLED IN THE FIELD, THE RATING
PLUG MUST ALSO BE CHANGED. THE ASSOCIATED
RATING PLUG MUST MATCH THE CURRENT SENSOR
RATING SPECIFIED ON THE PLUG LABEL. THE CUR-
RENT SENSOR RATING CAN BE VIEWED THROUGH
OPENINGS IN THE BACK OF THE CIRCUIT BREAKER.
ALSO VERIFY DESIRED PROTECTION SETTINGS
AFTER THESE COMPONENTS ARE CHANGED.
2.5 Current Sensors for Double Wide Circuit Breakers
The six (3-pole) or eight (4-pole) current sensors installed
in the circuit breaker are located on the lower conductors.
The poles are paralleled and the corresponding current
sensors are also paralleled
(See Figure 2.3).
for example,
a 4000A circuit breaker phase rating has two 2000:1
current sensors wired in parallel, which provides an overall
ratio of 4000:2. The auxiliary current transformers have a
ratio of 20:1 in this size circuit breaker to futher step down
the full load current to 100 mA.
3.0 PRINCIPLES OF OPERATION
3.1 General
The Digitrip 1150 family of trip units is designed for indus-
trial circuit breaker environments where the ambient
temperatures can range from –20
°
C to +85
°
C, but rarely
exceed 70
°
to 75
°
C. If, however, ambient temperatures
exceed this range, the trip unit performance may be
degraded. In order to insure that the tripping function is not
compromised due to an over-temperature condition, the
Digitrip 1150 microcomputer chips have a built-in over-
temperature protection feature, factory set to trip the
