Effective 7/1/2003
Page 11
I.L. 70C1037H04
associated rating plug must match the current sensor
rating specified on the plug label. The current sensor rating
can be viewed through openings in the back of the breaker.
2.5 Current Sensors (Magnum Frames greater than 3200A)
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 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 for this size breaker which further steps down the
rated current to 100 milliamperes and is equivalent to
100% (
I
n) to the Digitrip.
3.0 PRINCIPLES OF OPERATION
3.1 General
All models of trip units are designed for industrial circuit
breaker environments where the ambient temperatures can
range from –20
°
C to +85
°
C but rarely exceed 70
°
to 75
°
C. If, however, temperatures in the neighborhood of the trip
unit 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
microcomputer chip has a built-in over-temperature
protection feature, factory set to trip the breaker if the chip
temperature is excessive. If over-temperature is the reason
for the trip the red Long Delay Time LED will flash.
The Digitrip uses the Eaton custom-designed
CHip
™
(Cutler Hammer Integrated Processor) chip, an integrated
circuit that includes a microcomputer to perform its
numeric and logic functions. The principles of operation of
the trip unit are shown in Figure 3.1.
All sensing and tripping power required to operate the
protection function is derived from the current sensors in
the circuit breaker. The secondary currents from these
sensors provide the correct input information for the
protection functions, as well as tripping power, 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
™ (Cutler Hammer Integrated Processor) 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 software then determines whether to initiate
protection functions, including tripping the breaker through
the Trip Actuator.
Table 2.2 Ground (Earth) Fault Current Settings
2.4 Current Sensors (Magnum Frames less than or equal to
3200A)
The three (3-pole) or four (4-pole) primary current sensors
are installed internally in the circuit breaker on the lower
conductors of the breaker. The current sensor rating
defines the breaker rating (
I
n). For example, 2000A:1A
sensors are used on a 2000A rated breaker. There are 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) to the Digitrip.
The primary current sensors produce an output proportional
to the load current and furnish the Digitrip with the informa-
tion and energy required to trip the circuit breaker when
functional protection settings are exceeded.
If a set of current sensors with a different ratio are installed
in the field, the rating plug must also be changed. The
Ground Fault Current Settings
(Amperes)
1
Installed
Sensor and
Rating Plug
(Amperes)
I
n
.25
.30
.35
.40
.50
.60
.75
1.0
100
25
30
35
40
50
60
75
100
200
50
60
70
80
100
120
150
200
250
63
75
88
100
125
150
188
250
300
75
90
105
120
150
180
225
300
400
100
120
140
160
200
240
300
400
600
150
180
210
240
300
360
450
600
630
158
189
221
252
315
378
473
630
800
200
240
280
320
400
480
600
800
1000
250
300
350
400
500
600
750 1000
1200
300
360
420
480
600
720
900 1200
1250
312
375
438
500
625
750
938
1250
1600
400
480
560
640
800
960
1200 1600
2
2000
500
600
700
800
1000
1200
1500
2
2000
2
2500
625
750
875
1000
1250
1500
1875 2500
3000
750
900
1050
1200
1500
2
1800
2
2250
2
3000
2
3200
800
960
1120
1200
1600
2
1920
2
2400
2
3200
2
4000
3
1000
1200
1400
2
1600
2
2000
2
2400
2
3000
2
4000
2
5000
3
1250
2
1500
2
1750
2
2000
2
2500
2
3000
2
3750
2
5000
2
6000
1500
2
1800
2
2100
2
2400
2
3000
2
3600
2
4500
2
6000
2
6300
3
1575
1890
2205
2520
3150
3780
4725 6300
1. Tolerance on settings are ±10% of values shown.
2. On Models 520 LSIG, 520M and 520MC LSIG, the shaded values are set to a
maximum trip value of 1200 amperes for NEC.
3. See Section 2.5.
