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Table 2.2 Ground (Earth) Fault Current Settings
Ground Fault Current Settings
(Amperes)
Installed
Sensor/
Rating Plug
(Amperes) In
.25
.30
.35
200
50
60
70
250
63
75
88
300
75
90
105
400
100
120
140
600
150
180
210
630
158
189
221
800
200
240
280
1000
250
300
350
1200
300
360
420
1250
312
375
438
1600
400
480
560
2000
500
600
700
2500
625
750
875
3000
750
900
1050
3200
800
960
1120
4000
3
1000
1200
1400
5000
3
1250
2
1500
2
1750
6300
3
1575
1890
2205
1. Tolerance on settings are ±10% of values shown.
2. On Models 520 LSIG and 520M LSIG, the shaded values are set to a maximum
trip value of 1200 amperes for NEC.
3. See Section 2.5.
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 milliam-
peres, which is equivalent to 100% ( I n) to the Digitrip.
The primary current sensors produce an output propor-
tional to the load current and furnish the Digitrip DT20
family with the information 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 in-
stalled in the field, the rating plug must also be changed.
The associated rating plug must match the current sensor
Courtesy of NationalSwitchgear.com
1
.40
.50
.60
.75
80
100
120
150
100
125
150
188
120
150
180
225
160
200
240
300
240
300
360
450
252
315
378
473
320
400
480
600
400
500
600
750
480
600
720
900
500
625
750
938
640
800
960
1200 1600
800
1000
1200
1500
2
2000
1000
1250
1500
1875
1200
1500
2
1800
2
2250
2
3000
1200
1600
2
1920
2
2400
2
3200
2
1600
2
2000
2
2400
2
3000
2
4000
2
2000
2
2500
2
3000
2
3750
2
5000
2520
3150
3780
4725
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
1.0
in the circuit breaker are located on the lower conductors.
200
The poles are paralleled and the corresponding current
250
sensors are also paralleled (see Figure 2.3). For ex-
ample, a 4000A breaker phase rating has two 2000:1
300
current sensors wired in parallel, which provides an
400
overall ratio of 4000:2. The auxiliary current transformers
600
have a ratio of 20:1 for this size breaker which further
steps down the rated current to 100 milliamperes and is
630
equivalent to 100% ( I n) to the Digitrip.
800
1000
3.0 PRINCIPLES OF OPERATION
1200
3.1 General
1250
2
The Digitrip DT20 family of trip units is designed for
2
industrial circuit breaker environments where the ambient
temperatures can range from –20
2500
°
°
exceed 70
to 75
C. If, however, temperatures in the
2
neighborhood of the trip unit exceed this range, the trip
2
unit performance may be degraded. In order to insure that
2
the tripping function is not compromised due to an over-
temperature condition, the Digitrip 520 family microcom-
2
puter chip has a built-in over-temperature protection
6300
feature, factory set to trip the breaker if the chip tempera-
ture is excessive. On the 520 family, if over-temperature
is the reason for the trip the red Long Delay Time LED will
flash.
The Digitrip uses the Cutler-Hammer custom-designed
µ
S
RE+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.
In the Digitrip DT20 family of trip units, 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 result-
ing analog voltages are digitized by the S
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
I.L. 70C1037H02
°
°
C to +85
C but rarely
µ
RE+chip™.
Effective 8/13/99
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