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I.L. 70C1037H02
memory. The embedded software then determines
whether to initiate protection functions, including tripping
the breaker through the Trip Actuator.
3.2 Trip and Operation Indicators
The LEDs on the face of the trip unit, shown in Fig-
ures 1.1 and 3.3 to 3.9, 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 continues to supply power
to the LEDs as shown in Figure 3.1. The LED pulse
circuit, shown in Figure 3.1, is provided to reduce battery
burden and will supply a quick flash of the trip LED
approximately every 4 seconds. It is therefore important
to view the unit for at least 5 seconds to detect a flashing
cause of trip indicator.
Following a trip operation, push the Reset\Battery Test
button, shown in Figure 1.1, to turn off the LEDs.
A green LED, shown in Figure 1.1, indicates the opera-
tional status of the trip unit. Once the load current through
the circuit breaker exceeds approximately 10 percent (3
phase power) of the current sensor rating, the green LED
will flash on and off once each second to indicate that the
trip unit is energized and operating properly.
NOTE: A steady green status LED typically indicates that
a low level of load current, on the order of 5% of full load,
exists.
3.3 Making Current Release
All Digitrip DT20 Trip Units have a Making Current
Release function. This safety feature prevents the circuit
breaker from being closed and latched-in on a faulted
circuit. The nonadjustable release is preset at to a peak
current of 25 x I n which correlates to approximately 11 x I n
(rms) with maximum asymmetry.
The Making Current Release is enabled only for the first
two cycles following an initial circuit breaker closing
operation. The Making Current Release will trip the circuit
breaker instantaneously and flash the Instantaneous LED.
Effective 8/13/99
Courtesy of NationalSwitchgear.com
3.4 Zone Interlocking (520 family only)
CAUTION
IF ZONE INTERLOCKING IS NOT TO BE USED (I.E.,
ONLY STANDARD TIME-DELAY COORDINATION IS
INTENDED), THE ZONE INTERLOCKING TERMINALS
MUST BE CONNECTED BY A JUMPER FROM TERMI-
NAL B8 TO B9 OF THE BREAKER SECONDARY
TERMINALS SO THAT THE TIME-DELAY SETTINGS
WILL PROVIDE THE INTENDED COORDINATION.
Zone Selective Interlocking (or Zone Interlocking) is
available for the Digitrip 520 family on the Short Delay and
Ground Fault protection functions (see Figure 3.1). The
zone interlocking signal is wired via a single set of wires
labeled Zone In (Zin) and Zone Out (Zout) along with a
Zone Common wire. The Zone Selective Interlocking
function on the Digitrip 520 family has combined the logic
interlocking of Short Delay and Ground Fault. A zone out
signal is sent whenever the ground fault pick-up is
exceeded or when the short delay value of 2 x ( I r) is
exceeded. Zone Selective Interlocking provides the
fastest possible tripping for faults within the zone of
protection of the breaker and yet also provides positive
coordination among all breakers in the system (mains,
ties, feeders, and downstream breakers) to limit a power
outage to only the affected parts of the system. When
Zone Interlocking is employed, a fault within the zone of
protection of the breaker will cause the Digitrip 520 family
of units to:
•
Trip the affected breaker immediately and, at the same
time,
•
Send a signal to upstream Digitrip units to restrain from
tripping immediately. The restraining signal causes the
upstream breakers to follow their set coordination
times, so that the service is only minimally disrupted
while the fault is cleared in the shortest time possible.
For an example of how Zone Selective Interlocking may
be used, see Appendix A of this Instructional Leaflet.
Page 15
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