Effective 8/2006
Page 12
I.L. 66A7534H04
3.0 PRINCIPLES OF OPERATION
3.1 General
The Digitrip 520V and 520MCV trip units are designed for
circuit breaker environments where the ambient tempera-
tures can range from –30
°
C to +85
°
C, but rarely exceed
70
°
to 75
°
C. If, however, temperatures in the neighborhood
of the trip unit exceed 85
°
C, 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 trip unit has a built-in over-temperature protection
feature, factory set to trip the breaker if the chip tempera-
ture is excessive. If over-temperature is the reason for the
trip the red “Long Delay Time” LED will flash.
The Digitrip 520V and 520MCV use 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 power required to operate the protection function is
derived from the CH Type-V current sensors in the enclo-
sure behind the circuit breaker.
(See Figure 1.4)
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 microprocessor.
The microcomputer continually digitizes these signals.
This data is used to calculate true RMS current values,
which are then continually compared with the protection
settings. The embedded software then determines whether
to initiate protection functions, including tripping the
breaker through the Trip Actuator.
3.3 Zone Interlocking
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 TERMI-
NALS SO THAT THE TIME-DELAY SETTINGS WILL
PROVIDE THE INTENDED COORDINATION.
Zone Selective Interlocking (or Zone Interlocking) is
standard for the Digitrip trip unit 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 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 pickup is exceeded. Zone Selective
Interlocking provides the fastest possible tripping for faults
within the zone of protection of the circuit breaker and yet
also provides positive coordination among all breakers in
the system (mains, ties, feeders, and downstream break-
ers) 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 circuit breaker will
cause the Digitrip 520V and 520 MCV to simultaneously:
1)
Trip the affected circuit breaker immediately
2)
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.
3.2 Trip and Operation Indicators
The LEDs on the face of the trip unit, shown in Figures 1.1
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 and reset
the trip unit.
The green
Unit Status
LED, shown in Figure 1.1, flashes at
one second intervals to indicate the operational 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
Unit Status
LED typically indicates
that a low level of load current, on the order of 5% of full
load, exists.
