Appendix A: Zone Selective Interlocking
Examples
Case 1: There is no zone selective interlocking. (Standard
time delay coordination is used.)
Assume that a ground fault of 2000A occurs in
Figure 24
Fault at location 3
-The branch circuit breaker will trip, clearing the fault
in 0.1 seconds
Fault at location 2
-The feeder circuit breaker will trip, clearing the fault in
0.3 seconds
Fault at location 1
-The main circuit breaker will trip, clearing the fault in
0.5 seconds
Case 2: There is zone selective interlocking.
Assume a ground fault of 2000A occurs and refer to
Figure 24
.
Fault at location 3
-The branch circuit breaker trip unit will initiate the
trip in 0.045 seconds to clear the fault and the branch
will send a restraint signal to the feeder trip unit; the
feeder will send a restraint interlocking signal to Z1.
-Main and feeder trip units will begin to time out
and, in the event that the branch circuit breaker does
not clear the fault, the feeder circuit breaker will clear
the fault in 0.3 seconds (as above). Similarly, in the
event that the feeder breaker does not clear the fault,
the main breaker will clear the fault in 0.5 seconds (as
above).
Fault at location 2
-The feeder circuit breaker trip unit will initiate the
trip in 0.045 seconds to clear the fault and will send
an interlocking signal to the main trip unit. The main
trip unit will begin to time out and, in the event that
the feeder circuit breaker. Z2 does not clear the fault,
the main breaker will clear the fault in 0.5 seconds (as
above).
Fault at location 1
-There are no interlocking signals. The main circuit
breaker trip unit will initiate the trip in 0.045 seconds.
Figure 25
presents a zone selective interlocking connection
diagram for a system with two main circuit breakers from
incoming sources and a bus tie circuit breaker.
ote:
N
The blocking diode D1 is needed so that the feeder breakers
can send interlocking signals to both the main and the tie breakers and
prevent the tie breaker from sending an interlocking signal to itself.
Operating Manual for Series NRX
Digitrip Trip Units 1150 / 1150i
Effective September 2013
Instructional Leaflet IL01301064E
38
EATON
www.eaton.com
