Effective 10/2004
Page 43
I.L. 70C1036H05
APPENDIX A Zone Interlocking Examples
NOTICE
THE PROVISION FOR ZONE INTERLOCKING IS STAN-
DARD ON CIRCUIT BREAKERS WITH DIGITRIP TRIP
UNITS FOR SHORT TIME AND GROUND FAULT FUNC-
TIONS. THE APPROPRIATE JUMPER TO TERMINAL B8
AND B9 MUST BE ADDED ON THE CIRCUIT BREAKER
IF ZONE INTERLOCKING IS NOT DESIRED OR IF FIELD
TESTING IS DESIRED.
EXAMPLE 1: There is no Zone Selective Interlocking.
(Standard time delay coordination is used.)
Assume that a ground fault of 2000 Amperes occurs and
refer to Figure A.1.
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.
EXAMPLE 2: There is Zone Selective Interlocking.
Assume a ground fault of 2000 Amperes occurs and refer
to Figure A.1.
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 circuit breaker does not clear the fault, the main
circuit 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 interlock-
ing 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 circuit 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 A.2 presents a Zone Selective Interlocking connec-
tion diagram for a system with two main circuit breakers
from incoming sources and a bus tie circuit breaker. Note
that the blocking diode D1 is needed so that the feeder
circuit breakers can send interlocking signals to both the
main and the tie circuit breakers and prevent the tie
breaker from sending an interlocking signal to itself.
