I.B. 17555
Page 31
Effective 03/97
SECTION 4: APPLICATION CONSIDERA-
TIONS
4-1 ZONE INTERLOCKING CAPABILITIES
To minimize damage to the system, faults should be
cleared as quickly as possible. Zone selective interlock-
ing provides this capability better than a system with
only selective coordination.
When the “Ground Zone Interlocking” feature is utilized,
an immediate trip is initiated when the fault is in the
breaker’s zone of protection, regardless of its preset
time delay. When the “Phase Zone Interlocking” feature
is utilized, the inverse time overcurrent and short delay
phase elements work as follows. The short delay phase
element will initiate an immediate trip when the fault is in
the breaker’s zone of protection, regardless of its preset
time delay. For the inverse time overcurrent phase
element, the current sensed by the Digitrip 3000
must exceed 300 percent (3 x I
n
) for the zone selec-
tive interlocking to initiate an immediate trip signal.
This interlocking signal requires only a pair of wires from
the downstream breaker to the upstream breaker.
When a Digitrip 3000 initiates a trip signal, the zone
interlocking signal stays active for an additional 175 mil-
liseconds. Therefore, if a downstream Digitrip 3000 is
zone interlocked to an upstream Digitrip 3000, the
downstream breaker will have 175 milliseconds to clear
the fault before the upstream Digitrip 3000 is allowed to
react to that same fault.
Zone interlocking, therefore, provides fast tripping in the
zone of protection, but gives positive coordination
between mains, feeders and downstream breakers. For
faults outside the zone of protection, the Digitrip 3000
on the breaker nearest the fault sends an interlocking
signal to the Digitrip 3000 protective devices of the
upstream breakers. This interlocking signal restrains
tripping of the upstream breakers until their set coordi-
nation times are reached. Thus zone interlocking,
applied correctly, can result in minimum damage with a
resultant minimum disruption of service.
Zone selective interlocking is available on Digitrip 3000
Protective Relays for the inverse time and short time
functions on the phase and ground elements. Refer to
Figure 4-1 for a typical phase zone selection interlocking
wiring diagram or refer to Figure 4-2 for a typical ground
zone selection interlocking wiring diagram.