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Magnum PXR applications
MAGNUM PXR PRODUCT GUIDE
PA013006EN September 2022 www.eaton.com
5
Zone selective interlocking
Zone selective interlocking (ZSI) provides positive system
coordination by allowing the breaker closest to the fault
to trip without any preset time delays. This is achieved
by setting up the distribution system as shown in
Figure
75
. The hardwired connection between the trip units
sends a restraining signal upstream, allowing the breaker
closest to the fault to act instantaneously. The two Branch
breakers are in the lower Zone and selectively interlocked
to the Feeder breaker above them. By tripping the
breaker closest to the fault and bypassing any selective
coordination time delays reduces the stress and damage
on the distribution system by isolating and clearing the
fault faster.
By definition, a selectively coordinated system is one
whereby adjusting trip unit pickup and time delay
settings, the circuit breaker closest to the fault trips first.
The upstream breaker serves two functions: (1) backup
protection to the downstream breaker and (2) protection
of the conductors between the upstream and downstream
breakers. These elements are provided for on the PXR trip
units.
For faults that occur on the conductors between the
upstream and downstream breakers, it is ideally desir-
able for the upstream breaker to trip with no time delay.
Because that breaker is closest to the fault. This is the
feature provided by zone selective interlocking.
Zone selective interlocking is a communication signal
between trip units applied on upstream and downstream
breakers. The protection settings on each trip unit must be
applied as if zone selective interlocking were not employed
and set for selective coordination.
During fault conditions, each trip unit that senses the
fault sends a restraining signal to all upstream trip units.
This restraining signal results in the upstream trip unit to
continue timing as it is set. In the absence of a restraining
signal, the trip unit trips the associated breaker with no
intentional time delay, minimizing damage to the fault
point. This restraining signal is a low-level signal. To
minimize the potential for induced noise, and provide a
low impedance interface between trip units, twisted pair
conductors are used for interconnection.
Ground fault and short delay pickup on PXR trip units can
also have zone selective interlocking.
Zone selective interlocking may be applied as a type of
bus differential protection. It must be recognized; however,
that one must accept the minimum pickup of the trip unit
for sensitivity.
As shown
Figure 76
, multiple sources can be interlocked.
The use of “Steering Diodes” help to prevent signals
from going in the wrong direction. PXR zone interlocking
has been tested with over three levels with up to 20 trip
units per level. All Eaton circuit breaker and protective
relays employ the same circuits to make the ZSI systems
compatible with each other. For more information, see
application note AP02602002EN.
Figure 75. Twisted pair cabling
Figure 76. Main-Tie-Main system
