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PT UNIT USER MANUAL
APRIL 2023 www.eaton.com.cn
4. PT Protection Settings
4.9 Zone Selective Interlocking (ZSI)
The Zone Selective Interlocking (ZSI) function is provided only
on PT25 trip units and can be enabled or disabled through
the menu system or Power Trip Protection Manager software.
The ZSI functions in conjunction with the Short Delay
and Ground Fault protection functions. The ZSI provides
the fastest possible tripping for faults within the zone of
protection of the circuit breaker, and also provides positive
coordination among all circuit breakers in the system (mains,
ties, feeders, and downstream circuit breaker).
When the ZSI is enabled, a fault within the zone of
protection will immediately trip the breaker and send a
signal to upstream trip units to restrain them from tripping
immediately. The restraining signal causes the upstream
circuit breakers to follow their set coordination time delays so
that the service is interrupted to the isolated fault area only
while the fault is cleared in the shortest time possible.
The ZSI is wired using a set of three wires labeled Zone
In (Zin), Zone Out (Zout), and Zone Common (Zcom). The
restraining signal is sent whenever the ground fault pickup
is exceeded or when the short delay pickup value of two
times I
r
is exceeded. This provides maximum selectivity for
coordination with upstream circuit breakers. For the furthest
downstream breaker, the use of a self-interlocking jumper
may or may not be needed depending on the application.
If immediate tripping is desired on the last breaker, the Zin
on that breaker can be left open while the Zout is wired to
the Zin of the breaker upstream from it. If a time delay is
desired on the last breaker, then a jumper from the Zout of
that breaker should wired to the Zin of the same breaker to
provide a self-interlocking feature. Refer to Eaton Application
Note AP02602002SC for detailed description and examples.
4.6.8 Phase Loss Protection (PT20/25)
Phase loss protection is used in case of complete absence
of one or two phases. If the difference between the
maximum and minimum current values in the three phases
is greater than 75% of the maximum current value, and the
corresponding time delay setting is exceeded, the trip unit
will act.
Note:
Phase loss protection is effective only when at least
one phase current is greater than 50% of the Long Delay
Pickup (LDPU or I
r
) value.
4.6.9 Reverse Power
The trip unit monitors the power value in real time. When the
reverse active power exceeds the set value, and the set time
delay is exceeded, the trip unit will take the corresponding
action (Alarm or Open) when the function is enabled.
4.7 High Instantaneous Protection
The PT unit provides a high instantaneous trip function that
will trip the circuit breaker at the withstand current rating
of the circuit breaker. This function is factory set within the
frame module and reacts to the peak current. It is always
active regardless of the user’s instantaneous adjustment
selection, including “OFF”. The instantaneous (“INST”) indicator
shows this cause of trip.
All Series IZM6 frame modules have an High Instantaneous
trip feature.
4.8 Making Current Release (MCR)
All PT unit styles have a Making Current Release (MCR)
function. This safety feature prevents the circuit breaker from
being closed and latched-in on a faulted circuit. The MCR is
enabled only for the first two cycles of current following the
initial circuit breaker closing operation. The circuit breaker will
trip with no delay and the instantaneous (“INST”) indicator will
blink.
This non-adjustable trip function is subject to the circuit
breaker’s frame. Refer to time current curves for specific
values.
