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3. Protection functions
POWER XPERT® RELEASE TRIP UNIT FOR MAGNUM PXR CIRCUIT BREAKERS MN013015EN January 2022 www.eaton.com
3.9 Zone selective interlocking (ZSI)
The zone selective interlocking (ZSI) functions in conjunction
with the short delay and ground fault protection functions.
ZSI provides the fastest possible tripping for faults within the
zone of protection of the circuit breaker and provides positive
coordination among all circuit breakers in the system (mains,
ties, feeders, and downstream circuit breakers).
When ZSI is enabled and a fault occurs, all circuit breakers
that see the fault current will send a signal upstream that
they “see” the fault. When the fault current is above the
pickup level of the ground fault or the short delay settings,
the circuit breaker will check its input to check if anyone
below it has sent a restraining signal. If there is an input
signal, the circuit breaker will follow its normal time delay
waiting for the downstream breaker to clear the fault. If
there is no input signal, and after a double check of the
signal, the PXR breaker will immediately trip since it is the
closest circuit breaker to the fault.
The ZSI is wired using a set of three wires labeled Zone In
(ZI), Zone Out (ZO), and Zone Common (ZC) on the second-
ary terminals above the circuit breaker. These signals are
compatible with all Eaton circuit breakers which have the ZSI
function. The zone out signal is sent whenever the ground
fault pickup is exceeded or when the short delay pickup
value of 2 x (I
r
) is exceeded. This provides maximum selec-
tivity for coordination with larger upstream circuit breakers.
Figure 12. Typical ZSI wiring.
The PXR trip unit display provides unique new features to
provide confidence that ZSI is enabled, working, and to test
ZSI. The display of the trip unit will show the letters ZSI on
the bottom when ZSI is enabled. When there is a ZSI input
signal detected on the input terminal, the display of the PXR
trip unit will show a checkmark (ü) at the bottom next to the
letters ZSI. The check mark will remain on the display until
the RESET pushbutton pushed to clear it.
Figure 13 shows an example of the feeder 11 breaker being
tested. At the start of a test, press the RESET pushbutton
on all trip units and all trip units will start off with the check-
mark cleared.
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