Westinghouse KC-4, Installation, Operation & Maintenance Instructions Manual

Page: 90 / 178

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2. External Connections for the KC-4 Relay - Breaker Failure Protection of a Single Bus/Single Breaker Arrangement
- One Timer per Bus.
has been initiated. They energize the T D-5
breaker-failure timer (62) through the KC-4 (50)
contacts, which have already closed in response
to the same fault which operated the line relays.
If the breaker interrupts fault current as expected,
the KC-4 drops out and the T D-5 timer is de­
energized before it times out; no further action
is taken . But if current flow continues beyond the
expected time limit, the T D-5 delay expires and
the output (T R) contact of the T D-5 energizes
the 62A auxiliary relay for contact multiplication.
62A contact energizes the 868 lockout auxiliary
to strip the bus and block reclosing.
N ote that, in figure 2, each breaker has its own
line protection relays, BFI auxiliary and KC-4
current detector; but that all of these fun nel into a
single TD-5' timer. This is done for ecomony since
the same tripping device1 868, is energized regard­
less of which breaker fails. Targeting of the parti­
cular failed breaker is accomplished by the ICS
in the associated
KC-4
in conjunction with the 62X
and 62Y BFI contacts for that line. The zener
blocking diode inside each KC-4, connected to
4
terminal 50-2, isolates the multiple trip circuits
of the
KC-4 's from one another so that only the
target of the failed breaker will drop.
After the TD-5 has operated and picked up.
62A, the 62A contact also energizes the 86BF
auxiliary relay. Refer to the primary break(:r
tripping circuit. If a fault occurs and the break(:r
operating mechanism is stuck (the most frequent
type of breaker failure), line relays energize th e
trip coil with no effect. The breaker failure schem e
will operate the initiate backup tripping, elimin­
ating threats to power system integrity. H owever,
de current will continue to flow in the primary
relay
ICS and in the breaker trip coil, uninter­
rupted by the stuck 52a contact. The trip coil, and
other devices in the tripping path, are only inter­
mittently rated and may disintegrate or ignite
before operators or repair crews can interven(:.
For this reason, a normally-closed 86BF contact
is shown in series with the trip circuit, to interrupt
de current flow.
After the TD-5 has operated, a sneak JJdth may
be created through the supervising lamp for the
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