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ETL 1110-8-5(FR)
26 Jul 91
pickup voltage connected in series with the trip coil
of Figure 2 will trip the unit breaker. This 41"b"
should be considered for red light indication in lieu of
would also cause the breaker to trip if the unit was
an auxiliary contact, to allow remote supervision of
brought on-line without excitation, but the 41"a" in
the trip coil. Figure 3 suggests such a scheme. When
Paragraph a prevents the unit from closing under such
only a single red lamp is required without alarm
conditions. In the case where a 41 field breaker
annunciation, the red light can be put in place of the
contact is not present, such as in some static exciter
74TC relay coil. Problems have occurred where
applications, a 52E (exciter AC supply breaker) or
Projects have attempted to run parallel loads in series
other comparable contact should be substituted.
with the breaker coil. It is recommended that the
parallel loads be driven from contacts of Relay 74TC
c. The field breaker control circuit in Figure 2 al-
as shown. Since the 74TC coil is more reliable than
lows time for normal schemes to open the unit break-
most light filaments, and functions in the same man-
er. If normal shutdown or the previously described
ner, the reliability is not significantly affected. In
schemes fail to open the unit breaker, then the unit is
addition, a remote and local alarm can be added to
likely to end up with the field breaker open, and the
annunciate trip coil failure initiated by dropout of
unit breaker remaining closed. This condition will
Alarm Relay 74TC when the breaker is closed. The
cause Device 2GT in Figure 2 to time-out. Device
time delay (Device 2) is necessary to allow the break-
2GTX will operate Device 86L to trip the line break-
er to open during normal tripping without initiating
er so that the line cannot backfeed the failed unit.
an alarm. This paragraph describes only supervision
Tripping the line breaker will then trip the remaining
and alarm for the trip coil. Paragraph 7a describes
units on that line. This is necessary so that the other
backup for the trip coil.
units on the bus do not feed the failed unit. The
failed unit will be isolated, and its governor will drive
7. Fail-Safe Breaker Tripping
it to approximate speed-no-load.
6. Breaker Trip Coil Supervision and Alarm
a. The schemes presented previously have de-
scribed ways to backup the unit breakers after a fail-
Some control systems have used breaker auxiliary
ure. Paragraph 6 describes supervision and alarm for
contacts for remote red light indication of breaker
the trip coil. The failures described in Paragraphs 4,
status. Relays with high coil resistance and suitable
5, and 6 could be caused by a mechanical failure or a
Figure 3. Scheme to allow remote supervision of the trip coil
3
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