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

Page: 94 / 178

T Y P E
allowed since they cause no difficulties and result
in the simplest scheme. Recall that the transfer­
tripping of the faulted line isn't really redundant
since it provides the critical reclose-blocking
function at the far terminal, as in the previous
cases.
SETTING THE KC-4 FOR BREAKER
FAILURE PROTECTION
In all of these schemes, the KC-4 phase-unit
pickup should be set above maximum load but
below minimum fault current levels. The residual
current unit should be set below the minimum
ground fault current. If the largest load exceeds
the minimum fault current, the KC-4 must be set
below the minimum fault, and the user must ac­
cept a slight reduction in security. At times of
heavy load, the KC-4 contacts are closed even
though no fault is present.
In the past, 52a contacts have occasionally
been connected in parallel with the KC-4 to allow
timer starting for light faults below the KC-4 set­
ting. This practice is not recommended, since the
52a is closed whenever the line is in service and the
security benefits of KC-4 supervision are com­
pletely lost. Also, breaker-failure protection is
incomplete since opening of the 52a contact after
relay operation doesn't show that fault current was
actually interrupted -it only shows that the trip
mechanism cycled. A lower KC-4 setting is a
better alternative.
Sometimes, a transformer connected to one
side of the protected breaker can present a parti­
cular setting problem. Light-current faults may
occur inside the transformer for which the KC-4
relay won't pick up. Thus, no breaker-failure pro­
tection is provided when differential or sudden­
pressure relays initiate tripping. To remedy this,
connect a 52a contact in series with an 86T contact
from the breaker-failure de positive supply to ter­
minal
9 of the TD-5 timer 62 (86T is a contact of
the lockout switch associated with the transformer
differential relay). Repeat this connection for each
breaker which is adjacent to the transformer.
The 86T contact supervision insures that
security is not reduced by the added connection.
If a transformer fault results in 86T tripping, the
8
timer is energized and is stopped only when 52a
opens. This won't happen if the breaker is stuck,
and backup tripping is initiated. The only com­
bination of circumstances for which this connec:­
tion doesn't help is a light transformer fault, and
a breaker which opens but doesn't interrupt.
Many utilities consider this unlikely and provide
no further protection aganist it. The only alter­
native is to replace 52a with a separate low-se:t
current detector, which can energize the timer
only when 86T closes.
C HARACTERISTICS
Phase & Ground Overcurrent U nits are
available in the following current ranges:
Range
Taps
0.5-2 Amps 0.5 0.75
1 .0 1 .25
1 .5 2
1 -4 1 .0 1 .5 2.0
2.5
3.0 4.0
2-8 2
3 4 5 6 8
4- 16
4
6 8 9 1 2 1 6
10-40 10 15 20
24
30 40
20-80 20 30
40 48 60 80
The tap value is the minimum current required
to just close the overcurrent relay contacts. For
pickup settings in between taps refer to the section
under adjustments. The pickup and dropout time
curves for the overcurrent units shown in Fig. 6.
TRIP CIRC U IT
The main contacts will safely close 30 amperes
at 250 volts de and the seal-in contacts of the!
indicating contactor switch will safely carry this
current long enough to trip a circuit breaker.
The indicating contactor switch has two taps
that provide a pickup setting of 0.2 or 2 amperes.
To change taps requires connecting of lead located
in front of the tap block to the desired setting by
means of a screw connection .
T R I P CIRCUIT CONSTANTS
Indicating Contactor Switch -
0.2 amp. tap 6.5 ohms d-e resistanc(:
2.0 amp. tap 0. 1 5 ohms de resistance
1 .0 amp. tap (when supplied) 0. 1 ohm de
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