TM5-811 - 1 / A F J M A N 3 2 - 1 0 8 0
Table 4-4. Current Transformer (CT) Accuracy Class Ratings for Outdoor Circuit Breakers
"C" or "T"
Nominal Voltage Class
CT Ratio
Accuracy Class
RELAYING SERVICE
(MULTI-RATIO UNITS)
1,200:5
115kV
800
2,000:5
800
3,000:5
800
4,000:5
800
800
5,000:5
400
69 kV
1,200:5
800
2,000:5
3,000:5
800
100
46 kV
600:5
200
1,200:5
2,000:5
400
3,000:5
400
4,000:5
800
5,000:5
800
METERING SERVICE
(SINGLE-RATIO UNITS)
115 kV to 46 kV
300:5
0.6B-0.5
600:5
0.6B-0.5
800:5
0.6B-0.5
1,200:5
0.3B-0.5
1,500:5
0.3B-0.5
2,000:5
0.3B-0.5
3,000:5
0.3B-0.5
metering VTs are provided, a 0.3 accuracy class
The battery, charger and panelboard should be
will be specified, if available for the voltage rating
installed in a separate enclosure only when the
and burden needed.
capacity, voltage rating and consequent size of the
c. Control power. An ac power source will be
battery warrants a separate housing, or when a
provided to supply power to station equipment
separate control house is required to house the
requiring an alternating-current source of power.
battery, charger, panelboard, annunciators, carrier
An ac power source will also be provided when the
current, supervisory, telemetering, relaying or
other instrumentation or control equipment.
utility or Using Agency requires a capacitor-
d. Control buses. When a tie circuit breaker is
tripping scheme for circuit breakers or other power
provided in a switchgear lineup, a control-power
switching apparatus to permit tripping following a
power outage to the station. Otherwise, and be-
automatic-transfer-system will be provided to al-
low full control function even with the loss of
cause of greater reliability, a dc power source will
either source. To accomplish this, the designer will
be provided for the close and trip operations of
circuit switching equipment, and for other equip-
specify that each bus be provided with a control
ment rated for direct-current applications. The dc
power transformer (CPT) connected, via overcur-
power source will be a lead-calcium type of battery
rent protection, to the source side of the main
rated at 48 V, or at 125 V when the additional
circuit breaker or switch. Each CPT will be sized
to easily handle the total control power require-
cost is warranted because of the ampere-hour
capacity required to supply station loads. A
ments of both buses. The secondaries of the CPTs
will be connected to the input terminals of a
battery-charger will be provided to ensure that the
transfer relay, transfer contactor, or automatic
battery is fully-charged at all times. The battery
transfer switch, depending upon the size of the
charger will be equipped with separate alarm
load and the specific installation requirements. All
lights to indicate low ac source input voltage and
the load for both buses will be connected to the
low dc output voltage. In addition, alarm contacts
output terminals of the transfer device. Upon loss
will be provided for remote annunciation of low ac
of the "Normal" source, transfer to the "Backup"
input and low dc output voltages. The battery and
source should be instantaneous and retransfer
charger and a direct-current panelboard should be
back to the "Normal" source should be automatic.
installed in the station switchgear assembly, to
Features such as selectability of which source is
avoid the additional cost for a separate enclosure.
4-12
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