[1.5] [2.0] [3.0] [4.0]. Other thermal and mechanical ratings of current
transformers and their primary leads shall be coordinated with the design
of the circuit breaker and shall be not less than the momentary rating of
the associated circuit breaker. Circuit protectors shall be provided
across secondary leads of the current transformers to prevent the accident
open-circuiting of the transformers while energized. Each terminal of each
current transformer shall be connected to a short-circuiting terminal block
in the circuit interrupting mechanism cabinet, power transformer terminal
cabinet, and in the associated instrument and relay cabinets.
2.10.4.3
Current Transformers for Power Transformers
**************************************************************************
NOTE: ANSI C57.12.10, Table 20 gives recommended
values.
**************************************************************************
[Single-ratio] [Multi-ratio] bushing type current transformers shall be
provided internally around power transformer bushings as shown.
[Single-ratio units shall have a minimum relaying accuracy class of
[0.6B-0.5] [0.3B-0.5].] [Multi-ratio units shall have a minimum relaying
accuracy voltage class of [_____] for either a C or T classification.]
2.10.4.4
Current Transformers for Metal-Enclosed Switchgear
Single-ratio units, used for metering and relaying, shall have a metering
accuracy class rating of [_____] [B._____]. Single-ratio units, used only
for relaying, shall have a relaying accuracy class rating of [_____] for
[either] a C [or T] classification.
2.10.4.5
Current Transformers for Kwh and Demand Metering (Low-Voltage)
**************************************************************************
NOTE: Use the following guidelines for specifying
current transformers.
1. Select the standard current transformer (CT)
primary rating which is just below the full load
current of the serving power transformer, i.e., for
a 500 kVA transformer with a full load of 1387 amps
at 208 volts-select a 1200/5 CT ratio; for a 750 kVA
transformer with a full load of 902 amps at 480
volts-select a 800/5 CT ratio.
2. Select a continuous-thermal-current rating
factor (RF) in accordance with the following Table:
RATIO
RF at 30 degrees C
200/5
4.0
300/5
3.0
400/5
4.0
600/5
3.0
800/5
2.0
1200/5
1.5
1500/5
1.5
2000/5
1.5
3000/5
1.33
SECTION 16375A
Page 41
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