transformers (CTs) at standard burdens are listed in
IEEE C57.13. The minimum standard current
transformer accuracies for metal-clad switchgear are
listed in IEEE C37.20.2. In general, ANSI C12.11
requires a 0.3 accuracy class for up to a B-0.5
burden, except for some 200 and 400 ampere units.
Where metering current transformers are provided,
this accuracy class should be specified, if
available for the ampere rating and burden needed.
A "C" classification means the ratio error can be
calculated, whereas a "T" classification is one
which has to be derived by testing. IEEE C37.20.2
permits either classification up to the indicated
ratings.
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Unless otherwise indicated, bar, wound, or window-type transformers are
acceptable; and except for window-type units installed over insulated
buses, transformers shall have a BIL rating consistent with the rated BIL
of the associated switchgear or electric power apparatus bushings, buses or
conductors. Current transformers shall have the indicated ratios. The
continuous thermal-current rating factor shall be not less than [1.0] [1.2]
[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
accidental 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.
2.8.2.1
Current Transformers for Power Transformers
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NOTE: ANSI C57.12.10, Table 20 gives recommended
values.
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[Single-ratio] [Multi-ratio] bushing type current transformers shall be
provided in circuit breaker bushing wells as indicated. [Single-ratio
units shall have a minimum metering accuracy class rating 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.]
2.8.2.2
Current Transformers for Metal-Clad 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.
2.8.2.3
Current Transformers for Kilowatthour and Demand Metering
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NOTE: Use the following guidelines for specifying
current transformers.
1.
Select the standard current transformer (CT)
SECTION 16311A
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