NOTE: Refer to TM 5-811-1/AFM 88-9 Chapter 1 for
guidance regarding control buses.
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Control buses shall be provided as necessary to supply power to control
devices. [Buses shall be supplied from low-voltage panelboards. Where one
panelboard serves more than one bus, each group of units on each bus shall
be served by different branch circuit breakers.] For double-ended buses,
both buses shall be supplied from one low-voltage panelboard and each bus
shall be served by different branch circuit breakers. The low-voltage
panelboard shall be served from an automatic transfer [relay] [contactor]
[switch], which, in turn, shall be served from two control power
transformers (CPT). One CPT shall be connected via fuses ahead of each
main circuit breaker. Each CPT, fuse, transfer device, panelboard, and
wiring system shall be sized to handle 125 percent of the total load of
both buses. The "Normal" and "Backup" sources shall be as indicated. Upon
the loss of the "Normal" source, transfer to the "Backup" source shall be
instantaneous. Retransfer back to the "Normal" source shall be [automatic
upon the restoration of the "Normal" source] [automatic after a [_____]
time delay once the "Normal" source is restored]. [The "Normal" and
"Backup" source shall be selectable.] [An alarm shall be provided to
indicate a transfer operation.] [An alarm shall be provided to indicate
loss of a source.] Insulated wire buses shall be wired to interface
terminal blocks for connection between switchgear units and exterior
components. Wire bus shall not be less than [No. 8 AWG] [_____], nor less
than required to serve the complete switchgear lineup plus 25 percent spare
capacity.
2.7.4
Control Power Transformers
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NOTE: Where an outdoor structure-mounted
oil-immersed distribution transformer is used for
control power, such as when metal-clad switchgear is
not provided, specify requirements using data from
Section 16370A ELECTRICAL DISTRIBUTION SYSTEM AERIAL
and protect such transformers with power fuse
disconnecting units.
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Control power transformers shall comply with IEEE C57.12.01, shall be of
the ventilated dry type, and shall provide [240/120-volt, single-phase]
[208Y/120-volt, 3-phase] electric power for station ac control power
requirements. The transformer primary voltage rating shall be [_____] kV
and the transformer capacity shall be [_____] kVA [as indicated]. The BIL
rating shall equal or exceed the BIL rating of the switchgear. Transformer
current-limiting primary fuses shall be drawout type and shall be
interlocked with a secondary molded case circuit breaker provided as a part
of the transformer installation. Molded case circuit breakers shall comply
with NEMA AB 1. It shall not be possible to open the primary fuse
compartment unless this secondary circuit breaker is in the open position.
Construction shall be of the drawout type for either the complete assembly
or for primary fuses only, according to the manufacturer's standard.
Mechanical interlocks shall prevent removal of primary fuses, unless the
associated assembly is in a drawout or disconnected position. Transformer
compartments shall have hinged doors.
SECTION 16311A
Page 43
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