TM
5-811-1/AFJMAN
32-1080
will be interlocked with the smoke and rate-of-rise
circuit and protective device coordination con-
detectors in the transformer vault and/or vault
straints are met.
(5) Service for large loads. Unit substations and
rooms. The smoke and rate-of-rise detectors will
integral and/or non-integral load-center transform-
de-energize the HVAC air intake if a fire occurs in
ers will be used for industrial, heavy commercial,
the transformer vault and/or vault rooms. This is
and institutional applications involving large
not necessary if the HVAC intake is located on the
loads, multiple building serves, and the require-
roof or the opposite side of the building from the
ments for secondary bus protective devices.
transformer vault. A sign will be posted on the
(6) Secondary unit substations. Secondary unit
access doors to a transformer vault which indicates
substations with integral or nonintegral outgoing
the appropriate fire fighting equipment and proce-
dures. Pipes and ducts will not be routed above
sections will be used. Since neither type is tamper-
indoor transformers.
proof, fencing is required. Fence height will be in
(c) Dry-type transformers. Dry-type trans-
accordance with the NESC. Transformers larger
former applications will comply with the NEC and
than 1,500 kVA for 480Y/277 V service and 500
kVA for 280Y/120 V service should be avoided,
MIL-HDBK-1008A. Dry-type transformers will be
because of the magnitude of their secondary fault
located to preclude any reasonable chance that
products of combustion from a transformer fire
currents. However, in some cases, it may be more
will be drawn into the building's HVAC system.
feasible and cost effective to provide 2,000 kVA
transformers for 480Y/277 V service and use
Dry-type transformers in ventilated enclosures
current-limiting fuses in conjunction with circuit
may be specified for indoor applications in clean,
dry environments (not subject to blowing dust,
breakers to limit secondary fault currents. The
using agency (for Air Force, the Host/REQ CMD)
excessive humidity, or corrosive atmosphere) and
will be rated for the highest temperature of the
will stipulate demand meter requirements.
(7) Location. Exterior installations are pre-
location. Dry type transformers in non-ventilated
ferred over interior installations because space
enclosures may be used for indoor or outdoor
costs are less; however, secondary feeder lengths
applications, if the enclosure is rated for the
environment. Hermetically-sealed dry transform-
may require an interior location or make interior
ers (filled with dry nitrogen) will not be specified
installations economical in some cases.
(a) Liquid-filled transformers installed out-
due to their large size, the inability to add supple-
doors. Outdoor installations will comply with the
mentary cooling, and problems associated with loss
NEC, MIL-HDBK-1008A, and the NESC. Trans-
o f gas. Transformers protected by cast- or
formers will be located to preclude any reasonable
impregnanted-epoxy-resin may be used indoors and
chance that products of combustion from a trans-
outdoors, where environmental conditions would
former tire will be drawn into the HVAC air
shorten the life of a conventional dry transformer.
intake of adjacent buildings. The location of pad-
Specifications will include epoxy encapsulation for
mounted transformers will be compatible with the
the core and for both the high and low voltage
windings. Each connection insulation must be
architectural concept and protected from vehicular
compatible with the environment. The specifica-
traffic. Architectural compatibility will be ob-
tion for medium-voltage primary transformers will
tained by the proper location in relation to land-
scaping, the addition of shrubbery around the
require the basic insulation level to be the same as
for liquid-filled transformers in the same voltage
transformer, or the use of screened fence enclo-
sures. The primary service will be underground
class, and will require the transformers to be
tested in accordance with applicable ANSI tests for
from pad-mounted transformers. Secondary build-
liquid-filled transformers.
ing connections will use underground cables or bus
c. Underground Mounting. Transformers in un-
duct; however, the use of more than six under-
ground cables per phase in parallel will not be
derground vaults are not permitted except where
required to meet airfield clearances. Requirements
permitted since complexity of connection can lead
are given in chapter 7 for equipment in under-
to maintenance and space problems.
(b) Liquid-filled transformers installed in-
ground vaults.
doors. Indoor installations will comply with the
8-3. Installation of Transmission-to-Distribution
NEC and MIL-HDBK-1008A. Transformer vaults
Voltage Transformers.
will be located on the exterior wall of the building,
These transformers are installed as part of the
vented to the outside and accessible only from
main electric supply station, since the secondary
outside the building under normal design condi-
voltage will be of the 15 kV class or larger.
tions. Air intake fans and louvers of heating,
Primary unit substation transformers will be used
ventilating, and air conditioning (HVAC) systems
8-4
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