UFC 3 -520-01
June 10, 2002
4-2.6
Do not use autotransformers unless necessary for a special application.
Explain why the special application requires an autotransformer as part of the design
analysis.
4-2.7
Encapsulated transformers are not usually required. Most transformers are
continuously energized and the resultant core losses keep the transformer warm,
usually above dew point, so that encapsulation is not required. If necessary because of
corrosive or condensing environments, encapsulated transformers can be used. These
types of environments should not be observed for most interior applications.
4-3
MEDIUM VOLTAGE TRANSFORMERS.
4-3.1
Consider medium voltage transformers for use in large facilities when low
voltage transformers are no longer economically viable. Medium voltage transformers
can be located within large buildings near load centers to avoid long low voltage feeders
or to attain a more economical installation.
4-3.2
Ventilated dry-type or encaps ulated cast coil distribution transformers with
copper windings should be preferentially used. If liquid-insulated or oil-insulated
transformers are specified for interior applications, refer to MIL -HDBK-1008C, Fire
Protection for Facilities Engineering, Design, and Construction, for guidance regarding
installation criteria.
4-3.3
Hermetically sealed dry-type transformers should not be specified because of
their large size and the problems associated with loss of gas.
4-3.4
Provide a transformer economic analysis, including the rationale supporting
the decision, as part of the design analysis. Include life cycle cost comparisons
between units, including all building features required to accommodate each
transformer type, such as a vault, drainage system, and fire protection where required.
Ensure all transformers and equipment addressed in the analysis are equal in every
electrical respect, including but not limited to capacity, voltage, overload capability, and
BIL to obtain a fair comparison.
4-3.5
Do not use autotransformers unless necessary for a special application.
Explain why the special application requires an autotransformer as part of the design
analysis.
4-3.6
Fire protection criteria for transformer installations are provided in MIL -HDBK-
1008C.
4-4
OTHER TRANSFORMERS.
4-4.1
Isolation Transformers.
4-4.1.1 Isolation transformers are commonly used to establish a separately derived
system. A separately derived system as defined in the NEC is a wiring system whose
4-7
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