MIL-HDBK-1004/2A
functional justification for the type of unit provided. Select the insulation
type on the basis of surrounding atmosphere and basic impulse insulation level
required. Check loading characteristics from ANSI C57.91, C57.92, or C57.96
making sure that the lower impulse level standardly provided for dry-type
units are adequately protected. Ensure equipment conforms to NFGS-16465.
5.2.4.1 Dry-Type Units. Conventional-ventilated, epoxy-encased-ventilated
(cast resin), or gas-filled are all considered dry-type units. Normally the
conventional-ventilated type, as the least costly, shall be installed.
However, consider using the epoxy-encased type for units which serve in a
stand-by status. Conventional units readily absorb moisture when not
energized and need special handling when re-energized; epoxy-encased types do
not. Epoxy-encased types can be considered to be better braced for short-
circuit duty since the epoxy entirely supports the coils. Restrict gas-filled
types to hazardous area locations, if such units cannot be located outside the
dangerous area.
5.2.4.2 Nondry-Type Units. Install only less-flammable, liquid-insulated
and oil-insulated units. Do not install askarel-insulated and nonflammable,
fluid-insulated umits.
5.2.4.3 Insulation Comparisons. For a comparison of the various transformer
insulations refer to Table 6. Cost comparison is based on the cost of oil
filled outdoor type transformers. Cost percentages given first are for indoor
locations. Second cost percentages given in parentheses are for outdoor
locations. Costs are based upon transformers with cores made of silicon-steel
materials. Amorphous Core Transformers, with cores made of amorphous metal,
costs approximately twice the cost shown on Table 6. Cost of amorphous core
transformers is based upon Year 1991 price. Refer to section paragraph 1.4.3
for more information on Amorphous Core Transformers.
5.2.5
Unit Substation Rooms. Ensure unit substation rooms do not contain
any pipes, ducts, or other foreign systems, except those required for fire
protection, ventilation, and drainage of the room; however, dry-type units may
be installed without a separate room if piping is not run immediately over
such units.
5.2.5.1 Drainage. Provide rooms containing liquid-insulated transformers
with concrete curbs and an appropriate drainage system to prevent liquid from
spreading to adjacent rooms. Use the criteria in NFPA 70 for unit substation
rooms.
5.2.5.2 Access. Design equipment rooms where primary switching equipment,
other than transformer switching, is installed with not less than two separate
access doors for use by authorized personnel only.
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