MIL-HDBK-1004/2A
5.2.3
Arrangements. Use standard arrangements where possible. Standard
unit substations have the primary switch on the left, the transformer in the
middle, the circuit breakers on the right, and space for future additions.
5.2.3.1 Reversed. Where conditions dictate otherwise, the reversed
arrangement may be adopted.
5.2.3.2 Double-Ended. Use double-ended unit substations where two primary
sources are required to provide reliability. See Figure 6 for a layout of a
secondary-selective-type double-ended arrangement. Each transformer and its
associated equipment shall be capable of carrying the essential loads of both
sections. Refer to MIL-HDBK-1004/1 for additional policy on selection of
electric power sources for standby service. In sizing the transformers, take
into consideration allowable overloading. Refer to ANSI C57.91, C57.92, or
C57.96, for overloading. Where the normal double-ended unit substation is
used, provide for contemplated future expansion; this arrangement does not
permit addition of unexpected future circuit breakers. For applications where
future expansion may be greater than normal, or unpredictable, consider two
other arrangements requiring more floor area and initial cost, but which
permit easy addition of sections, or replacement of transformers with larger
units. One arrangement has the primary, transforming, and secondary sections
for each half in separate lineups, with a tie bus or conduit and wire
connecting the two. The tie breaker may be in either lineup. The other
arrangement has the primary and transforming sections, along with the
secondary main breakers separate from the secondary lineup.
5.2.3.3 Secondary Spot-Network. Use secondary spot-network unit substations
for loads requiring an unusually high service reliability where the increased
cost can be justified. Figure 2 would represent a secondary spot-network
substation with a two-transformer input to a multiple-feeder output, if the
tie circuit breaker were eliminated and the secondary main circuit breakers
were shown with network protectors. This system is the most reliable power
supply for large loads, since any fault on the primary input to the secondary
bus is automatically disconnected by the network protector operating at a
speed which greatly minimizes voltage dips from faults or large transient
loads. The extra cost of the additional transformers and network protectors
plus possible increased secondary duty ratings from the increased short-
circuit capacity of parallel transformers makes this a very expensive
installation, especially when in the usual case, three or four transformers
are paralled.
5.2.4
Transformer Insulations. Indoor unit substation transformers of the
dry-type may be used and need not be installed in vaults. Less-flammable,
liquid-insulated units may be installed without a vault where permitted by
NFPA 70. Where vaults are required for less-flammable, liquid-insulated
transformers in some cases and in all cases for oil-insulated transformers,
their installation shall conform to NFPA 70. Provide an economic or
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