MIL-HDBK-1004/2A
connection is impracticable. Where larger transformer capacities are
justified, provide current-limiting fuses in conjunction with primary or
secondary protective devices or other methods of limiting the available fault
current to an acceptable value.
5.2.1.3 Safety. All current-carrying parts of unit substations must be
dead-front construction, usually in grounded metal enclosures. In some cases,
nonmetallic enclosures, such as fiberglass may be functionally required.
Provide interlocking to prevent accidental contact with live parts when
working within the switchgear enclosure. For example, interlock access doors
to primary fuse compartments so that access is possible only when the primary
switch is open.
5.2.2
Design. A unit substation, by its definition, has coordinated
sections; therefore, provisions for future addition are possible at minimum
cost, if adequate space is provided.
5.2.2.1 Mounting. Provide a concrete base. Mount substations on channels
or other supports which are flush with the top of the base.
5.2.2.2 Short-Circuit Duty. Coordinate circuit interrupting devices with
each other. Each device shall be capable of clearing the maximum fault
available. Bracing for bus bars and other current-carrying parts should be
capable of withstanding the mechanical stresses produced by the maximum
available short-circuit current.
5.2.2.3 Primary Protection. Use air-type load interrupter switches with
current-limiting fuses, unless other protection such as circuit breakers or
liquid-type switches are more appropriate. Use of circuit breakers may be
required to provide sufficient interrupting duty, or because coordinated
tripping is required. Circuit breakers are more reliable than switches, but
their additional cost must be justified. Use of liquid-immersed switches is
appropriate for certain installations, such as in underground vaults.
5.2.2.4 Lightning Protection. Coordinate lightning protection with other
components of the distribution system that may be exposed to surge voltages.
Provide arresters when necessary. When dry-type transformers with lower
BIL???? ratings are used, extra protection may be required.
5.2.2.5 Secondary Protection. Use either metal-enclosed, low-voltage power
or molded-case circuit breakers as protection. Normally for building
services, molded-case units are used for less than 1,200 A and low-voltage
power units are used for higher amperages. For shore-to-ship service, low
voltage power units are required.
5.2.2.6 Instrumentation. Provide unit substations of more than 500 kVA with
at least an ammeter and a secondary voltmeter. The use of wattmeters
(recording or indicating) and voltmeters, for primary service metering, will
depend on the individual installation. For provision of watthour demand
meters with pulse initiators, see requirements for energy monitoring in this
section.
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