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TM 5-685/NAVFAC MO-912
2-1. Emergency power.
ally is started manually; a class B plant may have
either a manual or an automatic start system. Ac-
Emergency power is defined as an independent re-
cordingly, a class B plant is almost as costly to
serve source of electric energy which, upon failure
construct and operate as a primary power plant of
or outage of the normal source, automatically pro-
similar size. U s u a l l y , a class B plant is a
vides reliable electric power within a specified time.
permanent-type unit capable of operating between
a. A reliable and adequate source of electric
1000 and 4000 hours annually. The class C plant
power is necessary for the operation of active mili-
always has an autostart control system (set to start
tary installations. Power must also be available at
the plant when the primary power voltage varies or
inactive installations to provide water for fire pro-
the frequency changes more than the specified op-
tection, energy for automatic fire alarms, light for
erational requirements).
security purposes, heat for preservation of critical
(1) A class B plant (considered a standby long-
tactical communications and power equipment, and
term power source) is used where multiple commer-
for other operations.
cial power feeders are not available or extended and
b. Power, supplied by either the local utility com-
frequent power outages may occur. Total fuel stor-
pany or generated on-site, is distributed over the
age must be enough for at least 15 days continuous
activity. The source of distribution may be subject to
brownout, interruption or extended outage. Mis-
(2) A class C plant is used where rapid restora-
tion of power is necessary to feed the load. More
sion, safety, and health requirements may require
than one class C unit is usually used when the
a n uninterruptible power supply (UPS) or
technical load exceeds 300 kW at 208Y/120 volts or
standby/emergency supply for specific critical loads.
600 kilowatts (kW) at 48OY/277 volts. Spare class C
Justifiable applications for auxiliary generator are:
units are sometimes provided for rotational mainte-
(1) Hospitals (life support, operating room,
nance service. The autostart control system ensures
emergency lighting and communication, refrigera-
that the load is assumed as rapidly as possible.
tion, boiler plant, etc.).
Diesel engine prime movers may be equipped with
(2) Airfields (control tower, communications,
coolant and lubricating oil heaters to ensure quick
traffic control, engine start, security, etc.).
starting. Recommended total fuel storage must be
(3) Data processing plant systems.
enough for at least seven days continuous opera-
(4) Critical machinery
(5) Communication and security.
c. Emergency generators must provide adequate
c. It is essential that a schematic showing the
power for critical loads of a building or a limited
loads to be carried by an auxiliary generator be
group of buildings, heating plants, utility pumping
available for reference. Do not add loads until it is
plant, communication centers, or other such instal-
approved by responsible authority.
lations where interruption of normal service would
2-2. Types of power generation sources.
be serious enough to justify installation of an auxil-
iary power plant. The plant must be reliable and
a. The critical uses of electric power at a site
easily started in all seasons of the year. The plant
demand an emergency source of power whenever an
building should be completely fireproof with heating
outage occurs. Selection of the type of auxiliary gen-
and ventilation facilities that satisfy the plant's re-
erating plant is based on the mission of the particu-
quirements. The space around the units should per-
lar site and its anticipated power consumption rate
mit easy access for maintenance and repair. Space
during an emergency. The cost of plant operation
should be provided within the building for safe stor-
(fuel, amortized purchase price, depreciation, and
age of fuel such as a grounded and vented "day"
insurance) and operation and maintenance person-
tank. Type and grade of fuel should be identified on
nel requirements must be analyzed. Future load
the tank. Important considerations for these plants
growth requirements of the site must be considered
included the following:
for size selection.
(1) Selection of generators (size and quantity,
b. Auxiliary power generating plants are desig-
type of prime mover, and load requirements).
nated as either class B or class C. The design crite-
(2) Determination of need for instrumentation
ria for a class B plant is comparable to those of a
(meters, gauges, and indicator lights).
primary power plant. A primary power plant usu-

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