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 TM 55-1925-273-10-1
OPERATOR'S MANUAL
INLAND AND COASTAL LARGE TUG (LT)
THEORY OF OPERATION
ELECTRICAL SYSTEM
GENERAL
The 128' Large Tug (LT) uses alternating current (440 Vac, 220 Vac, and 120 Vac) and direct current (24 Vdc) electrical
systems. These electrical systems supply all of the electrical needs of the LT vessel.
The alternating current electrical system uses a 3-phase system. The 440 Vac (volts alternating current) system has two
power sources. Power may be derived from the diesel generator or from an outside power source via the shore power
receptacle and the shore power cable. The 220 and 110 Vac systems receive power through step-down transformers.
The direct current electrical system is a two-wire isolated ground system. A 24 Vdc (volts direct current) supplies many of
the lighting, communication, and accessory systems for the LT.
The main components of the vessel's electrical system consist of the ship's service main switchboard, the emergency switch-
board, load centers, power panels, motor controllers, and related wiring. The system distributes 440 Vac, 220 Vac, 120 Vac,
and emergency power throughout the vessel. Power (440 Vac, 3-phase, 60 Hz) is received from the power generation system
and routed via the main switchboard and emergency switchboard to the selected equipment and panels.
ALTERNATING CURRENT ELECTRICAL SYSTEM
The alternating current electrical system consists of the ship's service main switchboard, the emergency switchboard, trans-
formers, distribution panels, motor controllers, and related wiring.
MAIN SWITCHBOARD
The main switchboard is located in the EOS and provides the generator selection, shore power selection, and power distri-
bution for the ship's service 440 Vac. Refer to figure 1 for a block diagram of the main switchboard. The main switchboard
consists of transformers, as well as ammeters, voltmeters, kilowatt meters, frequency meters, a phase rotation meter, and a
synchronization meter that provides power monitoring.
Controls are provided for the automatic generator voltage regulation and the generator engine speed regulation. The switch-
board motorized bus tie breaker also serves as an emergency 440 Vac power feedback source for the main switchboard from
the emergency switchboard to the power selected equipment systems during emergency power conditions. An interlock
system is incorporated into the switchboard to prevent damage to the switchboard by not allowing power from two sources
to be supplied at the same time (see table 1).
0034 00-1
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