connection) to ensure that three-phase, ac motors will
three-phase circuits. For explanation purposes, the
always run in the correct direction.
three-phase unit will be discussed. As you read this
section, refer to figures 3-4 and 3-5.
Phase identification is shown by the letters A, B, and
C in a three-phase system. Switchboard and distribution
The A-2 ABT is designed to shift automatically
panel bus bars and terminals on the back of switchboards
from normal to the alternate or emergency source of
are marked to identify the phase with the appropriate
power when the source voltage drops to the dropout
letters, A, B, or C. The standard arrangement of phases
range (81 -69 volts) across any two of the three phases.
in power and lighting switchboards, distribution panels,
Upon restoration of normal power (98-109 volts), the
feeder distribution boxes, feeder junction boxes, and
unit will transfer back to the normal power supply. An
feeder connection boxes is in the order A, B, and C from
intentional time delay of 0.3 to 0.5 seconds in both the
top to bottom, front to back, or right to left when facing
transfer and retransfer operations is built in to prevent
the front of the switchboard, panel, or box, and left to
right when facing the rear of the switchboard, panel, or
unnecessary transfer of power during line voltage surges
box.
and very short duration losses of power.
BUS TRANSFER SWITCHES
Operation
Bus transfer equipment is used to provide two
sources of power to equipment that is vital to the ship.
Table 3-1 lists the sequence of events in transferring
(NOTE: Vital equipment is that equipment needed to
from the normal to the alternate source of power through
operate safely or that could cause the ship to become
the A-2 ABT switch:
disabled if it became de-energized.) Depending upon
the application, the transfer from one source to another
may be done manually, by a manual bus transfer switch,
or automatically by an automatic bus transfer switch.
Manual Bus Transfer (MBT)
Switches
When normal power to vital equipment is lost,
power must be restored as soon as possible to ensure the
safety of the ship. MBTs may be used to switch from
normal to alternate or emergency power for those loads
that draw a large starting current or that must meed some
condition before energizing.
By having a manual transfer of the power source,
the electrician on watch can make sure that all
conditions are met before energizing the equipment
after a loss of power.
Automatic Bus Transfer (ABT)
Switches
ABTs are used to provide two sources of power to
those loads that MUST be re-energized as soon as
possible. Examples of loads that must be re-energized
include lighting in main engineering spaces, the ship's
steering motors and controls, motor driven fuel pumps
and lubricating oil pumps in the engineering spaces.
The Model A-2 ABT switch operates on 120-volt,
60Hz circuits. It is usually used to handle small
Figure 3-4.--A pictorial view of the A-2 ABT.
lighting circuits. It may be used on single- or
3-4
