also has a manual switch for selecting the
capable of transferring the load between two
normal or emergency power sources and a test
sources of power available to the ABT. The two
sources are identified as normal (ship's service)
For purposes of explanation, the 3-phase
and alternate (ship's service) or normal (ship's
model will be discussed. The ABT-1A2 is
service) and emer-gency (service).
designed to transfer automatically from normal
Three-way transfers indicate that the
to emergency supply upon a decrease in voltage
ABT is capable of transferring the load between
to within the 81- to 69-volt range across any two
three sources of power available to the ABT.
of its three phases in a 120-volt system. Upon
These three sources of power are identified as
restoration of the voltage to the range of 98 to
normal, alternate, and emergency. Either the
109 volts, the unit is adjusted to retransfer to
normal or alternate source may be selected as
the normal source of supply. An intentional time
the preferred source.
delay of 0.3 to 0.5 seconds is included in the
One of the more common ABTs used with IC
circuitry for both transfer and retransfer. This
switchboards is the ABT-1A2. This model operates on
allows for surges in line voltage and short
120-volt, 60-Hz, single- or 3-phase systems.
duration losses of power.
The ABT-1A2 (fig. 3-6) has a control
Automatic operation (refer to fig. 3-7) is
disconnect switch that allows the ABT to be
accomplished when the normal supply voltage drops
operated in the manual or automatic mode. It
Figure 3-7.--Schematic and wiring diagram of ABT-1A2.