2. Open the normal power CB.
The ABT is designed to transfer automatically
3. Shift the interlock to prevent closure of the
from normal to alternate supply. It does this upon
normal power breaker and enable closure
a decrease in voltage across any two of its three
of the alternate power breaker.
phases. Upon restoration of the voltage, the unit is
4. Close the alternate power CB.
adjusted to retransfer to the normal source of
supply.
NOTE: If you are shifting MBTs while power
The ABT unit shown in figure 9-17 is equipped
is available on both sources, you should obtain
for manual operation. To manually operate the
permission from the EOOW before starting the
ABT, place the control disconnect switch in the
procedure. Failure to do so could cause loss of
manual position and operate the manual switch.
equipment that is on the line.
When in AUTO, test the automatic operation of
the ABT by turning the spring-loaded test switch
Transformers
to the test position.
A transformer is a device that has no moving
You must be careful when testing the ABT
parts. It transfers energy from one circuit to
units to ensure they do not include in their load
another by electromagnetic induction. The energy
vital and sensitive electronic circuitry that will be
is always transferred without a change in
adversely affected by the loss and almost instant
frequency. Usually there are changes in voltage
return of power. You must ensure all other groups
and current. A step-up transformer receives
are adequately informed of power supply system
electrical energy at one voltage and delivers it at
tests to be performed.
a higher voltage. Conversely, a step-down
transformer receives energy at one voltage and
Manual Bus Transfers
delivers it at a lower voltage. Transformers require
The MBTs are devices located within power
little care and maintenance because of their
panels that enable manual shifting from normal
simple, rugged, and durable construction. The
to alternate power. The MBTs are used on loads
efficiency of transformers is high. Because of
that are vital but do not require automatic
this, transformers are responsible for the more
recovery upon loss of power. An example of a
extensive use of ac than dc. The conventional
circuit with an MBT would be engine-room
constant-potential transformer operates with the
ventilation power. While it is desirable to have
primary connected across a constant-potential
ventilation all the time, its loss for a short time
source. It provides a secondary voltage that is
would not adversely affect plant operation. When
substantially constant from no load to full load.
time permits, an operator can switch the MBT and
Various types of small single-phase trans-
restart the vent fans.
formers are used in electrical equipment. In many
Most MBTs are built into the power panels
installations, transformers are used on SWBDs
they serve. They are constructed of two CBs with
to step down the voltage for indicating lights.
some type of mechanical safety interlock.
Low-voltage transformers are included in some
Indicator lights are also provided to show the
motor control panels to supply control circuits or
sources of power available. The CBs are type
to operate overload relays.
NQB or CBs without trip elements.
Instrument transformers include potential, or
The mechanical interlock is usually a movable
voltage, transformers and current transformers
bar that prevents closing both CBs at the same
(CTs). Instrument transformers are commonly
time. NEVER TAMPER WITH OR BYPASS
used with ac instruments when high voltages or
THE INTERLOCK ON AN MBT. If both source
large currents are to be measured.
breakers are closed at the same time, severe
The power-supply transformer used in
damage could result. If you are operating the
electronic circuits is a single-phase, constant-
electric plant in split-plant mode, you could end
potential transformer. It has one or more
up paralleling the ship's load across cables and
secondary windings, or a single secondary with
breakers rated much lower than the bus tie
several tap connections. These transformers have
capacity.
a low volt-ampere capacity. They are less efficient
The following is the correct procedure for you
than large constant-potential power transformers.
to use when shifting an MBT from normal to
The typical transformer: has two windings
alternate power.
insulated electrically from each other. These
windings are wound around a common magnetic
1. Be sure the alternate power indicator lamp
core made of laminated sheet steel. The principal
is on, showing that power is available.
9-18
