Figure 4-2.--Block diagram of lighting distribution system.
circuit breaker will close allowing the
emergency/alternate switchboard loads to be
Three small single-phase transformers are used
instead of one large three-phase transformer because the
loss of a composite unit would result in a loss of power.
AUTOMATIC BUS TRANSFER
banks of the three single-phase transformers occurs, the
ABTs are used to keep vital lighting loads energized
remaining two will still carry about 58 percent of the
by shifting to the alternate power source when the
initial load capacity. The remaining two transformers
will be connected open delta by disconnecting the
normal power, the ABT will automatically shift back to
defective transformer. In an open delta connection, the
the normal source. Their operation is described in
line current must be reduced so that it will not exceed
greater detail in chapter 3.
the rated current of the individual transformers. Each