There are three general types of single-phase
distribution transformers. The conventional type (fig.
2-6) requires a lightning arrester and fused cutout on the
primary phase conductor feeding it. The self-protected
(SP) type has a built-in lightning protector. The
completely self-protected (CSP) type has the lightning
arrester and current-overload devices connected to the
transformer. It requires no separate protective devices.
Secondary Mains
Secondary mains or circuits are the lines that carry
the electric power from the secondary side of the
transformer through a distribution system to supply the
electrical loads. They may or may not be on the same
pole with the feeder lines. If on the same pole, they may
be either on a crossarm below the feeder lines or, as
shown in figure 2-6, on spool racks attached to the side
of the pole below the feeder lines. The secondary circuits
may have several wires (service drops) connected to
various buildings to serve their electrical needs. Where
a large load is in demand, a transformer or transformer
bank may be located at the building site.
SINGLE PHASE.— Single-phase secondary
circuits usually supply current for electrical lighting
loads, small electric appliances, and small (1 horse-
power and under) single-phase electric motors. The
secondaries consist of two hot conductors and one
neutral conductor. In overhead construction, these
conductors are mounted on the bottom crossarm on a
pole or on spools attached to the side of a pole. (See fig.
2-6.) One transformer will feed this circuit if the
required load to be served is not too heavy. Where the
load is heavy or where several buildings are served, a
bank of three transformers may feed the circuit.
The normal voltage of a single-phase circuit is 120
volts from either one of the energized conductors to the
neutral or 240 volts across the two energized
conductors.
THREE PHASE.— Some facilities, such as motor
pools, industrial shops, and water and sewage plants,
may have equipment using three-phase motors, which
require three-phase power. Transformer banks are
installed to supply this power. If a number of buildings
in the area require three-phase power, cluster mount
may be installed with the three-phase secondaries
extending in two or three directions and with service
drops extending from the secondary to the buildings.
Service Drops
As you learned in the EA3 TRAMAN, each
building requiring electric current must have lead-in
conductors, known as service drops. These may be
two-, three-, or four-wire conductors or a single cable
containing the required number of conductors. A service
drop may be connected to a secondary main to provide
service to a small load. Where a transformer bank
services a building requiring a large power load, the
secondary becomes the service drop, since it feeds
current to one load only.
Most Navy buildings are not metered. However,
where it is desired to know how much electricity is being
consumed, a meter is installed ahead of the main switch
to the building. In this case, the service drop is connected
to the meter before it is connected to the mains.
CONTROL AND PROTECTIVE DEVICES
A power-distribution circuit, like any other
electrical circuit, requires the use of special devices to
provide control and to protect the system from internal
or external influences that may damage the circuit or
injure personnel.
Distribution Cutouts, Switches,
Reclosers, and Circuit Breakers
A distribution cutout is used to protect the
distribution system or the equipment connected to it.
Distribution cutouts are used with the installation of
transformers (fig. 2-6), capacitors, cable circuits, and at
sectionalizing points on overhead circuits.
Two types of switches used in power distribution
are the air switch and the oil switch. Both devices are
used to connect or disconnect a portion of the power
distribution system. The air switch is used for the
overhead section of the distribution system, and the oil
switch is used with underground portions.
Reclosers are for overload protection and are
designed to open a circuit in an overload condition and
then automatically reclose the circuit. Reclosers come
in single-or three-phase models and can either be pole
mounted or installed in a substation.
Oil, air, gas, and vacuum circuit breakers are used
to switch electric circuits and equipment in and out of
the system. They may be operated manually, by remote
control, or automatically under predetermined
conditions or when electrical failures in the system
occur.
Lightning Arresters
The purpose of installing a lightning arrester (fig.
2-6) on primary lines is twofold: first, to provide a point
in the circuit at which a lightning impulse can pass to
earth, through a ground wire, without injuring line
insulators, transformers, or other connected equipment;
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